nbody.c File Reference

#include "nbody.h"
#include "dvec3.h"
#include "utils.h"
#include <cglm/affine.h>
#include <cglm/mat4.h>
#include <cglm/vec3.h>
#include <math.h>

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Data Structures
struct	OrbiterDef

Functions
static void	add_body (NBodySim *sim, const double pos[3], const double vel[3], double mass, float radius, const vec3 albedo, float metallic, float roughness)
static double	pair_softening_sq (double radius_i, double radius_j)
static double	softened_orbital_vel (double grav, double central_mass, double orbit_r, double star_r, double body_r)
void	nbody_init_preset (NBodySim *sim)
	Initializes the simulation with a preset solar system configuration. More...
float	nbody_total_energy (const NBodySim *sim)
	Computes the total energy (kinetic + gravitational potential). More...
static void	compute_accelerations (const NBodySim *sim, double accel[][3])
static void	integrate_step (NBodySim *sim, float delta_time)
void	nbody_step (NBodySim *sim, float delta_time)
	Advances the simulation by the given wall-clock delta time. More...
void	nbody_write_instances (const NBodySim *sim, SphereInstance *out)
	Writes current body positions and materials into a SphereInstance array suitable for the instanced rendering pipeline. More...
int	nbody_get_count (const NBodySim *sim)
	Returns the number of active bodies. More...
float	nbody_kinetic_energy (const NBodySim *sim)
	Computes kinetic energy only: Σ ½ m v². More...
float	nbody_energy_drift (const NBodySim *sim)
	Returns the relative energy drift |E(t) - E₀| / |E₀|. More...
float	nbody_energy_drift_signed (const NBodySim *sim)
void	nbody_update_time_scale (NBodySim *sim, float delta_time)
	Smoothly transitions time_scale toward target_time_scale. More...

Variables
static const float	EPSILON = 1e-6F
static const double	MIN_DIST = 1e-9
static const double	CENTRAL_STAR_MASS = 100.0
static const float	CENTRAL_STAR_RADIUS = 1.5F
static const vec3	CENTRAL_STAR_ALBEDO = {1.0F, 0.76F, 0.34F}
static const float	CENTRAL_STAR_METALLIC = 1.0F
static const float	CENTRAL_STAR_ROUGHNESS = 0.2F
static const double	HALF = 0.5
static const struct OrbiterDef	ORBITERS []
static const int	ORBITER_COUNT = (int)(sizeof(ORBITERS) / sizeof(ORBITERS[0]))

Function Documentation

add_body()

static void add_body ( NBodySim *  sim, const double  pos[3], const double  vel[3], double  mass, float  radius, const vec3  albedo, float  metallic, float  roughness  )

static

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compute_accelerations()

static void compute_accelerations ( const NBodySim *  sim, double  accel[][3]  )

static

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integrate_step()

static void integrate_step ( NBodySim *  sim, float  delta_time  )

static

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nbody_energy_drift()

float nbody_energy_drift

(

const NBodySim *

sim

)

Returns the relative energy drift |E(t) - E₀| / |E₀|.

Returns 0 when initial_energy is zero (no reference).

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nbody_energy_drift_signed()

float nbody_energy_drift_signed

(

const NBodySim *

sim

)

Signed energy drift: negative = energy lost (damping), positive = energy gained (divergence).

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nbody_get_count()

int nbody_get_count

(

const NBodySim *

sim

)

Returns the number of active bodies.

nbody_init_preset()

void nbody_init_preset

(

NBodySim *

sim

)

Initializes the simulation with a preset solar system configuration.

Parameters

sim	Pointer to the simulation state.

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nbody_kinetic_energy()

float nbody_kinetic_energy

(

const NBodySim *

sim

)

Computes kinetic energy only: Σ ½ m v².

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nbody_step()

void nbody_step	(	NBodySim *	sim,
		float	delta_time
	)

Advances the simulation by the given wall-clock delta time.

Uses a fixed-timestep accumulator with Velocity Verlet integration.

Parameters

sim	Pointer to the simulation state.
delta_time	Wall-clock time elapsed since last call (seconds).

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nbody_total_energy()

float nbody_total_energy

(

const NBodySim *

sim

)

Computes the total energy (kinetic + gravitational potential).

Useful for stability diagnostics. With a symplectic integrator, this quantity oscillates with bounded amplitude around E₀.

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nbody_update_time_scale()

void nbody_update_time_scale	(	NBodySim *	sim,
		float	delta_time
	)

Smoothly transitions time_scale toward target_time_scale.

