add helper to create fancy underlines

src/core/SkGlyph.h

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkGlyph_DEFINED
 #define SkGlyph_DEFINED
 
@@ skipping 16 matching lines @@
     enum {
         kSubBits = 2,
         kSubMask = ((1 << kSubBits) - 1),
         kSubShift = 24, // must be large enough for glyphs and unichars
         kCodeMask = ((1 << kSubShift) - 1),
         // relative offsets for X and Y subpixel bits
         kSubShiftX = kSubBits,
         kSubShiftY = 0
     };
 
- public:
+    // Support horizontal and vertical skipping strike-through / underlines.
+    // The caller walks the linked list looking for a match. For a horizontal underline,
+    // the fBounds contains the top and bottom of the underline. The fInterval pair contains the
+    // beginning and end of of the intersection of the bounds and the glyph's path.
+    // If interval[0] >= interval[1], no intesection was found.
+    struct Intercept {
+        Intercept* fNext;
+        SkScalar fBounds[2];    // for horz underlines, the boundaries in Y
+        SkScalar fInterval[2];  // the outside intersections of the axis and the glyph
+    };
+
+    Intercept*  fIntercept;

[f(malita), 2016/02/08 14:20:27]

Caching this is nice, but may be overkill at this stage.  Since we don't know
how widely this feature is going to be used in Blink, is this something we can
defer for later (if needed)?

[caryclark, 2016/02/09 19:10:57]

If the same glyphs are underlined widely by the caller, the cache could make an
enormous performance difference. To mitigate the concerns about increasing the
size of the cache, I restructured the CL so that the same slot contains the
cached path and the cached underline.

+public:
     static const SkFixed kSubpixelRound = SK_FixedHalf >> SkGlyph::kSubBits;
     // A value that can never be generated by MakeID.
     static const uint32_t kImpossibleID = ~0;
     void*       fImage;
     SkPath*     fPath;
     SkFixed     fAdvanceX, fAdvanceY;
 
     uint16_t    fWidth, fHeight;
     int16_t     fTop, fLeft;
 
@@ skipping 78 matching lines @@
         }
     };
 
  private:
     // TODO(herb) remove friend statement after SkGlyphCache cleanup.
     friend class SkGlyphCache;
 
     void initCommon(uint32_t id) {
         fID             = id;
         fImage          = nullptr;
+        fIntercept      = nullptr;
         fPath           = nullptr;
         fMaskFormat     = MASK_FORMAT_UNKNOWN;
         fForceBW        = 0;
     }
 
     static unsigned ID2Code(uint32_t id) {
         return id & kCodeMask;
     }
 
     static unsigned ID2SubX(uint32_t id) {
@@ skipping 23 matching lines @@
         SkASSERT(code <= kCodeMask);
         x = FixedToSub(x);
         y = FixedToSub(y);
         uint32_t ID = (x << (kSubShift + kSubShiftX)) |
                       (y << (kSubShift + kSubShiftY)) |
                       code;
         SkASSERT(ID != kImpossibleID);
         return ID;
     }
 
-  // FIXME - This is needed because the Android frame work directly
+    // FIXME - This is needed because the Android frame work directly
   // accesses fID. Remove when fID accesses are cleaned up.
 #ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
   public:
 #endif
     uint32_t    fID;
 };
 SK_END_REQUIRE_DENSE
 
 #endif