Russian peasant multiplication

package mult

import "testing"

func TestMultNegative(t *testing.T) {
    if v := Mult(-1, -4); v != 4 {
        t.Fatalf("Expected %v to be 4", v)
    }
}

func TestMultPositive(t *testing.T) {
    if v := Mult(7, 9); v != 63 {
        t.Fatalf("Expected %v to be 63", v)
    }
}

func TestMultPositiveAndNegative(t *testing.T) {
    if v := Mult(-8, 9); v != -72 {
        t.Fatalf("Expected %v to be -72", v)
    }
}

package mult

func sign(x int) int {
    if x < 0 {
        return -1
    }
    return 1
}

func abs(x int) int {
    if x < 0 {
        return -x
    }
    return x
}

func Mult(a, b int) int {
    // Handle the case with negative values for a and b
    as := sign(a)
    a = abs(a)
    bs := sign(b)
    b = abs(b)

    // Actual multiplication
    x := uint(a)
    y := uint(b)
    z := uint(0)
    for x > 0 {
        if x%2 == 1 {
            z += uint(y)
        }
        x >>= 1
        y <<= 1
    }
    return int(z) * as * bs
}