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'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)
};
};
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