Fix measurement to respect grapheme and span boundaries

When measuring in TextLine (important for cursor positioning), the
substring measured must neither be too short (it can't just be the
substring up to the measure limit, but must include additional
characters if they form a ligature) nor too long (it can't extend
beyond the end of the CharacterStyle span, otherwise the measurement
fails to account for the fact that the CharacterStyle span boundary
breaks the ligature). This patch gets it just right.

Bug: 25375561
Change-Id: I36e1c4bdc66424d3b611cf54031756cf54cf3fec
diff --git a/core/java/android/text/TextLine.java b/core/java/android/text/TextLine.java
index 39e8694..3592187 100644
--- a/core/java/android/text/TextLine.java
+++ b/core/java/android/text/TextLine.java
@@ -718,13 +718,14 @@
      * @param bottom the bottom of the line
      * @param fmi receives metrics information, can be null
      * @param needWidth true if the width of the run is needed
+     * @param offset the offset for the purpose of measuring
      * @return the signed width of the run based on the run direction; only
      * valid if needWidth is true
      */
     private float handleText(TextPaint wp, int start, int end,
             int contextStart, int contextEnd, boolean runIsRtl,
             Canvas c, float x, int top, int y, int bottom,
-            FontMetricsInt fmi, boolean needWidth) {
+            FontMetricsInt fmi, boolean needWidth, int offset) {
 
         // Get metrics first (even for empty strings or "0" width runs)
         if (fmi != null) {
@@ -742,11 +743,11 @@
         if (needWidth || (c != null && (wp.bgColor != 0 || wp.underlineColor != 0 || runIsRtl))) {
             if (mCharsValid) {
                 ret = wp.getRunAdvance(mChars, start, end, contextStart, contextEnd,
-                        runIsRtl, end);
+                        runIsRtl, offset);
             } else {
                 int delta = mStart;
                 ret = wp.getRunAdvance(mText, delta + start, delta + end,
-                        delta + contextStart, delta + contextEnd, runIsRtl, delta + end);
+                        delta + contextStart, delta + contextEnd, runIsRtl, delta + offset);
             }
         }
 
@@ -895,8 +896,8 @@
             TextPaint wp = mWorkPaint;
             wp.set(mPaint);
             final int mlimit = measureLimit;
-            return handleText(wp, start, mlimit, start, limit, runIsRtl, c, x, top,
-                    y, bottom, fmi, needWidth || mlimit < measureLimit);
+            return handleText(wp, start, limit, start, limit, runIsRtl, c, x, top,
+                    y, bottom, fmi, needWidth || mlimit < measureLimit, mlimit);
         }
 
         mMetricAffectingSpanSpanSet.init(mSpanned, mStart + start, mStart + limit);
@@ -940,13 +941,14 @@
             }
 
             for (int j = i, jnext; j < mlimit; j = jnext) {
-                jnext = mCharacterStyleSpanSet.getNextTransition(mStart + j, mStart + mlimit) -
+                jnext = mCharacterStyleSpanSet.getNextTransition(mStart + j, mStart + inext) -
                         mStart;
+                int offset = Math.min(jnext, mlimit);
 
                 wp.set(mPaint);
                 for (int k = 0; k < mCharacterStyleSpanSet.numberOfSpans; k++) {
                     // Intentionally using >= and <= as explained above
-                    if ((mCharacterStyleSpanSet.spanStarts[k] >= mStart + jnext) ||
+                    if ((mCharacterStyleSpanSet.spanStarts[k] >= mStart + offset) ||
                             (mCharacterStyleSpanSet.spanEnds[k] <= mStart + j)) continue;
 
                     CharacterStyle span = mCharacterStyleSpanSet.spans[k];
@@ -958,7 +960,7 @@
                     wp.setHyphenEdit(0);
                 }
                 x += handleText(wp, j, jnext, i, inext, runIsRtl, c, x,
-                        top, y, bottom, fmi, needWidth || jnext < measureLimit);
+                        top, y, bottom, fmi, needWidth || jnext < measureLimit, offset);
             }
         }