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diff --git a/gfx/thebes/gfxSkipChars.h b/gfx/thebes/gfxSkipChars.h
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+/* -*- Mode: C++; tab-width: 20; 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/. */
+
+#ifndef GFX_SKIP_CHARS_H
+#define GFX_SKIP_CHARS_H
+
+#include "mozilla/Attributes.h"
+#include "nsTArray.h"
+
+/*
+ * gfxSkipChars is a data structure representing a list of characters that
+ * have been skipped. The initial string is called the "original string"
+ * and after skipping some characters, the result is called the "skipped
+ * string". gfxSkipChars provides efficient ways to translate between offsets in
+ * the original string and the skipped string. It is used by textrun code to
+ * keep track of offsets before and after text transformations such as
+ * whitespace compression and control code deletion.
+ */
+
+/**
+ * The gfxSkipChars is represented as a sorted array of skipped ranges.
+ *
+ * A freshly-created gfxSkipChars means "all chars kept".
+ */
+class gfxSkipChars {
+  friend struct SkippedRangeStartComparator;
+  friend struct SkippedRangeOffsetComparator;
+
+ private:
+  class SkippedRange {
+   public:
+    SkippedRange(uint32_t aOffset, uint32_t aLength, uint32_t aDelta)
+        : mOffset(aOffset), mLength(aLength), mDelta(aDelta) {}
+
+    uint32_t Start() const { return mOffset; }
+
+    uint32_t End() const { return mOffset + mLength; }
+
+    uint32_t Length() const { return mLength; }
+
+    uint32_t SkippedOffset() const { return mOffset - mDelta; }
+
+    uint32_t Delta() const { return mDelta; }
+
+    uint32_t NextDelta() const { return mDelta + mLength; }
+
+    void Extend(uint32_t aChars) { mLength += aChars; }
+
+   private:
+    uint32_t mOffset;  // original-string offset at which we want to skip
+    uint32_t mLength;  // number of skipped chars at this offset
+    uint32_t mDelta;   // sum of lengths of preceding skipped-ranges
+  };
+
+ public:
+  gfxSkipChars() : mCharCount(0) {}
+
+  void SkipChars(uint32_t aChars) {
+    NS_ASSERTION(mCharCount + aChars > mCharCount, "Character count overflow");
+    uint32_t rangeCount = mRanges.Length();
+    uint32_t delta = 0;
+    if (rangeCount > 0) {
+      SkippedRange& lastRange = mRanges[rangeCount - 1];
+      if (lastRange.End() == mCharCount) {
+        lastRange.Extend(aChars);
+        mCharCount += aChars;
+        return;
+      }
+      delta = lastRange.NextDelta();
+    }
+    mRanges.AppendElement(SkippedRange(mCharCount, aChars, delta));
+    mCharCount += aChars;
+  }
+
+  void KeepChars(uint32_t aChars) {
+    NS_ASSERTION(mCharCount + aChars > mCharCount, "Character count overflow");
+    mCharCount += aChars;
+  }
+
+  void SkipChar() { SkipChars(1); }
+
+  void KeepChar() { KeepChars(1); }
+
+  void TakeFrom(gfxSkipChars* aSkipChars) {
+    mRanges = std::move(aSkipChars->mRanges);
+    mCharCount = aSkipChars->mCharCount;
+    aSkipChars->mCharCount = 0;
+  }
+
+  int32_t GetOriginalCharCount() const { return mCharCount; }
+
+  const SkippedRange& LastRange() const {
+    // this is only valid if mRanges is non-empty; no assertion here
+    // because nsTArray will already assert if we abuse it
+    return mRanges[mRanges.Length() - 1];
+  }
+
+  friend class gfxSkipCharsIterator;
+
+ private:
+  nsTArray<SkippedRange> mRanges;
+  uint32_t mCharCount;
+};
+
+/**
+ * A gfxSkipCharsIterator represents a position in the original string. It lets
+ * you map efficiently to and from positions in the string after skipped
+ * characters have been removed. You can also specify an offset that is added to
+ * all incoming original string offsets and subtracted from all outgoing
+ * original string offsets --- useful when the gfxSkipChars corresponds to
+ * something offset from the original DOM coordinates, which it often does for
+ * gfxTextRuns.
+ *
+ * The current positions (in both the original and skipped strings) are
+ * always constrained to be >= 0 and <= the string length. When the position
+ * is equal to the string length, it is at the end of the string. The current
+ * positions do not include any aOriginalStringToSkipCharsOffset.
+ *
+ * When the position in the original string corresponds to a skipped character,
+ * the skipped-characters offset is the offset of the next unskipped character,
+ * or the skipped-characters string length if there is no next unskipped
+ * character.
