#version 450

layout (local_size_x = 1, local_size_y = 1)  in;

struct Particle {
	vec4        pos;
	vec4        vel;
	vec4        color;
	float       tex;
	float       ramp;
	float       scale;
	float       live;
};

struct Parameters {
	vec4        drag;	// [dx, dy, dz, grav scale]
	vec4        ramp;	// [rate, max, alpha rate, scale rate]
};

layout(std140, set = 0, binding = 0) buffer ParticleStates {
	Particle particles[];
};

layout(std140, set = 0, binding = 1) buffer ParticleParameters {
	Parameters parameters[];
};

//doubles as VkDrawIndirectCommand
layout(std140, set = 0, binding = 2) buffer ParticleSystem {
	uint        vertexCount;
	uint        particleCount;	//instanceCount
	uint        firstVertex;
	uint        firstInstance;
};

layout (push_constant) uniform PushConstants {
	vec4        gravity;
	float       dT;
};

void
main ()
{
	uint        ind = gl_GlobalInvocationID.x;
	if (ind >= particleCount) {
		return;
	}
	Particle    part = particles[ind];
	Parameters  parm = parameters[ind];

	part.pos += dT * part.vel;
	part.vel += dT * (part.vel * parm.drag + gravity * parm.drag.w);

	part.ramp += dT * parm.ramp.x;
	part.scale += dT * parm.ramp.z;
	part.color.a -= dT * parm.ramp.a;
	part.live -= dT;

	particles[ind] = part;
}