Permutations and Combinations With Groovy

fac: work out the factorial of any positive integer

comobs: given n amount of things, and picking r amount from it, where order does not matter, how many combos are there?

permutations: give n amount of things, picking r amount from it, where order matters, how many permutations are there?

def fac = {it = it as BigInteger;  it <= 1 ? 1 : it * call(it - 1)}

// c(n,r) = n!/r!(n-r)!
// n = 52 cards  ; r = 5 cards per hand
// order doesnt matter!!
def combos = { n, r -> fac(n).divide(fac(r)*fac(n-r))}

// P(n,r) = n!/(n-r)!
// order matters!!
def permutations = { n, r-> fac(n).divide(fac(n-r))}

combos(52,5)
2598960

permutations(10, 2)
90