Suppress linebreaks in emoji ZWJ sequences - DO NOT MERGE

Due to the way emoji ZWJ sequences are defined, the ICU line breaking
algorithm determines that there are valid line breaks inside the
sequence. This patch suppresses these line breaks.

Bug: 25433289
Change-Id: I225ebebc0f4186e4b8f48fee399c4a62b3f0218a

libs/minikin/LineBreaker.cpp

@@ -17,6 +17,7 @@
 #define VERBOSE_DEBUG 0
 
 #include <limits>
+#include <unicode/utf16.h>
 
 #define LOG_TAG "Minikin"
 #include <cutils/log.h>
@@ -30,6 +31,7 @@ namespace android {
 
 const int CHAR_TAB = 0x0009;
 const uint16_t CHAR_SOFT_HYPHEN = 0x00AD;
+const uint16_t CHAR_ZWJ = 0x200D;
 
 // Large scores in a hierarchy; we prefer desperate breaks to an overfull line. All these
 // constants are larger than any reasonable actual width score.
@@ -123,6 +125,32 @@ static bool isLineBreakingHyphen(uint16_t c) {
             c == 0x2E40);  // DOUBLE HYPHEN
 }
 
+/**
+ * Determine whether a line break at position i within the buffer buf is valid. This
+ * represents customization beyond the ICU behavior, because plain ICU provides some
+ * line break opportunities that we don't want.
+ **/
+static bool isBreakValid(uint16_t codeUnit, const uint16_t* buf, size_t bufEnd, size_t i) {
+    if (codeUnit == CHAR_SOFT_HYPHEN) {
+        return false;
+    }
+    if (codeUnit == CHAR_ZWJ) {
+        // Possible emoji ZWJ sequence
+        uint32_t next_codepoint;
+        U16_NEXT(buf, i, bufEnd, next_codepoint);
+        if (next_codepoint == 0x2764 ||       // HEAVY BLACK HEART
+                next_codepoint == 0x1F466 ||  // BOY
+                next_codepoint == 0x1F467 ||  // GIRL
+                next_codepoint == 0x1F468 ||  // MAN
+                next_codepoint == 0x1F469 ||  // WOMAN
+                next_codepoint == 0x1F48B ||  // KISS MARK
+                next_codepoint == 0x1F5E8) {  // LEFT SPEECH BUBBLE
+            return false;
+        }
+    }
+    return true;
+}
+
 // Ordinarily, this method measures the text in the range given. However, when paint
 // is nullptr, it assumes the widths have already been calculated and stored in the
 // width buffer.
@@ -175,8 +203,9 @@ float LineBreaker::addStyleRun(MinikinPaint* paint, const FontCollection* typefa
         }
         if (i + 1 == current) {
             // Override ICU's treatment of soft hyphen as a break opportunity, because we want it
-            // to be a hyphen break, with penalty and drawing behavior.
-            if (c != CHAR_SOFT_HYPHEN) {
+            // to be a hyphen break, with penalty and drawing behavior. Also, suppress line
+            // breaks within emoji ZWJ sequences.
+            if (isBreakValid(c, mTextBuf.data(), end, i + 1)) {
                 // TODO: Add a new type of HyphenEdit for breaks whose hyphen already exists, so
                 // we can pass the whole word down to Hyphenator like the soft hyphen case.
                 bool wordEndsInHyphen = isLineBreakingHyphen(c);