// Execute with dmd arguments: -unittest -main

​

import std.range;

import std.traits;

​

/**

 * Calls fun with the previous result and the current iteration state.

 */

struct WithPrevious(alias fun, StateType) {

    StateType _state;

    ElementType!StateType current;

​

    void popFront()

    {

        current = front();

        _state.popFront();

    }

​

    @property ElementType!StateType front()

    {

        return fun(current, _state.front);

    }

​

    @property typeof(this) save()

    {

        auto ans = this;

        ans._state = this._state.save();

        return ans;

    }

​

    static if(isInfinite!StateType)

        enum bool empty = false;

    else

        bool empty() const {

            return _state.empty;

        }

}

///

unittest {

    import std.path;

    import std.algorithm;

    enum path = r"foo/bar/foobar".pathSplitter;

    static assert(path.withPrevious!buildPath

      .equal([

          buildPath("foo"),

          buildPath("foo","bar"),

          buildPath("foo","bar","foobar")]));

}

​

WithPrevious!(fun, CommonType!(State))

withPrevious(alias fun, State)(State initial) if(isInputRange!State) {

    CommonType!(State) state = initial;

    return typeof(return)(state);

}