#version 450

layout (constant_id = 0) const int MaxParticles = 2048;

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 OutStates {
	Particle particles[];
} outStates;

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

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

layout(std140, set = 1, binding = 0) buffer InStates {
	Particle particles[];
} inStates;

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

//doubles as VkDrawIndirectCommand
layout(std140, set = 1, binding = 2) buffer InSystem {
	uint        vertexCount;
	uint        particleCount;	//instanceCount
	uint        firstVertex;
	uint        firstInstance;
} inSystem;

layout(std140, set = 2, binding = 0) buffer NewStates {
	Particle particles[];
} newStates;

layout(std140, set = 2, binding = 1) buffer NewParameters {
	Parameters parameters[];
} newParameters;

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

bool
is_dead (in Particle part, in Parameters parm)
{
	if (part.live <= 0) {
		return true;
	}
	if (part.ramp >= parm.ramp.y || part.color.a <= 0 || part.scale <= 0) {
		return true;
	}
	return false;
}

void
main ()
{
	uint        j = 0;
	// compact existing partles removing dead particles
	for (uint i = 0; i < inSystem.particleCount; i++) {
		if (is_dead (inStates.particles[i], inParameters.parameters[i])) {
			continue;
		}
		outStates.particles[j] = inStates.particles[i];
		outParameters.parameters[j] = inParameters.parameters[i];
		j++;
	}
	// inject any new particles that aren't DOA
	for (uint i = 0; i < newSystem.particleCount && j < MaxParticles; i++) {
		if (is_dead (newStates.particles[i], newParameters.parameters[i])) {
			continue;
		}
		outStates.particles[j] = newStates.particles[i];
		outParameters.parameters[j] = newParameters.parameters[i];
		j++;
	}
	outSystem.vertexCount = newSystem.vertexCount;
	outSystem.particleCount = j;
	outSystem.firstVertex = newSystem.firstVertex;
	outSystem.firstInstance = newSystem.firstInstance;
}