+ */
+class MOZ_STACK_CLASS gfxSkipCharsIterator {
+ public:
+  /**
+   * @param aOriginalStringToSkipCharsOffset add this to all incoming and
+   * outgoing original string offsets
+   */
+  gfxSkipCharsIterator(const gfxSkipChars& aSkipChars,
+                       int32_t aOriginalStringToSkipCharsOffset,
+                       int32_t aOriginalStringOffset)
+      : mSkipChars(&aSkipChars),
+        mOriginalStringOffset(0),
+        mSkippedStringOffset(0),
+        mCurrentRangeIndex(-1),
+        mOriginalStringToSkipCharsOffset(aOriginalStringToSkipCharsOffset) {
+    SetOriginalOffset(aOriginalStringOffset);
+  }
+
+  explicit gfxSkipCharsIterator(const gfxSkipChars& aSkipChars,
+                                int32_t aOriginalStringToSkipCharsOffset = 0)
+      : mSkipChars(&aSkipChars),
+        mOriginalStringOffset(0),
+        mSkippedStringOffset(0),
+        mOriginalStringToSkipCharsOffset(aOriginalStringToSkipCharsOffset) {
+    mCurrentRangeIndex =
+        mSkipChars->mRanges.IsEmpty() || mSkipChars->mRanges[0].Start() > 0 ? -1
+                                                                            : 0;
+  }
+
+  gfxSkipCharsIterator(const gfxSkipCharsIterator& aIterator) = default;
+
+  /**
+   * The empty constructor creates an object that is useless until it is
+   * assigned.
+   */
+  gfxSkipCharsIterator()
+      : mSkipChars(nullptr),
+        mOriginalStringOffset(0),
+        mSkippedStringOffset(0),
+        mCurrentRangeIndex(0),
+        mOriginalStringToSkipCharsOffset(0) {}
+
+  /**
+   * Return true if this iterator is properly initialized and usable.
+   */
+  bool IsInitialized() const { return mSkipChars != nullptr; }
+
+  /**
+   * Set the iterator to aOriginalStringOffset in the original string.
+   * This can efficiently move forward or backward from the current position.
+   * aOriginalStringOffset is clamped to [0,originalStringLength].
+   */
+  void SetOriginalOffset(int32_t aOriginalStringOffset);
+
+  /**
+   * Set the iterator to aSkippedStringOffset in the skipped string.
+   * This can efficiently move forward or backward from the current position.
+   * aSkippedStringOffset is clamped to [0,skippedStringLength].
+   */
+  void SetSkippedOffset(uint32_t aSkippedStringOffset);
+
+  uint32_t ConvertOriginalToSkipped(int32_t aOriginalStringOffset) {
+    SetOriginalOffset(aOriginalStringOffset);
+    return GetSkippedOffset();
+  }
+
+  int32_t ConvertSkippedToOriginal(uint32_t aSkippedStringOffset) {
+    SetSkippedOffset(aSkippedStringOffset);
+    return GetOriginalOffset();
+  }
+
+  /**
+   * Test if the character at the current position in the original string
+   * is skipped or not. If aRunLength is non-null, then *aRunLength is set
+   * to a number of characters all of which are either skipped or not, starting
+   * at this character. When the current position is at the end of the original
+   * string, we return true and *aRunLength is set to zero.
+   */
+  bool IsOriginalCharSkipped(int32_t* aRunLength = nullptr) const;
+
+  void AdvanceOriginal(int32_t aDelta) {
+    SetOriginalOffset(GetOriginalOffset() + aDelta);
+  }
+
+  void AdvanceSkipped(int32_t aDelta) {
+    SetSkippedOffset(GetSkippedOffset() + aDelta);
+  }
+
+  /**
+   * @return the offset within the original string
+   */
+  int32_t GetOriginalOffset() const {
+    return mOriginalStringOffset - mOriginalStringToSkipCharsOffset;
+  }
+
+  /**
+   * @return the offset within the skipped string corresponding to the
+   * current position in the original string. If the current position
+   * in the original string is a character that is skipped, then we return
+   * the position corresponding to the first non-skipped character in the
+   * original string after the current position, or the length of the skipped
+   * string if there is no such character.
+   */
+  uint32_t GetSkippedOffset() const { return mSkippedStringOffset; }
+
+  int32_t GetOriginalEnd() const {
+    return mSkipChars->GetOriginalCharCount() -
+           mOriginalStringToSkipCharsOffset;
+  }
+
+ private:
+  const gfxSkipChars* mSkipChars;
+
+  // Current position
+  int32_t mOriginalStringOffset;
+  uint32_t mSkippedStringOffset;
+
+  // Index of the last skippedRange that precedes or contains the current
+  // position in the original string.
+  // If index == -1 then we are before the first skipped char.
+  int32_t mCurrentRangeIndex;
+
+  // This offset is added to map from "skipped+unskipped characters in
+  // the original DOM string" character space to "skipped+unskipped
+  // characters in the textrun's gfxSkipChars" character space
+  int32_t mOriginalStringToSkipCharsOffset;
+};
+
+#endif /*GFX_SKIP_CHARS_H*/