Base-10 digits in factorial

factdigits.go

package main

import (
	"fmt"
	"math"
)

// http://www.johndcook.com/blog/2015/10/06/number-of-digits-in-n/
func digitsInFactorial(x int) int {
	if x <= 0 {
		panic("bad argument")
	}
	lg, _ := math.Lgamma(float64(x) + 1)
	return int(math.Floor(lg/math.Log(10)) + 1)
}

func main() {
	fmt.Println("x -> digits in x! (matches?)")
	fmt.Println("===================")
	for x := 1; x <= 100; x++ {
		lg := digitsInFactorial(x)
		fmt.Printf("%v -> %v (%v)\n", x, lg, x == lg)
	}
}