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// Copyright 2022-2024 NeTT. All rights reserved. MIT license.
/** * Data fed to the constructor. * The `result` property holds the result that will be returned after rolling. * `chance` is the weight of the result. */export interface GachaChoice<T> {  result: T;  chance: number;}/** * Gacha system. */export class GachaMachine<T> {  #items: T[];  #weights: number[];  #alias: number[];  #totalChance: number;  constructor(items: GachaChoice<T>[]) {    this.#items = new Array(items.length);    this.#alias = new Array(items.length);    this.#weights = new Array(items.length);    this.#totalChance = 0;    this.#configItems(items);  }  set items(items: GachaChoice<T>[]) {    this.#items = new Array(items.length);    this.#configItems(items);  }  /** Setup items for rolling. */  #configItems(items: GachaChoice<T>[]) {    this.#alias.fill(-1);    this.#weights.fill(-1);    let i = 0;    while (i < items.length) {      this.#totalChance += items[i].chance;      i += 1;    }    i = 0;    const multiplier = items.length / this.#totalChance;    const long = [],      small = [];    while (i < items.length) {      const prob = items[i].chance * multiplier;      if (prob > 1) long.push(i);      else small.push(i);      this.#items[i] = items[i].result;      this.#weights[i] = prob;      i += 1;    }    while (long.length && small.length) {      const j = small.pop();      const k = long.at(-1);      if (j === undefined || k === undefined) {        throw new Error(          "This definitely shouldn't happen. Open an issue at https://github.com/retraigo/fortuna"        ); // this will hopefully never happen      }      this.#alias[j] = k;      this.#weights[k] -= 1 - this.#weights[j];    }  }  /**   * Roll items from the gacha machine.   * ```ts   * const machine = new GachaMachine(items);   * machine.get(11)   * ```   *   * If you are looking for the `distinct` rolls,   * try importing GachaMachine3 instead of GachaMachine.   */  get(count: number): T[] {    let i = 0;    const res = new Array(count);    while (i < count) {      res[i] = this.#items[this.#roll()];      i += 1;    }    return res;  }  #roll(): number {    const inter = Math.random() * this.#items.length;    const i = ~~inter;    const y = inter - i;    return y < this.#weights[i]      ? i      : this.#alias[i] !== -1      ? this.#alias[i]      : i;  }}