import type { CreateReducerReturn, PayloadAction } from './createReducer'

type Reducers = Record<string, CreateReducerReturn<any>>
type State<TReducers extends Reducers> = {
    [P in keyof TReducers]: TReducers[P]['initialState']
}

type StateKey<TReducers extends Reducers> = keyof TReducers

function applyReducers<TState>(
    state: TState,
    newState: Partial<TState>
): TState {
    for (const [key, value] of Object.entries(newState)) {
        const lookup = key as keyof typeof newState
        if (state[lookup] !== value) {
            return {
                ...state,
                ...newState,
            }
        }
    }

    return state
}

export function combineReducers<TReducers extends Reducers>(
    reducers: TReducers
) {
    const initialState: State<TReducers> = {} as any

    /* Prepare initial state as well as action mapping */
    for (const [key, reducerConfig] of Object.entries(reducers)) {
        initialState[key as keyof typeof reducers] = reducerConfig.initialState
    }

    const reducer = (state: State<TReducers>, action: PayloadAction<any>) => {
        const updatedState: Partial<State<TReducers>> = {}
        for (const [key, reducerConfig] of Object.entries(reducers)) {
            const typedKey: StateKey<TReducers> = key
            updatedState[typedKey] = reducerConfig.reducer(
                state[typedKey],
                action
            )
        }

        return applyReducers(state, updatedState)
    }

    return {
        initialState,
        reducer,
    }
}