big-factorial.go

package main

import (
	"fmt"
	"log"
	"math/big"
)

// readInt reads an integer from standard input.
func readInt() (int, error) {
	var a int
	_, err := fmt.Scan(&a)
	return a, err
}

// bigFactorial calculates the factorial of an arbitrarily large integer.
func bigFactorial(n int64) *big.Int {
	ret := big.NewInt(1)
	if n == 0 || n == 1 {
		return ret
	}
	return ret.MulRange(1, n)
}

func main() {
	n, err := readInt()
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println(bigFactorial(int64(n)))
}