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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: 4 -*-
+ * 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 "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.SwapElements(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 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)
+        : mSkipChars(aIterator.mSkipChars),
+          mOriginalStringOffset(aIterator.mOriginalStringOffset),
+          mSkippedStringOffset(aIterator.mSkippedStringOffset),
+          mCurrentRangeIndex(aIterator.mCurrentRangeIndex),
+          mOriginalStringToSkipCharsOffset(aIterator.mOriginalStringToSkipCharsOffset)
+    { }
+
+    /**
+     * The empty constructor creates an object that is useless until it is assigned.
+     */
+    gfxSkipCharsIterator()
+        : mSkipChars(nullptr)
+    { }
+
+    /**
+     * Return true if this iterator is properly initialized and usable.
+     */
+    bool IsInitialized()
+    {
+        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*/