import std.stdio;

import std.range: isInputRange;

​

struct Difference(alias pred = "a", R1, R2)

if (isInputRange!R1 && isInputRange!R2)

{

    import std.range: ElementType;

    import std.functional: unaryFun, binaryFun;

​

    enum isUnary = is(typeof(unaryFun!pred(ElementType!R1.init)));

    static if (isUnary)

    {

        alias transform = unaryFun!pred;

        alias compare = binaryFun!"a == b";

    }

    else

    {

        alias transform = unaryFun!"a";

        alias compare = binaryFun!pred;

    }

​

    R1 r1;

    R2 r2;

​

    private void moveToNextElement() {

        import std.traits: TemplateOf;

        import std.range: SortedRange;

        enum r1Sorted = __traits(isSame, TemplateOf!(R1), SortedRange);

        enum r2Sorted = __traits(isSame, TemplateOf!(R2), SortedRange);

        static if (r1Sorted && r2Sorted)

        {

            // r1.popFront while:

            // r1.front == r2.front

        }

        else if (r2Sorted)

        {

            // r1.popFront while:

            // binary sarch in r2 for r1.front

        }

        else

        {

            // r1.popFront while:

            // linear search in r2 for r1.front

        }

    }

​

    this(R1 r1, R2 r2) { /* this.moveToNextElement; */ }

    bool empty() @property { return true; }

    auto front() @property { return 0; }

    void popFront() { /* this.moveToNextElement; */ }

}

​

auto difference(alias pred = "a", R1, R2)(R1 r1, R2 r2)

if (isInputRange!R1 && isInputRange!R2)

{

    import std.range: ElementType;

    import std.algorithm: sort;

    import std.traits: TemplateOf;

    enum r1Sorted 

        = is(TemplateOf!(typeof(r1)) == SortedRange);

    enum r2Sorted 

        = is(TemplateOf!(typeof(r2)) == SortedRange);

​

    static if (r1Sorted)

    {

        auto i1 = r1;

    }

    else

    {

        auto i1 = r1.sort;

    }

​

    static if (r2Sorted)

    {

        auto i2 = r2;

    }

    else

    {

        auto i2 = r2.sort;

    }

​

    return Difference!(pred, typeof(i1), typeof(i2))(i1, i2);

}

​

void main(string[] args)

{

    [1, 2, 3].difference([1, 2, 3]).writeln;

}