diff --git a/src/applications/differential/controller/DifferentialChangesetViewController.php b/src/applications/differential/controller/DifferentialChangesetViewController.php
index 1dac4c137d..576e0acb2b 100644
--- a/src/applications/differential/controller/DifferentialChangesetViewController.php
+++ b/src/applications/differential/controller/DifferentialChangesetViewController.php
@@ -1,480 +1,459 @@
 <?php
 
 final class DifferentialChangesetViewController extends DifferentialController {
 
   public function shouldAllowPublic() {
     return true;
   }
 
   public function handleRequest(AphrontRequest $request) {
     $viewer = $this->getViewer();
 
     $rendering_reference = $request->getStr('ref');
     $parts = explode('/', $rendering_reference);
     if (count($parts) == 2) {
       list($id, $vs) = $parts;
     } else {
       $id = $parts[0];
       $vs = 0;
     }
 
     $id = (int)$id;
     $vs = (int)$vs;
 
     $load_ids = array($id);
     if ($vs && ($vs != -1)) {
       $load_ids[] = $vs;
     }
 
     $changesets = id(new DifferentialChangesetQuery())
       ->setViewer($viewer)
       ->withIDs($load_ids)
       ->needHunks(true)
       ->execute();
     $changesets = mpull($changesets, null, 'getID');
 
     $changeset = idx($changesets, $id);
     if (!$changeset) {
       return new Aphront404Response();
     }
 
     $vs_changeset = null;
     if ($vs && ($vs != -1)) {
       $vs_changeset = idx($changesets, $vs);
       if (!$vs_changeset) {
         return new Aphront404Response();
       }
     }
 
     $view = $request->getStr('view');
     if ($view) {
       $phid = idx($changeset->getMetadata(), "$view:binary-phid");
       if ($phid) {
         return id(new AphrontRedirectResponse())->setURI("/file/info/$phid/");
       }
       switch ($view) {
         case 'new':
           return $this->buildRawFileResponse($changeset, $is_new = true);
         case 'old':
           if ($vs_changeset) {
             return $this->buildRawFileResponse($vs_changeset, $is_new = true);
           }
           return $this->buildRawFileResponse($changeset, $is_new = false);
         default:
           return new Aphront400Response();
       }
     }
 
     $old = array();
     $new = array();
     if (!$vs) {
       $right = $changeset;
       $left  = null;
 
       $right_source = $right->getID();
       $right_new = true;
       $left_source = $right->getID();
       $left_new = false;
 
       $render_cache_key = $right->getID();
 
       $old[] = $changeset;
       $new[] = $changeset;
     } else if ($vs == -1) {
       $right = null;
       $left = $changeset;
 
       $right_source = $left->getID();
       $right_new = false;
       $left_source = $left->getID();
       $left_new = true;
 
       $render_cache_key = null;
 
       $old[] = $changeset;
       $new[] = $changeset;
     } else {
       $right = $changeset;
       $left = $vs_changeset;
 
       $right_source = $right->getID();
       $right_new = true;
       $left_source = $left->getID();
       $left_new = true;
 
       $render_cache_key = null;
 
       $new[] = $left;
       $new[] = $right;
     }
 
     if ($left) {
-      $left_data = $left->makeNewFile();
-      $left_properties = $left->getNewProperties();
-      if ($right) {
-        $right_data = $right->makeNewFile();
-        $right_properties = $right->getNewProperties();
-      } else {
-        $right_data = $left->makeOldFile();
-        $right_properties = $left->getOldProperties();
-      }
-
-      $engine = new PhabricatorDifferenceEngine();
-      $synthetic = $engine->generateChangesetFromFileContent(
-        $left_data,
-        $right_data);
-
-      $choice = clone nonempty($left, $right);
-      $choice->attachHunks($synthetic->getHunks());
-
-      $choice->setOldProperties($left_properties);
-      $choice->setNewProperties($right_properties);
-
-      $changeset = $choice;
+      $changeset = $left->newComparisonChangeset($right);
     }
 
     if ($left_new || $right_new) {
       $diff_map = array();
       if ($left) {
         $diff_map[] = $left->getDiff();
       }
       if ($right) {
         $diff_map[] = $right->getDiff();
       }
       $diff_map = mpull($diff_map, null, 'getPHID');
 
       $buildables = id(new HarbormasterBuildableQuery())
         ->setViewer($viewer)
         ->withBuildablePHIDs(array_keys($diff_map))
         ->withManualBuildables(false)
         ->needBuilds(true)
         ->needTargets(true)
         ->execute();
       $buildables = mpull($buildables, null, 'getBuildablePHID');
       foreach ($diff_map as $diff_phid => $changeset_diff) {
         $changeset_diff->attachBuildable(idx($buildables, $diff_phid));
       }
     }
 
     $coverage = null;
     if ($right_new) {
       $coverage = $this->loadCoverage($right);
     }
 
     $spec = $request->getStr('range');
     list($range_s, $range_e, $mask) =
       DifferentialChangesetParser::parseRangeSpecification($spec);
 
     $diff = $changeset->getDiff();
     $revision_id = $diff->getRevisionID();
 
     $can_mark = false;
     $object_owner_phid = null;
     $revision = null;
     if ($revision_id) {
       $revision = id(new DifferentialRevisionQuery())
         ->setViewer($viewer)
         ->withIDs(array($revision_id))
         ->executeOne();
       if ($revision) {
         $can_mark = ($revision->getAuthorPHID() == $viewer->getPHID());
         $object_owner_phid = $revision->getAuthorPHID();
       }
     }
 
     if ($revision) {
       $container_phid = $revision->getPHID();
     } else {
       $container_phid = $diff->getPHID();
     }
 
     $viewstate_engine = id(new PhabricatorChangesetViewStateEngine())
       ->setViewer($viewer)
       ->setObjectPHID($container_phid)
       ->setChangeset($changeset);
 
     $viewstate = $viewstate_engine->newViewStateFromRequest($request);
 
     if ($viewstate->getDiscardResponse()) {
       return new AphrontAjaxResponse();
     }
 
     $parser = id(new DifferentialChangesetParser())
       ->setViewer($viewer)
       ->setViewState($viewstate)
       ->setCoverage($coverage)
       ->setChangeset($changeset)
       ->setRenderingReference($rendering_reference)
       ->setRenderCacheKey($render_cache_key)
       ->setRightSideCommentMapping($right_source, $right_new)
       ->setLeftSideCommentMapping($left_source, $left_new);
 
     if ($left && $right) {
       $parser->setOriginals($left, $right);
     }
 
     // Load both left-side and right-side inline comments.
     if ($revision) {
       $query = id(new DifferentialInlineCommentQuery())
         ->setViewer($viewer)
         ->needHidden(true)
         ->withRevisionPHIDs(array($revision->getPHID()));
       $inlines = $query->execute();
       $inlines = $query->adjustInlinesForChangesets(
         $inlines,
         $old,
         $new,
         $revision);
     } else {
       $inlines = array();
     }
 
     if ($left_new) {
       $inlines = array_merge(
         $inlines,
         $this->buildLintInlineComments($left));
     }
 
     if ($right_new) {
       $inlines = array_merge(
         $inlines,
         $this->buildLintInlineComments($right));
     }
 
     $phids = array();
     foreach ($inlines as $inline) {
       $parser->parseInlineComment($inline);
       if ($inline->getAuthorPHID()) {
         $phids[$inline->getAuthorPHID()] = true;
       }
     }
     $phids = array_keys($phids);
 
     $handles = $this->loadViewerHandles($phids);
     $parser->setHandles($handles);
 
     $engine = new PhabricatorMarkupEngine();
     $engine->setViewer($viewer);
 
     foreach ($inlines as $inline) {
       $engine->addObject(
         $inline,
         PhabricatorInlineCommentInterface::MARKUP_FIELD_BODY);
     }
 
     $engine->process();
 
     $parser
       ->setViewer($viewer)
       ->setMarkupEngine($engine)
       ->setShowEditAndReplyLinks(true)
       ->setCanMarkDone($can_mark)
       ->setObjectOwnerPHID($object_owner_phid)
       ->setRange($range_s, $range_e)
       ->setMask($mask);
 
     if ($request->isAjax()) {
       // NOTE: We must render the changeset before we render coverage
       // information, since it builds some caches.
       $response = $parser->newChangesetResponse();
 
       $mcov = $parser->renderModifiedCoverage();
 
       $coverage_data = array(
         'differential-mcoverage-'.md5($changeset->getFilename()) => $mcov,
       );
 
       $response->setCoverage($coverage_data);
 
       return $response;
     }
 
     $detail = id(new DifferentialChangesetListView())
       ->setUser($this->getViewer())
       ->setChangesets(array($changeset))
       ->setVisibleChangesets(array($changeset))
       ->setRenderingReferences(array($rendering_reference))
       ->setRenderURI('/differential/changeset/')
       ->setDiff($diff)
       ->setTitle(pht('Standalone View'))
       ->setBackground(PHUIObjectBoxView::BLUE_PROPERTY)
       ->setIsStandalone(true)
       ->setParser($parser);
 
     if ($revision_id) {
       $detail->setInlineCommentControllerURI(
         '/differential/comment/inline/edit/'.$revision_id.'/');
     }
 
     $crumbs = $this->buildApplicationCrumbs();
 
     if ($revision_id) {
       $crumbs->addTextCrumb('D'.$revision_id, '/D'.$revision_id);
     }
 
     $diff_id = $diff->getID();
     if ($diff_id) {
       $crumbs->addTextCrumb(
         pht('Diff %d', $diff_id),
         $this->getApplicationURI('diff/'.$diff_id));
     }
 
     $crumbs->addTextCrumb($changeset->getDisplayFilename());
     $crumbs->setBorder(true);
 
     $header = id(new PHUIHeaderView())
       ->setHeader(pht('Changeset View'))
       ->setHeaderIcon('fa-gear');
 
     $view = id(new PHUITwoColumnView())
       ->setHeader($header)
       ->setFooter($detail);
 
     return $this->newPage()
       ->setTitle(pht('Changeset View'))
       ->setCrumbs($crumbs)
       ->appendChild($view);
   }
 
   private function buildRawFileResponse(
     DifferentialChangeset $changeset,
     $is_new) {
 
     $viewer = $this->getViewer();
 
     if ($is_new) {
       $key = 'raw:new:phid';
     } else {
       $key = 'raw:old:phid';
     }
 
     $metadata = $changeset->getMetadata();
 
     $file = null;
     $phid = idx($metadata, $key);
     if ($phid) {
       $file = id(new PhabricatorFileQuery())
         ->setViewer($viewer)
         ->withPHIDs(array($phid))
         ->execute();
       if ($file) {
         $file = head($file);
       }
     }
 
     if (!$file) {
       // This is just building a cache of the changeset content in the file
       // tool, and is safe to run on a read pathway.
       $unguard = AphrontWriteGuard::beginScopedUnguardedWrites();
 
       if ($is_new) {
         $data = $changeset->makeNewFile();
       } else {
         $data = $changeset->makeOldFile();
       }
 
       $diff = $changeset->getDiff();
 
       $file = PhabricatorFile::newFromFileData(
         $data,
         array(
           'name' => $changeset->getFilename(),
           'mime-type' => 'text/plain',
           'ttl.relative' => phutil_units('24 hours in seconds'),
           'viewPolicy' => PhabricatorPolicies::POLICY_NOONE,
         ));
 
       $file->attachToObject($diff->getPHID());
 
       $metadata[$key] = $file->getPHID();
       $changeset->setMetadata($metadata);
       $changeset->save();
 
       unset($unguard);
     }
 
     return $file->getRedirectResponse();
   }
 
   private function buildLintInlineComments($changeset) {
     $diff = $changeset->getDiff();
 
     $target_phids = $diff->getBuildTargetPHIDs();
     if (!$target_phids) {
       return array();
     }
 
     $messages = id(new HarbormasterBuildLintMessage())->loadAllWhere(
       'buildTargetPHID IN (%Ls) AND path = %s',
       $target_phids,
       $changeset->getFilename());
 
     if (!$messages) {
       return array();
     }
 
     $change_type = $changeset->getChangeType();
     if (DifferentialChangeType::isDeleteChangeType($change_type)) {
       // If this is a lint message on a deleted file, show it on the left
       // side of the UI because there are no source code lines on the right
       // side of the UI so inlines don't have anywhere to render. See PHI416.
       $is_new = 0;
     } else {
       $is_new = 1;
     }
 
     $template = id(new DifferentialInlineComment())
       ->setChangesetID($changeset->getID())
       ->setIsNewFile($is_new)
       ->setLineLength(0);
 
     $inlines = array();
     foreach ($messages as $message) {
       $description = $message->getProperty('description');
 
       $inlines[] = id(clone $template)
         ->setSyntheticAuthor(pht('Lint: %s', $message->getName()))
         ->setLineNumber($message->getLine())
         ->setContent($description);
     }
 
     return $inlines;
   }
 
   private function loadCoverage(DifferentialChangeset $changeset) {
     $viewer = $this->getViewer();
 
     $target_phids = $changeset->getDiff()->getBuildTargetPHIDs();
     if (!$target_phids) {
       return null;
     }
 
     $unit = id(new HarbormasterBuildUnitMessageQuery())
       ->setViewer($viewer)
       ->withBuildTargetPHIDs($target_phids)
       ->execute();
     if (!$unit) {
       return null;
     }
 
