import std.stdio;

import std.range;

import std.algorithm;

​

struct ExtendedArray(T)

{

    T[] a;

    alias a this;

    T opIndex(int i)

    {

        if (i < 0) return a[0];

        else if (i >= a.length) return a[$-1];

        else return a[i];

    }

}

​

ExtendedArray!T extend(T)(T[] a)

{

    return ExtendedArray!T(a);

}

​

void main()

{

    int[] xs = [1, 2, 3, 4];

    auto extended = extend(xs);

    // can index past the end

    writeln(extended[5]);

    // can index before the start

    writeln(extended[-3]);

    // still works with range algorithms

    writeln(extended.map!(x => x^^2));

}