import { Reducer, Action } from 'redux';

export type IndexedState<State> = {
  [K in string]?: State;
};

type IndexedReducer<State> = Reducer<IndexedState<State>>;

/**
 * A utility to make a reducer collection-oriented. I.e., the resulting state
 * is indexed by some identifier derived from an `Action`. This utility reduces
 * boilerplate and manages immutable updates to the state.
 * 
 * Enhances a reducer by returning a parent state object that is keyed by an
 * index value. Actions are delegated to the provided reducer, using the return
 * value of `getIndexValue(action)` as the index value. If
 * `getIndexValue(action)` returns `undefined`, the reducer will be called with
 * it and all substates.
 */
export function indexByActionProperty<State>(
  reducer: Reducer<State>,
  getIndexValue: (action: Action) => string | undefined
): IndexedReducer<State> {
  return function(state: IndexedState<State> = {}, action: Action) {
    const index = getIndexValue(action);
    const indexes = index ? [ index ] : Object.keys(state);
    const nextState = { ...state };
    let stateDidChange = false;
    for (const index of indexes) {
      const currentSubstate = state[index];
      const nextSubstate = reducer(currentSubstate, action);
      if (currentSubstate !== nextSubstate) stateDidChange = true;
      nextState[index] = nextSubstate;
    }
    return stateDidChange ? nextState : state;
  }
}