/**
 * External dependencies
 */
import type { AnyAction, Reducer } from 'redux';

/**
 * Higher-order reducer creator which creates a combined reducer object, keyed
 * by a property on the action object.
 *
 * @param actionProperty Action property by which to key object.
 * @return Higher-order reducer.
 *
 * @example
 * ```js
 * import { keyedReducer } from '@wordpress/data';
 *
 * const itemsByContext = keyedReducer( 'context' )( ( state = [], action ) => {
 *   switch ( action.type ) {
 *     case 'ADD_ITEM':
 *       return [ ...state, action.item ];
 *   }
 *   return state;
 * } );
 * ```
 */
export const keyedReducer =
	< TState extends unknown, TAction extends AnyAction >(
		actionProperty: string
	) =>
	(
		reducer: Reducer< TState, TAction >
	): Reducer< Record< string, TState >, TAction > =>
	( state: Record< string, TState > = {}, action ) => {
		// Retrieve subkey from action. Do not track if undefined; useful for cases
		// where reducer is scoped by action shape.
		const key = action[ actionProperty ];
		if ( key === undefined ) {
			return state;
		}

		// Avoid updating state if unchanged. Note that this also accounts for a
		// reducer which returns undefined on a key which is not yet tracked.
		const nextKeyState = reducer( state[ key ], action );
		if ( nextKeyState === state[ key ] ) {
			return state;
		}

		return {
			...state,
			[ key ]: nextKeyState,
		};
	};