class Cache {
  // valid_for:     <Integer>; number of minutes until marked stale and removed.
  // cleanup_after: <Integer>; number of additions between cleanup process.
  constructor ({ valid_for, cleanup_after }) {
    this.valid_for = valid_for;
    this.cleanup_after = cleanup_after;
    this.cleanup_in_process = false;

    this.cleanup_idx = 0;
    this.map = {

    };
  }

  cleanup () {
    this.cleanup_in_process = true;
    // this guarantees that the previous thread and return will not be blocked by
    // this process.
    setTimeout(() => {
      Object.keys(this.map).forEach(o => {
        if (this.map[o].expires < +new Date()) delete this.map[o];
      });

      this.cleanup_in_process = false;
      this.cleanup_idx = 0;
    });
  }

  setItem (key, value) {
    // increment the index of insertions since cleanup.
    this.cleanup_idx++;
    this.map[key] = {
      value,
      expires: +new Date() + (1000 * 60 * this.valid_for),
    };

    // check if it is time to clean up (x number of arbitrary submissions).
    if (this.cleanup_idx === this.cleanup_after) {
      // run async at next thread availability.
      this.cleanup();
    }

    return this.map[key].value;
  }

  getItem (key) {
    return this.map[key].value;
  }
}