package q43

import "slices"

func multiply(num1, num2 string) string {
	if num1 == "0" || num2 == "0" {
		return "0"
	}

	l1, l2 := len(num1), len(num2)
	res := make([]byte, l1+l2)

	// Calculation
	for i1 := range l1 {
		n1 := num1[l1-1-i1] - '0'
		if n1 == 0 {
			continue
		}
		for i2 := range l2 {
			n2 := num2[l2-1-i2] - '0'
			j := i1 + i2
			res[j] += n1 * n2
			if res[j] > 9 {
				res[j+1] += res[j] / 10
				res[j] = res[j] % 10
			}
		}
		for i := l2 + i1; res[i] > 9; i++ {
			res[i+1] += res[i] / 10
			res[i] = res[i] % 10
		}
	}

	// Post process
	for i := len(res) - 1; i >= 0; i-- {
		if res[i] > 0 {
			res = res[:i+1]
			break
		}
	}

	slices.Reverse(res)
	for i := range res {
		res[i] = res[i] + '0'
	}
	return string(res)
}

var _ = multiply