Birthday Problem in Go

birthday.go

package main

import "fmt"
import "math"
import "math/big"

func bigFactorial(n int64) *big.Int {
	if (n > 0) {
		x := bigFactorial(n - 1)
		m := big.NewInt(n)

		return x.Mul(m, x)
	}

	return big.NewInt(1)
}

func birthdayProbability(num int64) float64 {
	allPermutations := bigFactorial(365)
	offsetPermutations := bigFactorial(365 - num)
	totalPermutations := new(big.Int).Div(allPermutations, offsetPermutations)
	totalBirthdays := big.NewFloat(math.Pow(365, float64(num)))

	notSameProbability, _ := new(big.Float).Quo(new(big.Float).SetInt(totalPermutations), totalBirthdays).Float64()

	return 1 - notSameProbability
}

func main() {
	result := birthdayProbability(70)

	fmt.Printf("Probability is %f\n", result)
}