Issue #1738 - Part 2: Implement well-formed JSON stringify

This implements the ES2019 spec for JSON stringification, including
lower-casing, properly escaping lone surrogates, etc.

js/src/json.cpp

@@ -66,26 +66,67 @@ InfallibleQuote(RangedPtr<const SrcCharT> srcBegin, RangedPtr<const SrcCharT> sr
     /* Step 1. */
     *dstPtr++ = '"';
 
+    // XXX: This is a rather ugly in-line definition. Move it somewhere better?
+    auto ToLowerHex = [](uint8_t u) {
+        MOZ_ASSERT(u <= 0xF);
+        return "0123456789abcdef"[u];
+    };
+
     /* Step 2. */
     while (srcBegin != srcEnd) {
-        SrcCharT c = *srcBegin++;
+        const SrcCharT c = *srcBegin++;
-        size_t escapeIndex = c % sizeof(escapeLookup);
+
-        Latin1Char escaped = escapeLookup[escapeIndex];
+        // Handle the Latin-1 cases.
-        if (MOZ_LIKELY((escapeIndex != size_t(c)) || !escaped)) {
+        if (MOZ_LIKELY(c < sizeof(escapeLookup))) {
+            Latin1Char escaped = escapeLookup[c];
+
+            // Directly copy non-escaped code points.
+            if (escaped == 0) {
                 *dstPtr++ = c;
                 continue;
             }
+
+            // Escape the rest, elaborating Unicode escapes when needed.
             *dstPtr++ = '\\';
             *dstPtr++ = escaped;
             if (escaped == 'u') {
-            MOZ_ASSERT(c < ' ');
-            MOZ_ASSERT((c >> 4) < 10);
-            uint8_t x = c >> 4, y = c % 16;
                 *dstPtr++ = '0';
                 *dstPtr++ = '0';
+
+                uint8_t x = c >> 4;
+                MOZ_ASSERT(x < 10);
                 *dstPtr++ = '0' + x;
-            *dstPtr++ = y < 10 ? '0' + y : 'a' + (y - 10);
+
+                *dstPtr++ = ToLowerHex(c & 0xF);
             }
+
+            continue;
+        }
+
+        // Non-ASCII non-surrogates are directly copied.
+        if (!unicode::IsSurrogate(c)) {
+            *dstPtr++ = c;
+            continue;
+        }
+
+        // So too for complete surrogate pairs.
+        if (MOZ_LIKELY(unicode::IsLeadSurrogate(c) &&
+                       srcBegin < srcEnd &&
+                       unicode::IsTrailSurrogate(*srcBegin)))
+        {
+            *dstPtr++ = c;
+            *dstPtr++ = *srcBegin++;
+            continue;
+        }
+
+        // But lone surrogates are Unicode-escaped.
+        char32_t as32 = char32_t(c);
+        *dstPtr++ = '\\';
+        *dstPtr++ = 'u';
+        *dstPtr++ = ToLowerHex(as32 >> 12);
+        *dstPtr++ = ToLowerHex((as32 >> 8) & 0xF);
+        *dstPtr++ = ToLowerHex((as32 >> 4) & 0xF);
+        *dstPtr++ = ToLowerHex(as32 & 0xF);
     }
 
     /* Steps 3-4. */

js/src/vm/Unicode.h

@@ -466,6 +466,19 @@ IsTrailSurrogate(uint32_t codePoint)
     return codePoint >= TrailSurrogateMin && codePoint <= TrailSurrogateMax;
 }
 
+/**
+ * Returns true if the given value is a UTF-16 surrogate.
+ *
+ * This function is intended for use in contexts where 32-bit values may need
+ * to be tested to see if they reside in the surrogate range, so it doesn't
+ * just take char16_t.
+ */
+inline bool
+IsSurrogate(uint32_t codePoint)
+{
+  return LeadSurrogateMin <= codePoint && codePoint <= TrailSurrogateMax;
+}
+
 inline char16_t
 LeadSurrogate(uint32_t codePoint)
 {