export class DefaultMap {
  constructor(defaultValue, ...rest) {
    this.defaultValue = defaultValue;
    this.m = new Map(rest);
  }
  getdefault(key, addToMapping = false) {
    return this.m.get(key) || (() => {
      const val = this.defaultValue(key);
      if (addToMapping)
        this.m.set(key, val);
      return val;
    })();
  }
  entries() {
    return this.m.entries();
  }
}