Repeater.js

class Repeater{
  static tracker_collect(tracker){
    switch(type(tracker)){
    case 'function':
      return tracker;
    case 'undefined':
    case 'null':
      return (value, index, list) => value;
    case 'number':
    case 'string':
      return (value, index, list) => value[tracker];
    default:
      throw Error(`unknown type as tracker ${type(tracker)}:${tracker}`);
    }
  }
  constructor(tracker, generator, sweeper){
    this.last = new Map();
    this.tracker = tracker_collect(tracker);
    this.generator = generator || ((value, index, list, key) => ({}));
    this.sweeper = sweeper || ((state, key) => true);
  }
  repeat(list){
    var next = new Map();
    var ret = list.map((value, index, list) => {
      var key = this.tracker(value, index, list);
      if(next.has(key)){
        throw Error(`key duplicate ${key}`);
      }
      var state;
      if(this.last.has(key)){
        state = this.last.get(key);
      }else{
        state = this.generator(value, index, list, key);
      }
      next.set(key, state);
      return {state, key, value, index, list};
    });
    this.last
      .filter((state, key) => !next.has(key) && !this.sweeper(state, key))
      .forEach(() => next.set(key, state));
    this.last = next;
    return ret;
  }
}