(gravity) added radial, Newtonian (1/r^2) force field

balls.c

@@ -48,11 +48,14 @@ void balls_init_state () {
 }
 
 void ball_update_state (struct ball * p) {
-    p->x += delta*p->v_x + delta*delta*g_x/2.0;
-    p->v_x += delta*g_x;
+    struct gravity_vector g;
+    gravity_get_vector (&g, p);
 
-    p->y += delta*p->v_y + delta*delta*g_y/2.0;
-    p->v_y += delta*g_y;
+    p->x += delta*p->v_x + delta*delta*g.x/2.0;
+    p->v_x += delta*g.x;
+
+    p->y += delta*p->v_y + delta*delta*g.y/2.0;
+    p->v_y += delta*g.y;
 
     if (p->x + p->radius > width) { /* right wall */
 	if (p->v_x > 0) {

gravity.c

@@ -4,13 +4,31 @@
 #include "gravity.h"
 #include "game.h"
 
-double g_y = 20;
-double g_x = 0;
+static double g_y = 20;
+static double g_x = 0;
+
+static double g_r = 50;
+static double g_g = 10000000;
+
+static int constant_field = 1;
 
 static int gravity_vector_countdown = 0;
 static int gravity_vector_init = 300;
 
+void gravity_constant_field (double x, double y) {
+    constant_field = 1;
+    g_x = x;
+    g_y = y;
+}
+    
+void gravity_newton_field (double r, double g) {
+    constant_field = 0;
+    g_r = r;
+    g_g = g;
+}
+
 void gravity_draw (cairo_t * cr) {
+    if (constant_field) {
 	if (gravity_vector_countdown != 0) {
 	    cairo_save(cr);
 	    cairo_new_path(cr);
@@ -25,15 +43,71 @@ void gravity_draw (cairo_t * cr) {
 		--gravity_vector_countdown;
 	    cairo_restore(cr);
 	}
+    } else {
+	cairo_set_source_rgb(cr, 1.0, 1.0, 1.0);
+	cairo_save (cr);
+	cairo_new_path (cr);
+	cairo_arc (cr, width/2, height/2, g_r, 0, 2*M_PI);
+	cairo_set_line_width(cr, 3.0);
+	cairo_stroke(cr);
+	cairo_restore(cr);
+    }
 }
 
 void gravity_show () {
+    if (constant_field)
 	gravity_vector_countdown = gravity_vector_init;
 };
 
 void gravity_change (double dx, double dy) {
+    if (constant_field) {
 	g_x += dx;
 	g_y += dy;
 	gravity_show ();
-};
+    } else {
+	g_r += dx;
+	g_g += dy;
+    }
+}
 
+void gravity_get_vector (struct gravity_vector * v, const struct ball * b) {
+    if (constant_field) {
+	v->x = g_x;
+	v->y = g_y;
+    } else {
+	double dx = width/2 - b->x;
+	double dy = height/2 - b->y;
+	double r2 = dx*dx+dy*dy;
+	if (r2 < g_r*g_r) {
+	    v->x = 0;
+	    v->y = 0;
+	} else {
+	    double r = sqrt(r2);
+	    v->x = g_g/r2/r*dx;
+	    v->y = g_g/r2/r*dy;
+	}
+    }
+}
+
+void gravity_collisions (struct ball * begin, struct ball * end) {
+    if (constant_field)
+	return;
+    for (struct ball * b = begin; b != end; ++b) {
+	double dx = b->x - width/2;
+	double dy = b->y - height/2;
+	double d2 = dx*dx + dy*dy;
+	double r = b->radius + g_r;
+	if (d2 <= r*r) {
+	    double dv_x = b->v_x;
+	    double dv_y = b->v_y;
+
+	    double f = dv_x*dx + dv_y*dy;
+
+	    if (f < 0) {
+		f /= d2;
+		b->v_x -= 2*f*dx;
+		b->v_y -= 2*f*dy;
+	    }
+	}
+    }
+}

gravity.h

@@ -1,11 +1,22 @@
 #ifndef GRAVITY_H_INCLUDED
 #define GRAVITY_H_INCLUDED
 
-extern double g_y;
-extern double g_x;
+#include "balls.h"
+
+struct gravity_vector {
+    double x;
+    double y;
+};
+
+extern void gravity_constant_field (double x, double y);
+extern void gravity_newton_field (double r, double g);
+
+extern void gravity_get_vector (struct gravity_vector * v, const struct ball * b);
 
 extern void gravity_draw (cairo_t * cr);
 extern void gravity_change (double dx, double dy);
 extern void gravity_show ();
 
+extern void gravity_collisions (struct ball * begin, struct ball * end);
+
 #endif

main.c

@@ -24,6 +24,8 @@ void check_collisions_simple () {
 	    ball_elastic_collision(balls + i, balls + j);
     for(int j = 0; j < n_balls; ++j)
 	ball_elastic_collision(&spaceship, balls + j);
+   gravity_collisions (balls, balls + n_balls);
+   gravity_collisions (&spaceship, &spaceship + 1);
 }
 
 void check_collisions_with_index () {
@@ -31,6 +33,8 @@ void check_collisions_with_index () {
     c_index_check_collisions(ball_elastic_collision);
    for(int j = 0; j < n_balls; ++j)
 	ball_elastic_collision(&spaceship, balls + j);
+   gravity_collisions (balls, balls + n_balls);
+   gravity_collisions (&spaceship, &spaceship + 1);
 }
 
 void (*check_collisions)() = 0;
@@ -137,7 +141,8 @@ void print_usage (const char * progname) {
 	    "options:\n"
 	    "\t<width>x<height>\n"
 	    "\tn=<number of balls>\n"
-	    "\tfx=<x-force>\n"
+	    "\tfconst=<x-force>,<y-force> :: constant force field\n"
+	    "\tfnewt=<radius>,<g> :: radial, Newtonian force field\n"
 	    "\tfy=<y-force>\n"
 	    "\tradius=<min-radius>-<max-radius>\n"
 	    "\tv=<min-velocity>-<max-velocity>\n"
@@ -193,16 +198,21 @@ gboolean timeout (gpointer user_data) {
 int main (int argc, const char *argv[]) {
     int w = DEFAULT_WIDTH;
     int h = DEFAULT_HEIGHT;
+    double fa, fb;
     
     for (int i = 1; i < argc; ++i) {
 	if (sscanf(argv[i], "%dx%d", &w, &h) == 2)
 	    continue;
 	if (sscanf(argv[i], "n=%u", &n_balls) == 1)
 	    continue;
-	if (sscanf(argv[i], "fx=%lf", &g_x) == 1)
+	if (sscanf(argv[i], "fconst=%lf,%lf", &fa, &fb) == 2) {
+	    gravity_constant_field (fa,fb);
 	    continue;
-	if (sscanf(argv[i], "fy=%lf", &g_y) == 1)
+	}
+	if (sscanf(argv[i], "fnewt=%lf,%lf", &fa, &fb) == 2) {
+	    gravity_newton_field (fa,fb);
 	    continue;
+	}
 	if (sscanf(argv[i], "radius=%u-%u", &radius_min, &radius_max) == 2)
 	    continue;
 	if (sscanf(argv[i], "v=%u-%u", &v_min, &v_max) == 2)