98 lines
2.5 KiB
JavaScript
Executable File
98 lines
2.5 KiB
JavaScript
Executable File
/**
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* Pseudo random number generator
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* using the xorshift32 method.
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*/
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export class Xorshift32 {
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/* @type {number} */
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initialSeed;
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/**
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* State holds a single uint32.
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* It's useful for staying within modulo 2**32.
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*
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* @type {Uint32Array}
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*/
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state;
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/** @param {number} seed */
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constructor(seed) {
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if (seed === undefined) {
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const maxInt32 = 2 ** 32;
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seed = Math.floor(Math.random() * (maxInt32 - 1)) + 1;
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}
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seed = seed | 0;
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this.state = Uint32Array.of(seed);
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}
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/** @protected Shuffle the internal state. */
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shuffle() {
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this.state[0] ^= this.state[0] << 13;
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this.state[0] ^= this.state[0] >>> 17;
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this.state[0] ^= this.state[0] << 5;
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// We could also do something like this:
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// x ^= x << 13;
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// x ^= x >> 17;
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// x ^= x << 5;
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// return x;
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// But we'd have to xor the x with 2^32 after every op,
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// we get that "for free" by using the uint32array
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}
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/**
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* Get a random number and shuffle the internal state.
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* @returns {number} a pseudo-random positive integer.
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*/
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get() {
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this.shuffle();
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return this.state[0];
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}
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/** @param {number} x @returns {number} a positive integer lower than x */
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lowerThan(x) {
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return this.get() % x;
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}
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/** @param {number} x @returns {number} a positive integer lower than or equal to x */
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lowerThanOrEqual(x) {
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return this.get() % (x + 1);
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}
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/**
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* @template T
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* @param {T[]} arr - The array to pick from.
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* @returns {T} One element from the array. * @return {<T>}
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*/
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randomElement(arr) {
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if (arr instanceof Set) {
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arr = [...arr];
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}
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const idx = this.lowerThan(arr.length);
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return arr[idx];
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}
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/**
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* @param {...<T>} ... pick random function argument
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* @returns {<T>}
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*/
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oneOf(...args) {
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const idx = this.lowerThan(args.length);
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return args[idx];
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}
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/**
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* @param {number} lowerThanOrEqual a positive integer
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* @param {number} greaterThanOrEqual a positive integer greater than lowerThanOrEqual
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* @returns {number} a pseudo-random integer
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*/
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within(greaterThanOrEqual, lowerThanOrEqual) {
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const range = lowerThanOrEqual - greaterThanOrEqual;
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const num = this.lowerThanOrEqual(range);
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return num + greaterThanOrEqual;
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}
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}
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