140 lines
4.6 KiB
JavaScript
140 lines
4.6 KiB
JavaScript
/* -*- Mode: js; js-indent-level: 2; -*- */
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/*
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* Copyright 2011 Mozilla Foundation and contributors
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* Licensed under the New BSD license. See LICENSE or:
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* http://opensource.org/licenses/BSD-3-Clause
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*
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* Based on the Base 64 VLQ implementation in Closure Compiler:
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* https://code.google.com/p/closure-compiler/source/browse/trunk/src/com/google/debugging/sourcemap/Base64VLQ.java
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*
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* Copyright 2011 The Closure Compiler Authors. All rights reserved.
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are
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* met:
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*
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* * Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* * Redistributions in binary form must reproduce the above
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* copyright notice, this list of conditions and the following
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* disclaimer in the documentation and/or other materials provided
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* with the distribution.
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* * Neither the name of Google Inc. nor the names of its
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* contributors may be used to endorse or promote products derived
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* from this software without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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var base64 = require('./base64');
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// A single base 64 digit can contain 6 bits of data. For the base 64 variable
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// length quantities we use in the source map spec, the first bit is the sign,
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// the next four bits are the actual value, and the 6th bit is the
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// continuation bit. The continuation bit tells us whether there are more
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// digits in this value following this digit.
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//
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// Continuation
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// | Sign
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// | |
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// V V
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// 101011
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var VLQ_BASE_SHIFT = 5;
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// binary: 100000
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var VLQ_BASE = 1 << VLQ_BASE_SHIFT;
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// binary: 011111
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var VLQ_BASE_MASK = VLQ_BASE - 1;
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// binary: 100000
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var VLQ_CONTINUATION_BIT = VLQ_BASE;
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/**
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* Converts from a two-complement value to a value where the sign bit is
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* placed in the least significant bit. For example, as decimals:
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* 1 becomes 2 (10 binary), -1 becomes 3 (11 binary)
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* 2 becomes 4 (100 binary), -2 becomes 5 (101 binary)
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*/
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function toVLQSigned(aValue) {
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return aValue < 0
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? ((-aValue) << 1) + 1
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: (aValue << 1) + 0;
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}
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/**
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* Converts to a two-complement value from a value where the sign bit is
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* placed in the least significant bit. For example, as decimals:
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* 2 (10 binary) becomes 1, 3 (11 binary) becomes -1
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* 4 (100 binary) becomes 2, 5 (101 binary) becomes -2
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*/
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function fromVLQSigned(aValue) {
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var isNegative = (aValue & 1) === 1;
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var shifted = aValue >> 1;
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return isNegative
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? -shifted
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: shifted;
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}
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/**
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* Returns the base 64 VLQ encoded value.
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*/
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exports.encode = function base64VLQ_encode(aValue) {
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var encoded = "";
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var digit;
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var vlq = toVLQSigned(aValue);
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do {
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digit = vlq & VLQ_BASE_MASK;
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vlq >>>= VLQ_BASE_SHIFT;
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if (vlq > 0) {
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// There are still more digits in this value, so we must make sure the
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// continuation bit is marked.
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digit |= VLQ_CONTINUATION_BIT;
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}
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encoded += base64.encode(digit);
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} while (vlq > 0);
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return encoded;
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};
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/**
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* Decodes the next base 64 VLQ value from the given string and returns the
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* value and the rest of the string via the out parameter.
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*/
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exports.decode = function base64VLQ_decode(aStr, aIndex, aOutParam) {
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var strLen = aStr.length;
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var result = 0;
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var shift = 0;
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var continuation, digit;
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do {
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if (aIndex >= strLen) {
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throw new Error("Expected more digits in base 64 VLQ value.");
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}
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digit = base64.decode(aStr.charCodeAt(aIndex++));
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if (digit === -1) {
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throw new Error("Invalid base64 digit: " + aStr.charAt(aIndex - 1));
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}
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continuation = !!(digit & VLQ_CONTINUATION_BIT);
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digit &= VLQ_BASE_MASK;
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result = result + (digit << shift);
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shift += VLQ_BASE_SHIFT;
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} while (continuation);
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aOutParam.value = fromVLQSigned(result);
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aOutParam.rest = aIndex;
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};
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