Allegro Programming Tutorial, by Richard G Baldwin

Beginning of the moveBall function

Purpose of moveBall (see Listing 4):

Move the ball one increment along one of four directions, and
Handle collisions between the ball and the walls.

Listing 4. Beginning of the moveBall function.

void moveBall(){
  //Save current location of ball.
  tempX = x;
  tempY = y;

  switch(dir){
    case 0:
      //Direction is northwest.
      if((x <= radius) || (y <= radius)){
        dir = rand() % 4;;
      }else{
        //No collision, set new
        // location for the ball
        --x;
        --y;
      }//end else
    break;