gecko-dev/accessible/base/TextUpdater.cpp

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "TextUpdater.h"

#include "LocalAccessible-inl.h"
#include "CacheConstants.h"
#include "DocAccessible-inl.h"
#include "nsAccessibilityService.h"
#include "TextLeafAccessible.h"
#include <algorithm>

using namespace mozilla::a11y;

void TextUpdater::Run(DocAccessible* aDocument, TextLeafAccessible* aTextLeaf,
                      const nsAString& aNewText) {
  NS_ASSERTION(aTextLeaf, "No text leaf accessible?");

  const nsString& oldText = aTextLeaf->Text();
  uint32_t oldLen = oldText.Length(), newLen = aNewText.Length();
  uint32_t minLen = std::min(oldLen, newLen);

  // Skip coinciding begin substrings.
  uint32_t skipStart = 0;
  for (; skipStart < minLen; skipStart++) {
    if (aNewText[skipStart] != oldText[skipStart]) break;
  }

  // The text was changed. Do update.
  if (skipStart != minLen || oldLen != newLen) {
    TextUpdater updater(aDocument, aTextLeaf);
    updater.DoUpdate(aNewText, oldText, skipStart);
    aDocument->QueueCacheUpdate(aTextLeaf, CacheDomain::Text);
  }
}

void TextUpdater::DoUpdate(const nsAString& aNewText, const nsAString& aOldText,
                           uint32_t aSkipStart) {
  LocalAccessible* parent = mTextLeaf->LocalParent();
  if (!parent) return;

  mHyperText = parent->AsHyperText();
  if (!mHyperText) {
    MOZ_ASSERT_UNREACHABLE("Text leaf parent is not hypertext!");
    return;
  }

  // Get the text leaf accessible offset and invalidate cached offsets after it.
  mTextOffset = mHyperText->GetChildOffset(mTextLeaf, true);
  NS_ASSERTION(mTextOffset != -1, "Text leaf hasn't offset within hyper text!");

  // Don't bother diffing if the hypertext isn't editable. Diffing non-editable
  // text can lead to weird screen reader results with live regions, e.g.,
  // changing "text" to "testing" might read the diff "s ing" when we'd really
  // just like to hear "testing."
  if (!mHyperText->IsEditable()) {
    // Fire text change event for removal.
    RefPtr<AccEvent> textRemoveEvent =
        new AccTextChangeEvent(mHyperText, mTextOffset, aOldText, false);
    mDocument->FireDelayedEvent(textRemoveEvent);

    // Fire text change event for insertion if there's text to insert.
    if (!aNewText.IsEmpty()) {
      RefPtr<AccEvent> textInsertEvent =
          new AccTextChangeEvent(mHyperText, mTextOffset, aNewText, true);
      mDocument->FireDelayedEvent(textInsertEvent);
    }

    mDocument->MaybeNotifyOfValueChange(mHyperText);

    // Update the text.
    mTextLeaf->SetText(aNewText);
    return;
  }

  uint32_t oldLen = aOldText.Length(), newLen = aNewText.Length();
  uint32_t minLen = std::min(oldLen, newLen);

  // Trim coinciding substrings from the end.
  uint32_t skipEnd = 0;
  while (minLen - skipEnd > aSkipStart &&
         aNewText[newLen - skipEnd - 1] == aOldText[oldLen - skipEnd - 1]) {
    skipEnd++;
  }

  uint32_t strLen1 = oldLen - aSkipStart - skipEnd;
  uint32_t strLen2 = newLen - aSkipStart - skipEnd;

  const nsAString& str1 = Substring(aOldText, aSkipStart, strLen1);
  const nsAString& str2 = Substring(aNewText, aSkipStart, strLen2);

  // Increase offset of the text leaf on skipped characters amount.
  mTextOffset += aSkipStart;

