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diff --git a/layout/generic/JustificationUtils.h b/layout/generic/JustificationUtils.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 mozilla_JustificationUtils_h_
+#define mozilla_JustificationUtils_h_
+
+#include "mozilla/Attributes.h"
+#include "nsCoord.h"
+
+namespace mozilla {
+
+/**
+ * Jutification Algorithm
+ *
+ * The justification algorithm is based on expansion opportunities
+ * between justifiable clusters.  By this algorithm, there is one
+ * expansion opportunity at each side of a justifiable cluster, and
+ * at most one opportunity between two clusters. For example, if there
+ * is a line in a Chinese document is: "你好世界hello world", then
+ * the expansion opportunities (marked as '*') would be:
+ *
+ *                    你*好*世*界*hello*' '*world
+ *
+ * The spacing left in a line will then be distributed equally to each
+ * opportunities. Because we want that, only justifiable clusters get
+ * expanded, and the split point between two justifiable clusters would
+ * be at the middle of the spacing, each expansion opportunities will be
+ * filled by two justification gaps. The example above would be:
+ *
+ *              你 | 好 | 世 | 界  |hello|  ' '  |world
+ *
+ * In the algorithm, information about expansion opportunities is stored
+ * in structure JustificationInfo, and the assignment of justification
+ * gaps is in structure JustificationAssignment.
+ */
+
+struct JustificationInfo
+{
+  // Number of expansion opportunities inside a span. It doesn't include
+  // any opportunities between this span and the one before or after.
+  int32_t mInnerOpportunities;
+  // The justifiability of the start and end sides of the span.
+  bool mIsStartJustifiable;
+  bool mIsEndJustifiable;
+
+  constexpr JustificationInfo()
+    : mInnerOpportunities(0)
+    , mIsStartJustifiable(false)
+    , mIsEndJustifiable(false)
+  {
+  }
+
+  // Claim that the last opportunity should be cancelled
+  // because the trailing space just gets trimmed.
+  void CancelOpportunityForTrimmedSpace()
+  {
+    if (mInnerOpportunities > 0) {
+      mInnerOpportunities--;
+    } else {
+      // There is no inner opportunities, hence the whole frame must
+      // contain only the trimmed space, because any content before
+      // space would cause an inner opportunity. The space made each
+      // side justifiable, which should be cancelled now.
+      mIsStartJustifiable = false;
+      mIsEndJustifiable = false;
+    }
+  }
+};
+
+struct JustificationAssignment
+{
+  // There are at most 2 gaps per end, so it is enough to use 2 bits.
+  uint8_t mGapsAtStart : 2;
+  uint8_t mGapsAtEnd : 2;
+
+  constexpr JustificationAssignment()
+    : mGapsAtStart(0)
+    , mGapsAtEnd(0)
+  {
+  }
+
+  int32_t TotalGaps() const { return mGapsAtStart + mGapsAtEnd; }
+};
+
+struct JustificationApplicationState
+{
+  struct
+  {
+    // The total number of justification gaps to be processed.
+    int32_t mCount;
+    // The number of justification gaps which have been handled.
+    int32_t mHandled;
+  } mGaps;
+
+  struct
+  {
+    // The total spacing left in a line before justification.
+    nscoord mAvailable;
+    // The spacing has been consumed by handled justification gaps.
+    nscoord mConsumed;
+  } mWidth;
+
+  JustificationApplicationState(int32_t aGaps, nscoord aWidth)
+  {
+    mGaps.mCount = aGaps;
+    mGaps.mHandled = 0;
+    mWidth.mAvailable = aWidth;
+    mWidth.mConsumed = 0;
+  }
+
+  bool IsJustifiable() const
+  {
+    return mGaps.mCount > 0 && mWidth.mAvailable > 0;
+  }
+
+  nscoord Consume(int32_t aGaps)
+  {
+    mGaps.mHandled += aGaps;
+    nscoord newAllocate = (mWidth.mAvailable * mGaps.mHandled) / mGaps.mCount;
+    nscoord deltaWidth = newAllocate - mWidth.mConsumed;
+    mWidth.mConsumed = newAllocate;
+    return deltaWidth;
+  }
+};
+
+class JustificationUtils
+{
+public:
+  // Compute justification gaps should be applied on a unit.
+  static int32_t CountGaps(const JustificationInfo& aInfo,
+                           const JustificationAssignment& aAssign)
+  {
+    // Justification gaps include two gaps for each inner opportunities
+    // and the gaps given assigned to the ends.
+    return aInfo.mInnerOpportunities * 2 + aAssign.TotalGaps();
+  }
+};
+
+} // namespace mozilla
+
+#endif /* !defined(mozilla_JustificationUtils_h_) */