base64-vlq.js (3474B)
1 /* -*- Mode: js; js-indent-level: 2; -*- */ 2 /* 3 * Copyright 2011 Mozilla Foundation and contributors 4 * Licensed under the New BSD license. See LICENSE or: 5 * http://opensource.org/licenses/BSD-3-Clause 6 * 7 * Based on the Base 64 VLQ implementation in Closure Compiler: 8 * https://code.google.com/p/closure-compiler/source/browse/trunk/src/com/google/debugging/sourcemap/Base64VLQ.java 9 * 10 * Copyright 2011 The Closure Compiler Authors. All rights reserved. 11 * Redistribution and use in source and binary forms, with or without 12 * modification, are permitted provided that the following conditions are 13 * met: 14 * 15 * * Redistributions of source code must retain the above copyright 16 * notice, this list of conditions and the following disclaimer. 17 * * Redistributions in binary form must reproduce the above 18 * copyright notice, this list of conditions and the following 19 * disclaimer in the documentation and/or other materials provided 20 * with the distribution. 21 * * Neither the name of Google Inc. nor the names of its 22 * contributors may be used to endorse or promote products derived 23 * from this software without specific prior written permission. 24 * 25 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 26 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 27 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 28 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 29 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 30 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 31 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 32 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 33 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 34 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 35 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 36 */ 37 38 const base64 = require("./base64"); 39 40 // A single base 64 digit can contain 6 bits of data. For the base 64 variable 41 // length quantities we use in the source map spec, the first bit is the sign, 42 // the next four bits are the actual value, and the 6th bit is the 43 // continuation bit. The continuation bit tells us whether there are more 44 // digits in this value following this digit. 45 // 46 // Continuation 47 // | Sign 48 // | | 49 // V V 50 // 101011 51 52 const VLQ_BASE_SHIFT = 5; 53 54 // binary: 100000 55 const VLQ_BASE = 1 << VLQ_BASE_SHIFT; 56 57 // binary: 011111 58 const VLQ_BASE_MASK = VLQ_BASE - 1; 59 60 // binary: 100000 61 const VLQ_CONTINUATION_BIT = VLQ_BASE; 62 63 /** 64 * Converts from a two-complement value to a value where the sign bit is 65 * placed in the least significant bit. For example, as decimals: 66 * 1 becomes 2 (10 binary), -1 becomes 3 (11 binary) 67 * 2 becomes 4 (100 binary), -2 becomes 5 (101 binary) 68 */ 69 function toVLQSigned(aValue) { 70 return aValue < 0 ? (-aValue << 1) + 1 : (aValue << 1) + 0; 71 } 72 73 /** 74 * Returns the base 64 VLQ encoded value. 75 */ 76 exports.encode = function base64VLQ_encode(aValue) { 77 let encoded = ""; 78 let digit; 79 80 let vlq = toVLQSigned(aValue); 81 82 do { 83 digit = vlq & VLQ_BASE_MASK; 84 vlq >>>= VLQ_BASE_SHIFT; 85 if (vlq > 0) { 86 // There are still more digits in this value, so we must make sure the 87 // continuation bit is marked. 88 digit |= VLQ_CONTINUATION_BIT; 89 } 90 encoded += base64.encode(digit); 91 } while (vlq > 0); 92 93 return encoded; 94 };