///
struct EachWithEachOtherRange(R) {
  private R inputRange;

  this(R r) {
    inputRange = r;
  }

  private size_t y = 0;
  private size_t x = 1;

  auto front() {
    assert(!empty);

    return [inputRange[x], inputRange[y]];
  }

  void popFront() {
    x++;

    if(x >= inputRange.length) {
      y++;
      x = y+1;
    }
  }

  bool empty() const {
    return x >= inputRange.length;
  }

  size_t length() const {
    const len = inputRange.length;

    return (len * len - len) / 2;
  }
}

///
auto eachWithOther(T)(T inputRange) {
  return EachWithEachOtherRange!(T)(inputRange);
}

unittest {
  import std.algorithm.iteration: map;
  import std.algorithm.searching: count;

  int[] arr = [0, 1, 2, 3];

  assert(arr.eachWithOther.map!(a => a).count == 6);
  assert(arr.eachWithOther.length == 6);
}