'use strict';

/* Expose. */
module.exports = trough;

/* Methods. */
var slice = [].slice;

/* Create new middleware. */
function trough() {
  var fns = [];
  var middleware = {};

  middleware.run = run;
  middleware.use = use;

  return middleware;

  /* Run `fns`.  Last argument must be
   * a completion handler. */
  function run() {
    var index = -1;
    var input = slice.call(arguments, 0, -1);
    var done = arguments[arguments.length - 1];

    if (typeof done !== 'function') {
      throw new Error('Expected function as last argument, not ' + done);
    }

    next.apply(null, [null].concat(input));

    /* Run the next `fn`, if any. */
    function next(err) {
      var fn = fns[++index];
      var params = slice.call(arguments, 0);
      var values = params.slice(1);
      var length = input.length;
      var pos = -1;

      if (err) {
        done(err);
        return;
      }

      /* Copy non-nully input into values. */
      while (++pos < length) {
        if (values[pos] === null || values[pos] === undefined) {
          values[pos] = input[pos];
        }
      }

      input = values;

      /* Next or done. */
      if (fn) {
        wrap(fn, next).apply(null, input);
      } else {
        done.apply(null, [null].concat(input));
      }
    }
  }

  /* Add `fn` to the list. */
  function use(fn) {
    if (typeof fn !== 'function') {
      throw new Error('Expected `fn` to be a function, not ' + fn);
    }

    fns.push(fn);

    return middleware;
  }
}

/* Wrap `fn`.  Can be sync or async; return a promise,
 * receive a completion handler, return new values and
 * errors. */
function wrap(fn, next) {
  var invoked;

  return wrapped;

  function wrapped() {
    var params = slice.call(arguments, 0);
    var callback = fn.length > params.length;
    var result;

    if (callback) {
      params.push(done);
    }

    try {
      result = fn.apply(null, params);
    } catch (err) {
      /* Well, this is quite the pickle.  `fn` received
       * a callback and invoked it (thus continuing the
       * pipeline), but later also threw an error.
       * We’re not about to restart the pipeline again,
       * so the only thing left to do is to throw the
       * thing instea. */
      if (callback && invoked) {
        throw err;
      }

      return done(err);
    }

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

  /* Invoke `next`, only once. */
  function done() {
    if (!invoked) {
      invoked = true;

      next.apply(null, arguments);
    }
  }

  /* Invoke `done` with one value.
   * Tracks if an error is passed, too. */
  function then(value) {
    done(null, value);
  }
}