Called once per frame. Ramps at NBODY_TIME_SCALE_RATE units/s, producing a decelerate → pause → accelerate-in-reverse effect.

Parameters

sim	Pointer to the simulation state.
delta_time	Wall-clock frame delta (seconds).

nbody_write_instances()

void nbody_write_instances	(	const NBodySim *	sim,
		SphereInstance *	out
	)

Writes current body positions and materials into a SphereInstance array suitable for the instanced rendering pipeline.

Parameters

sim	Pointer to the simulation state.
out	Output array (must have capacity >= sim->body_count).

pair_softening_sq()

static double pair_softening_sq ( double  radius_i, double  radius_j  )

static

softened_orbital_vel()

static double softened_orbital_vel ( double  grav, double  central_mass, double  orbit_r, double  star_r, double  body_r  )

static

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Variable Documentation

CENTRAL_STAR_ALBEDO

const vec3 CENTRAL_STAR_ALBEDO = {1.0F, 0.76F, 0.34F}

static

CENTRAL_STAR_MASS

const double CENTRAL_STAR_MASS = 100.0

static

CENTRAL_STAR_METALLIC

const float CENTRAL_STAR_METALLIC = 1.0F

static

CENTRAL_STAR_RADIUS

const float CENTRAL_STAR_RADIUS = 1.5F

static

CENTRAL_STAR_ROUGHNESS

const float CENTRAL_STAR_ROUGHNESS = 0.2F

static

EPSILON

const float EPSILON = 1e-6F

static

Small epsilon to avoid division by zero.

HALF

const double HALF = 0.5

static

MIN_DIST

const double MIN_DIST = 1e-9

static

Minimum distance threshold to avoid division by near-zero.

ORBITER_COUNT

const int ORBITER_COUNT = (int)(sizeof(ORBITERS) / sizeof(ORBITERS[0]))

static

ORBITERS

const struct OrbiterDef ORBITERS[]

static

Initial value:

= {
    
     {{ 3, 0, 0},      { 0, 1, 0},     3,   0.30F, 0.3F, {1.0F,0.84F,0.0F},  1.0F, 0.10F, 1.00F},
     {{ 0, 5, 0.5F},   {-1, 0, 0},     5,   0.50F, 1.5F, {0.77F,0.78F,0.78F},1.0F, 0.05F, 1.00F},
     {{ 0,-1, 7},      { 1, 0, 0},     7,   0.60F, 2.0F, {0.85F,0.85F,0.90F},1.0F, 0.15F, 1.00F},
     {{-10, 0, 1.5F},  { 0,-1, 0},    10,   0.80F, 4.0F, {0.72F,0.45F,0.20F},1.0F, 0.30F, 1.00F},
     {{ 8, 2, 2},      { 0, 1,-0.3F},  8,   0.40F, 0.5F, {0.15F,0.85F,0.95F},0.1F, 0.05F, 0.85F},
     {{-3,11,-2},      { 0.5F, 0, 0.6F},11,  0.35F, 0.5F, {0.90F,0.20F,0.75F},0.1F, 0.05F, 0.85F},
     {{ 4, 0.5F, 0},   { 0, 0, 1},     4,   0.35F, 0.6F, {0.15F,0.85F,0.30F},0.8F, 0.15F, 1.00F},
     {{ 0,-10, 2},     { 1, 0, 0},    10,   0.55F, 1.8F, {0.90F,0.55F,0.65F},0.3F, 0.25F, 0.95F},
     {{ 0, 0,-3.5F},   { 0, 1, 0},     3.5F,0.30F, 0.4F, {0.10F,0.25F,0.95F},0.9F, 0.10F, 1.00F},
     {{ 9,-2, 0},      { 0, 1, 0},     9,   0.65F, 3.0F, {1.0F,0.60F,0.10F}, 0.7F, 0.30F, 1.00F},
     {{11, 2,-3},      {-0.4F, 0, 0.7F},11,  0.30F, 0.4F, {0.55F,0.15F,0.95F},0.2F, 0.08F, 0.92F},
     {{-6, 0,-1},      { 0, 1, 0},     6,   0.45F, 1.2F, {0.20F,0.80F,0.75F},0.6F, 0.12F, 1.00F},
     {{-12,-2, 1},     { 0.3F, 0.7F, 0},12,  0.50F, 2.5F, {0.90F,0.12F,0.15F},0.85F,0.35F, 0.95F},
}