type Listener = () => void

type OnChange<T> = (args: { newState: T; oldState: T }) => void

export type Store<T> = {

getState: () => T

setState: (updater: (prev: T) => T) => void

subscribe: (listener: Listener) => () => void

}

export function createStore<T>(

initialState: T,

onChange?: OnChange<T>,

): Store<T> {

let state = initialState

const listeners = new Set<Listener>()

return {

getState: () => state,

setState: (updater: (prev: T) => T) => {

const prev = state

const next = updater(prev)

if (Object.is(next, prev)) return

state = next

onChange?.({ newState: next, oldState: prev })

for (const listener of listeners) listener()

},

subscribe: (listener: Listener) => {

listeners.add(listener)

return () => listeners.delete(listener)

},

}

}