[esnext] implement String padding proposal

src/js/harmony-string-padding.js

Index: src/js/harmony-string-padding.js
diff --git a/src/js/harmony-string-padding.js b/src/js/harmony-string-padding.js
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index 0000000000000000000000000000000000000000..97b76981802f202535e5217438e5e1a71ad0f6ad
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+++ b/src/js/harmony-string-padding.js
@@ -0,0 +1,75 @@
+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+(function(global, utils) {
+
+%CheckIsBootstrapping();
+
+// -------------------------------------------------------------------
+// Imports
+
+var GlobalString = global.String;
+var MakeTypeError;
+
+utils.Import(function(from) {
+  MakeTypeError = from.MakeTypeError;
+});
+
+// -------------------------------------------------------------------
+// http://tc39.github.io/proposal-string-pad-start-end/
+
+macro STRING_PAD_COMMON(NAME, MAXLENGTH, FILLSTRING, S, FILLER)

[Dan Ehrenberg, 2016/03/23 15:59:50]

Can you use a function rather than a macro here?

[caitp (gmail), 2016/03/23 17:00:41]

I've made a function for the "repeat the filler" segment of the macro, and the
rest is duplicated between the two functions.

I think the macro works better, but this is okay too

[Dan Ehrenberg, 2016/03/23 17:30:17]

I think there could be a middle point here, which eliminates more of the
duplication but doesn't use a macro in such an unstructured way. Maybe you could
put almost all of this in a function, and simply let the caller do the
CHECK_OBJECT_COERCIBLE part? this can be taken as a parameter.

[caitp (gmail), 2016/03/23 20:45:45]

done

+  CHECK_OBJECT_COERCIBLE(this, "String.prototype.NAME")
+
+  var S = TO_STRING(TO_OBJECT(this));
+  MAXLENGTH = TO_LENGTH(maxLength);
+  var stringLength = S.length;
+
+  if (MAXLENGTH <= stringLength) return S;
+
+  if (IS_UNDEFINED(FILLSTRING)) {
+    FILLSTRING = " ";
+  } else {
+    FILLSTRING = TO_STRING(FILLSTRING);
+    if (FILLSTRING === "") {
+      FILLSTRING = " ";
+    }
+  }
+
+  var fillLength = MAXLENGTH - stringLength;
+
+  var repetitions = (fillLength / fillString.length) | 0;
+  var remainingChars = (fillLength - fillString.length * repetitions) | 0;
+
+  var FILLER = "";
+  while (true) {
+    if (repetitions & 1) FILLER += FILLSTRING;
+    repetitions >>= 1;
+    if (repetitions === 0) break;
+    FILLSTRING += FILLSTRING;
+  }
+
+  if (remainingChars) {
+    FILLER += %_SubString(FILLSTRING, 0, remainingChars);
+  }
+endmacro
+
+function StringPadStart(maxLength, fillString) {
+  STRING_PAD_COMMON(padStart, maxLength, fillString, S, filler)
+  return filler + S;
+}
+%FunctionSetLength(StringPadStart, 1);
+
+function StringPadEnd(maxLength, fillString) {
+  STRING_PAD_COMMON(padEnd, maxLength, fillString, S, filler)
+  return S + filler;
+}
+%FunctionSetLength(StringPadEnd, 1);
+
+utils.InstallFunctions(GlobalString.prototype, DONT_ENUM, [
+  "padStart", StringPadStart,
+  "padEnd", StringPadEnd
+]);
+
+});