Maximum Combination Calculator

combinations.go

package main

import (
	"fmt"
	"math/big"
)

func factorial(n *big.Int) (result *big.Int) {
	result = big.NewInt(1)
	var one big.Int
	one.SetInt64(1)
	for n.Cmp(&big.Int{}) == 1 {
		result.Mul(result, n)
		n.Sub(n, &one)
	}
	return
}

func combinations(r, n int64) *big.Int {
	rBig := big.NewInt(r)
	nBig := big.NewInt(n)
	nSubR := new(big.Int)
	div := new(big.Int)
	mul := new(big.Int)
	nSubR.Sub(rBig, nBig)
	rFactorial := factorial(rBig)
	nFactorial := factorial(nBig)
	nSubRFactorial := factorial(nSubR)
	mul.Mul(nFactorial, nSubRFactorial)
	div.Div(rFactorial, mul)
	return div
}

func main() {
	fmt.Println(combinations(52, 13))
}