Move codePointAt() to separate CodePointSequence class.

src/vm/wren_core.c

@@ -144,6 +144,7 @@ static const char* coreLibSource =
 "\n"
 "class String is Sequence {\n"
 "  bytes { StringByteSequence.new(this) }\n"
+"  codePoints { StringCodePointSequence.new(this) }\n"
 "}\n"
 "\n"
 "class StringByteSequence is Sequence {\n"
@@ -158,6 +159,18 @@ static const char* coreLibSource =
 "  count { _string.byteCount_ }\n"
 "}\n"
 "\n"
+"class StringCodePointSequence is Sequence {\n"
+"  construct new(string) {\n"
+"    _string = string\n"
+"  }\n"
+"\n"
+"  [index] { _string.codePointAt_(index) }\n"
+"  iterate(iterator) { _string.iterate(iterator) }\n"
+"  iteratorValue(iterator) { _string.codePointAt_(iterator) }\n"
+"\n"
+"  count { _string.count }\n"
+"}\n"
+"\n"
 "class List is Sequence {\n"
 "  addAll(other) {\n"
 "    for (element in other) {\n"
@@ -1418,7 +1431,7 @@ void wrenInitializeCore(WrenVM* vm)
   PRIMITIVE(vm->stringClass, "[_]", string_subscript);
   PRIMITIVE(vm->stringClass, "byteAt_(_)", string_byteAt);
   PRIMITIVE(vm->stringClass, "byteCount_", string_byteCount);
-  PRIMITIVE(vm->stringClass, "codePointAt(_)", string_codePointAt);
+  PRIMITIVE(vm->stringClass, "codePointAt_(_)", string_codePointAt);
   PRIMITIVE(vm->stringClass, "contains(_)", string_contains);
   PRIMITIVE(vm->stringClass, "count", string_count);
   PRIMITIVE(vm->stringClass, "endsWith(_)", string_endsWith);

src/vm/wren_value.c

@@ -772,8 +772,18 @@ Value wrenStringCodePointAt(WrenVM* vm, ObjString* string, uint32_t index)
 {
   ASSERT(index < string->length, "Index out of bounds.");
   
-  int numBytes = wrenUtf8DecodeNumBytes(string->value[index]);
-  return wrenNewString(vm, string->value + index, numBytes);
+  int codePoint = wrenUtf8Decode((uint8_t*)string->value + index,
+                                 string->length - index);
+  if (codePoint == -1)
+  {
+    // If it isn't a valid UTF-8 sequence, treat it as a single raw byte.
+    char bytes[2];
+    bytes[0] = string->value[index];
+    bytes[1] = '\0';
+    return wrenNewString(vm, bytes, 1);
+  }
+  
+  return wrenStringFromCodePoint(vm, codePoint);
 }
 
 // Uses the Boyer-Moore-Horspool string matching algorithm.