import * as I from 'immutable';
import immutablediff from 'immutablediff';

export class StateSynchronizer {

  constructor(statefuls) {
    this._state = I.Map();
    this._statefuls = statefuls;
    this.updateState();
  }

  acceptStateChange(change) {
    for (const key in change) {
      const stateful = this._statefuls[key];
      if (!stateful) {
        continue;
      }
      stateful.acceptStateChange(change[key]);
    }
  }

  diff() {
    const state = this.state();
    if (this.previousState === state) {
      return StateSynchronizer.noChange;
    }
    // Take a pure JS diff.
    const steps = immutablediff(this.previousState, state).toJS();
    let diff = {};
    for (const {op, path, value} of steps) {
      if ('replace' === op || 'add' === op) {
        if ('/' === path) {
          diff = value;
        }
        else {
          const parts = path.split('/');
          parts.shift();
          let walk = diff;
          for (let i = 0; i < parts.length; ++i) {
            const part = parts[i];
            walk[part] = walk[part] || {};
            if (i === parts.length - 1) {
              walk[part] = value;
            }
            else {
              walk = walk[part];
            }
          }
        }
      }
    }
    // Side-effect.
    this.previousState = this.state();
    return diff;
  }

  state() {
    this.updateState();
    return this._state;
  }

  updateState() {
    for (const key in this._statefuls) {
      const stateful = this._statefuls[key];
      this._state = this._state.set(key, stateful.state());
    }
  }

}

StateSynchronizer.noChange = {};