diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -12397,7 +12397,7 @@
     'PhutilPHPCodeSnippetContextFreeGrammar' => 'PhutilCLikeCodeSnippetContextFreeGrammar',
     'PhutilPhabricatorAuthAdapter' => 'PhutilOAuthAuthAdapter',
     'PhutilProseDiff' => 'Phobject',
-    'PhutilProseDiffTestCase' => 'PhutilTestCase',
+    'PhutilProseDiffTestCase' => 'PhabricatorTestCase',
     'PhutilProseDifferenceEngine' => 'Phobject',
     'PhutilQueryString' => 'Phobject',
     'PhutilRealNameContextFreeGrammar' => 'PhutilContextFreeGrammar',
diff --git a/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php b/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php
--- a/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php
+++ b/src/applications/files/diff/PhabricatorDocumentEngineBlocks.php
@@ -50,20 +50,35 @@
 
       if ($old_line) {
         $old_hash = rtrim($old_line['text'], "\n");
-        $old_block = array_shift($old_map[$old_hash]);
-        $old_block->setDifferenceType($old_line['type']);
+        if (!strlen($old_hash)) {
+          // This can happen when one of the sources has no blocks.
+          $old_block = null;
+        } else {
+          $old_block = array_shift($old_map[$old_hash]);
+          $old_block->setDifferenceType($old_line['type']);
+        }
       } else {
         $old_block = null;
       }
 
       if ($new_line) {
         $new_hash = rtrim($new_line['text'], "\n");
-        $new_block = array_shift($new_map[$new_hash]);
-        $new_block->setDifferenceType($new_line['type']);
+        if (!strlen($new_hash)) {
+          $new_block = null;
+        } else {
+          $new_block = array_shift($new_map[$new_hash]);
+          $new_block->setDifferenceType($new_line['type']);
+        }
       } else {
         $new_block = null;
       }
 
+      // If both lists are empty, we may generate a row which has two empty
+      // blocks.
+      if (!$old_block && !$new_block) {
+        continue;
+      }
+
       $rows[] = array(
         $old_block,
         $new_block,
diff --git a/src/infrastructure/diff/prose/PhutilProseDifferenceEngine.php b/src/infrastructure/diff/prose/PhutilProseDifferenceEngine.php
--- a/src/infrastructure/diff/prose/PhutilProseDifferenceEngine.php
+++ b/src/infrastructure/diff/prose/PhutilProseDifferenceEngine.php
@@ -10,47 +10,14 @@
     $u_parts = $this->splitCorpus($u, $level);
     $v_parts = $this->splitCorpus($v, $level);
 
-    $matrix = id(new PhutilEditDistanceMatrix())
-      ->setMaximumLength(128)
-      ->setSequences($u_parts, $v_parts)
-      ->setComputeString(true);
-
-    // For word-level and character-level changes, smooth the output string
-    // to reduce the choppiness of the diff.
-    if ($level > 1) {
-      $matrix->setApplySmoothing(PhutilEditDistanceMatrix::SMOOTHING_FULL);
-    }
-
-    $u_pos = 0;
-    $v_pos = 0;
-
-    $edits = $matrix->getEditString();
-    $edits_length = strlen($edits);
-
-    $diff = new PhutilProseDiff();
-    for ($ii = 0; $ii < $edits_length; $ii++) {
-      $c = $edits[$ii];
-      if ($c == 's') {
-        $diff->addPart('=', $u_parts[$u_pos]);
-        $u_pos++;
-        $v_pos++;
-      } else if ($c == 'd') {
-        $diff->addPart('-', $u_parts[$u_pos]);
-        $u_pos++;
-      } else if ($c == 'i') {
-        $diff->addPart('+', $v_parts[$v_pos]);
-        $v_pos++;
-      } else if ($c == 'x') {
-        $diff->addPart('-', $u_parts[$u_pos]);
-        $diff->addPart('+', $v_parts[$v_pos]);
-        $u_pos++;
-        $v_pos++;
-      } else {
-        throw new Exception(
-          pht(
-            'Unexpected character ("%s") in edit string.',
-            $c));
-      }
+    if ($level === 0) {
+      $diff = $this->newHashDiff($u_parts, $v_parts);
+      $too_large = false;
+    } else {
+      list($diff, $too_large) = $this->newEditDistanceMatrixDiff(
+        $u_parts,
+        $v_parts,
+        $level);
     }
 
