import std.stdio, std.algorithm, std.typecons, std.container,std.array;

auto encode(T)(Group!("a == b", T[]) sf) {
    auto heap = sf.map!(s => tuple(s[1], [tuple(s[0], "")]))
                .array.heapify!q{b < a};

    while (heap.length > 1) {
        auto lo = heap.front; heap.removeFront;
        auto hi = heap.front; heap.removeFront;
        foreach (ref pair; lo[1]) pair[1] = '0' ~ pair[1];
        foreach (ref pair; hi[1]) pair[1] = '1' ~ pair[1];
        heap.insert(tuple(lo[0] + hi[0], lo[1] ~ hi[1]));
    }
    return heap.front[1].schwartzSort!q{tuple(a[1].length, a[0])};
}

void main() {
    auto s = "this is an example for huffman encoding"d;
    foreach (p; s.dup.sort().release.group.encode)
        writefln("'%s'  %s", p[]);
}