// On entry: elements & choices assumed valid, picks = elements.Length.
private static long CalcRank(int[] elements, int choices)
{
long result = 0;
var isUsed = new bool[choices];
//
// Perform ranking:
//
for (var ei1 = 0; ei1 < elements.Length; ++ei1)
{
isUsed[elements[ei1]] = true;
var digit = 0;
for (var ei2 = 0; ei2 < elements[ei1]; ++ei2)
{
if (!isUsed[ei2])
{
++digit;
}
}
result += digit * Combinatoric.Factorial(choices - ei1 - 1);
}
if (elements.Length < choices)
{
result = result / Combinatoric.Factorial(choices - elements.Length);
}
return(result);
}
// On entry: elements allocated for choices.
// On exit: result will be in elements.
private static void CalcPlainUnrank(int[] elements, long plainRank)
{
elements[0] = 0;
for (var ei = 1; ei < elements.Length; ++ei)
{
var yd = (int)(Combinatoric.Factorial(elements.Length) / Combinatoric.Factorial(ei + 1));
var yi = (int)((plainRank / yd) % ((ei + 1) * 2));
var ip = yi <= ei ? ei - yi : yi - ei - 1;
for (var si = ei; si > ip; --si)
{
elements[si] = elements[si - 1];
}
elements[ip] = ei;
}
}
// On entry: choices, picks assumed valid.
private static long CalcCount(int choices, int picks)
{
if (picks == 0)
{
return(0);
}
var result = Combinatoric.Factorial(choices);
if (picks < choices)
{
result = result / Combinatoric.Factorial(choices - picks);
}
return(result);
}
