encode.js.flow

/**
 * Portions Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 * @format
 */

/**
 * Copyright 2011 Mozilla Foundation and contributors
 * Licensed under the New BSD license. See LICENSE or:
 * http://opensource.org/licenses/BSD-3-Clause
 *
 * Based on the Base 64 VLQ implementation in Closure Compiler:
 * https://git.io/vymuA
 *
 * Copyright 2011 The Closure Compiler Authors. All rights reserved.
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *  * Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *  * Redistributions in binary form must reproduce the above
 *    copyright notice, this list of conditions and the following
 *    disclaimer in the documentation and/or other materials provided
 *    with the distribution.
 *  * Neither the name of Google Inc. nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * @copyright
 */

/* eslint-disable no-bitwise */

'use strict';

// A map of values to characters for the b64 encoding
const CHAR_MAP = [
  0x41,
  0x42,
  0x43,
  0x44,
  0x45,
  0x46,
  0x47,
  0x48,
  0x49,
  0x4a,
  0x4b,
  0x4c,
  0x4d,
  0x4e,
  0x4f,
  0x50,
  0x51,
  0x52,
  0x53,
  0x54,
  0x55,
  0x56,
  0x57,
  0x58,
  0x59,
  0x5a,
  0x61,
  0x62,
  0x63,
  0x64,
  0x65,
  0x66,
  0x67,
  0x68,
  0x69,
  0x6a,
  0x6b,
  0x6c,
  0x6d,
  0x6e,
  0x6f,
  0x70,
  0x71,
  0x72,
  0x73,
  0x74,
  0x75,
  0x76,
  0x77,
  0x78,
  0x79,
  0x7a,
  0x30,
  0x31,
  0x32,
  0x33,
  0x34,
  0x35,
  0x36,
  0x37,
  0x38,
  0x39,
  0x2b,
  0x2f,
];

// A single base 64 digit can contain 6 bits of data. For the base 64 variable
// length quantities we use in the source map spec, the first bit is the sign,
// the next four bits are the actual value, and the 6th bit is the
// continuation bit. The continuation bit tells us whether there are more
// digits in this value following this digit.
//
//   Continuation
//   |    Sign
//   |    |
//   V    V
//   101011

const VLQ_BASE_SHIFT = 5;

// binary: 100000
const VLQ_BASE = 1 << VLQ_BASE_SHIFT;

// binary: 011111
const VLQ_BASE_MASK = VLQ_BASE - 1;

// binary: 100000
const VLQ_CONTINUATION_BIT = VLQ_BASE;

/**
 * Converts from a two-complement value to a value where the sign bit is
 * placed in the least significant bit.  For example, as decimals:
 *   1 becomes 2 (10 binary), -1 becomes 3 (11 binary)
 *   2 becomes 4 (100 binary), -2 becomes 5 (101 binary)
 */
function toVLQSigned(value) {
  return value < 0 ? (-value << 1) + 1 : (value << 1) + 0;
}

/**
 * Encodes a number to base64 VLQ format and appends it to the passed-in buffer
 *
 * DON'T USE COMPOUND OPERATORS (eg `>>>=`) ON `let`-DECLARED VARIABLES!
 * V8 WILL DEOPTIMIZE THIS FUNCTION AND MAP CREATION WILL BE 25% SLOWER!
 *
 * DON'T ADD MORE COMMENTS TO THIS FUNCTION TO KEEP ITS LENGTH SHORT ENOUGH FOR
 * V8 OPTIMIZATION!
 */
function encode(value: number, buffer: Buffer, position: number): number {
  let vlq = toVLQSigned(value);
  let digit;
  do {
    digit = vlq & VLQ_BASE_MASK;
    vlq = vlq >>> VLQ_BASE_SHIFT;
    if (vlq > 0) {
      // There are still more digits in this value, so we must make sure the
      // continuation bit is marked.
      digit = digit | VLQ_CONTINUATION_BIT;
    }
    buffer[position++] = CHAR_MAP[digit];
  } while (vlq > 0);

  return position;
}

module.exports = encode;