sum of two large numbers

main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func main() {
	reader := bufio.NewReader(os.Stdin)
	line, _, _ := reader.ReadLine()
	nums := strings.Split(string(line), " ")
	num1 := nums[0]
	num2 := nums[1]
	result := addition(num1[:len(num1)], num2[:len(num2)])
	fmt.Print(result)
}

func addition(num1 string, num2 string) string {
	nums := [2]string{num1, num2}
	index0 := 0
	index1 := 1
	if len(num1) < len(num2) {
		index0 = 1
		index1 = 0
	}
	end1 := len(nums[index0]) - 1
	end2 := len(nums[index1]) - 1
	sum := []string{}
	carry := 0
	for i := 0; i <= end1 || carry > 0; i++ {
		num1 := 0
		num2 := 0
		if i <= end1 {
			num1, _ = strconv.Atoi(string(nums[index0][end1-i]))
			num2 = 0
			if i <= end2 {
				num2, _ = strconv.Atoi(string(nums[index1][end2-i]))
			}
		}

		s := num1 + num2 + carry
		sum = append(sum, strconv.Itoa(s%10))
		carry = s / 10
	}
	endSum := len(sum)
	rSum := make([]string, endSum, endSum)
	for i := 0; i < endSum; i++ {
		rSum[i] = sum[endSum-i-1]
	}
	return strings.Join(rSum, "")
}

main_test.go

package main

import (
	"fmt"
	"log"
	"math/rand"
	"testing"
)

func TestAddition(t *testing.T) {
	rand.Seed(5000)
	for i := 0; i < 1000; i++ {
		i1 := rand.Intn(100000)
		i2 := rand.Intn(10000000)

		result := addition(fmt.Sprint(i1), fmt.Sprint(i2))
		s := i1 + i2
		log.Printf("%v + %v = %v (%v)\n", i1, i2, s, result)
		if result != fmt.Sprint(s) {
			t.Errorf("result != sum => %v", s)
		}
	}
}