import { defineProperty } from 'cosmokit'
import type { Promisify } from 'cosmokit'
import { Context } from './context.ts'
import { Fiber, FiberState } from './fiber.ts'
import { DisposableList, symbols } from './utils.ts'

/**
 * Return whether an event result should stop a bail-style dispatch.
 *
 * @param value — a listener's return value.
 * @returns `true` unless `value` is `null`, `false`, or `undefined`.
 */
export function isBailed(value: any) {
  return value !== null && value !== false && value !== undefined
}

/** Extract the parameter tuple from a function type. */
export type Parameters<F> = F extends (...args: infer P) => any ? P : never
/** Extract the return type from a function type. */
export type ReturnType<F> = F extends (...args: any) => infer R ? R : never
/** Extract the explicit `this` type from a function type. */
export type ThisType<F> = F extends (this: infer T, ...args: any) => any ? T : never

/**
 * Event dispatch strategy used by the event service.
 *
 * `emit` runs synchronous listeners without awaiting them, `parallel` awaits
 * all listeners together, `serial` awaits them in order until one bails,
 * `bail` stops on the first synchronous bail value, and `waterfall` composes
 * listeners around a final `next` callback.
 */
export type DispatchMode = 'emit' | 'parallel' | 'serial' | 'bail' | 'waterfall'

declare module './context.ts' {
  export interface Context {
    /* eslint-disable max-len */
    /**
     * Dispatch an event, running all listeners concurrently.
     *
     * @param name — the event name.
     * @param args — arguments passed to every listener.
     * @returns a promise resolving once every listener has settled.
     */
    parallel<K extends keyof Events>(name: K, ...args: Parameters<Events[K]>): Promise<void>
    /** Same as above, with an explicit `this` for listeners (also used for filtering). */
    parallel<K extends keyof Events>(thisArg: NoInfer<ThisType<Events[K]>>, name: K, ...args: Parameters<Events[K]>): Promise<void>
    /**
     * Dispatch an event synchronously, ignoring listener return values.
     *
     * @param name — the event name.
     * @param args — arguments passed to every listener.
     */
    emit<K extends keyof Events>(name: K, ...args: Parameters<Events[K]>): void
    /** Same as above, with an explicit `this` for listeners (also used for filtering). */
    emit<K extends keyof Events>(thisArg: NoInfer<ThisType<Events[K]>>, name: K, ...args: Parameters<Events[K]>): void
    /**
     * Dispatch an event, awaiting listeners in order until one bails.
     *
     * @param name — the event name.
     * @param args — arguments passed to each listener.
     * @returns the first bail value (non-null, non-false, non-undefined), if any.
     */
    serial<K extends keyof Events>(name: K, ...args: Parameters<Events[K]>): Promisify<ReturnType<Events[K]>>
    /** Same as above, with an explicit `this` for listeners (also used for filtering). */
    serial<K extends keyof Events>(thisArg: NoInfer<ThisType<Events[K]>>, name: K, ...args: Parameters<Events[K]>): Promisify<ReturnType<Events[K]>>
    /**
     * Dispatch an event, calling listeners in order until one bails.
     *
     * @param name — the event name.
     * @param args — arguments passed to each listener.
     * @returns the first bail value (non-null, non-false, non-undefined), if any.
     */
    bail<K extends keyof Events>(name: K, ...args: Parameters<Events[K]>): ReturnType<Events[K]>
    /** Same as above, with an explicit `this` for listeners (also used for filtering). */
    bail<K extends keyof Events>(thisArg: NoInfer<ThisType<Events[K]>>, name: K, ...args: Parameters<Events[K]>): ReturnType<Events[K]>
    /**
     * Dispatch an event whose last argument is a `next` continuation.
     *
     * Each listener wraps the rest of the chain: calling `next()` invokes the
     * next listener (finally the built-in behavior); not calling it vetoes.
     *
     * @param name — the event name.
     * @param args — listener arguments; the final one is the innermost `next`.
     * @returns the outermost listener's return value.
     */
    waterfall<K extends keyof Events>(name: K, ...args: Parameters<Events[K]>): ReturnType<Events[K]>
    /** Same as above, with an explicit `this` for listeners (also used for filtering). */
    waterfall<K extends keyof Events>(thisArg: NoInfer<ThisType<Events[K]>>, name: K, ...args: Parameters<Events[K]>): ReturnType<Events[K]>
    /**
     * Register an event listener owned by the current fiber.
     *
     * @param name — the event name to listen for.
     * @param listener — called with the dispatch arguments.
     * @param options — listener options; a boolean is shorthand for `prepend`.
     * @returns a disposer removing the listener; `true` if it was still registered.
     */
    on<K extends keyof Events>(name: K, listener: Events[K], options?: boolean | EventOptions): () => boolean
    /**
     * Same as `on()`, but the listener disposes itself after its first call.
     *
     * @param name — the event name to listen for.
     * @param listener — called at most once with the dispatch arguments.
     * @param options — listener options; a boolean is shorthand for `prepend`.
     * @returns a disposer removing the listener; `true` if it was still registered.
     */
    once<K extends keyof Events>(name: K, listener: Events[K], options?: boolean | EventOptions): () => boolean
    /* eslint-enable max-len */
  }
}

