import { Action, Reducer } from 'redux'

// -----------------
// STATE - This defines the type of data maintained in the Redux store.

export interface CounterState {
  count: number
}

// -----------------
// ACTIONS - These are serializable (hence replayable) descriptions of state transitions.
// They do not themselves have any side-effects they just describe something that is going to happen.
// Use @typeName and isActionType for type detection that works even after serialization/deserialization.

export interface IncrementCountAction { type: 'INCREMENT_COUNT' }
export interface DecrementCountAction { type: 'DECREMENT_COUNT' }

// Declare a 'discriminated union' type. This guarantees that all references to 'type' properties contain one of the
// declared type strings (and not any other arbitrary string).
export type KnownAction = IncrementCountAction | DecrementCountAction

// ----------------
// ACTION CREATORS - These are functions exposed to UI components that will trigger a state transition.
// They don't directly mutate state, but they can have external side-effects (such as loading data).

export const actionCreators = {
  increment: () => ({ type: 'INCREMENT_COUNT' } as IncrementCountAction),
  decrement: () => ({ type: 'DECREMENT_COUNT' } as DecrementCountAction)
}

// ----------------
// REDUCER - For a given state and action, returns the new state. To support time travel, this must not mutate the old state.

export const reducer: Reducer<CounterState> = (state: CounterState | undefined, incomingAction: Action): CounterState => {
  if (state === undefined) {
    return { count: 0 }
  }

  const action = incomingAction as KnownAction
  switch (action.type) {
    case 'INCREMENT_COUNT':
      return { count: state.count + 1 }
    case 'DECREMENT_COUNT':
      return { count: state.count - 1 }
    default:
      return state
  }
}
