Add support for shaper-driven line breaking to HarfBuzzShaper

third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp

Index: third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp
diff --git a/third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp b/third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp
index 08180a69c22f7ec9a4fa04359f1845ab00049c83..a01ba87a573ec81b21183cfafb0931a47c438310 100644
--- a/third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp
+++ b/third_party/WebKit/Source/platform/fonts/shaping/HarfBuzzShaper.cpp
@@ -31,6 +31,12 @@
 
 #include "platform/fonts/shaping/HarfBuzzShaper.h"
 
+#include <hb.h>
+#include <unicode/uchar.h>
+#include <unicode/uscript.h>
+#include <algorithm>
+#include <memory>
+#include "platform/LayoutUnit.h"
 #include "platform/fonts/Font.h"
 #include "platform/fonts/FontDescription.h"
 #include "platform/fonts/FontFallbackIterator.h"
@@ -46,11 +52,6 @@
 #include "wtf/MathExtras.h"
 #include "wtf/PtrUtil.h"
 #include "wtf/text/Unicode.h"
-#include <algorithm>
-#include <hb.h>
-#include <memory>
-#include <unicode/uchar.h>
-#include <unicode/uscript.h>
 
 namespace blink {
 enum HolesQueueItemAction { HolesQueueNextFont, HolesQueueRange };
@@ -708,4 +709,169 @@ PassRefPtr<ShapeResult> HarfBuzzShaper::shape(const Font* font,
                                               TextDirection direction) const {
   return shape(font, direction, 0, m_textLength);
 }
+
+namespace {
+
+unsigned previousBreakOpportunity(unsigned start,
+                                  unsigned offset,
+                                  unsigned textLength,
+                                  LazyLineBreakIterator& breakIterator,
+                                  LineBreakType breakType) {
+  unsigned pos = std::min(start + offset, textLength);
+  for (; pos > start; pos--) {
+    int nextBreak = 0;
+    if (breakIterator.isBreakable(pos, nextBreak, breakType))
+      return pos;
+  }
+  return start;
+}
+
+unsigned nextBreakOpportunity(unsigned offset,
+                              LazyLineBreakIterator& breakIterator,
+                              LineBreakType breakType) {
+  int nextBreak = 0;
+  breakIterator.isBreakable(offset, nextBreak, breakType);
+  return nextBreak;
+}
+
+unsigned previousSafeToBreakAfter(const UChar* text,
+                                  unsigned start,
+                                  unsigned offset) {
+  // TODO: This should use harf-buzz safe to break info when available. For now

[drott, 2017/03/22 21:01:11]

s/harf-buzz/HarfBuzz/

+  // it'll scan back until the previous space character or something.
+  for (; offset > start; offset--) {
+    if (text[offset - 1] == spaceCharacter)
+      break;
+  }
+  return offset;
+}
+
+unsigned nextSafeToBreakBefore(const UChar* text,
+                               unsigned end,
+                               unsigned offset) {
+  for (; offset < end; offset++) {
+    if (text[offset] == spaceCharacter)
+      break;
+  }
+  return offset;
+}
+
+}  // namespace
+
+// Shapes a line of text by finding a valid and appropriate break opportunity
+// based on the shaping results for the entire paragraph. Re-shapes the start
+// and end of the line as needed.
+//
+// Definitions:
+//   Candidate break opportunity: Ideal point to break, disregarding line
+//                                breaking rules. May be in the middle of a word
+//                                or inside a ligature.
+//    Valid break opportunity:    A point where a break is allowed according to
+//                                the relevant breaking rules.
+//    Safe-to-break:              A point where a break may occur without
+//                                affecting the rendering or metrics of the
+//                                text. Breaking at safe-to-break point does not
+//                                require reshaping.
+//
+// For example:
+//   Given the string "Line breaking example", an available space of 100px and a
+//   mono-space font where each glyph is 10px wide.
+//
+//   Line breaking example
+//   |        |
+//   0       100px
+//
+//   The candidate (or ideal) break opportunity would be at an offset of 10 as
+//   the break would happen at exactly 100px in that case.
+//   The previous valid break opportunity though is at an offset of 5.
+//   If we further assume that the font kerns with space then even though it's a
+//   valid break opportunity reshaping is required as the combined width of the
+//   two segments "Line " and "breaking" may be different from "Line breaking".
+PassRefPtr<ShapeResult> HarfBuzzShaper::shapeLine(const Font* font,
+                                                  const ShapeResult* result,
+                                                  unsigned startOffset,
+                                                  const AtomicString locale,
+                                                  LineBreakType breakType,
+                                                  LayoutUnit availableSpace,
+                                                  unsigned* breakOffset) const {
+  // The start position in the original shape results.
+  LayoutUnit startPosition = result->snappedStartPositionForOffset(startOffset);

[drott, 2017/03/22 21:01:11]

IIUC, this one is pixel snapped, why is that the case? Does it mean that words
(or every piece of string between breaks) can only be positioned at full pixels?

