// To do: remove `void`s
// To do: remove `null` from output of our APIs, allow it as user APIs.

/**
 * @typedef {(error?: Error | null | undefined, ...output: Array<any>) => void} Callback
 *   Callback.
 *
 * @typedef {(...input: Array<any>) => any} Middleware
 *   Ware.
 *
 * @typedef Pipeline
 *   Pipeline.
 * @property {Run} run
 *   Run the pipeline.
 * @property {Use} use
 *   Add middleware.
 *
 * @typedef {(...input: Array<any>) => void} Run
 *   Call all middleware.
 *
 *   Calls `done` on completion with either an error or the output of the
 *   last middleware.
 *
 *   > 👉 **Note**: as the length of input defines whether async functions get a
 *   > `next` function,
 *   > it’s recommended to keep `input` at one value normally.

 *
 * @typedef {(fn: Middleware) => Pipeline} Use
 *   Add middleware.
 */

/**
 * Create new middleware.
 *
 * @returns {Pipeline}
 *   Pipeline.
 */
export function trough() {
  /** @type {Array<Middleware>} */
  const fns = []
  /** @type {Pipeline} */
  const pipeline = {run, use}

  return pipeline

  /** @type {Run} */
  function run(...values) {
    let middlewareIndex = -1
    /** @type {Callback} */
    const callback = values.pop()

    if (typeof callback !== 'function') {
      throw new TypeError('Expected function as last argument, not ' + callback)
    }

    next(null, ...values)

    /**
     * Run the next `fn`, or we’re done.
     *
     * @param {Error | null | undefined} error
     * @param {Array<any>} output
     */
    function next(error, ...output) {
      const fn = fns[++middlewareIndex]
      let index = -1

      if (error) {
        callback(error)
        return
      }

      // Copy non-nullish input into values.
      while (++index < values.length) {
        if (output[index] === null || output[index] === undefined) {
          output[index] = values[index]
        }
      }

      // Save the newly created `output` for the next call.
      values = output

      // Next or done.
      if (fn) {
        wrap(fn, next)(...output)
      } else {
        callback(null, ...output)
      }
    }
  }

  /** @type {Use} */
  function use(middelware) {
    if (typeof middelware !== 'function') {
      throw new TypeError(
        'Expected `middelware` to be a function, not ' + middelware
      )
    }

    fns.push(middelware)
    return pipeline
  }
}

/**
 * Wrap `middleware` into a uniform interface.
 *
 * You can pass all input to the resulting function.
 * `callback` is then called with the output of `middleware`.
 *
 * If `middleware` accepts more arguments than the later given in input,
 * an extra `done` function is passed to it after that input,
 * which must be called by `middleware`.
 *
 * The first value in `input` is the main input value.
 * All other input values are the rest input values.
 * The values given to `callback` are the input values,
 * merged with every non-nullish output value.
 *
 * * if `middleware` throws an error,
 *   returns a promise that is rejected,
 *   or calls the given `done` function with an error,
 *   `callback` is called with that error
 * * if `middleware` returns a value or returns a promise that is resolved,
 *   that value is the main output value
 * * if `middleware` calls `done`,
 *   all non-nullish values except for the first one (the error) overwrite the
 *   output values
 *
 * @param {Middleware} middleware
 *   Function to wrap.
 * @param {Callback} callback
 *   Callback called with the output of `middleware`.
 * @returns {Run}
 *   Wrapped middleware.
 */
export function wrap(middleware, callback) {
  /** @type {boolean} */
  let called

  return wrapped

  /**
   * Call `middleware`.
   * @this {any}
   * @param {Array<any>} parameters
   * @returns {void}
   */
  function wrapped(...parameters) {
    const fnExpectsCallback = middleware.length > parameters.length
    /** @type {any} */
    let result

    if (fnExpectsCallback) {
      parameters.push(done)
    }

    try {
      result = middleware.apply(this, parameters)
    } catch (error) {
      const exception = /** @type {Error} */ (error)

      // Well, this is quite the pickle.
      // `middleware` received a callback and called it synchronously, but that
      // threw an error.
      // The only thing left to do is to throw the thing instead.
      if (fnExpectsCallback && called) {
        throw exception
      }

      return done(exception)
    }

    if (!fnExpectsCallback) {
      if (result && result.then && typeof result.then === 'function') {
        result.then(then, done)
      } else if (result instanceof Error) {
        done(result)
      } else {
        then(result)
      }
    }
  }

  /**
   * Call `callback`, only once.
   *
   * @type {Callback}
   */
  function done(error, ...output) {
    if (!called) {
      called = true
      callback(error, ...output)
    }
  }

  /**
   * Call `done` with one value.
   *
   * @param {any} [value]
   */
  function then(value) {
    done(null, value)
  }
}