87c
#include <xsimple.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>

#define RND ((double)rand() / RAND_MAX)

#define WIN_X 110
#define WIN_Y 75
#define WIN_W 800
#define WIN_H 600

#define HZ 30

#define NBALLS 5

typedef struct {
    int x, y;
    int oldx, oldy;
    int dx, dy;
    int color;
} BALL;

BALL ball[NBALLS] = {
    {300, 100, -1, -1, 10, -5, 58},
    {100, 300, -1, -1, -5, -10, 59},
    {130, 200, -1, -1, -10, 10, 60},
    {350, 370, -1, -1, 10, 5, 61},
    {600, 350, -1, -1, -5, 10, 62}
};


static int itstime(struct timeval *t)
{
    struct timeval now;
    double s1, s2;

    gettimeofday(&now, NULL);
    s1 = (double)t->tv_sec + (double)t->tv_usec / 1000000;
    s2 = (double)now.tv_sec + (double)now.tv_usec / 1000000;
    if (s2 - s1 >= 1./HZ) {
	*t = now;
	return 1;
    }
    return 0;
}

static void do_balls(void)
{
    int i;

    for (i = 0; i < NBALLS; i++) {
	if (ball[i].oldx != -1)
	    fillellipse(ball[i].oldx, ball[i].oldy, 50, 50, BLACK);
    }

    for (i = 0; i < NBALLS; i++)
	fillellipse(ball[i].x, ball[i].y, 50, 50,
		xv.egacolor[ball[i].color]);

    update(0, 0, 0, 0);

    for (i = 0; i < NBALLS; i++) {
	ball[i].oldx = ball[i].x; ball[i].oldy = ball[i].y;

	ball[i].x += ball[i].dx; ball[i].y += ball[i].dy;
	if (ball[i].x == WIN_W - 50 || ball[i].x == 0)
	    ball[i].dx = -ball[i].dx;
	if (ball[i].y == WIN_H - 50 || ball[i].y == 0)
	    ball[i].dy = -ball[i].dy;
    }
}

int main(int argc, char **argv)
{
    struct timeval t;

    srand(time(NULL));

    xinit("balls", "Balls", "Bouncing Balls", WIN_X, WIN_Y, WIN_W, WIN_H, 0,
	    "black", "black", 0);

    gettimeofday(&t, NULL);

    cls(BLACK);

    for (;;) {
	XEvent e;

	if (itstime(&t))
	    do_balls();

	while (getevent(&e)) {
	    switch (e.type) {
		case KeyPress:
		    switch (XLookupKeysym((XKeyEvent *)&e, 0)) {
			int i;
			case XK_Down:
			    for (i = 0; i < NBALLS; i++)
				ball[i].color = (int)(1 + RND * 15);
			    break;
			case XK_q:
			    exit(0);
		    }
		    break;
	    }
	}
    }
}
.

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