line ray collision detection SDL

//http://www.youtube.com/user/thecplusplusguy
#include <SDL/SDL.h>
#include <iostream>


//put a pixel to the screen
void putPixel(int x,int y)
{
    if(x<0 || x>640 || y<0 || y>480)
        return;
    Uint32* pixels=(Uint32*)SDL_GetVideoSurface()->pixels;
    Uint32* pixel=pixels+y*SDL_GetVideoSurface()->pitch/4+x;
    *pixel=SDL_MapRGB(SDL_GetVideoSurface()->format,0xff,0xff,0xff);
}

//get the absolute value of a number (the abs() works only for ints I think
double absolut(double d)
{
    return (d>0 ? d : -d);
}


//draw a line
void drawline(SDL_Surface* screen,int x1,int y1,int x2,int y2)
{
    if(x2>x1)
    {
        int x=x1,y=y1;
        double deltaerr=(float)(y2-y1)/(x2-x1);
        double error=0.0;
        while(x<x2)
        {
            putPixel(x,y);
            error+=deltaerr;
            if(error>=0.5)
            {
                y++;
                error-=1.;
            }
            x++;
        }
    }else if(x1>x2)
    {
        int x=x1,y=y1;
        double deltaerr=absolut((double)(y2-y1)/(x2-x1));
        double error=0.0;
        while(x>x2)
        {
            putPixel(x,y);
            error+=deltaerr;
            if(error>=0.5)
            {
                y++;
                error-=1.;
            }
            x--;
        }
    }else if(x1==x2)
    {
        for(int i=y1;i<y2;i++)
        {
            putPixel(x1,i);
        }
    }else if(y1>y2)
    {

    }
}

//check collision between the ray and the line
bool lineray(float xs,float ys,float xd,float yd,int x1,int y1,int x2,int y2)
{
    float t=t = (xs*(y2 - y1) + ys*(x1 - x2) + y1*x2 - y2*x1)/(yd*x2 - x1 *yd - y2*xd + y1*xd);
    if(t<0)
        return false;
    int x=xs+t*xd;
    int y=ys+t*yd;
    if(x>=x1 && x<=x2 && y>=y1 && y<=y2)
    {
        drawline(SDL_GetVideoSurface(),x,y,x,y+100);
        return 1;
    }
    return 0;

}



int main(int argc,char** argv)
{
    SDL_Init(SDL_INIT_EVERYTHING);
    SDL_Surface* screen=SDL_SetVideoMode(640,480,32,SDL_SWSURFACE);
    bool running=true;
    SDL_Event event;
    Uint32 start;
    bool b[4]={0,0,0,0};
    int x=100,y=100;
    while(running)
    {
        start=SDL_GetTicks();
        while(SDL_PollEvent(&event))
        {
            switch(event.type)
            {
                case SDL_QUIT:
                    running=false;
                    break;
                case SDL_KEYDOWN:
                    switch(event.key.keysym.sym)
                    {
                        case SDLK_UP:
                            b[0]=true;
                            break;
                        case SDLK_LEFT:
                            b[1]=true;
                            break;
                        case SDLK_DOWN:
                            b[2]=true;
                            break;
                        case SDLK_RIGHT:
                            b[3]=true;
                            break;
                    }
                    break;
                case SDL_KEYUP:
                    switch(event.key.keysym.sym)
                    {
                        case SDLK_UP:
                            b[0]=false;
                            break;
                        case SDLK_LEFT:
                            b[1]=false;
                            break;
                        case SDLK_DOWN:
                            b[2]=false;
                            break;
                        case SDLK_RIGHT:
                            b[3]=false;
                            break;
                    }
                    break;
            }
        }
        //render
        if(b[0])
            y--;
        if(b[1])
            x--;
        if(b[2])
            y++;
        if(b[3])
            x++;
        SDL_FillRect(screen,&screen->clip_rect,SDL_MapRGB(screen->format,0,0,0));
        putPixel(50,50);
        drawline(screen,10,10,200,100);
        drawline(screen,x,y,x+0.70710*100,y+0.70710*100);
        lineray(x,y,0.70710,0.70710,10,10,200,100);
        SDL_Flip(screen);
        if(1000/30>(SDL_GetTicks()-start))
            SDL_Delay(1000/30-(SDL_GetTicks()-start));
    }
    SDL_Quit();
}