     $coverage = array();
     foreach ($unit as $message) {
       $test_coverage = $message->getProperty('coverage');
       if ($test_coverage === null) {
         continue;
       }
       $coverage_data = idx($test_coverage, $changeset->getFileName());
       if (!strlen($coverage_data)) {
         continue;
       }
       $coverage[] = $coverage_data;
     }
 
     if (!$coverage) {
       return null;
     }
 
     return ArcanistUnitTestResult::mergeCoverage($coverage);
   }
 
 }
diff --git a/src/applications/differential/parser/DifferentialChangesetParser.php b/src/applications/differential/parser/DifferentialChangesetParser.php
index 63a6c606c5..686979bca0 100644
--- a/src/applications/differential/parser/DifferentialChangesetParser.php
+++ b/src/applications/differential/parser/DifferentialChangesetParser.php
@@ -1,1928 +1,1893 @@
 <?php
 
 final class DifferentialChangesetParser extends Phobject {
 
   const HIGHLIGHT_BYTE_LIMIT = 262144;
 
   protected $visible      = array();
   protected $new          = array();
   protected $old          = array();
   protected $intra        = array();
   protected $depthOnlyLines = array();
   protected $newRender    = null;
   protected $oldRender    = null;
 
   protected $filename     = null;
   protected $hunkStartLines = array();
 
   protected $comments     = array();
   protected $specialAttributes = array();
 
   protected $changeset;
 
   protected $renderCacheKey = null;
 
   private $handles = array();
   private $user;
 
   private $leftSideChangesetID;
   private $leftSideAttachesToNewFile;
 
   private $rightSideChangesetID;
   private $rightSideAttachesToNewFile;
 
   private $originalLeft;
   private $originalRight;
 
   private $renderingReference;
   private $isSubparser;
 
   private $isTopLevel;
 
   private $coverage;
   private $markupEngine;
   private $highlightErrors;
   private $disableCache;
   private $renderer;
   private $highlightingDisabled;
   private $showEditAndReplyLinks = true;
   private $canMarkDone;
   private $objectOwnerPHID;
   private $offsetMode;
 
   private $rangeStart;
   private $rangeEnd;
   private $mask;
   private $linesOfContext = 8;
 
   private $highlightEngine;
   private $viewer;
 
   private $viewState;
 
   public function setRange($start, $end) {
     $this->rangeStart = $start;
     $this->rangeEnd = $end;
     return $this;
   }
 
   public function setMask(array $mask) {
     $this->mask = $mask;
     return $this;
   }
 
   public function renderChangeset() {
     return $this->render($this->rangeStart, $this->rangeEnd, $this->mask);
   }
 
   public function setShowEditAndReplyLinks($bool) {
     $this->showEditAndReplyLinks = $bool;
     return $this;
   }
 
   public function getShowEditAndReplyLinks() {
     return $this->showEditAndReplyLinks;
   }
 
   public function setViewState(PhabricatorChangesetViewState $view_state) {
     $this->viewState = $view_state;
     return $this;
   }
 
   public function getViewState() {
     return $this->viewState;
   }
 
   public function setRenderer(DifferentialChangesetRenderer $renderer) {
     $this->renderer = $renderer;
     return $this;
   }
 
   public function getRenderer() {
     return $this->renderer;
   }
 
   public function setDisableCache($disable_cache) {
     $this->disableCache = $disable_cache;
     return $this;
   }
 
   public function getDisableCache() {
     return $this->disableCache;
   }
 
   public function setCanMarkDone($can_mark_done) {
     $this->canMarkDone = $can_mark_done;
     return $this;
   }
 
   public function getCanMarkDone() {
     return $this->canMarkDone;
   }
 
   public function setObjectOwnerPHID($phid) {
     $this->objectOwnerPHID = $phid;
     return $this;
   }
 
   public function getObjectOwnerPHID() {
     return $this->objectOwnerPHID;
   }
 
   public function setOffsetMode($offset_mode) {
     $this->offsetMode = $offset_mode;
     return $this;
   }
 
   public function getOffsetMode() {
     return $this->offsetMode;
   }
 
   public function setViewer(PhabricatorUser $viewer) {
     $this->viewer = $viewer;
     return $this;
   }
 
   public function getViewer() {
     return $this->viewer;
   }
 
   private function newRenderer() {
     $viewer = $this->getViewer();
     $viewstate = $this->getViewstate();
 
     $renderer_key = $viewstate->getRendererKey();
 
     if ($renderer_key === null) {
       $is_unified = $viewer->compareUserSetting(
         PhabricatorUnifiedDiffsSetting::SETTINGKEY,
         PhabricatorUnifiedDiffsSetting::VALUE_ALWAYS_UNIFIED);
 
       if ($is_unified) {
         $renderer_key = '1up';
       } else {
         $renderer_key = $viewstate->getDefaultDeviceRendererKey();
       }
     }
 
     switch ($renderer_key) {
       case '1up':
         $renderer = new DifferentialChangesetOneUpRenderer();
         break;
       default:
         $renderer = new DifferentialChangesetTwoUpRenderer();
         break;
     }
 
     return $renderer;
   }
 
   const CACHE_VERSION = 14;
   const CACHE_MAX_SIZE = 8e6;
 
   const ATTR_GENERATED  = 'attr:generated';
   const ATTR_DELETED    = 'attr:deleted';
   const ATTR_UNCHANGED  = 'attr:unchanged';
   const ATTR_MOVEAWAY   = 'attr:moveaway';
 
   public function setOldLines(array $lines) {
     $this->old = $lines;
     return $this;
   }
 
   public function setNewLines(array $lines) {
     $this->new = $lines;
     return $this;
   }
 
   public function setSpecialAttributes(array $attributes) {
     $this->specialAttributes = $attributes;
     return $this;
   }
 
   public function setIntraLineDiffs(array $diffs) {
     $this->intra = $diffs;
     return $this;
   }
 
   public function setDepthOnlyLines(array $lines) {
     $this->depthOnlyLines = $lines;
     return $this;
   }
 
   public function getDepthOnlyLines() {
     return $this->depthOnlyLines;
   }
 
   public function setVisibleLinesMask(array $mask) {
     $this->visible = $mask;
     return $this;
   }
 
   public function setLinesOfContext($lines_of_context) {
     $this->linesOfContext = $lines_of_context;
     return $this;
   }
 
   public function getLinesOfContext() {
     return $this->linesOfContext;
   }
 
 
   /**
    * Configure which Changeset comments added to the right side of the visible
    * diff will be attached to. The ID must be the ID of a real Differential
    * Changeset.
    *
    * The complexity here is that we may show an arbitrary side of an arbitrary
    * changeset as either the left or right part of a diff. This method allows
    * the left and right halves of the displayed diff to be correctly mapped to
    * storage changesets.
    *
    * @param id    The Differential Changeset ID that comments added to the right
    *              side of the visible diff should be attached to.
    * @param bool  If true, attach new comments to the right side of the storage
    *              changeset. Note that this may be false, if the left side of
    *              some storage changeset is being shown as the right side of
    *              a display diff.
    * @return this
    */
   public function setRightSideCommentMapping($id, $is_new) {
     $this->rightSideChangesetID = $id;
     $this->rightSideAttachesToNewFile = $is_new;
     return $this;
   }
 
   /**
    * See setRightSideCommentMapping(), but this sets information for the left
    * side of the display diff.
    */
   public function setLeftSideCommentMapping($id, $is_new) {
     $this->leftSideChangesetID = $id;
     $this->leftSideAttachesToNewFile = $is_new;
     return $this;
   }
 
   public function setOriginals(
     DifferentialChangeset $left,
     DifferentialChangeset $right) {
 
     $this->originalLeft = $left;
     $this->originalRight = $right;
     return $this;
   }
 
   public function diffOriginals() {
     $engine = new PhabricatorDifferenceEngine();
     $changeset = $engine->generateChangesetFromFileContent(
       implode('', mpull($this->originalLeft->getHunks(), 'getChanges')),
       implode('', mpull($this->originalRight->getHunks(), 'getChanges')));
 
     $parser = new DifferentialHunkParser();
 
     return $parser->parseHunksForHighlightMasks(
       $changeset->getHunks(),
       $this->originalLeft->getHunks(),
       $this->originalRight->getHunks());
   }
 
   /**
    * Set a key for identifying this changeset in the render cache. If set, the
    * parser will attempt to use the changeset render cache, which can improve
    * performance for frequently-viewed changesets.
    *
    * By default, there is no render cache key and parsers do not use the cache.
    * This is appropriate for rarely-viewed changesets.
    *
    * NOTE: Currently, this key must be a valid Differential Changeset ID.
    *
    * @param   string  Key for identifying this changeset in the render cache.
    * @return  this
    */
   public function setRenderCacheKey($key) {
     $this->renderCacheKey = $key;
     return $this;
   }
 
   private function getRenderCacheKey() {
     return $this->renderCacheKey;
   }
 
   public function setChangeset(DifferentialChangeset $changeset) {
     $this->changeset = $changeset;
 
     $this->setFilename($changeset->getFilename());
 
     return $this;
   }
 
   public function setRenderingReference($ref) {
     $this->renderingReference = $ref;
     return $this;
   }
 
   private function getRenderingReference() {
     return $this->renderingReference;
   }
 
   public function getChangeset() {
     return $this->changeset;
   }
 
   public function setFilename($filename) {
     $this->filename = $filename;
     return $this;
   }
 
   public function setHandles(array $handles) {
     assert_instances_of($handles, 'PhabricatorObjectHandle');
     $this->handles = $handles;
     return $this;
   }
 
   public function setMarkupEngine(PhabricatorMarkupEngine $engine) {
     $this->markupEngine = $engine;
     return $this;
   }
 
   public function setCoverage($coverage) {
     $this->coverage = $coverage;
     return $this;
   }
   private function getCoverage() {
     return $this->coverage;
   }
 
   public function parseInlineComment(
     PhabricatorInlineCommentInterface $comment) {
 
     // Parse only comments which are actually visible.
     if ($this->isCommentVisibleOnRenderedDiff($comment)) {
       $this->comments[] = $comment;
     }
     return $this;
   }
 
   private function loadCache() {
     $render_cache_key = $this->getRenderCacheKey();
     if (!$render_cache_key) {
       return false;
     }
 
     $data = null;
 
     $changeset = new DifferentialChangeset();
     $conn_r = $changeset->establishConnection('r');
     $data = queryfx_one(
       $conn_r,
       'SELECT * FROM %T WHERE id = %d',
       $changeset->getTableName().'_parse_cache',
       $render_cache_key);
 
     if (!$data) {
       return false;
     }
 
     if ($data['cache'][0] == '{') {
       // This is likely an old-style JSON cache which we will not be able to
       // deserialize.
       return false;
     }
 
     $data = unserialize($data['cache']);
     if (!is_array($data) || !$data) {
       return false;
     }
 
     foreach (self::getCacheableProperties() as $cache_key) {
       if (!array_key_exists($cache_key, $data)) {
         // If we're missing a cache key, assume we're looking at an old cache
         // and ignore it.
         return false;
       }
     }
 
     if ($data['cacheVersion'] !== self::CACHE_VERSION) {
       return false;
     }
 
     // Someone displays contents of a partially cached shielded file.
     if (!isset($data['newRender']) && (!$this->isTopLevel || $this->comments)) {
       return false;
     }
 
     unset($data['cacheVersion'], $data['cacheHost']);
     $cache_prop = array_select_keys($data, self::getCacheableProperties());
     foreach ($cache_prop as $cache_key => $v) {
       $this->$cache_key = $v;
     }
 
     return true;
   }
 
   protected static function getCacheableProperties() {
     return array(
       'visible',
       'new',
       'old',
       'intra',
       'depthOnlyLines',
       'newRender',
       'oldRender',
       'specialAttributes',
       'hunkStartLines',
       'cacheVersion',
       'cacheHost',
       'highlightingDisabled',
     );
   }
 
   public function saveCache() {
     if (PhabricatorEnv::isReadOnly()) {
       return false;
     }
 
     if ($this->highlightErrors) {
       return false;
     }
 
     $render_cache_key = $this->getRenderCacheKey();
     if (!$render_cache_key) {
       return false;
     }
 
     $cache = array();
     foreach (self::getCacheableProperties() as $cache_key) {
       switch ($cache_key) {
         case 'cacheVersion':
           $cache[$cache_key] = self::CACHE_VERSION;
           break;
         case 'cacheHost':
           $cache[$cache_key] = php_uname('n');
           break;
         default:
           $cache[$cache_key] = $this->$cache_key;
           break;
       }
     }
     $cache = serialize($cache);
 
     // We don't want to waste too much space by a single changeset.
     if (strlen($cache) > self::CACHE_MAX_SIZE) {
       return;
     }
 
     $changeset = new DifferentialChangeset();
     $conn_w = $changeset->establishConnection('w');
 
     $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
       try {
         queryfx(
           $conn_w,
           'INSERT INTO %T (id, cache, dateCreated) VALUES (%d, %B, %d)
             ON DUPLICATE KEY UPDATE cache = VALUES(cache)',
           DifferentialChangeset::TABLE_CACHE,
           $render_cache_key,
           $cache,
           time());
       } catch (AphrontQueryException $ex) {
         // Ignore these exceptions. A common cause is that the cache is
         // larger than 'max_allowed_packet', in which case we're better off
         // not writing it.
 
