Fix ZWJ not working for Indic fonts

This is a fix for bug 15185229 ZWJ not working in Sinhala and Kannada.

Indic fonts (unlike Arabic) require the entire string, including ZWJ,
to be passed to Harfbuzz; it's not enough for the ZWJ to be present in
the context. The solution is to be "sticky" in font itemization,
continuing to use the same font as long as it has Unicode coverage.

Change-Id: I7673bc56fbda09f1e1a4582e8d88342343b706f1

2 files changed, 25 insertions, 27 deletions

diff --git a/include/minikin/FontCollection.h b/include/minikin/FontCollection.h
index a350237..78ab2aa 100644
--- a/include/minikin/FontCollection.h
+++ b/include/minikin/FontCollection.h
@@ -32,8 +32,6 @@ public:
 
     ~FontCollection();
 
-    const FontFamily* getFamilyForChar(uint32_t ch, FontLanguage lang, int variant) const;
-
     class Run {
     public:
         // Do copy constructor, assignment, destructor so it can be used in vectors
@@ -74,6 +72,8 @@ private:
         size_t end;
     };
 
+    const FontInstance* getInstanceForChar(uint32_t ch, FontLanguage lang, int variant) const;
+
     // static for allocating unique id's
     static uint32_t sNextId;
 
diff --git a/libs/minikin/FontCollection.cpp b/libs/minikin/FontCollection.cpp
index 89086ee..1713b47 100644
--- a/libs/minikin/FontCollection.cpp
+++ b/libs/minikin/FontCollection.cpp
@@ -114,13 +114,8 @@ FontCollection::~FontCollection() {
 // 3. If a font matches just language, it gets a score of 2.
 // 4. Matching the "compact" or "elegant" variant adds one to the score.
 // 5. Highest score wins, with ties resolved to the first font.
-
-// Note that we may want to make the selection more dependent on
-// context, so for example a sequence of Devanagari, ZWJ, Devanagari
-// would get itemized as one run, even though by the rules the ZWJ
-// would go to the Latin font.
-const FontFamily* FontCollection::getFamilyForChar(uint32_t ch, FontLanguage lang,
-            int variant) const {
+const FontCollection::FontInstance* FontCollection::getInstanceForChar(uint32_t ch,
+            FontLanguage lang, int variant) const {
     if (ch >= mMaxChar) {
         return NULL;
     }
@@ -128,7 +123,7 @@ const FontFamily* FontCollection::getFamilyForChar(uint32_t ch, FontLanguage lan
 #ifdef VERBOSE_DEBUG
     ALOGD("querying range %d:%d\n", range.start, range.end);
 #endif
-    FontFamily* bestFamily = NULL;
+    const FontInstance* bestInstance = NULL;
     int bestScore = -1;
     for (size_t i = range.start; i < range.end; i++) {
         const FontInstance* instance = mInstanceVec[i];
@@ -136,7 +131,7 @@ const FontFamily* FontCollection::getFamilyForChar(uint32_t ch, FontLanguage lan
             FontFamily* family = instance->mFamily;
             // First font family in collection always matches
             if (mInstances[0].mFamily == family) {
-                return family;
+                return instance;
             }
             int score = lang.match(family->lang()) * 2;
             if (variant != 0 && variant == family->variant()) {
@@ -144,18 +139,18 @@ const FontFamily* FontCollection::getFamilyForChar(uint32_t ch, FontLanguage lan
             }
             if (score > bestScore) {
                 bestScore = score;
-                bestFamily = family;
+                bestInstance = instance;
             }
         }
     }
-    return bestFamily;
+    return bestInstance;
 }
 
 void FontCollection::itemize(const uint16_t *string, size_t string_size, FontStyle style,
         vector<Run>* result) const {
     FontLanguage lang = style.getLanguage();
     int variant = style.getVariant();
-    const FontFamily* lastFamily = NULL;
+    const FontInstance* lastInstance = NULL;
     Run* run = NULL;
     int nShorts;
     for (size_t i = 0; i < string_size; i += nShorts) {
@@ -168,20 +163,23 @@ void FontCollection::itemize(const uint16_t *string, size_t string_size, FontSty
                 nShorts = 2;
             }
         }
-        const FontFamily* family = getFamilyForChar(ch, lang, variant);
-        if (i == 0 || family != lastFamily) {
-            Run dummy;
-            result->push_back(dummy);
-            run = &result->back();
-            if (family == NULL) {
-                run->font = NULL;  // maybe we should do something different here
-            } else {
-                run->font = family->getClosestMatch(style);
-                // TODO: simplify refcounting (FontCollection lifetime dominates)
-                run->font->RefLocked();
+        // Continue using existing font as long as it has coverage.
+        if (lastInstance == NULL || !lastInstance->mCoverage->get(ch)) {
+            const FontInstance* instance = getInstanceForChar(ch, lang, variant);
+            if (i == 0 || instance != lastInstance) {
+                Run dummy;
+                result->push_back(dummy);
+                run = &result->back();
+                if (instance == NULL) {
+                    run->font = NULL;  // maybe we should do something different here
+                } else {
+                    run->font = instance->mFamily->getClosestMatch(style);
+                    // TODO: simplify refcounting (FontCollection lifetime dominates)
+                    run->font->RefLocked();
+                }
+                lastInstance = instance;
+                run->start = i;
             }
-            lastFamily = family;
-            run->start = i;
         }
         run->end = i + nShorts;
     }