layout/generic/JustificationUtils.h

   1 /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef mozilla_JustificationUtils_h_
   8 #define mozilla_JustificationUtils_h_
   9
  10 #include "mozilla/Attributes.h"
  11
  12 namespace mozilla {
  13
  14 /**
  15  * Jutification Algorithm
  16  *
  17  * The justification algorithm is based on expansion opportunities
  18  * between justifiable clusters.  By this algorithm, there is one
  19  * expansion opportunity at each side of a justifiable cluster, and
  20  * at most one opportunity between two clusters. For example, if there
  21  * is a line in a Chinese document is: "你好世界hello world", then
  22  * the expansion opportunities (marked as '*') would be:
  23  *
  24  *                    你*好*世*界*hello*' '*world
  25  *
  26  * The spacing left in a line will then be distributed equally to each
  27  * opportunities. Because we want that, only justifiable clusters get
  28  * expanded, and the split point between two justifiable clusters would
  29  * be at the middle of the spacing, each expansion opportunities will be
  30  * filled by two justification gaps. The example above would be:
  31  *
  32  *              你 | 好 | 世 | 界  |hello|  ' '  |world
  33  *
  34  * In the algorithm, information about expansion opportunities is stored
  35  * in structure JustificationInfo, and the assignment of justification
  36  * gaps is in structure JustificationAssignment.
  37  */
  38
  39 struct JustificationInfo
  40 {
  41   // Number of expansion opportunities inside a span. It doesn't include
  42   // any opportunities between this span and the one before or after.
  43   int32_t mInnerOpportunities;
  44   // The justifiability of the start and end sides of the span.
  45   bool mIsStartJustifiable;
  46   bool mIsEndJustifiable;
  47
  48   MOZ_CONSTEXPR JustificationInfo()
  49     : mInnerOpportunities(0)
  50     , mIsStartJustifiable(false)
  51     , mIsEndJustifiable(false)
  52   {
  53   }
  54
  55   // Claim that the last opportunity should be cancelled
  56   // because the trailing space just gets trimmed.
  57   void CancelOpportunityForTrimmedSpace()
  58   {
  59     if (mInnerOpportunities > 0) {
  60       mInnerOpportunities--;
  61     } else {
  62       // There is no inner opportunities, hence the whole frame must
  63       // contain only the trimmed space, because any content before
  64       // space would cause an inner opportunity. The space made each
  65       // side justifiable, which should be cancelled now.
  66       mIsStartJustifiable = false;
  67       mIsEndJustifiable = false;
  68     }
  69   }
  70 };
  71
  72 struct JustificationAssignment
  73 {
  74   // There are at most 2 gaps per end, so it is enough to use 2 bits.
  75   uint8_t mGapsAtStart : 2;
  76   uint8_t mGapsAtEnd : 2;
  77
  78   MOZ_CONSTEXPR JustificationAssignment()
  79     : mGapsAtStart(0)
  80     , mGapsAtEnd(0)
  81   {
  82   }
  83
  84   int32_t TotalGaps() const { return mGapsAtStart + mGapsAtEnd; }
  85 };
  86
  87 struct JustificationApplicationState
  88 {
  89   struct
  90   {
  91     // The total number of justification gaps to be processed.
  92     int32_t mCount;
  93     // The number of justification gaps which have been handled.
  94     int32_t mHandled;
  95   } mGaps;
  96
  97   struct
  98   {
  99     // The total spacing left in a line before justification.
 100     nscoord mAvailable;
 101     // The spacing has been consumed by handled justification gaps.
 102     nscoord mConsumed;
 103   } mWidth;
 104
 105   JustificationApplicationState(int32_t aGaps, nscoord aWidth)
 106   {
 107     mGaps.mCount = aGaps;
 108     mGaps.mHandled = 0;
 109     mWidth.mAvailable = aWidth;
 110     mWidth.mConsumed = 0;
 111   }
 112
 113   bool IsJustifiable() const
 114   {
 115     return mGaps.mCount > 0 && mWidth.mAvailable > 0;
 116   }
 117
 118   nscoord Consume(int32_t aGaps)
 119   {
 120     mGaps.mHandled += aGaps;
 121     nscoord newAllocate = (mWidth.mAvailable * mGaps.mHandled) / mGaps.mCount;
 122     nscoord deltaWidth = newAllocate - mWidth.mConsumed;
 123     mWidth.mConsumed = newAllocate;
 124     return deltaWidth;
 125   }
 126 };
 127
 128 class JustificationUtils
 129 {
 130 public:
 131   // Compute justification gaps should be applied on a unit.
 132   static int32_t CountGaps(const JustificationInfo& aInfo,
 133                            const JustificationAssignment& aAssign)
 134   {
 135     // Justification gaps include two gaps for each inner opportunities
 136     // and the gaps given assigned to the ends.
 137     return aInfo.mInnerOpportunities * 2 + aAssign.TotalGaps();
 138   }
 139 };
 140
 141 }
 142
 143 #endif /* !defined(mozilla_JustificationUtils_h_) */