         // TODO: It would be nice to tailor this more narrowly.
       }
     unset($unguarded);
   }
 
   private function markGenerated($new_corpus_block = '') {
     $generated_guess = (strpos($new_corpus_block, '@'.'generated') !== false);
 
     if (!$generated_guess) {
       $generated_path_regexps = PhabricatorEnv::getEnvConfig(
         'differential.generated-paths');
       foreach ($generated_path_regexps as $regexp) {
         if (preg_match($regexp, $this->changeset->getFilename())) {
           $generated_guess = true;
           break;
         }
       }
     }
 
     $event = new PhabricatorEvent(
       PhabricatorEventType::TYPE_DIFFERENTIAL_WILLMARKGENERATED,
       array(
         'corpus' => $new_corpus_block,
         'is_generated' => $generated_guess,
       )
     );
     PhutilEventEngine::dispatchEvent($event);
 
     $generated = $event->getValue('is_generated');
 
     $attribute = $this->changeset->isGeneratedChangeset();
     if ($attribute) {
       $generated = true;
     }
 
     $this->specialAttributes[self::ATTR_GENERATED] = $generated;
   }
 
   public function isGenerated() {
     return idx($this->specialAttributes, self::ATTR_GENERATED, false);
   }
 
   public function isDeleted() {
     return idx($this->specialAttributes, self::ATTR_DELETED, false);
   }
 
   public function isUnchanged() {
     return idx($this->specialAttributes, self::ATTR_UNCHANGED, false);
   }
 
   public function isMoveAway() {
     return idx($this->specialAttributes, self::ATTR_MOVEAWAY, false);
   }
 
   private function applyIntraline(&$render, $intra, $corpus) {
 
     foreach ($render as $key => $text) {
       $result = $text;
 
       if (isset($intra[$key])) {
         $result = PhabricatorDifferenceEngine::applyIntralineDiff(
           $result,
           $intra[$key]);
       }
 
       $result = $this->adjustRenderedLineForDisplay($result);
 
       $render[$key] = $result;
     }
   }
 
   private function getHighlightFuture($corpus) {
     $language = $this->getViewState()->getHighlightLanguage();
 
     if (!$language) {
       $language = $this->highlightEngine->getLanguageFromFilename(
         $this->filename);
 
       if (($language != 'txt') &&
           (strlen($corpus) > self::HIGHLIGHT_BYTE_LIMIT)) {
         $this->highlightingDisabled = true;
         $language = 'txt';
       }
     }
 
     return $this->highlightEngine->getHighlightFuture(
       $language,
       $corpus);
   }
 
   protected function processHighlightedSource($data, $result) {
 
     $result_lines = phutil_split_lines($result);
     foreach ($data as $key => $info) {
       if (!$info) {
         unset($result_lines[$key]);
       }
     }
     return $result_lines;
   }
 
   private function tryCacheStuff() {
     $viewstate = $this->getViewState();
 
     $skip_cache = false;
 
     if ($this->disableCache) {
       $skip_cache = true;
     }
 
     $character_encoding = $viewstate->getCharacterEncoding();
     if ($character_encoding !== null) {
       $skip_cache = true;
     }
 
     $highlight_language = $viewstate->getHighlightLanguage();
     if ($highlight_language !== null) {
       $skip_cache = true;
     }
 
     $changeset = $this->changeset;
 
     if ($changeset->getFileType() != DifferentialChangeType::FILE_TEXT &&
         $changeset->getFileType() != DifferentialChangeType::FILE_SYMLINK) {
 
       $this->markGenerated();
 
     } else {
       if ($skip_cache || !$this->loadCache()) {
         $this->process();
         if (!$skip_cache) {
           $this->saveCache();
         }
       }
     }
   }
 
   private function process() {
     $changeset = $this->changeset;
 
     $hunk_parser = new DifferentialHunkParser();
     $hunk_parser->parseHunksForLineData($changeset->getHunks());
 
     $this->realignDiff($changeset, $hunk_parser);
 
     $hunk_parser->reparseHunksForSpecialAttributes();
 
     $unchanged = false;
     if (!$hunk_parser->getHasAnyChanges()) {
       $filetype = $this->changeset->getFileType();
       if ($filetype == DifferentialChangeType::FILE_TEXT ||
           $filetype == DifferentialChangeType::FILE_SYMLINK) {
         $unchanged = true;
       }
     }
 
     $moveaway = false;
     $changetype = $this->changeset->getChangeType();
     if ($changetype == DifferentialChangeType::TYPE_MOVE_AWAY) {
       $moveaway = true;
     }
 
     $this->setSpecialAttributes(array(
       self::ATTR_UNCHANGED  => $unchanged,
       self::ATTR_DELETED    => $hunk_parser->getIsDeleted(),
       self::ATTR_MOVEAWAY   => $moveaway,
     ));
 
     $lines_context = $this->getLinesOfContext();
 
     $hunk_parser->generateIntraLineDiffs();
     $hunk_parser->generateVisibleLinesMask($lines_context);
 
     $this->setOldLines($hunk_parser->getOldLines());
     $this->setNewLines($hunk_parser->getNewLines());
     $this->setIntraLineDiffs($hunk_parser->getIntraLineDiffs());
     $this->setDepthOnlyLines($hunk_parser->getDepthOnlyLines());
     $this->setVisibleLinesMask($hunk_parser->getVisibleLinesMask());
     $this->hunkStartLines = $hunk_parser->getHunkStartLines(
       $changeset->getHunks());
 
     $new_corpus = $hunk_parser->getNewCorpus();
     $new_corpus_block = implode('', $new_corpus);
     $this->markGenerated($new_corpus_block);
 
     if ($this->isTopLevel &&
         !$this->comments &&
           ($this->isGenerated() ||
            $this->isUnchanged() ||
            $this->isDeleted())) {
       return;
     }
 
     $old_corpus = $hunk_parser->getOldCorpus();
     $old_corpus_block = implode('', $old_corpus);
     $old_future = $this->getHighlightFuture($old_corpus_block);
     $new_future = $this->getHighlightFuture($new_corpus_block);
     $futures = array(
       'old' => $old_future,
       'new' => $new_future,
     );
     $corpus_blocks = array(
       'old' => $old_corpus_block,
       'new' => $new_corpus_block,
     );
 
     $this->highlightErrors = false;
     foreach (new FutureIterator($futures) as $key => $future) {
       try {
         try {
           $highlighted = $future->resolve();
         } catch (PhutilSyntaxHighlighterException $ex) {
           $this->highlightErrors = true;
           $highlighted = id(new PhutilDefaultSyntaxHighlighter())
             ->getHighlightFuture($corpus_blocks[$key])
             ->resolve();
         }
         switch ($key) {
         case 'old':
           $this->oldRender = $this->processHighlightedSource(
             $this->old,
             $highlighted);
           break;
         case 'new':
           $this->newRender = $this->processHighlightedSource(
             $this->new,
             $highlighted);
           break;
         }
       } catch (Exception $ex) {
         phlog($ex);
         throw $ex;
       }
     }
 
     $this->applyIntraline(
       $this->oldRender,
       ipull($this->intra, 0),
       $old_corpus);
     $this->applyIntraline(
       $this->newRender,
       ipull($this->intra, 1),
       $new_corpus);
   }
 
   private function shouldRenderPropertyChangeHeader($changeset) {
     if (!$this->isTopLevel) {
       // We render properties only at top level; otherwise we get multiple
       // copies of them when a user clicks "Show More".
       return false;
     }
 
     return true;
   }
 
   public function render(
     $range_start  = null,
     $range_len    = null,
     $mask_force   = array()) {
 
     $renderer = $this->getRenderer();
     if (!$renderer) {
       $renderer = $this->newRenderer();
       $this->setRenderer($renderer);
     }
 
     // "Top level" renders are initial requests for the whole file, versus
     // requests for a specific range generated by clicking "show more". We
     // generate property changes and "shield" UI elements only for toplevel
     // requests.
     $this->isTopLevel = (($range_start === null) && ($range_len === null));
     $this->highlightEngine = PhabricatorSyntaxHighlighter::newEngine();
 
     $viewstate = $this->getViewState();
 
     $encoding = null;
 
     $character_encoding = $viewstate->getCharacterEncoding();
     if ($character_encoding) {
       // We are forcing this changeset to be interpreted with a specific
       // character encoding, so force all the hunks into that encoding and
       // propagate it to the renderer.
       $encoding = $character_encoding;
       foreach ($this->changeset->getHunks() as $hunk) {
         $hunk->forceEncoding($character_encoding);
       }
     } else {
       // We're just using the default, so tell the renderer what that is
       // (by reading the encoding from the first hunk).
       foreach ($this->changeset->getHunks() as $hunk) {
         $encoding = $hunk->getDataEncoding();
         break;
       }
     }
 
     $this->tryCacheStuff();
 
     // If we're rendering in an offset mode, treat the range numbers as line
     // numbers instead of rendering offsets.
     $offset_mode = $this->getOffsetMode();
     if ($offset_mode) {
       if ($offset_mode == 'new') {
         $offset_map = $this->new;
       } else {
         $offset_map = $this->old;
       }
 
       // NOTE: Inline comments use zero-based lengths. For example, a comment
       // that starts and ends on line 123 has length 0. Rendering considers
       // this range to have length 1. Probably both should agree, but that
       // ship likely sailed long ago. Tweak things here to get the two systems
       // to agree. See PHI985, where this affected mail rendering of inline
       // comments left on the final line of a file.
 
       $range_end = $this->getOffset($offset_map, $range_start + $range_len);
       $range_start = $this->getOffset($offset_map, $range_start);
       $range_len = ($range_end - $range_start) + 1;
     }
 
     $render_pch = $this->shouldRenderPropertyChangeHeader($this->changeset);
 
     $rows = max(
       count($this->old),
       count($this->new));
 
     $renderer = $this->getRenderer()
       ->setUser($this->getViewer())
       ->setChangeset($this->changeset)
       ->setRenderPropertyChangeHeader($render_pch)
       ->setIsTopLevel($this->isTopLevel)
       ->setOldRender($this->oldRender)
       ->setNewRender($this->newRender)
       ->setHunkStartLines($this->hunkStartLines)
       ->setOldChangesetID($this->leftSideChangesetID)
       ->setNewChangesetID($this->rightSideChangesetID)
       ->setOldAttachesToNewFile($this->leftSideAttachesToNewFile)
       ->setNewAttachesToNewFile($this->rightSideAttachesToNewFile)
       ->setCodeCoverage($this->getCoverage())
       ->setRenderingReference($this->getRenderingReference())
       ->setMarkupEngine($this->markupEngine)
       ->setHandles($this->handles)
       ->setOldLines($this->old)
       ->setNewLines($this->new)
       ->setOriginalCharacterEncoding($encoding)
       ->setShowEditAndReplyLinks($this->getShowEditAndReplyLinks())
       ->setCanMarkDone($this->getCanMarkDone())
       ->setObjectOwnerPHID($this->getObjectOwnerPHID())
       ->setHighlightingDisabled($this->highlightingDisabled)
       ->setDepthOnlyLines($this->getDepthOnlyLines());
 
     list($engine, $old_ref, $new_ref) = $this->newDocumentEngine();
     if ($engine) {
       $engine_blocks = $engine->newEngineBlocks(
         $old_ref,
         $new_ref);
     } else {
       $engine_blocks = null;
     }
 
     $has_document_engine = ($engine_blocks !== null);
 
     // See T13515. Sometimes, we collapse file content by default: for
     // example, if the file is marked as containing generated code.
 
     // If a file has inline comments, that normally means we never collapse
     // it. However, if the viewer has already collapsed all of the inlines,
     // it's fine to collapse the file.
 
     $expanded_comments = array();
     foreach ($this->comments as $comment) {
       if ($comment->isHidden()) {
         continue;
       }
       $expanded_comments[] = $comment;
     }
 
     $collapsed_count = (count($this->comments) - count($expanded_comments));
 
     $shield_raw = null;
     $shield_text = null;
     $shield_type = null;
     if ($this->isTopLevel && !$expanded_comments && !$has_document_engine) {
       if ($this->isGenerated()) {
         $shield_text = pht(
           'This file contains generated code, which does not normally '.
           'need to be reviewed.');
       } else if ($this->isMoveAway()) {
         // We put an empty shield on these files. Normally, they do not have
         // any diff content anyway. However, if they come through `arc`, they
         // may have content. We don't want to show it (it's not useful) and
         // we bailed out of fully processing it earlier anyway.
 
         // We could show a message like "this file was moved", but we show
         // that as a change header anyway, so it would be redundant. Instead,
         // just render an empty shield to skip rendering the diff body.
         $shield_raw = '';
       } else if ($this->isUnchanged()) {
         $type = 'text';
         if (!$rows) {
           // NOTE: Normally, diffs which don't change files do not include
           // file content (for example, if you "chmod +x" a file and then
           // run "git show", the file content is not available). Similarly,
           // if you move a file from A to B without changing it, diffs normally
           // do not show the file content. In some cases `arc` is able to
           // synthetically generate content for these diffs, but for raw diffs
           // we'll never have it so we need to be prepared to not render a link.
           $type = 'none';
         }
 
         $shield_type = $type;
 
         $type_add = DifferentialChangeType::TYPE_ADD;
         if ($this->changeset->getChangeType() == $type_add) {
           // Although the generic message is sort of accurate in a technical
           // sense, this more-tailored message is less confusing.
           $shield_text = pht('This is an empty file.');
         } else {
           $shield_text = pht('The contents of this file were not changed.');
         }
       } else if ($this->isDeleted()) {
         $shield_text = pht('This file was completely deleted.');
       } else if ($this->changeset->getAffectedLineCount() > 2500) {
         $shield_text = pht(
           'This file has a very large number of changes (%s lines).',
           new PhutilNumber($this->changeset->getAffectedLineCount()));
       }
     }
 
     $shield = null;
     if ($shield_raw !== null) {
       $shield = $shield_raw;
     } else if ($shield_text !== null) {
       if ($shield_type === null) {
         $shield_type = 'default';
       }
 
       // If we have inlines and the shield would normally show the whole file,
       // downgrade it to show only text around the inlines.
       if ($collapsed_count) {
         if ($shield_type === 'text') {
           $shield_type = 'default';
         }
 
         $shield_text = array(
           $shield_text,
           ' ',
           pht(
             'This file has %d collapsed inline comment(s).',
             new PhutilNumber($collapsed_count)),
         );
       }
 
       $shield = $renderer->renderShield($shield_text, $shield_type);
     }
 
     if ($shield !== null) {
       return $renderer->renderChangesetTable($shield);
     }
 
