16: power digit sum

16_power_digit_sum/main.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"fmt"
+	"math/big"
+	"strconv"
+)
+
+func main() {
+	n := &big.Int{}
+	n.SetInt64(1024)
+	p := (&big.Int{}).SetInt64(1)
+	for i := 0; i < 100; i++ {
+		p = p.Mul(p, n)
+	}
+	sum, err := SumDigits(p.String())
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println("product:", p.String(), "sum:", sum)
+}
+
+func SumDigits(input string) (int, error) {
+	l := len(input)
+	sum := 0
+	for i := 0; i < l; i++ {
+		n, err := strconv.Atoi(input[i : i+1])
+		if err != nil {
+			return 0, fmt.Errorf("invalid input: %s", input)
+		}
+		sum += n
+	}
+	return sum, nil
+}

16_power_digit_sum/power_digit_sum_test.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+func TestSumDigits(t *testing.T) {
+	tt := []struct {
+		name    string
+		input   string
+		want    int
+		wantErr error
+	}{
+		{
+			"invalid",
+			"abc",
+			0,
+			fmt.Errorf("invalid input: %s", "abc"),
+		},
+		{
+			"invalid_2",
+			"1a",
+			0,
+			fmt.Errorf("invalid input: %s", "1a"),
+		},
+		{
+			"1",
+			"1",
+			1,
+			nil,
+		},
+		{
+			"123",
+			"123",
+			6,
+			nil,
+		},
+		{
+			"2345",
+			"2345",
+			14,
+			nil,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := SumDigits(tc.input)
+			if !reflect.DeepEqual(err, tc.wantErr) {
+				t.Errorf("SumDigits(%s) err=%v want err %v", tc.input, err, tc.wantErr)
+				return
+			}
+			if got != tc.want {
+				t.Errorf("SumDigits(%s)=%d, want %d", tc.input, got, tc.want)
+			}
+		})
+	}
+}