export default class Query {

  $$criteria = {with: [], without: []};
  $$ecs;
  $$index = new Set();

  constructor(parameters, ecs) {
    this.$$ecs = ecs;
    for (let i = 0; i < parameters.length; ++i) {
      const parameter = parameters[i];
      switch (parameter.charCodeAt(0)) {
        case '!'.charCodeAt(0):
          this.$$criteria.without.push(ecs.Components[parameter.slice(1)]);
          break;
        default:
          this.$$criteria.with.push(ecs.Components[parameter]);
          break;
      }
    }
  }

  get count() {
    return this.$$index.size;
  }

  deindex(entityIds) {
    for (let i = 0; i < entityIds.length; ++i) {
      this.$$index.delete(entityIds[i]);
    }
  }

  reindex(entityIds) {
    if (0 === this.$$criteria.with.length && 0 === this.$$criteria.without.length) {
      for (const entityId of entityIds) {
        this.$$index.add(entityId);
      }
      return;
    }
    for (const entityId of entityIds) {
      let should = true;
      for (let j = 0; j < this.$$criteria.with.length; ++j) {
        if ('undefined' === typeof this.$$criteria.with[j].get(entityId)) {
          should = false;
          break;
        }
      }
      if (should) {
        for (let j = 0; j < this.$$criteria.without.length; ++j) {
          if ('undefined' !== typeof this.$$criteria.without[j].get(entityId)) {
            should = false;
            break;
          }
        }
      }
      if (should) {
        this.$$index.add(entityId);
      }
      else if (!should) {
        this.$$index.delete(entityId);
      }
    }
  }

  select() {
    const it = this.$$index.values();
    return {
      [Symbol.iterator]() {
        return this;
      },
      next: () => {
        const result = it.next();
        if (result.done) {
          return {done: true, value: undefined};
        }
        return {done: false, value: this.$$ecs.get(result.value)};
      },
    };
  }

}