     // This request should render the "undershield" headers if it's a top-level
     // request which made it this far (indicating the changeset has no shield)
     // or it's a request with no mask information (indicating it's the request
     // that removes the rendering shield). Possibly, this second class of
     // request might need to be made more explicit.
     $is_undershield = (empty($mask_force) || $this->isTopLevel);
     $renderer->setIsUndershield($is_undershield);
 
     $old_comments = array();
     $new_comments = array();
     $old_mask = array();
     $new_mask = array();
     $feedback_mask = array();
     $lines_context = $this->getLinesOfContext();
 
     if ($this->comments) {
       // If there are any comments which appear in sections of the file which
       // we don't have, we're going to move them backwards to the closest
       // earlier line. Two cases where this may happen are:
       //
       //   - Porting ghost comments forward into a file which was mostly
       //     deleted.
       //   - Porting ghost comments forward from a full-context diff to a
       //     partial-context diff.
 
       list($old_backmap, $new_backmap) = $this->buildLineBackmaps();
 
       foreach ($this->comments as $comment) {
         $new_side = $this->isCommentOnRightSideWhenDisplayed($comment);
 
         $line = $comment->getLineNumber();
 
         // See T13524. Lint inlines from Harbormaster may not have a line
         // number.
         if ($line === null) {
           $back_line = null;
         } else if ($new_side) {
           $back_line = idx($new_backmap, $line);
         } else {
           $back_line = idx($old_backmap, $line);
         }
 
         if ($back_line != $line) {
           // TODO: This should probably be cleaner, but just be simple and
           // obvious for now.
           $ghost = $comment->getIsGhost();
           if ($ghost) {
             $moved = pht(
               'This comment originally appeared on line %s, but that line '.
               'does not exist in this version of the diff. It has been '.
               'moved backward to the nearest line.',
               new PhutilNumber($line));
             $ghost['reason'] = $ghost['reason']."\n\n".$moved;
             $comment->setIsGhost($ghost);
           }
 
           $comment->setLineNumber($back_line);
           $comment->setLineLength(0);
         }
 
         $start = max($comment->getLineNumber() - $lines_context, 0);
         $end = $comment->getLineNumber() +
           $comment->getLineLength() +
           $lines_context;
         for ($ii = $start; $ii <= $end; $ii++) {
           if ($new_side) {
             $new_mask[$ii] = true;
           } else {
             $old_mask[$ii] = true;
           }
         }
       }
 
       foreach ($this->old as $ii => $old) {
         if (isset($old['line']) && isset($old_mask[$old['line']])) {
           $feedback_mask[$ii] = true;
         }
       }
 
       foreach ($this->new as $ii => $new) {
         if (isset($new['line']) && isset($new_mask[$new['line']])) {
           $feedback_mask[$ii] = true;
         }
       }
 
       $this->comments = id(new PHUIDiffInlineThreader())
         ->reorderAndThreadCommments($this->comments);
 
       foreach ($this->comments as $comment) {
         $final = $comment->getLineNumber() +
           $comment->getLineLength();
         $final = max(1, $final);
         if ($this->isCommentOnRightSideWhenDisplayed($comment)) {
           $new_comments[$final][] = $comment;
         } else {
           $old_comments[$final][] = $comment;
         }
       }
     }
     $renderer
       ->setOldComments($old_comments)
       ->setNewComments($new_comments);
 
     if ($engine_blocks !== null) {
       $reference = $this->getRenderingReference();
       $parts = explode('/', $reference);
       if (count($parts) == 2) {
         list($id, $vs) = $parts;
       } else {
         $id = $parts[0];
         $vs = 0;
       }
 
       // If we don't have an explicit "vs" changeset, it's the left side of
       // the "id" changeset.
       if (!$vs) {
         $vs = $id;
       }
 
       $renderer
         ->setDocumentEngine($engine)
         ->setDocumentEngineBlocks($engine_blocks);
 
       return $renderer->renderDocumentEngineBlocks(
         $engine_blocks,
         (string)$id,
         (string)$vs);
     }
 
     // If we've made it here with a type of file we don't know how to render,
     // bail out with a default empty rendering. Normally, we'd expect a
     // document engine to catch these changes before we make it this far.
     switch ($this->changeset->getFileType()) {
       case DifferentialChangeType::FILE_DIRECTORY:
       case DifferentialChangeType::FILE_BINARY:
       case DifferentialChangeType::FILE_IMAGE:
         $output = $renderer->renderChangesetTable(null);
         return $output;
     }
 
     if ($this->originalLeft && $this->originalRight) {
       list($highlight_old, $highlight_new) = $this->diffOriginals();
       $highlight_old = array_flip($highlight_old);
       $highlight_new = array_flip($highlight_new);
       $renderer
         ->setHighlightOld($highlight_old)
         ->setHighlightNew($highlight_new);
     }
     $renderer
       ->setOriginalOld($this->originalLeft)
       ->setOriginalNew($this->originalRight);
 
     if ($range_start === null) {
       $range_start = 0;
     }
     if ($range_len === null) {
       $range_len = $rows;
     }
     $range_len = min($range_len, $rows - $range_start);
 
     list($gaps, $mask) = $this->calculateGapsAndMask(
       $mask_force,
       $feedback_mask,
       $range_start,
       $range_len);
 
     $renderer
       ->setGaps($gaps)
       ->setMask($mask);
 
     $html = $renderer->renderTextChange(
       $range_start,
       $range_len,
       $rows);
 
     return $renderer->renderChangesetTable($html);
   }
 
   /**
    * This function calculates a lot of stuff we need to know to display
    * the diff:
    *
    * Gaps - compute gaps in the visible display diff, where we will render
    * "Show more context" spacers. If a gap is smaller than the context size,
    * we just display it. Otherwise, we record it into $gaps and will render a
    * "show more context" element instead of diff text below. A given $gap
    * is a tuple of $gap_line_number_start and $gap_length.
    *
    * Mask - compute the actual lines that need to be shown (because they
    * are near changes lines, near inline comments, or the request has
    * explicitly asked for them, i.e. resulting from the user clicking
    * "show more"). The $mask returned is a sparsely populated dictionary
    * of $visible_line_number => true.
    *
    * @return array($gaps, $mask)
    */
   private function calculateGapsAndMask(
     $mask_force,
     $feedback_mask,
     $range_start,
     $range_len) {
 
     $lines_context = $this->getLinesOfContext();
 
     $gaps = array();
     $gap_start = 0;
     $in_gap = false;
     $base_mask = $this->visible + $mask_force + $feedback_mask;
     $base_mask[$range_start + $range_len] = true;
     for ($ii = $range_start; $ii <= $range_start + $range_len; $ii++) {
       if (isset($base_mask[$ii])) {
         if ($in_gap) {
           $gap_length = $ii - $gap_start;
           if ($gap_length <= $lines_context) {
             for ($jj = $gap_start; $jj <= $gap_start + $gap_length; $jj++) {
               $base_mask[$jj] = true;
             }
           } else {
             $gaps[] = array($gap_start, $gap_length);
           }
           $in_gap = false;
         }
       } else {
         if (!$in_gap) {
           $gap_start = $ii;
           $in_gap = true;
         }
       }
     }
     $gaps = array_reverse($gaps);
     $mask = $base_mask;
 
     return array($gaps, $mask);
   }
 
   /**
    * Determine if an inline comment will appear on the rendered diff,
    * taking into consideration which halves of which changesets will actually
    * be shown.
    *
    * @param PhabricatorInlineCommentInterface Comment to test for visibility.
    * @return bool True if the comment is visible on the rendered diff.
    */
   private function isCommentVisibleOnRenderedDiff(
     PhabricatorInlineCommentInterface $comment) {
 
     $changeset_id = $comment->getChangesetID();
     $is_new = $comment->getIsNewFile();
 
     if ($changeset_id == $this->rightSideChangesetID &&
         $is_new == $this->rightSideAttachesToNewFile) {
         return true;
     }
 
     if ($changeset_id == $this->leftSideChangesetID &&
         $is_new == $this->leftSideAttachesToNewFile) {
         return true;
     }
 
     return false;
   }
 
 
   /**
    * Determine if a comment will appear on the right side of the display diff.
    * Note that the comment must appear somewhere on the rendered changeset, as
    * per isCommentVisibleOnRenderedDiff().
    *
    * @param PhabricatorInlineCommentInterface Comment to test for display
    *              location.
    * @return bool True for right, false for left.
    */
   private function isCommentOnRightSideWhenDisplayed(
     PhabricatorInlineCommentInterface $comment) {
 
     if (!$this->isCommentVisibleOnRenderedDiff($comment)) {
       throw new Exception(pht('Comment is not visible on changeset!'));
     }
 
     $changeset_id = $comment->getChangesetID();
     $is_new = $comment->getIsNewFile();
 
     if ($changeset_id == $this->rightSideChangesetID &&
         $is_new == $this->rightSideAttachesToNewFile) {
       return true;
     }
 
     return false;
   }
 
   /**
    * Parse the 'range' specification that this class and the client-side JS
    * emit to indicate that a user clicked "Show more..." on a diff. Generally,
    * use is something like this:
    *
    *   $spec = $request->getStr('range');
    *   $parsed = DifferentialChangesetParser::parseRangeSpecification($spec);
    *   list($start, $end, $mask) = $parsed;
    *   $parser->render($start, $end, $mask);
    *
    * @param string Range specification, indicating the range of the diff that
    *               should be rendered.
    * @return tuple List of <start, end, mask> suitable for passing to
    *               @{method:render}.
    */
   public static function parseRangeSpecification($spec) {
     $range_s = null;
     $range_e = null;
     $mask = array();
 
     if ($spec) {
       $match = null;
       if (preg_match('@^(\d+)-(\d+)(?:/(\d+)-(\d+))?$@', $spec, $match)) {
         $range_s = (int)$match[1];
         $range_e = (int)$match[2];
         if (count($match) > 3) {
           $start = (int)$match[3];
           $len = (int)$match[4];
           for ($ii = $start; $ii < $start + $len; $ii++) {
             $mask[$ii] = true;
           }
         }
       }
     }
 
     return array($range_s, $range_e, $mask);
   }
 
   /**
    * Render "modified coverage" information; test coverage on modified lines.
    * This synthesizes diff information with unit test information into a useful
    * indicator of how well tested a change is.
    */
   public function renderModifiedCoverage() {
     $na = phutil_tag('em', array(), '-');
 
     $coverage = $this->getCoverage();
     if (!$coverage) {
       return $na;
     }
 
     $covered = 0;
     $not_covered = 0;
 
     foreach ($this->new as $k => $new) {
       if (!$new['line']) {
         continue;
       }
 
       if (!$new['type']) {
         continue;
       }
 
       if (empty($coverage[$new['line'] - 1])) {
         continue;
       }
 
       switch ($coverage[$new['line'] - 1]) {
         case 'C':
           $covered++;
           break;
         case 'U':
           $not_covered++;
           break;
       }
     }
 
     if (!$covered && !$not_covered) {
       return $na;
     }
 
     return sprintf('%d%%', 100 * ($covered / ($covered + $not_covered)));
   }
 
   /**
    * Build maps from lines comments appear on to actual lines.
    */
   private function buildLineBackmaps() {
     $old_back = array();
     $new_back = array();
     foreach ($this->old as $ii => $old) {
       if ($old === null) {
         continue;
       }
       $old_back[$old['line']] = $old['line'];
     }
     foreach ($this->new as $ii => $new) {
       if ($new === null) {
         continue;
       }
       $new_back[$new['line']] = $new['line'];
     }
 
     $max_old_line = 0;
     $max_new_line = 0;
     foreach ($this->comments as $comment) {
       if ($this->isCommentOnRightSideWhenDisplayed($comment)) {
         $max_new_line = max($max_new_line, $comment->getLineNumber());
       } else {
         $max_old_line = max($max_old_line, $comment->getLineNumber());
       }
     }
 
     $cursor = 1;
     for ($ii = 1; $ii <= $max_old_line; $ii++) {
       if (empty($old_back[$ii])) {
         $old_back[$ii] = $cursor;
       } else {
         $cursor = $old_back[$ii];
       }
     }
 
     $cursor = 1;
     for ($ii = 1; $ii <= $max_new_line; $ii++) {
       if (empty($new_back[$ii])) {
         $new_back[$ii] = $cursor;
       } else {
         $cursor = $new_back[$ii];
       }
     }
 
     return array($old_back, $new_back);
   }
 
   private function getOffset(array $map, $line) {
     if (!$map) {
       return null;
     }
 
     $line = (int)$line;
     foreach ($map as $key => $spec) {
       if ($spec && isset($spec['line'])) {
         if ((int)$spec['line'] >= $line) {
           return $key;
         }
       }
     }
 
     return $key;
   }
 
   private function realignDiff(
     DifferentialChangeset $changeset,
     DifferentialHunkParser $hunk_parser) {
     // Normalizing and realigning the diff depends on rediffing the files, and
     // we currently need complete representations of both files to do anything
     // reasonable. If we only have parts of the files, skip realignment.
 