/** Options accepted by `ctx.on()` and `ctx.once()`. */
export interface EventOptions {
  /** Add the listener before existing listeners for the same event. */
  prepend?: boolean
  /** Receive the event regardless of context filter checks. */
  global?: boolean
}

/** Registered listener record stored by the event service. */
export interface Hook extends EventOptions {
  ctx: Context
  callback: (...args: any[]) => any
}

/**
 * Event bus installed as `ctx.events` and mixed into every context.
 *
 * The service supports concurrent, synchronous, serial, bail, and waterfall
 * dispatch and automatically disposes listeners with their owning fiber.
 */
export class EventsService {
  _hooks: Record<keyof any, Hook[]> = {}

  constructor(private ctx: Context) {
    defineProperty(this, symbols.tracker, {
      property: 'ctx',
      noShadow: true,
    })

    this.on('internal/listener', function (this: Context, name, listener, options: EventOptions) {
      if (name === 'internal/update' && !options.global) {
        const hooks = this.fiber._hooks['internal/update'] ??= new DisposableList()
        const method = options.prepend ? 'unshift' : 'push'
        return hooks[method](listener)
      }
    })

    this.on('internal/update', function (config, noSave, next) {
      const cbs = [...this._hooks['internal/update'] || []]
      const _next = () => {
        const cb = cbs.shift() ?? next
        return cb.call(this, config, noSave, _next)
      }
      return _next()
    }, { global: true, prepend: true })
  }

  /**
   * Resolve listeners for one dispatch and apply context filtering.
   *
   * @param type — the dispatch mode, reported on `internal/dispatch`.
   * @param args — the raw dispatch arguments; consumed up to the event name.
   * @returns the matching listener callbacks, bound to the dispatch `this`.
   */
  dispatch(type: string, args: any[]) {
    const thisArg = typeof args[0] === 'object' || typeof args[0] === 'function' ? args.shift() : null
    const name: string = args.shift()
    if (!name.startsWith('internal/')) {
      this.emit('internal/dispatch', type, name, args, thisArg)
    }
    const filter = thisArg?.[Context.filter]
    return (this._hooks[name] || [])
      .filter(hook => hook.global || !filter || filter.call(thisArg, hook.ctx))
      .map(hook => hook.callback.bind(thisArg))
  }

  /**
   * Run listeners concurrently and wait for all of them.
   *
   * @param args — optional `this`, the event name, then listener arguments.
   * @returns a promise resolving once every listener has settled.
   */
  async parallel(...args: any[]) {
    const results = await Promise.allSettled(this.dispatch('emit', args).map(async cb => cb(...args)))
    const errors = results.filter((result): result is PromiseRejectedResult => result.status === 'rejected')
    if (errors.length) throw new AggregateError(errors.map(error => error.reason))
  }

  /**
   * Run listeners synchronously without waiting for returned promises.
   *
   * @param args — optional `this`, the event name, then listener arguments.
   */
  emit(...args: any[]) {
    this.dispatch('emit', args).map(cb => cb(...args))
  }

  /**
   * Run listeners in order, awaiting each, until one returns a bail value.
   *
   * @param args — optional `this`, the event name, then listener arguments.
   * @returns the first bail value (see {@link isBailed}), if any.
   */
  async serial(...args: any[]) {
    for (const cb of this.dispatch('serial', args)) {
      const result = await cb(...args)
      if (isBailed(result)) return result
    }
  }

  /**
   * Run listeners synchronously until one returns a bail value.
   *
   * @param args — optional `this`, the event name, then listener arguments.
   * @returns the first bail value (see {@link isBailed}), if any.
   */
  bail(...args: any[]) {
    for (const cb of this.dispatch('bail', args)) {
      const result = cb(...args)
      if (isBailed(result)) return result
    }
  }

