export function shuffle(arr, rng = Math.random) {
  const a = [...arr];
  for (let i = a.length - 1; i > 0; i--) {
    const j = Math.floor(rng() * (i + 1));
    [a[i], a[j]] = [a[j], a[i]];
  }
  return a;
}

export class Bag {
  constructor(ids, { rng = Math.random, state = null } = {}) {
    this.rng = rng;
    if (state && Array.isArray(state.bag) && state.bag.length) {
      this.ids = [...ids];
      this.bag = state.bag;
      this.cursor = state.cursor ?? 0;
      this.round = state.round ?? 1;
      this.history = [];
      this.histPos = -1;
      this.lastShown = null;
    } else {
      this.setIds(ids);
    }
  }

  setIds(ids) {
    this.ids = [...ids];
    this.bag = shuffle(this.ids, this.rng);
    this.cursor = 0;
    this.round = 1;
    this.history = [];
    this.histPos = -1;
    this.lastShown = null;
  }

  _pop() {
    if (this.cursor >= this.bag.length) {
      // new round: reshuffle, avoid immediate repeat of lastShown
      this.round += 1;
      this.cursor = 0;
      if (this.ids.length > 1) {
        do { this.bag = shuffle(this.ids, this.rng); }
        while (this.bag[0] === this.lastShown);
      } else {
        this.bag = shuffle(this.ids, this.rng);
      }
    }
    return this.bag[this.cursor++];
  }

  next() {
    // if we stepped back, going next replays forward through history first
    if (this.histPos < this.history.length - 1) {
      this.histPos += 1;
      this.lastShown = this.history[this.histPos];
      return this.lastShown;
    }
    const id = this._pop();
    this.history.push(id);
    this.histPos = this.history.length - 1;
    this.lastShown = id;
    return id;
  }

  prev() {
    if (this.histPos <= 0) return null;
    this.histPos -= 1;
    this.lastShown = this.history[this.histPos];
    return this.lastShown;
  }

  progress() {
    return { seen: Math.min(this.cursor, this.bag.length), total: this.bag.length, round: this.round };
  }

  serialize() {
    return { bag: this.bag, cursor: this.cursor, round: this.round };
  }
}