     // We have more than one hunk, so we're definitely missing part of the file.
     $hunks = $changeset->getHunks();
     if (count($hunks) !== 1) {
       return null;
     }
 
     // The first hunk doesn't start at the beginning of the file, so we're
     // missing some context.
     $first_hunk = head($hunks);
     if ($first_hunk->getOldOffset() != 1 || $first_hunk->getNewOffset() != 1) {
       return null;
     }
 
     $old_file = $changeset->makeOldFile();
     $new_file = $changeset->makeNewFile();
     if ($old_file === $new_file) {
       // If the old and new files are exactly identical, the synthetic
       // diff below will give us nonsense and whitespace modes are
       // irrelevant anyway. This occurs when you, e.g., copy a file onto
       // itself in Subversion (see T271).
       return null;
     }
 
 
     $engine = id(new PhabricatorDifferenceEngine())
       ->setNormalize(true);
 
     $normalized_changeset = $engine->generateChangesetFromFileContent(
       $old_file,
       $new_file);
 
     $type_parser = new DifferentialHunkParser();
     $type_parser->parseHunksForLineData($normalized_changeset->getHunks());
 
     $hunk_parser->setNormalized(true);
     $hunk_parser->setOldLineTypeMap($type_parser->getOldLineTypeMap());
     $hunk_parser->setNewLineTypeMap($type_parser->getNewLineTypeMap());
   }
 
   private function adjustRenderedLineForDisplay($line) {
     // IMPORTANT: We're using "str_replace()" against raw HTML here, which can
     // easily become unsafe. The input HTML has already had syntax highlighting
     // and intraline diff highlighting applied, so it's full of "<span />" tags.
 
     static $search;
     static $replace;
     if ($search === null) {
       $rules = $this->newSuspiciousCharacterRules();
 
       $map = array();
       foreach ($rules as $key => $spec) {
         $tag = phutil_tag(
           'span',
           array(
             'data-copy-text' => $key,
             'class' => $spec['class'],
             'title' => $spec['title'],
           ),
           $spec['replacement']);
         $map[$key] = phutil_string_cast($tag);
       }
 
       $search = array_keys($map);
       $replace = array_values($map);
     }
 
     $is_html = false;
     if ($line instanceof PhutilSafeHTML) {
       $is_html = true;
       $line = hsprintf('%s', $line);
     }
 
     $line = phutil_string_cast($line);
 
     // TODO: This should be flexible, eventually.
     $tab_width = 2;
 
     $line = self::replaceTabsWithSpaces($line, $tab_width);
     $line = str_replace($search, $replace, $line);
 
     if ($is_html) {
       $line = phutil_safe_html($line);
     }
 
     return $line;
   }
 
   private function newSuspiciousCharacterRules() {
     // The "title" attributes are cached in the database, so they're
     // intentionally not wrapped in "pht(...)".
 
     $rules = array(
       "\xE2\x80\x8B" => array(
         'title' => 'ZWS',
         'class' => 'suspicious-character',
         'replacement' => '!',
       ),
       "\xC2\xA0" => array(
         'title' => 'NBSP',
         'class' => 'suspicious-character',
         'replacement' => '!',
       ),
       "\x7F" => array(
         'title' => 'DEL (0x7F)',
         'class' => 'suspicious-character',
         'replacement' => "\xE2\x90\xA1",
       ),
     );
 
     // Unicode defines special pictures for the control characters in the
     // range between "0x00" and "0x1F".
 
     $control = array(
       'NULL',
       'SOH',
       'STX',
       'ETX',
       'EOT',
       'ENQ',
       'ACK',
       'BEL',
       'BS',
       null, // "\t" Tab
       null, // "\n" New Line
       'VT',
       'FF',
       null, // "\r" Carriage Return,
       'SO',
       'SI',
       'DLE',
       'DC1',
       'DC2',
       'DC3',
       'DC4',
       'NAK',
       'SYN',
       'ETB',
       'CAN',
       'EM',
       'SUB',
       'ESC',
       'FS',
       'GS',
       'RS',
       'US',
     );
 
     foreach ($control as $idx => $label) {
       if ($label === null) {
         continue;
       }
 
       $rules[chr($idx)] = array(
         'title' => sprintf('%s (0x%02X)', $label, $idx),
         'class' => 'suspicious-character',
         'replacement' => "\xE2\x90".chr(0x80 + $idx),
       );
     }
 
     return $rules;
   }
 
   public static function replaceTabsWithSpaces($line, $tab_width) {
     static $tags = array();
     if (empty($tags[$tab_width])) {
       for ($ii = 1; $ii <= $tab_width; $ii++) {
         $tag = phutil_tag(
           'span',
           array(
             'data-copy-text' => "\t",
           ),
           str_repeat(' ', $ii));
         $tag = phutil_string_cast($tag);
         $tags[$ii] = $tag;
       }
     }
 
     // Expand all prefix tabs until we encounter any non-tab character. This
     // is cheap and often immediately produces the correct result with no
     // further work (and, particularly, no need to handle any unicode cases).
 
     $len = strlen($line);
 
     $head = 0;
     for ($head = 0; $head < $len; $head++) {
       $char = $line[$head];
       if ($char !== "\t") {
         break;
       }
     }
 
     if ($head) {
       if (empty($tags[$tab_width * $head])) {
         $tags[$tab_width * $head] = str_repeat($tags[$tab_width], $head);
       }
       $prefix = $tags[$tab_width * $head];
       $line = substr($line, $head);
     } else {
       $prefix = '';
     }
 
     // If we have no remaining tabs elsewhere in the string after taking care
     // of all the prefix tabs, we're done.
     if (strpos($line, "\t") === false) {
       return $prefix.$line;
     }
 
     $len = strlen($line);
 
     // If the line is particularly long, don't try to do anything special with
     // it. Use a faster approximation of the correct tabstop expansion instead.
     // This usually still arrives at the right result.
     if ($len > 256) {
       return $prefix.str_replace("\t", $tags[$tab_width], $line);
     }
 
     $in_tag = false;
     $pos = 0;
 
     // See PHI1210. If the line only has single-byte characters, we don't need
     // to vectorize it and can avoid an expensive UTF8 call.
 
     $fast_path = preg_match('/^[\x01-\x7F]*\z/', $line);
     if ($fast_path) {
       $replace = array();
       for ($ii = 0; $ii < $len; $ii++) {
         $char = $line[$ii];
         if ($char === '>') {
           $in_tag = false;
           continue;
         }
 
         if ($in_tag) {
           continue;
         }
 
         if ($char === '<') {
           $in_tag = true;
           continue;
         }
 
         if ($char === "\t") {
           $count = $tab_width - ($pos % $tab_width);
           $pos += $count;
           $replace[$ii] = $tags[$count];
           continue;
         }
 
         $pos++;
       }
 
       if ($replace) {
         // Apply replacements starting at the end of the string so they
         // don't mess up the offsets for following replacements.
         $replace = array_reverse($replace, true);
 
         foreach ($replace as $replace_pos => $replacement) {
           $line = substr_replace($line, $replacement, $replace_pos, 1);
         }
       }
     } else {
       $line = phutil_utf8v_combined($line);
       foreach ($line as $key => $char) {
         if ($char === '>') {
           $in_tag = false;
           continue;
         }
 
         if ($in_tag) {
           continue;
         }
 
         if ($char === '<') {
           $in_tag = true;
           continue;
         }
 
         if ($char === "\t") {
           $count = $tab_width - ($pos % $tab_width);
           $pos += $count;
           $line[$key] = $tags[$count];
           continue;
         }
 
         $pos++;
       }
 
       $line = implode('', $line);
     }
 
     return $prefix.$line;
   }
 
   private function newDocumentEngine() {
     $changeset = $this->changeset;
     $viewer = $this->getViewer();
 
-    // TODO: This should probably be made non-optional in the future.
-    if (!$viewer) {
-      return null;
-    }
-
-    $old_file = null;
-    $new_file = null;
-
-    switch ($changeset->getFileType()) {
-      case DifferentialChangeType::FILE_IMAGE:
-      case DifferentialChangeType::FILE_BINARY:
-        list($old_file, $new_file) = $this->loadFileObjectsForChangeset();
-        break;
-    }
-
-    $type_delete = DifferentialChangeType::TYPE_DELETE;
-    $type_add = DifferentialChangeType::TYPE_ADD;
-    $change_type = $changeset->getChangeType();
+    list($old_file, $new_file) = $this->loadFileObjectsForChangeset();
 
-    $no_old = ($change_type == $type_add);
-    $no_new = ($change_type == $type_delete);
+    $no_old = !$changeset->hasOldState();
+    $no_new = !$changeset->hasNewState();
 
     if ($no_old) {
       $old_ref = null;
     } else {
       $old_ref = id(new PhabricatorDocumentRef())
         ->setName($changeset->getOldFile());
       if ($old_file) {
         $old_ref->setFile($old_file);
       } else {
         $old_data = $this->getRawDocumentEngineData($this->old);
         $old_ref->setData($old_data);
       }
     }
 
     if ($no_new) {
       $new_ref = null;
     } else {
       $new_ref = id(new PhabricatorDocumentRef())
         ->setName($changeset->getFilename());
       if ($new_file) {
         $new_ref->setFile($new_file);
       } else {
         $new_data = $this->getRawDocumentEngineData($this->new);
         $new_ref->setData($new_data);
       }
     }
 
-
     $old_engines = null;
     if ($old_ref) {
       $old_engines = PhabricatorDocumentEngine::getEnginesForRef(
         $viewer,
         $old_ref);
     }
 
     $new_engines = null;
     if ($new_ref) {
       $new_engines = PhabricatorDocumentEngine::getEnginesForRef(
         $viewer,
         $new_ref);
     }
 
     if ($new_engines !== null && $old_engines !== null) {
       $shared_engines = array_intersect_key($new_engines, $old_engines);
       $default_engine = head_key($new_engines);
     } else if ($new_engines !== null) {
       $shared_engines = $new_engines;
       $default_engine = head_key($shared_engines);
     } else if ($old_engines !== null) {
       $shared_engines = $old_engines;
       $default_engine = head_key($shared_engines);
     } else {
       return null;
     }
 
     foreach ($shared_engines as $key => $shared_engine) {
       if (!$shared_engine->canDiffDocuments($old_ref, $new_ref)) {
         unset($shared_engines[$key]);
       }
     }
 
     $viewstate = $this->getViewState();
 
     $engine_key = $viewstate->getDocumentEngineKey();
     if (strlen($engine_key)) {
       if (isset($shared_engines[$engine_key])) {
         $document_engine = $shared_engines[$engine_key];
       } else {
         $document_engine = null;
       }
     } else {
       // If we aren't rendering with a specific engine, only use a default
       // engine if the best engine for the new file is a shared engine which
       // can diff files. If we're less picky (for example, by accepting any
       // shared engine) we can end up with silly behavior (like ".json" files
       // rendering as Jupyter documents).
 
       if (isset($shared_engines[$default_engine])) {
         $document_engine = $shared_engines[$default_engine];
       } else {
         $document_engine = null;
       }
     }
 
     if ($document_engine) {
       return array(
         $document_engine,
         $old_ref,
         $new_ref);
     }
 
     return null;
   }
 
   private function loadFileObjectsForChangeset() {
     $changeset = $this->changeset;
     $viewer = $this->getViewer();
 
+    $old_phid = $changeset->getOldFileObjectPHID();
+    $new_phid = $changeset->getNewFileObjectPHID();
+
     $old_file = null;
     $new_file = null;
 
-    // TODO: Improve the architectural issue as discussed in D955
-    // https://secure.phabricator.com/D955
-    $reference = $this->getRenderingReference();
-    $parts = explode('/', $reference);
-    if (count($parts) == 2) {
-      list($id, $vs) = $parts;
-    } else {
-      $id = $parts[0];
-      $vs = 0;
-    }
-    $id = (int)$id;
-    $vs = (int)$vs;
-
-    if (!$vs) {
-      $metadata = $this->changeset->getMetadata();
-      $old_phid = idx($metadata, 'old:binary-phid');
-      $new_phid = idx($metadata, 'new:binary-phid');
-    } else {
-      $vs_changeset = id(new DifferentialChangeset())->load($vs);
-      $old_phid = null;
-      $new_phid = null;
-
-      // TODO: This is spooky, see D6851
-      if ($vs_changeset) {
-        $vs_metadata = $vs_changeset->getMetadata();
-        $old_phid = idx($vs_metadata, 'new:binary-phid');
-      }
-
-      $changeset = id(new DifferentialChangeset())->load($id);
-      if ($changeset) {
-        $metadata = $changeset->getMetadata();
-        $new_phid = idx($metadata, 'new:binary-phid');
-      }
-    }
-
     if ($old_phid || $new_phid) {
       $file_phids = array();
       if ($old_phid) {
         $file_phids[] = $old_phid;
       }
       if ($new_phid) {
         $file_phids[] = $new_phid;
       }
 
       $files = id(new PhabricatorFileQuery())
         ->setViewer($viewer)
         ->withPHIDs($file_phids)
         ->execute();
+      $files = mpull($files, null, 'getPHID');
+
+      if ($old_phid) {
+        $old_file = idx($files, $old_phid);
+        if (!$old_file) {
+          throw new Exception(
+            pht(
+              'Failed to load file data for changeset ("%s").',
+              $old_phid));
+        }
+        $changeset->attachOldFileObject($old_file);
+      }
 
-      foreach ($files as $file) {
-        if ($file->getPHID() == $old_phid) {
-          $old_file = $file;
-        } else if ($file->getPHID() == $new_phid) {
-          $new_file = $file;
+      if ($new_phid) {
+        $new_file = idx($files, $new_phid);
+        if (!$new_file) {
+          throw new Exception(
+            pht(
+              'Failed to load file data for changeset ("%s").',
+              $new_phid));
         }
+        $changeset->attachNewFileObject($new_file);
       }
     }
 
     return array($old_file, $new_file);
   }
 
   public function newChangesetResponse() {
     // NOTE: This has to happen first because it has side effects. Yuck.
     $rendered_changeset = $this->renderChangeset();
 
     $renderer = $this->getRenderer();
     $renderer_key = $renderer->getRendererKey();
 
     $viewstate = $this->getViewState();
 
     $undo_templates = $renderer->renderUndoTemplates();
     foreach ($undo_templates as $key => $undo_template) {
       $undo_templates[$key] = hsprintf('%s', $undo_template);
     }
 
