libdot/js/lib_codec.js

// Copyright 2018 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

'use strict';

lib.codec = {};

/**
 * Join an array of code units to a string.
 *
 * The code units must not be larger than 65535.  The individual code units may
 * be for UTF-8 or UTF-16 -- it doesn't matter since UTF-16 can handle all UTF-8
 * code units.
 *
 * The input array type may be an Array or a typed Array (e.g. Uint8Array).
 *
 * @param {!Uint8Array|!Array<number>} array The code units to generate for
 *     the string.
 * @return {string} A UTF-16 encoded string.
 */
lib.codec.codeUnitArrayToString = function(array) {
  // String concat is faster than Array.join.
  //
  // String.fromCharCode.apply is faster than this if called less frequently
  // and with smaller array sizes (like <32K).  But it's a recursive call so
  // larger arrays will blow the stack and fail.  We also seem to be faster
  // (or at least more constant time) when called frequently.
  let ret = '';
  for (let i = 0; i < array.length; ++i) {
    ret += String.fromCharCode(array[i]);
  }
  return ret;
};

/**
 * Create an array of code units from a UTF-16 encoded string.
 *
 * @param {string} str The string to extract code units from.
 * @param {!ArrayBufferView=} ret The buffer to hold the result.  If not set, a
 *     new Uint8Array is created.
 * @return {!ArrayBufferView} The array of code units.
 */
lib.codec.stringToCodeUnitArray = function(
    str, ret = new Uint8Array(str.length)) {
  // Indexing string directly is faster than Array.map.
  for (let i = 0; i < str.length; ++i) {
    ret[i] = str.charCodeAt(i);
  }
  return ret;
};