base64-vlq.js #1

/* -*- Mode: js; js-indent-level: 2; -*- */
/*
 * 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://code.google.com/p/closure-compiler/source/browse/trunk/src/com/google/debugging/sourcemap/Base64VLQ.java
 *
 * 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.
 */
{
  var base64 = require('./base64');

  // 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

  var VLQ_BASE_SHIFT = 5;

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

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

  // binary: 100000
  var 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(aValue) {
    return aValue < 0
      ? ((-aValue) << 1) + 1
      : (aValue << 1) + 0;
  }

  /**
   * Converts to a two-complement value from a value where the sign bit is
   * placed in the least significant bit.  For example, as decimals:
   *   2 (10 binary) becomes 1, 3 (11 binary) becomes -1
   *   4 (100 binary) becomes 2, 5 (101 binary) becomes -2
   */
  function fromVLQSigned(aValue) {
    var isNegative = (aValue & 1) === 1;
    var shifted = aValue >> 1;
    return isNegative
      ? -shifted
      : shifted;
  }

  /**
   * Returns the base 64 VLQ encoded value.
   */
  exports.encode = function base64VLQ_encode(aValue) {
    var encoded = "";
    var digit;

    var vlq = toVLQSigned(aValue);

    do {
      digit = vlq & VLQ_BASE_MASK;
      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 |= VLQ_CONTINUATION_BIT;
      }
      encoded += base64.encode(digit);
    } while (vlq > 0);

    return encoded;
  };

  /**
   * Decodes the next base 64 VLQ value from the given string and returns the
   * value and the rest of the string via the out parameter.
   */
  exports.decode = function base64VLQ_decode(aStr, aIndex, aOutParam) {
    var strLen = aStr.length;
    var result = 0;
    var shift = 0;
    var continuation, digit;

    do {
      if (aIndex >= strLen) {
        throw new Error("Expected more digits in base 64 VLQ value.");
      }

      digit = base64.decode(aStr.charCodeAt(aIndex++));
      if (digit === -1) {
        throw new Error("Invalid base64 digit: " + aStr.charAt(aIndex - 1));
      }

      continuation = !!(digit & VLQ_CONTINUATION_BIT);
      digit &= VLQ_BASE_MASK;
      result = result + (digit << shift);
      shift += VLQ_BASE_SHIFT;
    } while (continuation);

    aOutParam.value = fromVLQSigned(result);
    aOutParam.rest = aIndex;
  };
}