DifferentialChangesetParser.php

<?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;
  private $availableDocumentEngines;

  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.
   *
   * @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(
    PhabricatorInlineComment $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(
      $conn_r,
      'SELECT * FROM %T WHERE cacheIndex = %s',
      DifferentialChangeset::TABLE_CACHE,
      PhabricatorHash::digestForIndex($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(
          $conn_w,
          'INSERT INTO %T (cacheIndex, cache, dateCreated) VALUES (%s, %B, %d)
            ON DUPLICATE KEY UPDATE cache = VALUES(cache)',
          DifferentialChangeset::TABLE_CACHE,
          PhabricatorHash::digestForIndex($render_cache_key),
          $cache,
          PhabricatorTime::getNow());
      } 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() {
    $changeset = $this->getChangeset();
    if (!$changeset->hasSourceTextBody()) {

      // TODO: This isn't really correct (the change is not "generated"), the
      // intent is just to not render a text body for Subversion directory
      // changes, etc.
      $this->markGenerated();

      return;
    }

    $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;
    }

    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()) {

    $viewer = $this->getViewer();

    $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())
      ->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());

    if ($this->markupEngine) {
      $renderer->setMarkupEngine($this->markupEngine);
    }

    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);

    // Remove empty comments that don't have any unsaved draft data.
    PhabricatorInlineComment::loadAndAttachVersionedDrafts(
      $viewer,
      $this->comments);
    foreach ($this->comments as $key => $comment) {
      if ($comment->isVoidComment($viewer)) {
        unset($this->comments[$key]);
      }
    }

    // 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);

      $old_max_display = 1;
      foreach ($this->old as $old) {
        if (isset($old['line'])) {
          $old_max_display = $old['line'];
        }
      }

      $new_max_display = 1;
      foreach ($this->new as $new) {
        if (isset($new['line'])) {
          $new_max_display = $new['line'];
        }
      }

      foreach ($this->comments as $comment) {
        $display_line = $comment->getLineNumber() + $comment->getLineLength();
        $display_line = max(1, $display_line);

        if ($this->isCommentOnRightSideWhenDisplayed($comment)) {
          $display_line = min($new_max_display, $display_line);
          $new_comments[$display_line][] = $comment;
        } else {
          $display_line = min($old_max_display, $display_line);
          $old_comments[$display_line][] = $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;
      }

      if ($mask_force) {
        $engine_blocks->setRevealedIndexes(array_keys($mask_force));
      }

      if ($range_start !== null || $range_len !== null) {
        $range_min = $range_start;

        if ($range_len === null) {
          $range_max = null;
        } else {
          $range_max = (int)$range_start + (int)$range_len;
        }

        $engine_blocks->setRange($range_min, $range_max);
      }

      $renderer
        ->setDocumentEngine($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 PhabricatorInlineComment Comment to test for visibility.
   * @return bool True if the comment is visible on the rendered diff.
   */
  private function isCommentVisibleOnRenderedDiff(
    PhabricatorInlineComment $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 PhabricatorInlineComment Comment to test for display
   *              location.
   * @return bool True for right, false for left.
   */
  private function isCommentOnRightSideWhenDisplayed(
    PhabricatorInlineComment $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 === null) {
        continue;
      }

      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();

    list($old_file, $new_file) = $this->loadFileObjectsForChangeset();

    $no_old = !$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]);
      }
    }

    $this->availableDocumentEngines = $shared_engines;

    $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;

    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);
      }

      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);
    }

    $document_engine = $renderer->getDocumentEngine();
    if ($document_engine) {
      $document_engine_key = $document_engine->getDocumentEngineKey();
    } else {
      $document_engine_key = null;
    }

    $available_keys = array();
    $engines = $this->availableDocumentEngines;
    if (!$engines) {
      $engines = array();
    }

    $available_keys = mpull($engines, 'getDocumentEngineKey');

    // TODO: Always include "source" as a usable engine to default to
    // the buitin rendering. This is kind of a hack and does not actually
    // use the source engine. The source engine isn't a diff engine, so
    // selecting it causes us to fall through and render with builtin
    // behavior. For now, overall behavir is reasonable.

    $available_keys[] = PhabricatorSourceDocumentEngine::ENGINEKEY;
    $available_keys = array_fuse($available_keys);
    $available_keys = array_values($available_keys);

    $state = array(
      'undoTemplates' => $undo_templates,
      'rendererKey' => $renderer_key,
      'highlight' => $viewstate->getHighlightLanguage(),
      'characterEncoding' => $viewstate->getCharacterEncoding(),
      'requestDocumentEngineKey' => $viewstate->getDocumentEngineKey(),
      'responseDocumentEngineKey' => $document_engine_key,
      'availableDocumentEngineKeys' => $available_keys,
      '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);
  }

}