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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;
};
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