factorial.go

package main

import (
	"fmt"
	"math"
	"math/big"
	"os"
	"strconv"
)

func numThreads(n int64) int64 {
	if n <= 5000 {
		return 2
	}
	return 8 * int64(math.Log2(float64(n)))
}

func prodNToM(n int64, m int64, c chan *big.Int) {
	var net = new(big.Int)
	net.MulRange(n, m)
	c <- net
}

func min(a int64, b int64) int64 {
	if a > b {
		return b
	}
	return a
}

func reduce(a *big.Int, b *big.Int, c chan *big.Int) {
	a.Mul(a, b)
	c <- a
}

func Factorial(i int64) *big.Int {
	var n, parts, d int64

	parts = 0
	d = i / numThreads(i)

	c := make(chan *big.Int)

	for n = 1; n <= i; n += min(i-n+1, d+1) {
		go prodNToM(n, min(i, n+d), c)
		parts++
	}

	for parts > 1 {
		for n = 0; n < parts; n += 2 {
			go reduce(<-c, <-c, c)
		}
		parts >>= 1
	}

	return <-c
}

func main() {
	i, err := strconv.ParseInt(os.Args[1], 10, 64)
	if err != nil {
		panic(err)
	}
	fmt.Println(Factorial(i))
}