nqueen.d

uint inner_nqueen(int all_bits, int left, int depth, int right) pure nothrow @safe @nogc
{
  int results, bitmap = ~(left | depth | right) & all_bits;
  while (bitmap != 0) {
    int last_bit = -bitmap & bitmap;
    bitmap ^= last_bit;
    results += inner_nqueen(all_bits, (left | last_bit) << 1, depth | last_bit, (right | last_bit) >> 1);
  }
  return results + (depth == all_bits ? 1 : 0);
}

uint nqueen(uint n) pure nothrow @safe @nogc { return inner_nqueen((1 << n) - 1, 0, 0, 0); }

unittest {
  import std.conv : to; foreach (i, elem; [1, 1, 0, 0, 2, 10, 4, 40, 92]) assert( nqueen(i.to!uint) == elem );
}