// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/// @assertion Stream<T> distinct([bool equals(T previous, T next)])
///    Skips data events if they are equal to the previous data event.
///    The returned stream provides the same events as this stream, except that it
/// never provides two consecutive data events that are equal.
///    Equality is determined by the provided equals method. If that is omitted,
/// the '==' operator on the last provided data element is used.
/// @description Checks that if parameter is present, returned stream filters
/// events according to the supplied function.
/// @author kaigorodov

library distinct_A01_t02;
import "dart:async";
import "../../../Utils/expect.dart";

void check<T>(Stream<T> s, bool equals(T previous, T next)) {
  Stream<T> d = s.distinct(equals);
  bool first = true;
  var previous;
  asyncStart();
  d.listen(
    (T event) {
      Expect.isTrue(first || !equals(previous, event), "p=$previous, e=$event");
      first = false;
      previous = event;
    },
    onDone:() {
      asyncEnd();
    }
  );
}

int abs(x) => x < 0 ? -x: x;

int sign(x) => (x < 0) ? -1: (x == 0 ? 0 : 1);

void test(CreateStreamFunction create) {
  check<int>(
      create([1, 2, 2, 3]),
      (previous, next) => previous == next
  );
  check<int>(
      create([2, 4, 3, 1]),
      (previous, next) => previous % 2 == next % 2
  );
  check<int>(
      create(new Iterable.generate(10, (int index) => index)),
      (var previous, var next) => abs(previous - next) <= 1
  );
  check(
      create(new Iterable.generate(10, (int index) => -5 + index)),
      (var previous, var next) => sign(previous) == sign(next)
  );
}