#!/usr/bin/env node 'use strict';

const USAGE = USAGE: $ cat files* | npx reservoir-sample-stdin NUMBER-OF-LINES ;

/**

• Apply reservoir sampling on an array: randomly sample a fixed number of
• elements from an array, in such a way that new data can be incorporated.
• Iterative usage example:
• var reservoir = [];
• var linesRead = 0;
• var maxSize = 3;
• [reservoir, linesRead] = reservoirSample(reservoir, linesRead, maxSize,

•                                      [1, 2, 3, 4, 5]);

• [reservoir, linesRead] = reservoirSample(reservoir, linesRead, maxSize,

•                                      [10, 20, 30, 40, 50]);

• console.log(reservoir);
• @param {Array} reservoir current reservoir
• @param {number} linesRead number of lines read to achieve reservoir
• @param {number} maxSize maximum size of reservoir
• @param {Array} lines new lines to evaluate
• @returns {[Array, number]} the new reservoir and linesRead. */ function reservoirSample(reservoir, linesRead, maxSize, lines) { for (const line of lines) { if ((++linesRead) <= maxSize) { reservoir.push(line); } else { if (Math.random() <= maxSize / linesRead) { const idx = Math.floor(Math.random() * maxSize); reservoir[idx] = line; } } } return [reservoir, linesRead]; } module.export = reservoirSample;

if (module === require.main) { // Parse inputs const maxSize = parseInt(process.argv[2]); if (!maxSize) { console.error(USAGE); process.exit(1); }

// Prepare stdin process.stdin.setEncoding('utf8');

// Parse stdin let tailBuffer = ''; let reservoir = []; let linesRead = 0; function processData(data) { const lines = (tailBuffer + data).split('\n'); tailBuffer = lines.pop(); [reservoir, linesRead] = reservoirSample(reservoir, linesRead, maxSize, lines); } process.stdin.on('data', processData);

// Finalize trailing chunk of stdin and print reservoir process.stdin.on('end', () => { processData(''); console.log(reservoir.join('\n')); }); }