Moduleexact-combinatorics-0.2.1.0Haskell2010

Math.Combinatorics.Exact.Factorial

The factorial numbers (http://oeis.org/A000142). For negative inputs, all functions return 0 (rather than throwing an exception or using Maybe).

Notable limits:

12! is the largest factorial that can fit into Int32.
20! is the largest factorial that can fit into Int64.
170! is the largest factorial that can fit into 64-bit Double.

1 value

Packageexact-combinatorics-0.2.1.0
Exports1
LanguageHaskell2010
LicenceBSD-3-Clause
SourceFactorial.hs

valuefactorial :: (Integral a, Bits a) => Int -> a

Exact factorial numbers. For a fast approximation see math-functions:Numeric.SpecFunctions.factorial instead. The naive definition of the factorial numbers is:

factorial n
    | n < 0     = 0
    | otherwise = product [1..n]

However, we use a fast algorithm based on the split-recursive form:

factorial n =
    2^(n - popCount n) * product [(q k)^k | forall k, k >= 1]
    where
    q k = product [j | forall j, n*2^(-k) < j <= n*2^(-k+1), odd j]