fact.go

package main

import (
	"big"
	"fmt"
)

func fak(i int64) *big.Int {
	if i < 0 {
		return nil
	}
	r := big.NewInt(1)
	var f big.Int
	for k := int64(2); k <= i; k++ {
		r.Mul(r, f.SetInt64(k))
	}
	return r
}

func fak1(i int64) *big.Int {
	var z big.Int
	return z.MulRange(1, i)
}

func main() {
	fmt.Println(fak(5))
	fmt.Println(fak1(6))
}

random.go

package main

import (
	"fmt"
	"rand"
	"time"
)

func main() {
	rand.Seed(time.Seconds())
	fmt.Println(rand.Int())
}