     $state = array(
       'undoTemplates' => $undo_templates,
       'rendererKey' => $renderer_key,
       'highlight' => $viewstate->getHighlightLanguage(),
       'characterEncoding' => $viewstate->getCharacterEncoding(),
       'documentEngine' => $viewstate->getDocumentEngineKey(),
       'isHidden' => $viewstate->getHidden(),
     );
 
     return id(new PhabricatorChangesetResponse())
       ->setRenderedChangeset($rendered_changeset)
       ->setChangesetState($state);
   }
 
   private function getRawDocumentEngineData(array $lines) {
     $text = array();
 
     foreach ($lines as $line) {
       if ($line === null) {
         continue;
       }
 
       // If this is a "No newline at end of file." annotation, don't hand it
       // off to the DocumentEngine.
       if ($line['type'] === '\\') {
         continue;
       }
 
       $text[] = $line['text'];
     }
 
     return implode('', $text);
   }
 
 }
diff --git a/src/applications/differential/render/DifferentialChangesetRenderer.php b/src/applications/differential/render/DifferentialChangesetRenderer.php
index 6b1ed8fa27..a81a04cefb 100644
--- a/src/applications/differential/render/DifferentialChangesetRenderer.php
+++ b/src/applications/differential/render/DifferentialChangesetRenderer.php
@@ -1,757 +1,767 @@
 <?php
 
 abstract class DifferentialChangesetRenderer extends Phobject {
 
   private $user;
   private $changeset;
   private $renderingReference;
   private $renderPropertyChangeHeader;
   private $isTopLevel;
   private $isUndershield;
   private $hunkStartLines;
   private $oldLines;
   private $newLines;
   private $oldComments;
   private $newComments;
   private $oldChangesetID;
   private $newChangesetID;
   private $oldAttachesToNewFile;
   private $newAttachesToNewFile;
   private $highlightOld = array();
   private $highlightNew = array();
   private $codeCoverage;
   private $handles;
   private $markupEngine;
   private $oldRender;
   private $newRender;
   private $originalOld;
   private $originalNew;
   private $gaps;
   private $mask;
   private $originalCharacterEncoding;
   private $showEditAndReplyLinks;
   private $canMarkDone;
   private $objectOwnerPHID;
   private $highlightingDisabled;
   private $scopeEngine = false;
   private $depthOnlyLines;
 
   private $documentEngine;
   private $documentEngineBlocks;
 
   private $oldFile = false;
   private $newFile = false;
 
   abstract public function getRendererKey();
 
   public function setShowEditAndReplyLinks($bool) {
     $this->showEditAndReplyLinks = $bool;
     return $this;
   }
 
   public function getShowEditAndReplyLinks() {
     return $this->showEditAndReplyLinks;
   }
 
   public function setHighlightingDisabled($highlighting_disabled) {
     $this->highlightingDisabled = $highlighting_disabled;
     return $this;
   }
 
   public function getHighlightingDisabled() {
     return $this->highlightingDisabled;
   }
 
   public function setOriginalCharacterEncoding($original_character_encoding) {
     $this->originalCharacterEncoding = $original_character_encoding;
     return $this;
   }
 
   public function getOriginalCharacterEncoding() {
     return $this->originalCharacterEncoding;
   }
 
   public function setIsUndershield($is_undershield) {
     $this->isUndershield = $is_undershield;
     return $this;
   }
 
   public function getIsUndershield() {
     return $this->isUndershield;
   }
 
   public function setMask($mask) {
     $this->mask = $mask;
     return $this;
   }
   protected function getMask() {
     return $this->mask;
   }
 
   public function setGaps($gaps) {
     $this->gaps = $gaps;
     return $this;
   }
   protected function getGaps() {
     return $this->gaps;
   }
 
   public function setDepthOnlyLines(array $lines) {
     $this->depthOnlyLines = $lines;
     return $this;
   }
 
   public function getDepthOnlyLines() {
     return $this->depthOnlyLines;
   }
 
   public function attachOldFile(PhabricatorFile $old = null) {
     $this->oldFile = $old;
     return $this;
   }
 
   public function getOldFile() {
     if ($this->oldFile === false) {
       throw new PhabricatorDataNotAttachedException($this);
     }
     return $this->oldFile;
   }
 
   public function hasOldFile() {
     return (bool)$this->oldFile;
   }
 
   public function attachNewFile(PhabricatorFile $new = null) {
     $this->newFile = $new;
     return $this;
   }
 
   public function getNewFile() {
     if ($this->newFile === false) {
       throw new PhabricatorDataNotAttachedException($this);
     }
     return $this->newFile;
   }
 
   public function hasNewFile() {
     return (bool)$this->newFile;
   }
 
   public function setOriginalNew($original_new) {
     $this->originalNew = $original_new;
     return $this;
   }
   protected function getOriginalNew() {
     return $this->originalNew;
   }
 
   public function setOriginalOld($original_old) {
     $this->originalOld = $original_old;
     return $this;
   }
   protected function getOriginalOld() {
     return $this->originalOld;
   }
 
   public function setNewRender($new_render) {
     $this->newRender = $new_render;
     return $this;
   }
   protected function getNewRender() {
     return $this->newRender;
   }
 
   public function setOldRender($old_render) {
     $this->oldRender = $old_render;
     return $this;
   }
   protected function getOldRender() {
     return $this->oldRender;
   }
 
   public function setMarkupEngine(PhabricatorMarkupEngine $markup_engine) {
     $this->markupEngine = $markup_engine;
     return $this;
   }
   public function getMarkupEngine() {
     return $this->markupEngine;
   }
 
   public function setHandles(array $handles) {
     assert_instances_of($handles, 'PhabricatorObjectHandle');
     $this->handles = $handles;
     return $this;
   }
   protected function getHandles() {
     return $this->handles;
   }
 
   public function setCodeCoverage($code_coverage) {
     $this->codeCoverage = $code_coverage;
     return $this;
   }
   protected function getCodeCoverage() {
     return $this->codeCoverage;
   }
 
   public function setHighlightNew($highlight_new) {
     $this->highlightNew = $highlight_new;
     return $this;
   }
   protected function getHighlightNew() {
     return $this->highlightNew;
   }
 
   public function setHighlightOld($highlight_old) {
     $this->highlightOld = $highlight_old;
     return $this;
   }
   protected function getHighlightOld() {
     return $this->highlightOld;
   }
 
   public function setNewAttachesToNewFile($attaches) {
     $this->newAttachesToNewFile = $attaches;
     return $this;
   }
   protected function getNewAttachesToNewFile() {
     return $this->newAttachesToNewFile;
   }
 
   public function setOldAttachesToNewFile($attaches) {
     $this->oldAttachesToNewFile = $attaches;
     return $this;
   }
   protected function getOldAttachesToNewFile() {
     return $this->oldAttachesToNewFile;
   }
 
   public function setNewChangesetID($new_changeset_id) {
     $this->newChangesetID = $new_changeset_id;
     return $this;
   }
   protected function getNewChangesetID() {
     return $this->newChangesetID;
   }
 
   public function setOldChangesetID($old_changeset_id) {
     $this->oldChangesetID = $old_changeset_id;
     return $this;
   }
   protected function getOldChangesetID() {
     return $this->oldChangesetID;
   }
 
   public function setDocumentEngine(PhabricatorDocumentEngine $engine) {
     $this->documentEngine = $engine;
     return $this;
   }
 
   public function getDocumentEngine() {
     return $this->documentEngine;
   }
 
   public function setDocumentEngineBlocks(
     PhabricatorDocumentEngineBlocks $blocks) {
     $this->documentEngineBlocks = $blocks;
     return $this;
   }
 
   public function getDocumentEngineBlocks() {
     return $this->documentEngineBlocks;
   }
 
   public function setNewComments(array $new_comments) {
     foreach ($new_comments as $line_number => $comments) {
       assert_instances_of($comments, 'PhabricatorInlineCommentInterface');
     }
     $this->newComments = $new_comments;
     return $this;
   }
   protected function getNewComments() {
     return $this->newComments;
   }
 
   public function setOldComments(array $old_comments) {
     foreach ($old_comments as $line_number => $comments) {
       assert_instances_of($comments, 'PhabricatorInlineCommentInterface');
     }
     $this->oldComments = $old_comments;
     return $this;
   }
   protected function getOldComments() {
     return $this->oldComments;
   }
 
   public function setNewLines(array $new_lines) {
     $this->newLines = $new_lines;
     return $this;
   }
   protected function getNewLines() {
     return $this->newLines;
   }
 
   public function setOldLines(array $old_lines) {
     $this->oldLines = $old_lines;
     return $this;
   }
   protected function getOldLines() {
     return $this->oldLines;
   }
 
   public function setHunkStartLines(array $hunk_start_lines) {
     $this->hunkStartLines = $hunk_start_lines;
     return $this;
   }
 
   protected function getHunkStartLines() {
     return $this->hunkStartLines;
   }
 
   public function setUser(PhabricatorUser $user) {
     $this->user = $user;
     return $this;
   }
   protected function getUser() {
     return $this->user;
   }
 
   public function setChangeset(DifferentialChangeset $changeset) {
     $this->changeset = $changeset;
     return $this;
   }
   protected function getChangeset() {
     return $this->changeset;
   }
 
   public function setRenderingReference($rendering_reference) {
     $this->renderingReference = $rendering_reference;
     return $this;
   }
   protected function getRenderingReference() {
     return $this->renderingReference;
   }
 
   public function setRenderPropertyChangeHeader($should_render) {
     $this->renderPropertyChangeHeader = $should_render;
     return $this;
   }
 
   private function shouldRenderPropertyChangeHeader() {
     return $this->renderPropertyChangeHeader;
   }
 
   public function setIsTopLevel($is) {
     $this->isTopLevel = $is;
     return $this;
   }
 
   private function getIsTopLevel() {
     return $this->isTopLevel;
   }
 
   public function setCanMarkDone($can_mark_done) {
     $this->canMarkDone = $can_mark_done;
     return $this;
   }
 
   public function getCanMarkDone() {
     return $this->canMarkDone;
   }
 
   public function setObjectOwnerPHID($phid) {
     $this->objectOwnerPHID = $phid;
     return $this;
   }
 
   public function getObjectOwnerPHID() {
     return $this->objectOwnerPHID;
   }
 
   final public function renderChangesetTable($content) {
     $props = null;
     if ($this->shouldRenderPropertyChangeHeader()) {
       $props = $this->renderPropertyChangeHeader();
     }
 
     $notice = null;
     if ($this->getIsTopLevel()) {
       $force = (!$content && !$props);
 
       // If we have DocumentEngine messages about the blocks, assume they
       // explain why there's no content.
       $blocks = $this->getDocumentEngineBlocks();
       if ($blocks) {
         if ($blocks->getMessages()) {
           $force = false;
         }
       }
 
       $notice = $this->renderChangeTypeHeader($force);
     }
 
     $undershield = null;
     if ($this->getIsUndershield()) {
       $undershield = $this->renderUndershieldHeader();
     }
 
     $result = array(
       $notice,
       $props,
       $undershield,
       $content,
     );
 
     return hsprintf('%s', $result);
   }
 
   abstract public function isOneUpRenderer();
   abstract public function renderTextChange(
     $range_start,
     $range_len,
     $rows);
 
   public function renderDocumentEngineBlocks(
     PhabricatorDocumentEngineBlocks $blocks,
     $old_changeset_key,
     $new_changeset_key) {
     return null;
   }
 
   abstract protected function renderChangeTypeHeader($force);
   abstract protected function renderUndershieldHeader();
 
   protected function didRenderChangesetTableContents($contents) {
     return $contents;
   }
 
   /**
    * Render a "shield" over the diff, with a message like "This file is
    * generated and does not need to be reviewed." or "This file was completely
    * deleted." This UI element hides unimportant text so the reviewer doesn't
    * need to scroll past it.
    *
    * The shield includes a link to view the underlying content. This link
    * may force certain rendering modes when the link is clicked:
    *
    *    - `"default"`: Render the diff normally, as though it was not
    *      shielded. This is the default and appropriate if the underlying
    *      diff is a normal change, but was hidden for reasons of not being
    *      important (e.g., generated code).
    *    - `"text"`: Force the text to be shown. This is probably only relevant
    *      when a file is not changed.
    *    - `"none"`: Don't show the link (e.g., text not available).
    *
    * @param   string        Message explaining why the diff is hidden.
    * @param   string|null   Force mode, see above.
    * @return  string        Shield markup.
    */
   abstract public function renderShield($message, $force = 'default');
 
   abstract protected function renderPropertyChangeHeader();
 
   protected function buildPrimitives($range_start, $range_len) {
     $primitives = array();
 
     $hunk_starts = $this->getHunkStartLines();
 
     $mask = $this->getMask();
     $gaps = $this->getGaps();
 
     $old = $this->getOldLines();
     $new = $this->getNewLines();
     $old_render = $this->getOldRender();
     $new_render = $this->getNewRender();
     $old_comments = $this->getOldComments();
     $new_comments = $this->getNewComments();
 
     $size = count($old);
     for ($ii = $range_start; $ii < $range_start + $range_len; $ii++) {
       if (empty($mask[$ii])) {
         list($top, $len) = array_pop($gaps);
         $primitives[] = array(
           'type' => 'context',
           'top' => $top,
           'len' => $len,
         );
 
         $ii += ($len - 1);
         continue;
       }
 
       $ospec = array(
         'type' => 'old',
         'htype' => null,
         'cursor' => $ii,
         'line' => null,
         'oline' => null,
         'render' => null,
       );
 
       $nspec = array(
         'type' => 'new',
         'htype' => null,
         'cursor' => $ii,
         'line' => null,
         'oline' => null,
         'render' => null,
         'copy' => null,
         'coverage' => null,
       );
 
       if (isset($old[$ii])) {
         $ospec['line'] = (int)$old[$ii]['line'];
         $nspec['oline'] = (int)$old[$ii]['line'];
         $ospec['htype'] = $old[$ii]['type'];
         if (isset($old_render[$ii])) {
           $ospec['render'] = $old_render[$ii];
         }
       }
 
       if (isset($new[$ii])) {
         $nspec['line'] = (int)$new[$ii]['line'];
         $ospec['oline'] = (int)$new[$ii]['line'];
         $nspec['htype'] = $new[$ii]['type'];
         if (isset($new_render[$ii])) {
           $nspec['render'] = $new_render[$ii];
         }
       }
 
       if (isset($hunk_starts[$ospec['line']])) {
         $primitives[] = array(
           'type' => 'no-context',
         );
       }
 
       $primitives[] = $ospec;
       $primitives[] = $nspec;
 
       if ($ospec['line'] !== null && isset($old_comments[$ospec['line']])) {
         foreach ($old_comments[$ospec['line']] as $comment) {
           $primitives[] = array(
             'type' => 'inline',
             'comment' => $comment,
             'right' => false,
           );
         }
       }
 
       if ($nspec['line'] !== null && isset($new_comments[$nspec['line']])) {
         foreach ($new_comments[$nspec['line']] as $comment) {
           $primitives[] = array(
             'type' => 'inline',
             'comment' => $comment,
             'right' => true,
           );
         }
       }
 
       if ($hunk_starts && ($ii == $size - 1)) {
         $primitives[] = array(
           'type' => 'no-context',
         );
       }
     }
 
     if ($this->isOneUpRenderer()) {
       $primitives = $this->processPrimitivesForOneUp($primitives);
     }
 
     return $primitives;
   }
 
   private function processPrimitivesForOneUp(array $primitives) {
     // Primitives come out of buildPrimitives() in two-up format, because it
     // is the most general, flexible format. To put them into one-up format,
     // we need to filter and reorder them. In particular:
     //
     //   - We discard unchanged lines in the old file; in one-up format, we
     //     render them only once.
     //   - We group contiguous blocks of old-modified and new-modified lines, so
     //     they render in "block of old, block of new" order instead of
     //     alternating old and new lines.
 