  /**
   * Compose listeners around the final `next` callback.
   *
   * The last dispatch argument is treated as the innermost `next`. Listeners
   * run outermost-first; a listener that does not call `next()` vetoes the
   * rest of the chain, including the built-in behavior.
   *
   * @param args — optional `this`, the event name, listener arguments, then `next`.
   * @returns the outermost listener's return value.
   */
  waterfall(...args: any[]) {
    const cbs = this.dispatch('waterfall', args)
    const inner = args.pop()
    const next = () => {
      const cb = cbs.shift() ?? inner
      return cb(...args)
    }
    args.push(next)
    return next()
  }

  /**
   * Store a listener record as an effect on the current fiber.
   *
   * @param label — effect label shown in fiber diagnostics.
   * @param hooks — the listener list for one event.
   * @param callback — the listener to store.
   * @param options — placement and filtering options.
   * @returns a disposer that unregisters the listener.
   */
  register(label: string, hooks: Hook[], callback: any, options: EventOptions): () => void {
    const method = options.prepend ? 'unshift' : 'push'
    return this.ctx.fiber.effect(() => {
      hooks[method]({ ctx: this.ctx, callback, ...options })
      return () => this.unregister(hooks, callback)
    }, label)
  }

  /**
   * Remove a stored listener record.
   *
   * @param hooks — the listener list for one event.
   * @param callback — the listener to remove.
   * @returns `true` if the listener was found and removed.
   */
  unregister(hooks: Hook[], callback: any) {
    const index = hooks.findIndex(hook => hook.callback === callback)
    if (index >= 0) {
      hooks.splice(index, 1)
      return true
    }
  }

  /**
   * Register an event listener owned by the current fiber.
   *
   * The listener is removed automatically when the fiber unloads. Throws
   * `CordisError('INACTIVE_EFFECT')` if the fiber is already disposed.
   *
   * @param name — the event name to listen for.
   * @param listener — called with the dispatch arguments.
   * @param options — listener options; a boolean is shorthand for `prepend`.
   * @returns a disposer removing the listener; `true` if it was still registered.
   */
  on(name: string | symbol, listener: (...args: any) => any, options?: boolean | EventOptions) {
    if (typeof options !== 'object') {
      options = { prepend: options }
    }

    // handle special events
    this.ctx.fiber.assertActive()
    listener = this.ctx.reflect.bind(listener)
    const result = this.bail(this.ctx, 'internal/listener', name, listener, options)
    if (result) return result

    const hooks = this._hooks[name] ||= []
    const label = `ctx.on(${typeof name === 'string' ? JSON.stringify(name) : name.toString()})`
    return this.register(label, hooks, listener, options)
  }

  /**
   * Register an event listener that disposes itself after the first call.
   *
   * @param name — the event name to listen for.
   * @param listener — called at most once with the dispatch arguments.
   * @param options — listener options; a boolean is shorthand for `prepend`.
   * @returns a disposer removing the listener; `true` if it was still registered.
   */
  once(name: string, listener: (...args: any) => any, options?: boolean | EventOptions) {
    const dispose = this.on(name, function (...args: any[]) {
      dispose()
      return listener.apply(this, args)
    }, options)
    return dispose
  }
}

/**
 * Built-in framework events used by core services and extension points.
 *
 * Plugin and status events track fiber lifecycle, service events observe
 * dependency registration, update/get/set/listener events allow core services
 * to intercept runtime operations, and `internal/dispatch` exposes event-bus
 * diagnostics before public events are delivered.
 */
export interface Events {
  /** A plugin fiber was created or its uid was cleared on disposal. */
  'internal/plugin'(fiber: Fiber): void
  /** A fiber changed lifecycle state; receives the fiber and its previous state. */
  'internal/status'(fiber: Fiber, oldValue: FiberState): void
  /** Interception hook for a service binding (no core producer). */
  'internal/service'(this: Context, name: string, value: any): void
  /** Waterfall: a fiber config update is being applied; skip `next()` to veto. */
  'internal/update'(this: Fiber, config: any, noSave: boolean, next: () => void): void
  /** Waterfall: a service is being read through the context proxy. */
  'internal/get'(ctx: Context, name: string, error: Error, next: () => any): any
  /** Waterfall: a service is being written through the context proxy. */
  'internal/set'(ctx: Context, name: string, value: any, error: Error, next: () => boolean): boolean
  /** Bail: a listener is being registered; a non-null result replaces registration. */
  'internal/listener'(this: Context, name: string, listener: any, prepend: boolean): void
  /** An event is being dispatched to listeners (fired for non-internal events only). */
  'internal/dispatch'(mode: DispatchMode, name: string, args: any[], thisArg: any): void
}