public static void FallingFactorial()
{
//this example shows how to get falling factorial
//use Combinatoric.FallingFactorial(n, r) for n!/r!
Console.WriteLine(Combinatoric.FallingFactorial(10, 5));
}
//constructors
//static methods
static long count(IEnumerable <T> source, int length, CombinatoricModel model)
{
switch (model)
{
case CombinatoricModel.Normal:
return(Combinatoric.FallingFactorial((long)source.Count(), length));
case CombinatoricModel.Repetition:
return(((long)source.Count()).Power(length));
case CombinatoricModel.Distinct:
var lookup = source.ToLookup(x => x);
return(distinctCount(lookup.Select(x => x.LongCount()).OrderBy(x => x).ToList(), length, lookup.Count, source.Count() - length));
case CombinatoricModel.RepetitionDistinct:
return(source.Distinct().LongCount().Power(length));
default:
return(0L);
}
}