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Original file line number | Diff line number | Diff line change |
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/* -------------------- | ||
* stream-chop | ||
* Functions to create test streams | ||
* ------------------*/ | ||
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/* eslint-disable no-bitwise */ | ||
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'use strict'; | ||
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// Modules | ||
const {GeneratorReadStream, GeneratorWriteStream} = require('stream-gen'), | ||
Random = require('@offirmo/random'); | ||
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// Constants | ||
const SEED = 0; | ||
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// Exports | ||
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module.exports = { | ||
createReadStream, | ||
createWriteStream | ||
}; | ||
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/** | ||
* Create readable byte stream of specified length. | ||
* Stream of bytes is drawn from MT pseudo-random generator. | ||
* Output for same `start` and `len` is deterministic. | ||
* | ||
* @param {number} start - Start bytes of sequence | ||
* @param {number} len - Length of sequence | ||
* @returns {Stream} - Readable data stream | ||
*/ | ||
function createReadStream(start, len) { | ||
const gen = createGenerator(start, len); | ||
return new GeneratorReadStream(gen); | ||
} | ||
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/** | ||
* Create writable stream. | ||
* It checks that data piped into it matches expected | ||
* i.e. same data as in stream from `createReadStream()`. | ||
* Callback is called with error if data does not match, or with `null` when stream ends without error. | ||
* | ||
* @param {number} start - Start bytes of sequence | ||
* @param {number} len - Length of sequence | ||
* @param {function} cb - Callback - called with `null` when stream ends, or an error if matching fail | ||
* @returns {Stream} - Readable data stream | ||
*/ | ||
function createWriteStream(start, len, cb) { | ||
const gen = createGenerator(start, len); | ||
return new GeneratorWriteStream(gen, cb); | ||
} | ||
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function createGenerator(start, len) { | ||
// Init MT pseudo-random number generator | ||
const mt = Random.engines.mt19937(); | ||
mt.seed(SEED); | ||
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// Init empty buffer | ||
let leftover = 0, | ||
leftoverBytes = 0; | ||
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// Discard start | ||
if (start > 0) { | ||
const discardBytes = start % 4, | ||
discardQuads = (start - discardBytes) / 4; | ||
if (discardQuads > 0) mt.discard(discardQuads); | ||
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if (discardBytes > 0) { | ||
// Get new 32-bit number, discard unneeded bytes, and put rest in leftover | ||
leftover = mt(); | ||
leftover >>= discardBytes * 8; | ||
leftoverBytes = 4 - discardBytes; | ||
} | ||
} | ||
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// Create generator to produce bytes | ||
let bytesRemaining = len; | ||
function next() { | ||
// If finished, end generator | ||
if (bytesRemaining === 0) return {value: undefined, done: true}; | ||
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// If no leftover, pull another 32-bit number from MT. | ||
if (leftoverBytes === 0) { | ||
leftover = mt(); | ||
leftoverBytes = 4; | ||
} | ||
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// Pull lowest byte from leftover | ||
const value = leftover & 0xFF; | ||
leftover >>= 8; | ||
leftoverBytes--; | ||
bytesRemaining--; | ||
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return {value, done: false}; | ||
} | ||
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return () => ({next}); | ||
} |