     $out = array();
 
     $old_buf = array();
     $new_buf = array();
     foreach ($primitives as $primitive) {
       $type = $primitive['type'];
 
       if ($type == 'old') {
         if (!$primitive['htype']) {
           // This is a line which appears in both the old file and the new
           // file, or the spacer corresponding to a line added in the new file.
           // Ignore it when rendering a one-up diff.
           continue;
         }
         $old_buf[] = $primitive;
       } else if ($type == 'new') {
         if ($primitive['line'] === null) {
           // This is an empty spacer corresponding to a line removed from the
           // old file. Ignore it when rendering a one-up diff.
           continue;
         }
         if (!$primitive['htype']) {
           // If this line is the same in both versions of the file, put it in
           // the old line buffer. This makes sure inlines on old, unchanged
           // lines end up in the right place.
 
           // First, we need to flush the line buffers if they're not empty.
           if ($old_buf) {
             $out[] = $old_buf;
             $old_buf = array();
           }
           if ($new_buf) {
             $out[] = $new_buf;
             $new_buf = array();
           }
           $old_buf[] = $primitive;
         } else {
           $new_buf[] = $primitive;
         }
       } else if ($type == 'context' || $type == 'no-context') {
         $out[] = $old_buf;
         $out[] = $new_buf;
         $old_buf = array();
         $new_buf = array();
         $out[] = array($primitive);
       } else if ($type == 'inline') {
 
         // If this inline is on the left side, put it after the old lines.
         if (!$primitive['right']) {
           $out[] = $old_buf;
           $out[] = array($primitive);
           $old_buf = array();
         } else {
           $out[] = $old_buf;
           $out[] = $new_buf;
           $out[] = array($primitive);
           $old_buf = array();
           $new_buf = array();
         }
 
       } else {
         throw new Exception(pht("Unknown primitive type '%s'!", $primitive));
       }
     }
 
     $out[] = $old_buf;
     $out[] = $new_buf;
     $out = array_mergev($out);
 
     return $out;
   }
 
   protected function getChangesetProperties($changeset) {
     $old = $changeset->getOldProperties();
     $new = $changeset->getNewProperties();
 
-    // When adding files, don't show the uninteresting 644 filemode change.
-    if ($changeset->getChangeType() == DifferentialChangeType::TYPE_ADD &&
-        $new == array('unix:filemode' => '100644')) {
-      unset($new['unix:filemode']);
-    }
+    // If a property has been changed, but is not present on one side of the
+    // change and has an uninteresting default value on the other, remove it.
+    // This most commonly happens when a change adds or removes a file: the
+    // side of the change with the file has a "100644" filemode in Git.
+
+    $defaults = array(
+      'unix:filemode' => '100644',
+    );
 
-    // Likewise when removing files.
-    if ($changeset->getChangeType() == DifferentialChangeType::TYPE_DELETE &&
-        $old == array('unix:filemode' => '100644')) {
-      unset($old['unix:filemode']);
+    foreach ($defaults as $default_key => $default_value) {
+      $old_value = idx($old, $default_key, $default_value);
+      $new_value = idx($new, $default_key, $default_value);
+
+      $old_default = ($old_value === $default_value);
+      $new_default = ($new_value === $default_value);
+
+      if ($old_default && $new_default) {
+        unset($old[$default_key]);
+        unset($new[$default_key]);
+      }
     }
 
     $metadata = $changeset->getMetadata();
 
     if ($this->hasOldFile()) {
       $file = $this->getOldFile();
       if ($file->getImageWidth()) {
         $dimensions = $file->getImageWidth().'x'.$file->getImageHeight();
         $old['file:dimensions'] = $dimensions;
       }
       $old['file:mimetype'] = $file->getMimeType();
       $old['file:size'] = phutil_format_bytes($file->getByteSize());
     } else {
       $old['file:mimetype'] = idx($metadata, 'old:file:mime-type');
       $size = idx($metadata, 'old:file:size');
       if ($size !== null) {
         $old['file:size'] = phutil_format_bytes($size);
       }
     }
 
     if ($this->hasNewFile()) {
       $file = $this->getNewFile();
       if ($file->getImageWidth()) {
         $dimensions = $file->getImageWidth().'x'.$file->getImageHeight();
         $new['file:dimensions'] = $dimensions;
       }
       $new['file:mimetype'] = $file->getMimeType();
       $new['file:size'] = phutil_format_bytes($file->getByteSize());
     } else {
       $new['file:mimetype'] = idx($metadata, 'new:file:mime-type');
       $size = idx($metadata, 'new:file:size');
       if ($size !== null) {
         $new['file:size'] = phutil_format_bytes($size);
       }
     }
 
     return array($old, $new);
   }
 
   public function renderUndoTemplates() {
     $views = array(
       'l' => id(new PHUIDiffInlineCommentUndoView())->setIsOnRight(false),
       'r' => id(new PHUIDiffInlineCommentUndoView())->setIsOnRight(true),
     );
 
     foreach ($views as $key => $view) {
       $scaffold = $this->getRowScaffoldForInline($view);
       $views[$key] = id(new PHUIDiffInlineCommentTableScaffold())
         ->addRowScaffold($scaffold);
     }
 
     return $views;
   }
 
   final protected function getScopeEngine() {
     if ($this->scopeEngine === false) {
       $hunk_starts = $this->getHunkStartLines();
 
       // If this change is missing context, don't try to identify scopes, since
       // we won't really be able to get anywhere.
       $has_multiple_hunks = (count($hunk_starts) > 1);
 
       $has_offset_hunks = false;
       if ($hunk_starts) {
         $has_offset_hunks = (head_key($hunk_starts) != 1);
       }
 
       $missing_context = ($has_multiple_hunks || $has_offset_hunks);
 
       if ($missing_context) {
         $scope_engine = null;
       } else {
         $line_map = $this->getNewLineTextMap();
         $scope_engine = id(new PhabricatorDiffScopeEngine())
           ->setLineTextMap($line_map);
       }
 
       $this->scopeEngine = $scope_engine;
     }
 
     return $this->scopeEngine;
   }
 
   private function getNewLineTextMap() {
     $new = $this->getNewLines();
 
     $text_map = array();
     foreach ($new as $new_line) {
       if (!isset($new_line['line'])) {
         continue;
       }
       $text_map[$new_line['line']] = $new_line['text'];
     }
 
     return $text_map;
   }
 
 }
diff --git a/src/applications/differential/storage/DifferentialChangeset.php b/src/applications/differential/storage/DifferentialChangeset.php
index 682dffdd88..9f5ad0d998 100644
--- a/src/applications/differential/storage/DifferentialChangeset.php
+++ b/src/applications/differential/storage/DifferentialChangeset.php
@@ -1,532 +1,669 @@
 <?php
 
 final class DifferentialChangeset
   extends DifferentialDAO
   implements
     PhabricatorPolicyInterface,
     PhabricatorDestructibleInterface {
 
   protected $diffID;
   protected $oldFile;
   protected $filename;
   protected $awayPaths;
   protected $changeType;
   protected $fileType;
   protected $metadata = array();
   protected $oldProperties;
   protected $newProperties;
   protected $addLines;
   protected $delLines;
 
   private $unsavedHunks = array();
   private $hunks = self::ATTACHABLE;
   private $diff = self::ATTACHABLE;
 
   private $authorityPackages;
   private $changesetPackages;
 
   private $newFileObject = self::ATTACHABLE;
   private $oldFileObject = self::ATTACHABLE;
 
+  private $hasOldState;
+  private $hasNewState;
+  private $oldStateMetadata;
+  private $newStateMetadata;
+
   const TABLE_CACHE = 'differential_changeset_parse_cache';
 
   const METADATA_TRUSTED_ATTRIBUTES = 'attributes.trusted';
   const METADATA_UNTRUSTED_ATTRIBUTES = 'attributes.untrusted';
   const METADATA_EFFECT_HASH = 'hash.effect';
 
   const ATTRIBUTE_GENERATED = 'generated';
 
   protected function getConfiguration() {
     return array(
       self::CONFIG_SERIALIZATION => array(
         'metadata'      => self::SERIALIZATION_JSON,
         'oldProperties' => self::SERIALIZATION_JSON,
         'newProperties' => self::SERIALIZATION_JSON,
         'awayPaths'     => self::SERIALIZATION_JSON,
       ),
       self::CONFIG_COLUMN_SCHEMA => array(
         'oldFile' => 'bytes?',
         'filename' => 'bytes',
         'changeType' => 'uint32',
         'fileType' => 'uint32',
         'addLines' => 'uint32',
         'delLines' => 'uint32',
 
         // T6203/NULLABILITY
         // These should all be non-nullable, and store reasonable default
         // JSON values if empty.
         'awayPaths' => 'text?',
         'metadata' => 'text?',
         'oldProperties' => 'text?',
         'newProperties' => 'text?',
       ),
       self::CONFIG_KEY_SCHEMA => array(
         'diffID' => array(
           'columns' => array('diffID'),
         ),
       ),
     ) + parent::getConfiguration();
   }
 
   public function getAffectedLineCount() {
     return $this->getAddLines() + $this->getDelLines();
   }
 
   public function attachHunks(array $hunks) {
     assert_instances_of($hunks, 'DifferentialHunk');
     $this->hunks = $hunks;
     return $this;
   }
 
   public function getHunks() {
     return $this->assertAttached($this->hunks);
   }
 
   public function getDisplayFilename() {
     $name = $this->getFilename();
     if ($this->getFileType() == DifferentialChangeType::FILE_DIRECTORY) {
       $name .= '/';
     }
     return $name;
   }
 
   public function getOwnersFilename() {
     // TODO: For Subversion, we should adjust these paths to be relative to
     // the repository root where possible.
 
