import identity from 'lodash.identity';

 * Create a higher-order reducer by combining a set of sub-reducer
 * functions that are indexed by the standard action.type.  When no
 * action.type is acted on, the original state is merely
 * passed-through (using the [identity
 * function](
 * This is one of the more prevalent composition reducers, and
 * provides an alternative to the switch statement (commonly used to
 * provide this control mechanism).
 * The **User Guide** discusses reducerHash() in more detail (see
 * {@tutorial conceptHash}), and additional examples can be found in
 * {@tutorial conceptJoin} and {@tutorial fullExample}.
 * **NOTE**: Because this function is so central to the rudimentary aspects of
 * reduction, it is common to provide a value-added {@tutorial logExt}.
 * @param {ActionReducerHash} actionHandlers - a hash of reducer functions,
 * indexed by the standard redux action.type.
 * @param {InitialState} [initialState] - the optional fall-back state
 * value used during the state initialization boot-strap process.
 * @returns {reducerFn} a newly created reducer function (described above).
export default function reducerHash(actionHandlers, initialState) {

  // TODO: consider validation of actionHandlers param.

  // internal function: locate handler from actionHandlers action.type hash lookup
  //                    ... default: identity pass-through
  const locateHandler = (action) => actionHandlers[action.type] || identity;

  // expose the new reducer fn, which resolves according the the supplied actionHandlers
  return (state=initialState, action, originalReducerState) => {

    // maintain the originalReducerState as the immutable state
    // at the time of the start of the reduction process
    // ... in support of joinReducers()
    // ... for more info, refer to the User Guide {@tutorial originalReducerState}
    if (originalReducerState === undefined) {
      originalReducerState = state;

    // execute the handler indexed by the action.type (or the identity pass-through)
    return locateHandler(action)(state, action, originalReducerState);


//*** Specification: ActionReducerHash

 * @typedef {Object} ActionReducerHash
 * A hash of reducer functions, indexed by the standard redux
 * action.type.
 * @property {reducerFn} actionType1 - The reducer function servicing: 'actionType1'.
 * @property {reducerFn} actionType2 - The reducer function servicing: 'actionType2'.
 * @property {reducerFn} ...more - ...etc.