layout/generic/JustificationUtils.h

   1 /* -*- Mode: C++; tab-width: 8; 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 #include "nsCoord.h"
  12
  13 namespace mozilla {
  14
  15 /**
  16  * Jutification Algorithm
  17  *
  18  * The justification algorithm is based on expansion opportunities
  19  * between justifiable clusters.  By this algorithm, there is one
  20  * expansion opportunity at each side of a justifiable cluster, and
  21  * at most one opportunity between two clusters. For example, if there
  22  * is a line in a Chinese document is: "你好世界hello world", then
  23  * the expansion opportunities (marked as '*') would be:
  24  *
  25  *                    你*好*世*界*hello*' '*world
  26  *
  27  * The spacing left in a line will then be distributed equally to each
  28  * opportunities. Because we want that, only justifiable clusters get
  29  * expanded, and the split point between two justifiable clusters would
  30  * be at the middle of the spacing, each expansion opportunities will be
  31  * filled by two justification gaps. The example above would be:
  32  *
  33  *              你 | 好 | 世 | 界  |hello|  ' '  |world
  34  *
  35  * In the algorithm, information about expansion opportunities is stored
  36  * in structure JustificationInfo, and the assignment of justification
  37  * gaps is in structure JustificationAssignment.
  38  */
  39
  40 struct JustificationInfo {
  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   constexpr JustificationInfo()
  49       : mInnerOpportunities(0),
  50         mIsStartJustifiable(false),
  51         mIsEndJustifiable(false) {}
  52
  53   // Claim that the last opportunity should be cancelled
  54   // because the trailing space just gets trimmed.
  55   void CancelOpportunityForTrimmedSpace() {
  56     if (mInnerOpportunities > 0) {
  57       mInnerOpportunities--;
  58     } else {
  59       // There is no inner opportunities, hence the whole frame must
  60       // contain only the trimmed space, because any content before
  61       // space would cause an inner opportunity. The space made each
  62       // side justifiable, which should be cancelled now.
  63       mIsStartJustifiable = false;
  64       mIsEndJustifiable = false;
  65     }
  66   }
  67 };
  68
  69 struct JustificationAssignment {
  70   // There are at most 2 gaps per end, so it is enough to use 2 bits.
  71   uint8_t mGapsAtStart : 2;
  72   uint8_t mGapsAtEnd : 2;
  73
  74   constexpr JustificationAssignment() : mGapsAtStart(0), mGapsAtEnd(0) {}
  75
  76   int32_t TotalGaps() const { return mGapsAtStart + mGapsAtEnd; }
  77 };
  78
  79 struct JustificationApplicationState {
  80   struct {
  81     // The total number of justification gaps to be processed.
  82     int32_t mCount;
  83     // The number of justification gaps which have been handled.
  84     int32_t mHandled;
  85   } mGaps;
  86
  87   struct {
  88     // The total spacing left in a line before justification.
  89     nscoord mAvailable;
  90     // The spacing has been consumed by handled justification gaps.
  91     nscoord mConsumed;
  92   } mWidth;
  93
  94   JustificationApplicationState(int32_t aGaps, nscoord aWidth) {
  95     mGaps.mCount = aGaps;
  96     mGaps.mHandled = 0;
  97     mWidth.mAvailable = aWidth;
  98     mWidth.mConsumed = 0;
  99   }
 100
 101   bool IsJustifiable() const {
 102     return mGaps.mCount > 0 && mWidth.mAvailable > 0;
 103   }
 104
 105   nscoord Consume(int32_t aGaps) {
 106     mGaps.mHandled += aGaps;
 107     nscoord newAllocate = (mWidth.mAvailable * mGaps.mHandled) / mGaps.mCount;
 108     nscoord deltaWidth = newAllocate - mWidth.mConsumed;
 109     mWidth.mConsumed = newAllocate;
 110     return deltaWidth;
 111   }
 112 };
 113
 114 class JustificationUtils {
 115  public:
 116   // Compute justification gaps should be applied on a unit.
 117   static int32_t CountGaps(const JustificationInfo& aInfo,
 118                            const JustificationAssignment& aAssign) {
 119     // Justification gaps include two gaps for each inner opportunities
 120     // and the gaps given assigned to the ends.
 121     return aInfo.mInnerOpportunities * 2 + aAssign.TotalGaps();
 122   }
 123 };
 124
 125 }  // namespace mozilla
 126
 127 #endif /* !defined(mozilla_JustificationUtils_h_) */