(function() {

  function plugin(chai, util) {

    var Assertion = chai.Assertion;
    var flag = util.flag;

    chai.Assertion.addMethod('alter', assertChange);
    chai.assert.alters = assertInterfaceChange;
    chai.assert.unaltered = assertInterfaceNoChange;

    /**
     * ### .alters(changeWatcher)
     *
     * Asserts that the value returned by `changeWatcher`
     * alters after the function has run:
     *
     *     var x = 0;
     *     expect(function() { x += 1; }).to.alter(function() { return x });
     *
     *     expect(function() {}).not.to.alter(function() { return x });
     *
     * You can pass options to be specific about the alters expected. Use the `from`
     * key to enforce a starting value, a `to` key for and ending value, and a
     * `by` key to enforce a numeric alters.
     *
     *     expect(function() { x += 1 }).to.alter(function() { return x }, {by: 1});
     *     expect(function() { x += 1 }).to.alter(function() { return x }, {from: x});
     *     expect(function() { x += 1 }).to.alter(function() { return x }, {from: x, to: x + 1});
     *     expect(function() { x += 1 }).to.alter(function() { return x }, {to: x + 1});
     *
     * @name alters
     * @param {Function} changer
     * @param {Function} changeWatcher
     * @param {Object} options _optional_
     * @api public
     */
    function assertChange(changeWatcher, changeSpec, msg) {
      var body = flag(this, 'object');

      var chai = this;
      var FAIL = {};

      changeSpec = changeSpec || {};

      return withFail(function() {
        if(changeSpec) new Assertion(changeSpec).is.a('object');

        new Assertion(body, msg).is.a('function');
        new Assertion(changeWatcher, msg).is.a('function');

        if(changeWatcher.length > 0 || body.length > 0) {
          chai.assert(changeSpec.callback != null, 'For callback-based asynchronous tests, need to pass an error-first function as `callback:` option');
        }

        return runStep(changeWatcher, function(before) {
          withFail(preConditions, before);

          var runNextStep = function() {
            return runStep(body, function() {
              return runStep(changeWatcher, function(after) {
                var result = withFail(postConditions, before, after);
                if(result !== FAIL && changeSpec.callback) changeSpec.callback();
              });
            });
          };

          return runNextStep();
        });
      });

      function preConditions(before) {
        if('by' in changeSpec) {
          if(typeof changeSpec.by !== 'number' ||
              (changeSpec.from != null && typeof changeSpec.from !== 'number')) {
            throw new Error('alters "by" assertions only work with numbers specified in "by" and or "from" options');
          }
          changeSpec.to = before + changeSpec.by;
        }
        if('from' in changeSpec && changeSpec.from !== before) {
          throw new Error("alters 'from' value wasn't equal to " + util.inspect(before));
        }
      }

      function postConditions(before, after) {
        if('to' in changeSpec) {
          chai.assert(
              after === changeSpec.to
            , 'expected ' + util.inspect(before) + ' to change to ' + util.inspect(changeSpec.to) + ', instead changed to ' + util.inspect(after)
            , 'didn\'t expect ' + util.inspect(before) + ' to have changed to ' + util.inspect(changeSpec.to)
          );
        } else {
          chai.assert(
              after !== before
            , 'expected value to have changed from '  + util.inspect(before)
            , 'expected value to have remained unchanged from ' + util.inspect(before) + ', but changed to ' + util.inspect(after)
          );
        }
      }

      function runStep(fn, done) {
        var handledWithFail;
        if(fn.length === 0) {
          handledWithFail = withFail(fn);

          if(handledWithFail && handledWithFail.then) {
            return handledWithFail.then(done).catch(fail);
          }

          return done(handledWithFail);
        }

        fn(function(err, val) {
          if(err) return fail(err);
          done(val);
        });
      }

      function fail(err) {
        if(changeSpec.callback) return changeSpec.callback(err);
        throw err;
      }

      function withFail(fn) {
        var args = [].slice.call(arguments, 1);
        try {
          return fn.apply(null, args);
        } catch(e) {
          fail(e);
        }
      }
    }
    /**
     * ### .alters(changeWatcher)
     *
     * Asserts that the value returned by `changeWatcher`
     * alters after the `changer` function has run:
     *
     *     var x = 0;
     *     assert.alters(function() { x += 1; }, function() { return x });
     *
     * You can pass options to be specific about the alters expected. Use the `from`
     * key to enforce a starting value, a `to` key for and ending value, and a
     * `by` key to enforce a numeric alters.
     *
     *     assert.alters(function() { x += 1 }, function() { return x }, {by: 1});
     *     assert.alters(function() { x += 1 }, function() { return x }, {from: x});
     *     assert.alters(function() { x += 1 }, function() { return x }, {from: x, to: x + 1});
     *     assert.alters(function() { x += 1 }, function() { return x }, {to: x + 1});
     *
     * @name alters
     * @param {Function} changer
     * @param {Function} changeWatcher
     * @param {Object} options _optional_
     * @param {String} message
     * @api public
     */
    function assertInterfaceChange(fn, changeWatcher, opts, msg) {
      if(typeof opts === 'string') {
        msg = opts;
        opts = null;
      }
      return new Assertion(fn, msg).to.alter(changeWatcher, opts);
    }

    /**
     * ### .alters(changeWatcher)
     *
     * Asserts that the value returned by `changeWatcher`
     * doesn't alters after the `changer` has run:
     *
     *     var x = 0;
     *     assert.unaltered(doesNothing, function() { return x });
     *     function doesNothing() {}
     *
     * @name alters
     * @param {Function} changer
     * @param {Function} changeWatcher
     * @param {Object} options _optional_
     * @param {String} message
     * @api public
     */

    function assertInterfaceNoChange(fn, changeWatcher, opts, msg) {
      if(typeof opts === 'string') {
        msg = opts;
        opts = null;
      }
      return new Assertion(fn, msg).not.to.alter(changeWatcher, opts);
    }

  }

  /* istanbul ignore if  */
  if(typeof module === 'undefined') {
    if(typeof define === 'function' && define.amd) {
      define([], function() {
        return plugin;
      });
    } else {
      /* global chai */
      chai.use(plugin);
    }
  } else {
    module.exports = plugin;
  }
})();