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Filename : base64-vlq.js
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/* -*- 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;
};