fisher_yates.js (1765B)
1 /** 2 * @license Apache-2.0 3 * 4 * Copyright (c) 2018 The Stdlib Authors. 5 * 6 * Licensed under the Apache License, Version 2.0 (the "License"); 7 * you may not use this file except in compliance with the License. 8 * You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 19 'use strict'; 20 21 // MODULES // 22 23 var floor = require( '@stdlib/math/base/special/floor' ); 24 25 26 // FUNCTIONS // 27 28 var slice = Array.prototype.slice; 29 30 31 // MAIN // 32 33 /** 34 * Samples uniformly without replacement using the Fisher-Yates shuffle. 35 * 36 * ## References 37 * 38 * - Knuth, Donald E. 1997. _The Art of Computer Programming, Volume 2 (3rd Ed.): Seminumerical Algorithms_. Boston, MA, USA: Addison-Wesley Longman Publishing Co., Inc. 39 * 40 * @private 41 * @param {ArrayLike} x - array-like object from which to sample 42 * @param {NonNegativeInteger} size - sample size 43 * @param {Function} rand - PRNG for generating uniformly distributed numbers 44 * @returns {Array} sample 45 */ 46 function fisherYates( x, size, rand ) { 47 var tmp; 48 var N; 49 var i; 50 var j; 51 52 N = x.length; 53 54 // Note: we skip the first element, as no further swaps are possible given that all other indices are excluded from swapping... 55 for ( i = N-1; i > 0; i-- ) { 56 // Generate an integer index on the interval: [0,i] 57 j = floor( rand()*(i+1) ); 58 59 // Swap elements: 60 tmp = x[ i ]; 61 x[ i ] = x[ j ]; 62 x[ j ] = tmp; 63 } 64 return slice.call( x, 0, size ); 65 } 66 67 68 // EXPORTS // 69 70 module.exports = fisherYates;