Collision Test Code

#include "SDL/SDL.h"
#include <stdlib.h>
#include <stdio.h>

//This is actually C code, so I'm setting up a bool
//type, like the one in C++.
#define true 1
#define false 0
typedef int bool;

//Sprite data structure.
typedef struct sprite_t {
  int x, y; //Horizontal  and vertical position, respectively. I assume these refer to the upper left corner of the sprite.
  int w, h; //Width and height of sprite, respectively.
  SDL_Surface *image;
}sprite_t;

//One of the few good reasons to use a global variable:
//the screen surface
SDL_Surface *screen;

//set sprite width and height from image.
void setup_sprite(sprite_t *sprite);

//collision test function. Returns true if sprites are touching.
bool collision(sprite_t sprite_1, sprite_t sprite_2);

//render put the sprite on the screen
void render(sprite_t sprite);

int main(){
    //sprites
    sprite_t s1,s2;
  //load images
    s1.image = SDL_LoadBMP("sprite-1.bmp");
    s2.image = SDL_LoadBMP("sprite-2.bmp");
    //check if image loaded correctly:
    if(s1.image == NULL || s2.image == NULL){
        printf("Error: image load failed: %s\n", SDL_GetError());
        return -1;
    }
    //set sprite width from surface
    setup_sprite(&s1);
    setup_sprite(&s2);
    //Init SDL
    if (SDL_Init(SDL_INIT_VIDEO)){
    printf("Error: Unable to init SDL: %s\n", SDL_GetError());
    return -1;
  }
  //Create window
    screen = SDL_SetVideoMode(640, 480, 24, SDL_SWSURFACE);
    if(screen == NULL){
        printf("Error: Unable to create window: %s\n", SDL_GetError());
        return -1;
    }
    //set sprite positions.
    s1.x = 200;
    s1.y = 200;
    s2.x = 150;
    s2.y = 150;
    //main loop
    int done = 0;
    SDL_Event event;
    while(!done){
        //use the arrow keys to move s2 around.
        while(SDL_PollEvent(&event)){
            switch(event.type){
                case SDL_KEYDOWN:
                    switch(event.key.keysym.sym){
                        case SDLK_UP: s2.y--; break;
                        case SDLK_DOWN: s2.y++; break;
                        case SDLK_RIGHT: s2.x++; break;
                        case SDLK_LEFT: s2.x--; break;
                        case SDLK_ESCAPE: done = 1; break;
                    }
                    //check if moving resulted in a collision.
                    if(collision(s1,s2)){
                        printf("collision!\n");
                    }
                    break;
                case SDL_QUIT:
                    done = 1;
                    break;
            }
        }
        //clear screen
        SDL_FillRect(screen, NULL, SDL_MapRGB(screen->format, 0, 0, 0));
        //render sprites
        render(s1);
        render(s2);
        SDL_Flip(screen);
    }
    return 0;
}

//collision test function. Returns true if sprites are touching.
bool collision(sprite_t sprite_1, sprite_t sprite_2){
  //Let's examine the x axis first:
  //If the leftmost or rightmost point of the first sprite lies somewhere inside the second, continue.
  if( (sprite_1.x >= sprite_2.x && sprite_1.x <= (sprite_2.x + sprite_2.w)) ||
      ((sprite_1.x + sprite_1.w) >= sprite_2.x && (sprite_1.x + sprite_1.w) <= (sprite_2.x + sprite_2.w)) ){
    //Now we look at the y axis:
    if( (sprite_1.y >= sprite_2.y && sprite_1.y <= (sprite_2.y + sprite_2.h)) ||
        ((sprite_1.y + sprite_1.h) >= sprite_2.y && (sprite_1.y + sprite_1.h) <= (sprite_2.y + sprite_2.h)) ){
      //The sprites appear to overlap.
      return true;
    }
  }
  //This algorithm will not detect collisions if sprite_2 is smaller than sprite_1.
    //To solve this problem, we'll check again, using sprite_2 in place of sprite_1.
    if( (sprite_2.x >= sprite_1.x && sprite_2.x <= (sprite_1.x + sprite_1.w)) ||
      ((sprite_2.x + sprite_2.w) >= sprite_1.x && (sprite_2.x + sprite_2.w) <= (sprite_1.x + sprite_1.w)) ){
    //Now we look at the y axis:
    if( (sprite_2.y >= sprite_1.y && sprite_2.y <= (sprite_1.y + sprite_1.h)) ||
        ((sprite_2.y + sprite_2.h) >= sprite_1.y && (sprite_2.y + sprite_2.h) <= (sprite_1.y + sprite_1.h)) ){
      //The sprites appear to overlap.
      return true;
    }
  }
  //The sprites do not overlap:
  return false;
}

//set sprite width and height from image.
void setup_sprite(sprite_t *sprite){
    //safety first:
    if(sprite == NULL)return;
    if(sprite->image == NULL)return;

    sprite->w = sprite->image->w;
    sprite->h = sprite->image->h;
}

//render put the sprite on the screen
void render(sprite_t sprite){
    if(sprite.image == NULL)return;
    SDL_Rect drect; //used to store location image to blit
    drect.x = sprite.x;
    drect.y = sprite.y;
    SDL_BlitSurface(sprite.image, NULL, screen, &drect);
}