type Listener<T> = (state: T, prevState: T) => void;

export type Store<T> = {
  getState: () => T;
  setState: (partial: Partial<T> | ((prev: T) => T), action?: string) => void;
  subscribe: (listener: Listener<T>) => () => void;
};

export function createStore<T extends object>(initialState: T): Store<T> {
  let state: T = initialState;
  const listeners = new Set<Listener<T>>();

  const getState = () => state;

  const setState: Store<T>['setState'] = (partial) => {
    const prevState = state;
    const nextState =
      typeof partial === 'function'
        ? (partial as (prev: T) => T)(prevState)
        : ({ ...prevState, ...partial } as T);
    if (Object.is(nextState, prevState)) return;
    state = nextState;
    listeners.forEach((l) => l(state, prevState));
  };

  const subscribe: Store<T>['subscribe'] = (listener) => {
    listeners.add(listener);
    return () => listeners.delete(listener);
  };

  return { getState, setState, subscribe };
}