#include <metal_stdlib>
using namespace metal;

struct Particle {
  float2 position;
  float  direction;
  float  speed;
  float3 color;
  float  life;
};

kernel void compute(texture2d<half, access::read_write>
output [[texture(0)]],
uint2 id [[thread_position_in_grid]]) {
  output.write(half4(0.0, 0.0, 0.0, 1.0), id);
}

kernel void particleKernel(texture2d<half, access::read_write>
                           output [[texture(0)]],
                           device Particle *particles [[buffer(0)]],
                           uint id [[thread_position_in_grid]]) {
  float xVelocity = particles[id].speed * cos(particles[id].direction);
  float yVelocity = particles[id].speed * sin(particles[id].direction)
  + 3.0;
  particles[id].position.x += xVelocity;
  particles[id].position.y += yVelocity;
  particles[id].life -= 1.0;
  half4 color;
  color.rgb = half3(particles[id].color * particles[id].life
                    / 255.0);
  color.a = 1.0;
  uint2 position = uint2(particles[id].position);
  output.write(color, position);
  output.write(color, position + uint2(0, 1));
  output.write(color, position - uint2(0, 1));
  output.write(color, position + uint2(1, 0));
  output.write(color, position - uint2(1, 0));
}