20: factorial digit sum

20_factorial_digit_sum/main.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"fmt"
+	"math/big"
+	"strconv"
+)
+
+func main() {
+	fmt.Println("sum:", sumDigits(findFactorial(100)))
+}
+
+func findFactorial(input int) string {
+	product, _ := (&big.Int{}).SetString("1", 0)
+	for input > 0 {
+		i, _ := (&big.Int{}).SetString(strconv.Itoa(input), 0)
+		product = product.Mul(product, i)
+		input--
+	}
+	return product.String()
+}
+
+func sumDigits(input string) int {
+	l := len(input)
+	if l == 0 {
+		return 0
+	}
+	sum := 0
+	for i := 0; i < l; i++ {
+		ns := input[i : i+1]
+		n, err := strconv.Atoi(ns)
+		if err != nil {
+			panic(err)
+		}
+		sum += n
+	}
+	return sum
+}