Add margins array to line widths object

In order to support layout in non-rectangular regions, the LineWidths
object needs to accept an arbitrary array of margins. This is
implemented in addition to the existing firstWidthLineCount/restWidth
mechanism for convenience, though using only arrays would have the
same expressive power.

Bug: 20182243
Change-Id: Iea96bca1a92012314ac27e617c67f306c1f1b2f2

include/minikin/LineBreaker.h

@@ -44,17 +44,29 @@ class LineWidths {
             mFirstWidthLineCount = firstWidthLineCount;
             mRestWidth = restWidth;
         }
+        void setMargins(const std::vector<float>& margins) {
+            mMargins = margins;
+        }
         bool isConstant() const {
             // technically mFirstWidthLineCount == 0 would count too, but doesn't actually happen
-            return mRestWidth == mFirstWidth;
+            return mRestWidth == mFirstWidth && mMargins.empty();
         }
         float getLineWidth(int line) const {
-            return (line < mFirstWidthLineCount) ? mFirstWidth : mRestWidth;
+            float width = (line < mFirstWidthLineCount) ? mFirstWidth : mRestWidth;
+            if (!mMargins.empty()) {
+                if ((size_t)line < mMargins.size()) {
+                    width -= mMargins[line];
+                } else {
+                    width -= mMargins.back();
+                }
+            }
+            return width;
         }
     private:
         float mFirstWidth;
         int mFirstWidthLineCount;
         float mRestWidth;
+        std::vector<float> mMargins;
 };
 
 class TabStops {
@@ -120,6 +132,8 @@ class LineBreaker {
 
         void setLineWidths(float firstWidth, int firstWidthLineCount, float restWidth);
 
+        void setMargins(const std::vector<float>& margins);
+
         void setTabStops(const int* stops, size_t nStops, int tabWidth) {
             mTabStops.set(stops, nStops, tabWidth);
         }

libs/minikin/LineBreaker.cpp

@@ -78,6 +78,11 @@ void LineBreaker::setLineWidths(float firstWidth, int firstWidthLineCount, float
     mLineWidths.setWidths(firstWidth, firstWidthLineCount, restWidth);
 }
 
+
+void LineBreaker::setMargins(const std::vector<float>& margins) {
+    mLineWidths.setMargins(margins);
+}
+
 // This function determines whether a character is a space that disappears at end of line.
 // It is the Unicode set: [[:General_Category=Space_Separator:]-[:Line_Break=Glue:]]
 // Note: all such characters are in the BMP, so it's ok to use code units for this.