sparkler: introduce a particle system

A while ago I made this particle system on SDL, and had the beginnings of
an octree implemented within it, but never finished actually using the
octree to accelerate the proximity searches.

This now has the octree completed and of course more particle interactions now
that neighbors could be found more quickly.

The simulation somewhat resembles a fireworks display.  Every particle is drawn
as a single pixel.  The visual effect is dominated by spontaneously spawned
rockets which explode into thousands of particles accompanied by bursts that
thrust particles away from the explosion radially in an expanding sphere
resembling a shock wave.  When the shock wave happens to strike another rocket,
it explodes, resulting in another shock wave.  This can produce spectacular
chain reactions, so it's worth running for some time and seeing what transpires.

1 files changed, 111 insertions, 0 deletions

diff --git a/modules/sparkler/burst.c b/modules/sparkler/burst.c
new file mode 100644
index 0000000..828ca02
--- /dev/null
+++ b/modules/sparkler/burst.c
@@ -0,0 +1,111 @@
+#include <stdlib.h>
+
+#include "bsp.h"
+#include "container.h"
+#include "particle.h"
+#include "particles.h"
+
+
+/* a "burst" (shockwave) particle type */
+/* this doesn't draw anything, it just pushes neighbors away in an increasing radius */
+
+#define BURST_FORCE		0.01f
+#define BURST_MAX_LIFETIME	8
+
+typedef struct _burst_ctxt_t {
+	int	longevity;
+	int	lifetime;
+} burst_ctxt_t;
+
+
+static int burst_init(particles_t *particles, particle_t *p)
+{
+	burst_ctxt_t	*ctxt = p->ctxt;
+
+	ctxt->longevity = ctxt->lifetime = BURST_MAX_LIFETIME;
+	p->props->velocity = 0; /* burst should be stationary */
+	p->props->mass = 0; /* no mass prevents gravity's effects */
+
+	return 1;
+}
+
+
+static inline void thrust_part(particle_t *burst, particle_t *victim, float distance_sq)
+{
+	v3f_t	direction = v3f_sub(&victim->props->position, &burst->props->position);
+
+	/* TODO: normalize is expensive, see about removing these. */
+	direction = v3f_normalize(&direction);
+	victim->props->direction = v3f_add(&victim->props->direction, &direction);
+	victim->props->direction = v3f_normalize(&victim->props->direction);
+
+	victim->props->velocity += BURST_FORCE;
+}
+
+
+typedef struct burst_sphere_t {
+	particle_t	*center;
+	float		radius_min;
+	float		radius_max;
+} burst_sphere_t;
+
+
+static void burst_cb(bsp_t *bsp, list_head_t *occupants, void *_s)
+{
+	burst_sphere_t	*s = _s;
+	bsp_occupant_t	*o;
+	float		rmin_sq = s->radius_min * s->radius_min;
+	float		rmax_sq = s->radius_max * s->radius_max;
+
+	/* XXX: to avoid having a callback per-particle, bsp_occupant_t was
+	 * moved to the public particle, and the particle-specific
+	 * implementations directly perform bsp-accelerated searches.  Another
+	 * wart caused by this is particles_bsp().
+	 */
+	list_for_each_entry(o, occupants, occupants) {
+		particle_t	*p = container_of(o, particle_t, occupant);
+		float		d_sq;
+		
+		if (p == s->center) {
+			/* leave ourselves alone */
+			continue;
+		}
+
+		d_sq = v3f_distance_sq(&s->center->props->position, &p->props->position);
+
+		if (d_sq > rmin_sq && d_sq < rmax_sq) {
+			/* displace the part relative to the burst origin */
+			thrust_part(s->center, p, d_sq);
+		}
+
+	}
+}
+
+
+static particle_status_t burst_sim(particles_t *particles, particle_t *p)
+{
+	burst_ctxt_t	*ctxt = p->ctxt;
+	bsp_t		*bsp = particles_bsp(particles);	/* XXX see note above about bsp_occupant_t */
+	burst_sphere_t	s;
+
+	if (!ctxt->longevity || (ctxt->longevity--) <= 0) {
+		return PARTICLE_DEAD;
+	}
+
+	/* affect neighbors for the shock-wave */
+	s.radius_min = (1.0f - ((float)ctxt->longevity / ctxt->lifetime)) * 0.075f;
+	s.radius_max = s.radius_min + .01f;
+	s.center = p;
+	bsp_search_sphere(bsp, &p->props->position, s.radius_min, s.radius_max, burst_cb, &s);
+
+	return PARTICLE_ALIVE;
+}
+
+
+particle_ops_t	burst_ops = {
+			.context_size = sizeof(burst_ctxt_t),
+			.sim = burst_sim,
+			.init = burst_init,
+			.draw = NULL,
+			.cleanup = NULL,
+		};