[eae, 2017/03/27 20:14:02]

No, this snaps to the nearest 1/64th of a pixel, not full pixels. It is needed
as the layout system operates entirely with fixed point LayoutUnits.

Any start and end position or length needs to (eventually) be snapped to
LayoutUnits. 

[drott, 2017/03/27 20:24:36]

I see, that makes more sense, thank you for the explanation.

+
+  // If the start offset is not at a safe-to-break boundary content between

[drott, 2017/03/22 21:01:11]

What do you mean by "content" here? Or probably there is a "the" missing?

[eae, 2017/03/27 20:14:02]

Missing "the", thank you.

+  // the start and the next safe-to-break boundary needs to be reshaped and the
+  // available space adjusted to take the reshaping into account.
+  RefPtr<ShapeResult> lineStartResult;
+  unsigned firstSafe = nextSafeToBreakBefore(m_text, m_textLength, startOffset);
+  if (firstSafe != startOffset) {
+    LayoutUnit originalWidth =
+        result->snappedEndPositionForOffset(firstSafe) - startPosition;
+    lineStartResult = shape(font, result->direction(), startOffset, firstSafe);
+    availableSpace += lineStartResult->snappedWidth() - originalWidth;
+  }
+
+  // Find a candidate break opportunity by identifying the last offset before
+  // exceeding the available space and the determine the closest valid break
+  // preceding the candidate.
+  LazyLineBreakIterator breakIterator(String(m_text, m_textLength), locale);
+  LayoutUnit endPosition = startPosition + availableSpace;
+  unsigned candidateBreak = result->offsetForPosition(endPosition, false);
+  unsigned breakOpportunity = previousBreakOpportunity(
+      startOffset, candidateBreak, m_textLength, breakIterator, breakType);
+  if (breakOpportunity <= startOffset) {
+    breakOpportunity =
+        nextBreakOpportunity(candidateBreak, breakIterator, breakType);
+  }
+
+  RefPtr<ShapeResult> lineEndResult;
+  unsigned lastSafe = breakOpportunity;
+  while (breakOpportunity > startOffset) {
+    // If the previous valid break opportunity is not at a safe-to-break
+    // boundary reshape between the safe-to-break offset and the valid break
+    // offset. If the resulting width exceeds the available space the
+    // preceding boundary is tried until the available space is sufficient.
+    unsigned previousSafe = std::max(

[drott, 2017/03/22 21:01:11]

Nice.

+        previousSafeToBreakAfter(m_text, startOffset, breakOpportunity),
+        startOffset);
+    if (previousSafe != breakOpportunity) {
+      LayoutUnit safePosition =
+          result->snappedStartPositionForOffset(previousSafe);
+      while (breakOpportunity > previousSafe && previousSafe > startOffset) {
+        lineEndResult =
+            shape(font, result->direction(), previousSafe, breakOpportunity);
+        if (safePosition + lineEndResult->snappedWidth() <= endPosition)
+          break;
+        lineEndResult = nullptr;
+        breakOpportunity =
+            previousBreakOpportunity(startOffset, breakOpportunity - 1,
+                                     m_textLength, breakIterator, breakType);
+      }
+    }
+
+    if (breakOpportunity > startOffset) {
+      lastSafe = previousSafe;
+      break;
+    }
+
+    // No suitable break opportunity, not exceeding the available space,
+    // found. Choose the next valid one even though it will overflow.
+    breakOpportunity =
+        nextBreakOpportunity(candidateBreak, breakIterator, breakType);
+  }
+
+  // Create shape results for the line by copying from the re-shaped result (if
+  // reshaping was needed) and the original shape results.
+  RefPtr<ShapeResult> lineResult =
+      ShapeResult::create(font, 0, result->direction());
+  unsigned maxLength = std::numeric_limits<unsigned>::max();
+  if (lineStartResult)
+    lineStartResult->copyRange(0, maxLength, lineResult.get());
+  if (lastSafe > firstSafe)
+    result->copyRange(firstSafe, lastSafe, lineResult.get());
+  if (lineEndResult)
+    lineEndResult->copyRange(lastSafe, maxLength, lineResult.get());
+
+  *breakOffset = breakOpportunity;
+  return lineResult.release();
+}
+
 }  // namespace blink