parse-base64vlq-mappings/base64-vlq.js

Parses out base64 VLQ encoded mappings.

'use strict';
/*
 * Copyright 2011 Mozilla Foundation and contributors
 * Licensed under the New BSD license. See LICENSE or:
 * http://opensource.org/licenses/BSD-3-Clause
 */

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
  * 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
  * 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.
  */
exports.decode = function base64VLQ_decode(aStr) {
  var i = 0;
  var strLen = aStr.length;
  var result = 0;
  var shift = 0;
  var continuation, digit;

  do {
    if (i >= strLen) {
      throw new Error("Expected more digits in base 64 VLQ value.");
    }
    digit = base64.decode(aStr.charAt(i++));
    continuation = !!(digit & VLQ_CONTINUATION_BIT);
    digit &= VLQ_BASE_MASK;
    result = result + (digit << shift);
    shift += VLQ_BASE_SHIFT;
  } while (continuation);

  return {
    value: fromVLQSigned(result),
    rest: aStr.slice(i)
  };
};