     $path = $this->getFilename();
 
     if (!isset($path[0])) {
       return '/';
     }
 
     if ($path[0] != '/') {
       $path = '/'.$path;
     }
 
     return $path;
   }
 
   public function addUnsavedHunk(DifferentialHunk $hunk) {
     if ($this->hunks === self::ATTACHABLE) {
       $this->hunks = array();
     }
     $this->hunks[] = $hunk;
     $this->unsavedHunks[] = $hunk;
     return $this;
   }
 
   public function setAuthorityPackages(array $authority_packages) {
     $this->authorityPackages = mpull($authority_packages, null, 'getPHID');
     return $this;
   }
 
   public function getAuthorityPackages() {
     return $this->authorityPackages;
   }
 
   public function setChangesetPackages($changeset_packages) {
     $this->changesetPackages = mpull($changeset_packages, null, 'getPHID');
     return $this;
   }
 
   public function getChangesetPackages() {
     return $this->changesetPackages;
   }
 
+  public function setHasOldState($has_old_state) {
+    $this->hasOldState = $has_old_state;
+    return $this;
+  }
+
+  public function setHasNewState($has_new_state) {
+    $this->hasNewState = $has_new_state;
+    return $this;
+  }
+
+  public function hasOldState() {
+    if ($this->hasOldState !== null) {
+      return $this->hasOldState;
+    }
+
+    $change_type = $this->getChangeType();
+    return !DifferentialChangeType::isCreateChangeType($change_type);
+  }
+
+  public function hasNewState() {
+    if ($this->hasNewState !== null) {
+      return $this->hasNewState;
+    }
+
+    $change_type = $this->getChangeType();
+    return !DifferentialChangeType::isDeleteChangeType($change_type);
+  }
+
   public function save() {
     $this->openTransaction();
       $ret = parent::save();
       foreach ($this->unsavedHunks as $hunk) {
         $hunk->setChangesetID($this->getID());
         $hunk->save();
       }
     $this->saveTransaction();
     return $ret;
   }
 
   public function delete() {
     $this->openTransaction();
 
       $hunks = id(new DifferentialHunk())->loadAllWhere(
         'changesetID = %d',
         $this->getID());
       foreach ($hunks as $hunk) {
         $hunk->delete();
       }
 
       $this->unsavedHunks = array();
 
       queryfx(
         $this->establishConnection('w'),
         'DELETE FROM %T WHERE id = %d',
         self::TABLE_CACHE,
         $this->getID());
 
       $ret = parent::delete();
     $this->saveTransaction();
     return $ret;
   }
 
   /**
    * Test if this changeset and some other changeset put the affected file in
    * the same state.
    *
    * @param DifferentialChangeset Changeset to compare against.
    * @return bool True if the two changesets have the same effect.
    */
   public function hasSameEffectAs(DifferentialChangeset $other) {
     if ($this->getFilename() !== $other->getFilename()) {
       return false;
     }
 
     $hash_key = self::METADATA_EFFECT_HASH;
 
     $u_hash = $this->getChangesetMetadata($hash_key);
     if ($u_hash === null) {
       return false;
     }
 
     $v_hash = $other->getChangesetMetadata($hash_key);
     if ($v_hash === null) {
       return false;
     }
 
     if ($u_hash !== $v_hash) {
       return false;
     }
 
     // Make sure the final states for the file properties (like the "+x"
     // executable bit) match one another.
     $u_props = $this->getNewProperties();
     $v_props = $other->getNewProperties();
     ksort($u_props);
     ksort($v_props);
 
     if ($u_props !== $v_props) {
       return false;
     }
 
     return true;
   }
 
   public function getSortKey() {
     $sort_key = $this->getFilename();
     // Sort files with ".h" in them first, so headers (.h, .hpp) come before
     // implementations (.c, .cpp, .cs).
     $sort_key = str_replace('.h', '.!h', $sort_key);
     return $sort_key;
   }
 
   public function makeNewFile() {
     $file = mpull($this->getHunks(), 'makeNewFile');
     return implode('', $file);
   }
 
   public function makeOldFile() {
     $file = mpull($this->getHunks(), 'makeOldFile');
     return implode('', $file);
   }
 
   public function makeChangesWithContext($num_lines = 3) {
     $with_context = array();
     foreach ($this->getHunks() as $hunk) {
       $context = array();
       $changes = explode("\n", $hunk->getChanges());
       foreach ($changes as $l => $line) {
         $type = substr($line, 0, 1);
         if ($type == '+' || $type == '-') {
           $context += array_fill($l - $num_lines, 2 * $num_lines + 1, true);
         }
       }
       $with_context[] = array_intersect_key($changes, $context);
     }
     return array_mergev($with_context);
   }
 
   public function getAnchorName() {
     return 'change-'.PhabricatorHash::digestForAnchor($this->getFilename());
   }
 
   public function getAbsoluteRepositoryPath(
     PhabricatorRepository $repository = null,
     DifferentialDiff $diff = null) {
 
     $base = '/';
     if ($diff && $diff->getSourceControlPath()) {
       $base = id(new PhutilURI($diff->getSourceControlPath()))->getPath();
     }
 
     $path = $this->getFilename();
     $path = rtrim($base, '/').'/'.ltrim($path, '/');
 
     $svn = PhabricatorRepositoryType::REPOSITORY_TYPE_SVN;
     if ($repository && $repository->getVersionControlSystem() == $svn) {
       $prefix = $repository->getDetail('remote-uri');
       $prefix = id(new PhutilURI($prefix))->getPath();
       if (!strncmp($path, $prefix, strlen($prefix))) {
         $path = substr($path, strlen($prefix));
       }
       $path = '/'.ltrim($path, '/');
     }
 
     return $path;
   }
 
   public function attachDiff(DifferentialDiff $diff) {
     $this->diff = $diff;
     return $this;
   }
 
   public function getDiff() {
     return $this->assertAttached($this->diff);
   }
 
   public function newFileTreeIcon() {
     $icon = $this->getPathIconIcon();
     $color = $this->getPathIconColor();
 
     return id(new PHUIIconView())
       ->setIcon("{$icon} {$color}");
   }
 
   public function getIsOwnedChangeset() {
     $authority_packages = $this->getAuthorityPackages();
     $changeset_packages = $this->getChangesetPackages();
 
     if (!$authority_packages || !$changeset_packages) {
       return false;
     }
 
     return (bool)array_intersect_key($authority_packages, $changeset_packages);
   }
 
   public function getIsLowImportanceChangeset() {
     $change_type = $this->getChangeType();
 
     $change_map = array(
       DifferentialChangeType::TYPE_DELETE => true,
       DifferentialChangeType::TYPE_MOVE_AWAY => true,
       DifferentialChangeType::TYPE_MULTICOPY => true,
     );
 
     if (isset($change_map[$change_type])) {
       return $change_map[$change_type];
     }
 
     if ($this->isGeneratedChangeset()) {
       return true;
     }
 
     return false;
   }
 
   public function getPathIconIcon() {
     return idx($this->getPathIconDetails(), 'icon');
   }
 
   public function getPathIconColor() {
     return idx($this->getPathIconDetails(), 'color');
   }
 
   private function getPathIconDetails() {
     $change_icons = array(
       DifferentialChangeType::TYPE_DELETE => array(
         'icon' => 'fa-times',
         'color' => 'delete-color',
       ),
       DifferentialChangeType::TYPE_ADD => array(
         'icon' => 'fa-plus',
         'color' => 'create-color',
       ),
       DifferentialChangeType::TYPE_MOVE_AWAY => array(
         'icon' => 'fa-circle-o',
         'color' => 'grey',
       ),
       DifferentialChangeType::TYPE_MULTICOPY => array(
         'icon' => 'fa-circle-o',
         'color' => 'grey',
       ),
       DifferentialChangeType::TYPE_MOVE_HERE => array(
         'icon' => 'fa-plus-circle',
         'color' => 'create-color',
       ),
       DifferentialChangeType::TYPE_COPY_HERE => array(
         'icon' => 'fa-plus-circle',
         'color' => 'create-color',
       ),
     );
 
     $change_type = $this->getChangeType();
     if (isset($change_icons[$change_type])) {
       return $change_icons[$change_type];
     }
 
     if ($this->isGeneratedChangeset()) {
       return array(
         'icon' => 'fa-cogs',
         'color' => 'grey',
       );
     }
 
     $file_type = $this->getFileType();
     $icon = DifferentialChangeType::getIconForFileType($file_type);
 
     return array(
       'icon' => $icon,
       'color' => 'bluetext',
     );
   }
 
   public function setChangesetMetadata($key, $value) {
     if (!is_array($this->metadata)) {
       $this->metadata = array();
     }
 
     $this->metadata[$key] = $value;
 
     return $this;
   }
 
   public function getChangesetMetadata($key, $default = null) {
     if (!is_array($this->metadata)) {
       return $default;
     }
 
     return idx($this->metadata, $key, $default);
   }
 
   private function setInternalChangesetAttribute($trusted, $key, $value) {
     if ($trusted) {
       $meta_key = self::METADATA_TRUSTED_ATTRIBUTES;
     } else {
       $meta_key = self::METADATA_UNTRUSTED_ATTRIBUTES;
     }
 
     $attributes = $this->getChangesetMetadata($meta_key, array());
     $attributes[$key] = $value;
     $this->setChangesetMetadata($meta_key, $attributes);
 
     return $this;
   }
 
   private function getInternalChangesetAttributes($trusted) {
     if ($trusted) {
       $meta_key = self::METADATA_TRUSTED_ATTRIBUTES;
     } else {
       $meta_key = self::METADATA_UNTRUSTED_ATTRIBUTES;
     }
 
     return $this->getChangesetMetadata($meta_key, array());
   }
 
   public function setTrustedChangesetAttribute($key, $value) {
     return $this->setInternalChangesetAttribute(true, $key, $value);
   }
 
   public function getTrustedChangesetAttributes() {
     return $this->getInternalChangesetAttributes(true);
   }
 
   public function getTrustedChangesetAttribute($key, $default = null) {
     $map = $this->getTrustedChangesetAttributes();
     return idx($map, $key, $default);
   }
 
   public function setUntrustedChangesetAttribute($key, $value) {
     return $this->setInternalChangesetAttribute(false, $key, $value);
   }
 
   public function getUntrustedChangesetAttributes() {
     return $this->getInternalChangesetAttributes(false);
   }
 
   public function getUntrustedChangesetAttribute($key, $default = null) {
     $map = $this->getUntrustedChangesetAttributes();
     return idx($map, $key, $default);
   }
 
   public function getChangesetAttributes() {
     // Prefer trusted values over untrusted values when both exist.
     return
       $this->getTrustedChangesetAttributes() +
       $this->getUntrustedChangesetAttributes();
   }
 
   public function getChangesetAttribute($key, $default = null) {
     $map = $this->getChangesetAttributes();
     return idx($map, $key, $default);
   }
 
   public function isGeneratedChangeset() {
     return $this->getChangesetAttribute(self::ATTRIBUTE_GENERATED);
   }
 
   public function getNewFileObjectPHID() {
     $metadata = $this->getMetadata();
     return idx($metadata, 'new:binary-phid');
   }
 
   public function getOldFileObjectPHID() {
     $metadata = $this->getMetadata();
     return idx($metadata, 'old:binary-phid');
   }
 
   public function attachNewFileObject(PhabricatorFile $file) {
     $this->newFileObject = $file;
     return $this;
   }
 
   public function getNewFileObject() {
     return $this->assertAttached($this->newFileObject);
   }
 
   public function attachOldFileObject(PhabricatorFile $file) {
     $this->oldFileObject = $file;
     return $this;
   }
 
   public function getOldFileObject() {
     return $this->assertAttached($this->oldFileObject);
   }
 
+  public function newComparisonChangeset(
+    DifferentialChangeset $against = null) {
+
+    $left = $this;
+    $right = $against;
+
+    $left_data = $left->makeNewFile();
+    $left_properties = $left->getNewProperties();
+    $left_metadata = $left->getNewStateMetadata();
+    $left_state = $left->hasNewState();
+    $shared_metadata = $left->getMetadata();
+    if ($right) {
+      $right_data = $right->makeNewFile();
+      $right_properties = $right->getNewProperties();
+      $right_metadata = $right->getNewStateMetadata();
+      $right_state = $right->hasNewState();
+      $shared_metadata = $right->getMetadata();
+    } else {
+      $right_data = $left->makeOldFile();
+      $right_properties = $left->getOldProperties();
+      $right_metadata = $left->getOldStateMetadata();
+      $right_state = $left->hasOldState();
+    }
+
+    $engine = new PhabricatorDifferenceEngine();
+
+    $synthetic = $engine->generateChangesetFromFileContent(
+      $left_data,
+      $right_data);
+
+    $comparison = id(new self())
+      ->makeEphemeral(true)
+      ->attachDiff($left->getDiff())
+      ->setOldFile($left->getFilename())
+      ->setFilename($right->getFilename());
+
+    // TODO: Change type?
+    // TODO: File type?
+    // TODO: Away paths?
+    // TODO: View state key?
+
+    $comparison->attachHunks($synthetic->getHunks());
+
+    $comparison->setOldProperties($left_properties);
+    $comparison->setNewProperties($right_properties);
+
+    $comparison
+      ->setOldStateMetadata($left_metadata)
+      ->setNewStateMetadata($right_metadata)
+      ->setHasOldState($left_state)
+      ->setHasNewState($right_state);
+
+    // NOTE: Some metadata is not stored statefully, like the "generated"
+    // flag. For now, use the rightmost "new state" metadata to fill in these
+    // values.
+
+    $metadata = $comparison->getMetadata();
+    $metadata = $metadata + $shared_metadata;
+    $comparison->setMetadata($metadata);
+
+    return $comparison;
+  }
+
+  public function getNewStateMetadata() {
+    return $this->getMetadataWithPrefix('new:');
+  }
+
+  public function setNewStateMetadata(array $metadata) {
+    return $this->setMetadataWithPrefix($metadata, 'new:');
+  }
+
+  public function getOldStateMetadata() {
+    return $this->getMetadataWithPrefix('old:');
+  }
+
+  public function setOldStateMetadata(array $metadata) {
+    return $this->setMetadataWithPrefix($metadata, 'old:');
+  }
+
+  private function getMetadataWithPrefix($prefix) {
+    $length = strlen($prefix);
+
+    $result = array();
+    foreach ($this->getMetadata() as $key => $value) {
+      if (strncmp($key, $prefix, $length)) {
+        continue;
+      }
+
+      $key = substr($key, $length);
+      $result[$key] = $value;
+    }
+
+    return $result;
+  }
+
+  private function setMetadataWithPrefix(array $metadata, $prefix) {
+    foreach ($metadata as $key => $value) {
+      $key = $prefix.$key;
+      $this->metadata[$key] = $value;
+    }
+
+    return $this;
+  }
+
 
 /* -(  PhabricatorPolicyInterface  )----------------------------------------- */
 
 
   public function getCapabilities() {
     return array(
       PhabricatorPolicyCapability::CAN_VIEW,
     );
   }
 
   public function getPolicy($capability) {
     return $this->getDiff()->getPolicy($capability);
   }
 
   public function hasAutomaticCapability($capability, PhabricatorUser $viewer) {
     return $this->getDiff()->hasAutomaticCapability($capability, $viewer);
   }
 
 
 /* -(  PhabricatorDestructibleInterface  )----------------------------------- */
 
 
   public function destroyObjectPermanently(
     PhabricatorDestructionEngine $engine) {
     $this->openTransaction();
 
       $hunks = id(new DifferentialHunk())->loadAllWhere(
         'changesetID = %d',
         $this->getID());
       foreach ($hunks as $hunk) {
         $engine->destroyObject($hunk);
       }
 
       $this->delete();
 
     $this->saveTransaction();
   }
 
 
 }