  // It could be single insertion or removal or the case of long strings. Do not
  // calculate the difference between long strings and prefer to fire pair of
  // insert/remove events as the old string was replaced on the new one.
  if (strLen1 == 0 || strLen2 == 0 || strLen1 > kMaxStrLen ||
      strLen2 > kMaxStrLen) {
    if (strLen1 > 0) {
      // Fire text change event for removal.
      RefPtr<AccEvent> textRemoveEvent =
          new AccTextChangeEvent(mHyperText, mTextOffset, str1, false);
      mDocument->FireDelayedEvent(textRemoveEvent);
    }

    if (strLen2 > 0) {
      // Fire text change event for insertion.
      RefPtr<AccEvent> textInsertEvent =
          new AccTextChangeEvent(mHyperText, mTextOffset, str2, true);
      mDocument->FireDelayedEvent(textInsertEvent);
    }

    mDocument->MaybeNotifyOfValueChange(mHyperText);

    // Update the text.
    mTextLeaf->SetText(aNewText);
    return;
  }

  // Otherwise find the difference between strings and fire events.
  // Note: we can skip initial and final coinciding characters since they don't
  // affect the Levenshtein distance.

  // Compute the flat structured matrix need to compute the difference.
  uint32_t len1 = strLen1 + 1, len2 = strLen2 + 1;
  uint32_t* entries = new uint32_t[len1 * len2];

  for (uint32_t colIdx = 0; colIdx < len1; colIdx++) entries[colIdx] = colIdx;

  uint32_t* row = entries;
  for (uint32_t rowIdx = 1; rowIdx < len2; rowIdx++) {
    uint32_t* prevRow = row;
    row += len1;
    row[0] = rowIdx;
    for (uint32_t colIdx = 1; colIdx < len1; colIdx++) {
      if (str1[colIdx - 1] != str2[rowIdx - 1]) {
        uint32_t left = row[colIdx - 1];
        uint32_t up = prevRow[colIdx];
        uint32_t upleft = prevRow[colIdx - 1];
        row[colIdx] = std::min(upleft, std::min(left, up)) + 1;
      } else {
        row[colIdx] = prevRow[colIdx - 1];
      }
    }
  }

  // Compute events based on the difference.
  nsTArray<RefPtr<AccEvent> > events;
  ComputeTextChangeEvents(str1, str2, entries, events);

  delete[] entries;

  // Fire events.
  for (int32_t idx = events.Length() - 1; idx >= 0; idx--) {
    mDocument->FireDelayedEvent(events[idx]);
  }

  mDocument->MaybeNotifyOfValueChange(mHyperText);

  // Update the text.
  mTextLeaf->SetText(aNewText);
}

void TextUpdater::ComputeTextChangeEvents(
    const nsAString& aStr1, const nsAString& aStr2, uint32_t* aEntries,
    nsTArray<RefPtr<AccEvent> >& aEvents) {
  int32_t colIdx = aStr1.Length(), rowIdx = aStr2.Length();

  // Point at which strings last matched.
  int32_t colEnd = colIdx;
  int32_t rowEnd = rowIdx;

  int32_t colLen = colEnd + 1;
  uint32_t* row = aEntries + rowIdx * colLen;
  uint32_t dist = row[colIdx];  // current Levenshtein distance
  while (rowIdx && colIdx) {    // stop when we can't move diagonally
    if (aStr1[colIdx - 1] == aStr2[rowIdx - 1]) {  // match
      if (rowIdx < rowEnd) {  // deal with any pending insertion
        FireInsertEvent(Substring(aStr2, rowIdx, rowEnd - rowIdx), rowIdx,
                        aEvents);
      }
      if (colIdx < colEnd) {  // deal with any pending deletion
        FireDeleteEvent(Substring(aStr1, colIdx, colEnd - colIdx), rowIdx,
                        aEvents);
      }

      colEnd = --colIdx;  // reset the match point
      rowEnd = --rowIdx;
      row -= colLen;
      continue;
    }
    --dist;
    if (dist == row[colIdx - 1 - colLen]) {  // substitution
      --colIdx;
      --rowIdx;
      row -= colLen;
      continue;
    }
    if (dist == row[colIdx - colLen]) {  // insertion
      --rowIdx;
      row -= colLen;
      continue;
    }
    if (dist == row[colIdx - 1]) {  // deletion
      --colIdx;
      continue;
    }
    MOZ_ASSERT_UNREACHABLE("huh?");
    return;
  }

  if (rowEnd) FireInsertEvent(Substring(aStr2, 0, rowEnd), 0, aEvents);
  if (colEnd) FireDeleteEvent(Substring(aStr1, 0, colEnd), 0, aEvents);
}