#pragma kernel CSMain

#define NUM_THREAD_X 16

struct Particle
{
	float3 velocity;
	float3 pos;
};

float _Time;
float _ForceRate;
float _Damping;
float _Gravity;
float3 _Bounds;
RWStructuredBuffer<Particle> _ParticleBuffer;
RWStructuredBuffer<float3> _InputBuffer;

float rand(float2 co){
    return frac(sin(dot(co.xy ,float2(12.9898,78.233))) * 43758.5453);
}

[numthreads(NUM_THREAD_X,1,1)]
void CSMain (uint3 id : SV_DispatchThreadID)
{
	uint idx = id.x;

	Particle particle = _ParticleBuffer[idx];
	float3 force = float3 (
			rand(float2(idx, _Time  )) - 0.5, 
			rand(float2(idx, _Time*2)) - 0.5, 
			rand(float2(idx, _Time*3)) - 0.5
			);

	float3 velocity = particle.velocity * _Damping +  force * _ForceRate;

	velocity.y -= _Gravity;

	float3 pos = particle.pos + velocity;

	float3 overPlus = pos - _Bounds;
	if ( overPlus.x > 0) { pos.x = _Bounds.x;  velocity.x *= -0.95;}
	if ( overPlus.y > 0) { pos.y = _Bounds.y;  velocity.y *= -0.95;}
	if ( overPlus.z > 0) { pos.z = _Bounds.z;  velocity.z *= -0.95;}

	float3 overMinus = pos + _Bounds;
	if ( overMinus.x < 0){ pos.x = -_Bounds.x; velocity.x *= -0.95;}
	if ( overMinus.y < 0){ pos.y = -_Bounds.y; velocity.y *= -0.95;}
	if ( overMinus.z < 0){ pos.z = -_Bounds.z; velocity.z *= -0.95;}

	particle.pos = pos;
	particle.velocity = velocity;
	_ParticleBuffer[idx] = particle;
	
	_InputBuffer[idx] = particle.pos;
}