     $diff->reorderParts();
@@ -119,7 +86,7 @@
         } else if (!strlen($new)) {
           $result->addPart('-', $old);
         } else {
-          if ($matrix->didReachMaximumLength()) {
+          if ($too_large) {
             // If this text was too big to diff, don't try to subdivide it.
             $result->addPart('-', $old);
             $result->addPart('+', $new);
@@ -206,4 +173,103 @@
     return $results;
   }
 
+  private function newEditDistanceMatrixDiff(
+    array $u_parts,
+    array $v_parts,
+    $level) {
+
+    $matrix = id(new PhutilEditDistanceMatrix())
+      ->setMaximumLength(128)
+      ->setSequences($u_parts, $v_parts)
+      ->setComputeString(true);
+
+    // For word-level and character-level changes, smooth the output string
+    // to reduce the choppiness of the diff.
+    if ($level > 1) {
+      $matrix->setApplySmoothing(PhutilEditDistanceMatrix::SMOOTHING_FULL);
+    }
+
+    $u_pos = 0;
+    $v_pos = 0;
+
+    $edits = $matrix->getEditString();
+    $edits_length = strlen($edits);
+
+    $diff = new PhutilProseDiff();
+    for ($ii = 0; $ii < $edits_length; $ii++) {
+      $c = $edits[$ii];
+      if ($c == 's') {
+        $diff->addPart('=', $u_parts[$u_pos]);
+        $u_pos++;
+        $v_pos++;
+      } else if ($c == 'd') {
+        $diff->addPart('-', $u_parts[$u_pos]);
+        $u_pos++;
+      } else if ($c == 'i') {
+        $diff->addPart('+', $v_parts[$v_pos]);
+        $v_pos++;
+      } else if ($c == 'x') {
+        $diff->addPart('-', $u_parts[$u_pos]);
+        $diff->addPart('+', $v_parts[$v_pos]);
+        $u_pos++;
+        $v_pos++;
+      } else {
+        throw new Exception(
+          pht(
+            'Unexpected character ("%s") in edit string.',
+            $c));
+      }
+    }
+
+    return array($diff, $matrix->didReachMaximumLength());
+  }
+
+  private function newHashDiff(array $u_parts, array $v_parts) {
+
+    $u_ref = new PhabricatorDocumentRef();
+    $v_ref = new PhabricatorDocumentRef();
+
+    $u_blocks = $this->newDocumentEngineBlocks($u_parts);
+    $v_blocks = $this->newDocumentEngineBlocks($v_parts);
+
+    $rows = id(new PhabricatorDocumentEngineBlocks())
+      ->addBlockList($u_ref, $u_blocks)
+      ->addBlockList($v_ref, $v_blocks)
+      ->newTwoUpLayout();
+
+    $diff = new PhutilProseDiff();
+    foreach ($rows as $row) {
+      list($u_block, $v_block) = $row;
+
+      if ($u_block && $v_block) {
+        if ($u_block->getDifferenceType() === '-') {
+          $diff->addPart('-', $u_block->getContent());
+          $diff->addPart('+', $v_block->getContent());
+        } else {
+          $diff->addPart('=', $u_block->getContent());
+        }
+      } else if ($u_block) {
+        $diff->addPart('-', $u_block->getContent());
+      } else {
+        $diff->addPart('+', $v_block->getContent());
+      }
+    }
+
+    return $diff;
+  }
+
+  private function newDocumentEngineBlocks(array $parts) {
+    $blocks = array();
+
+    foreach ($parts as $part) {
+      $hash = PhabricatorHash::digestForIndex($part);
+
+      $blocks[] = id(new PhabricatorDocumentEngineBlock())
+        ->setContent($part)
+        ->setDifferenceHash($hash);
+    }
+
+    return $blocks;
+  }
+
 }
diff --git a/src/infrastructure/diff/prose/__tests__/PhutilProseDiffTestCase.php b/src/infrastructure/diff/prose/__tests__/PhutilProseDiffTestCase.php
--- a/src/infrastructure/diff/prose/__tests__/PhutilProseDiffTestCase.php
+++ b/src/infrastructure/diff/prose/__tests__/PhutilProseDiffTestCase.php
@@ -1,6 +1,7 @@
 <?php
 
-final class PhutilProseDiffTestCase extends PhutilTestCase {
+final class PhutilProseDiffTestCase
+  extends PhabricatorTestCase {
 
   public function testProseDiffsDistance() {
     $this->assertProseParts(