Karatsuba test

// author: r57shell@uralweb.ru
#include <cstdio>

// standard add carry flag calculation
bool add_c(int a, int b)
{
    if ((unsigned int)a > 0xF || (unsigned int)b > 0xF)
        printf("Assertation failed!\n");
    int r = a + b;
    return ((a&0x8)&b)|(((~r)&0x8)&b)|(((~r)&0x8)&a);
}

// if you don't belive to previous function, use this
bool add_c1(int a, int b)
{
    if ((unsigned int)a > 0xF || (unsigned int)b > 0xF)
        printf("Assertation failed!\n");
    int r = a + b;
    return r>0xF;
}

// simple signed 4.4 fixed point multiplication
int mul(int a, int b)
{
    if ((unsigned int)a > 0xFF || (unsigned int)b > 0xFF)
        printf("Assertation failed!\n");
    return (((int)(char)a*(int)(char)b)>>4)&0xFF;
}

// karatsuba test signed 4.4 fixed point multiplication
int mul2(int a, int b)
{
    if ((unsigned int)a > 0xFF || (unsigned int)b > 0xFF)
        printf("Assertation failed!\n");
    int s=1;
    if ((char)a < 0)
    {
        s *= -1;
        a = -(char)a;
    }
    if ((char)b < 0)
    {
        s *= -1;
        b = -(char)b;
    }
    if ((unsigned int)a > 0xFF || (unsigned int)b > 0xFF)
        printf("Assertation failed!\n");
    int z1 = ((a&0xF)*(b&0xF))&0xFF; //mul low
    int z3 = ((a>>4)*(b>>4))&0xFF; // mul high
    int x = ((a>>4)-(a&0xF))*((b>>4)-(b&0xF))&0xFF; // mul :S
    int y = (z1-x)&0xFF;
    int z2 = (y+z3)&0xFF; // z2 now calculated
    int r1 = ((z1>>4)+z2&0xF); // sum low part
    int c = add_c((z1>>4),z2&0xF)?1:0; // check carry
    int r2 = ((z2>>4)+(z3&0xF)+c)&0xF; // sum high part
    int r = (r1)|(r2<<4); // combine result
    if (s == -1) // sign invert
        r = ((~r)+((z1)&0xF?0:1))&0xFF;
    return r; // hurray!
}

int main()
{
    for (int a=0; a<0x100; ++a)
        for (int b=0; b<0x100; ++b)
        {
            if (mul(a,b) != mul2(a,b))
                printf("mul(%02X,%02X)=%02X!=%02X=mul2(%02X,%02X)\n",a,b,mul(a,b),mul2(a,b),a,b);
        }
    return 0;
}