const Pre = '__pre__'
const Last = '__last__'

export type AsyncHandler<T = unknown> = (event: T) => any
export type AsyncEvents = Record<string, unknown>
export type AsyncFlush = { pre?: boolean; last?: boolean }
export type AsyncBus<T extends AsyncEvents> = {
    on<Key extends keyof T>(type: Key, handler: AsyncHandler<T[Key]>, flush?: AsyncFlush): void
    off<Key extends keyof T>(type: Key, handler?: AsyncHandler<T[Key]>, flush?: AsyncFlush): void
    emit<Key extends keyof T>(type: Key, args?: T[Key]): Promise<any>
}

export const asyncBusFactory = <T extends AsyncEvents>() => {
    const store = {} as { [key in keyof T]: Array<AsyncHandler> }
    const allKey = (type: keyof T): Array<keyof T> => [Pre + type.toString(), type, Last + type.toString()]
    const getKey = (type: keyof T, flush?: AsyncFlush): keyof T => {
        const keys = allKey(type)
        return flush ? (flush.pre ? keys[0] : flush.last ? keys[2] : keys[0]) : keys[0]
    }

    const bus: AsyncBus<T> = {
        on: (type, handler, flush) => {
            const key = getKey(type, flush)
            if (!store[key]) {
                store[key] = []
            }
            if (!store[key].includes(handler as any)) {
                store[key].push(handler as any)
            }
        },
        off: (type, handler, flush) => {
            const keys = flush ? [getKey(type, flush)] : allKey(type)
            for (const key of keys) {
                if (handler) {
                    if (store[key]) {
                        store[key] = store[key].filter(h => h !== handler)
                    }
                } else if (store[key]) {
                    delete store[key]
                }
            }
        },
        emit: async (type, args) => {
            const keys = allKey(type)
            for (const key of keys) {
                if (store[key]) {
                    for (const fn of store[key]) {
                        console.log(fn)
                        await fn(args)
                    }
                }
            }
        },
    }

    return bus
}