Untitled

module test;

import std.algorithm;
import std.stdio;
import std.traits;
import std.range;

struct Permutations(R) {
    ElementType!R[] _input, _perm;
    size_t k, n;

    this(R r) {
        _input = array(r);
        _perm = array(r);
        n = _perm.length;
        k = n;
    }

    this(R r, size_t elems) {
        _input = array(r);
        _perm = array(r);
        n = min(elems, _perm.length);
        k = n;
    }

    ElementType!R[] front() { return _perm;}
    bool empty() { return (n == 1 && k == 0 )|| (n>1 && k <=1);}
    @property Permutations save() { return this;}
    void popFront() {
        k = n;
        if (k==0) { n=1;} // permutation of an empty range or of zero elements
        else {
            C3: _perm = _perm[1..k] ~ _perm[0] ~ _perm[k..$];
            if (_perm[k-1] == _input[k-1]) {
                k--;
                if (k > 1) goto C3;
            }
        }
    }
}

/// ditto
Permutations!R permutations(R)(R r) if (isDynamicArray!R) {
    return Permutations!R(r);
}

/// ditto
Permutations!R permutations(R)(R r, size_t n) if (isDynamicArray!R) {
    return Permutations!R(r, n);
}

/// ditto
Permutations!(ElementType!R[]) permutations(R)(R r) if (!isDynamicArray!R && isForwardRange!R && !isInfinite!R) {
    return Permutations!(ElementType!R[])(array(r));
}

/// ditto
Permutations!(ElementType!R[]) permutations(R)(R r, size_t n) if (!isDynamicArray!R && isForwardRange!R && !isInfinite!R) {
    return Permutations!(ElementType!R[])(array(r), n);
}


void main()
{
    string[] words = ["foo", "bar", "doo"];
    auto p1 = permutations(words);
    writeln(p1);
}