102 lines
2.8 KiB
JavaScript
102 lines
2.8 KiB
JavaScript
/**
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* Implements `var_int` encoding/decoding.
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* @see {@link https://bitmessage.org/wiki/Protocol_specification#Variable_length_integer}
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* @module bitmessage/varint
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*/
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"use strict";
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var assert = require("assert");
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var bitmessage = require("./");
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/**
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* Decode var_int.
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* @param {Buffer} buf - Buffer that starts with encoded var_int
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* @return {{value: (number|Int64), length: number, rest: number}}
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* Decoded var_int structure.
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*/
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exports.decode = function(buf) {
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assert(buf.length > 0, "Empty buffer");
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var value, length;
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switch (buf[0]) {
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case 253:
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value = buf.readUInt16BE(1);
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assert(value >= 253, "Impractical var_int");
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length = 3;
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break;
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case 254:
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value = buf.readUInt32BE(1);
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assert(value >= 65536, "Impractical var_int");
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length = 5;
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break;
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case 255:
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var hi = buf.readUInt32BE(1);
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assert(hi !== 0, "Impractical var_int");
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var lo = buf.readUInt32BE(5);
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value = new bitmessage.Int64(hi, lo);
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length = 9;
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break;
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default:
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value = buf[0];
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length = 1;
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}
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var rest = buf.slice(length);
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return {value: value, length: length, rest: rest};
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};
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/**
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* Encode number into var_int.
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* @param {(number|Int64|Buffer)} value - Input number
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* @return {Buffer} Encoded var_int.
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*/
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exports.encode = function(value) {
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var buf, buf64, targetStart;
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if (typeof value === "number") {
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assert(value >= 0, "Value cannot be less than zero");
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if (value < 253) {
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buf = new Buffer([value]);
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} else if (value < 65536) {
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buf = new Buffer(3);
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buf[0] = 253;
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buf.writeUInt16BE(value, 1);
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} else if (value < 4294967296) {
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buf = new Buffer(5);
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buf[0] = 254;
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buf.writeUInt32BE(value, 1);
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} else {
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// Value may be inaccurate (max safe int = 2^53) but we will still
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// try to convert it.
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buf = new Buffer(9);
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buf[0] = 255;
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buf.writeUInt32BE(Math.floor(value / 4294967296), 1); // high32
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buf.writeUInt32BE(value % 4294967296, 5); // low32
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}
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} else {
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if (value.high32 && value.low32) {
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// int64-native instance.
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buf = new Buffer(9);
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buf[0] = 255;
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buf.writeUInt32BE(value.high32(), 1);
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buf.writeUInt32BE(value.low32(), 5);
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} else {
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// Try to convert to buffer.
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if (Buffer.isBuffer(value)) {
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buf64 = value;
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} else if (value.toBuffer) {
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// node-int64 instance, rawBuffer = true (prevents extra buffer
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// allocation since we copy it's value to new buffer anyway).
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buf64 = value.toBuffer(true);
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} else {
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throw new Error("Value encode error");
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}
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assert(buf64.length <= 8, "Buffer too big");
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buf = new Buffer(9);
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buf.fill(0);
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buf[0] = 255;
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targetStart = 1 + (8 - buf64.length);
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buf64.copy(buf, targetStart);
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}
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}
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return buf;
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};
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