Question

My friend gave me a bit of code for my SDL program, all I know that it makes a random color but I have no idea how it works , here is the code

 int unsigned temp = 10101;//seed
    for(int y = 0;y < times;y++){
        temp = temp*(y+y+1);
        temp = (temp^(0xffffff))>>2;
        //printf("%x\n",temp);
        SDL_FillRect(sprite[y],NULL,temp);
        SDL_BlitSurface(sprite[y],&paste[y],rScreen(),NULL);
        }

My question is , How does this code work and how does it make a random color

Answer 1

Your friend is creating a “random RGB value” ranging from 0x000000 to 0xFFFFFF out of some amateur PRNG he invented.

I’ll explain the code with comments:

This is the so called “seed”. The initial value that will generate the pseudo-random sequence of values.

 int unsigned temp = 10101; //seed 

then we got the for loop:

 for(int y = 0;y < times;y++)
 {
    temp = temp*(y+y+1);
    temp = (temp^(0xffffff))>>2;

each round your friend is trying to make complicated multiplications and sums to come up with a new temp value which is divided by 2 (the >>2 in the code above) and then masked with 0xFFFFFFF to get a value in the range of 0x000000 to 0xFFFFFFF (he wrongly used bitwise XOR ^ instead of bitwise AND &)

The resulting value is used as a RGB value for the SDL_FillRect() function.

    //printf("%x\n",temp);
    SDL_FillRect(sprite[y],NULL,temp);
    SDL_BlitSurface(sprite[y],&paste[y],rScreen(),NULL);
 }