[builtins] Move StringStartsWith to a C++ builtin.

src/builtins/builtins-string.cc

Index: src/builtins/builtins-string.cc
diff --git a/src/builtins/builtins-string.cc b/src/builtins/builtins-string.cc
index a195eb722358f4f5cee9eb0ef27fa015cbe8b71d..907efefbe1b3f1c7c7350d47e6bbfd8deda6a9e6 100644
--- a/src/builtins/builtins-string.cc
+++ b/src/builtins/builtins-string.cc
@@ -1202,6 +1202,55 @@ void Builtins::Generate_StringPrototypeSubstring(CodeStubAssembler* a) {
   }
 }
 
+BUILTIN(StringPrototypeStartsWith) {
+  HandleScope handle_scope(isolate);
+  TO_THIS_STRING(str, "String.prototype.startsWith");
+
+  // Check if the search string is a regExp and fail if it is.
+  Handle<Object> search = args.atOrUndefined(isolate, 1);
+  Maybe<bool> is_reg_exp = Object::IsRegExp(isolate, search);
+  if (is_reg_exp.IsNothing()) {
+    DCHECK(isolate->has_pending_exception());
+    return isolate->heap()->exception();
+  }
+  if (is_reg_exp.FromJust()) {
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kFirstArgumentNotRegExp,
+                              isolate->factory()->NewStringFromStaticChars(
+                                  "String.prototype.startsWith")));
+  }
+  Handle<String> search_string;
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, search_string,
+                                     Object::ToString(isolate, search));
+
+  Handle<Object> position = args.atOrUndefined(isolate, 2);
+  int start;
+
+  if (position->IsUndefined(isolate)) {
+    start = 0;
+  } else {
+    ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, position,
+                                       Object::ToInteger(isolate, position));
+    double index = std::max(position->Number(), 0.0);
+    index = std::min(index, static_cast<double>(str->length()));
+    start = static_cast<uint32_t>(index);
+  }
+
+  if (start + search_string->length() > str->length()) {
+    return *isolate->factory()->false_value();
+  }
+
+  FlatStringReader str_reader(isolate, String::Flatten(str));
+  FlatStringReader search_reader(isolate, String::Flatten(search_string));
+
+  for (int i = 0; i < search_string->length(); i++) {
+    if (str_reader.Get(start + i) != search_reader.Get(i)) {
+      return *isolate->factory()->false_value();
+    }
+  }
+  return *isolate->factory()->true_value();
+}
+
 // ES6 section 21.1.3.25 String.prototype.toString ()
 void Builtins::Generate_StringPrototypeToString(CodeStubAssembler* assembler) {
   typedef compiler::Node Node;