let NUM_PARTICLES: u32 = 1500u;

struct Particle {
  pos : vec2<f32>;
  vel : vec2<f32>;
};

[[block]]
struct SimParams {
  deltaT : f32;
  rule1Distance : f32;
  rule2Distance : f32;
  rule3Distance : f32;
  rule1Scale : f32;
  rule2Scale : f32;
  rule3Scale : f32;
};

[[block]]
struct Particles {
  particles : [[stride(16)]] array<Particle>;
};

[[group(0), binding(0)]] var<uniform> params : SimParams;
[[group(0), binding(1)]] var<storage> particlesSrc : [[access(read)]] Particles;
[[group(0), binding(2)]] var<storage> particlesDst : [[access(read_write)]] Particles;

// https://github.com/austinEng/Project6-Vulkan-Flocking/blob/master/data/shaders/computeparticles/particle.comp
[[stage(compute), workgroup_size(64)]]
fn main([[builtin(global_invocation_id)]] global_invocation_id : vec3<u32>) {
  let index : u32 = global_invocation_id.x;
  if (index >= NUM_PARTICLES) {
    return;
  }

  var vPos : vec2<f32> = particlesSrc.particles[index].pos;
  var vVel : vec2<f32> = particlesSrc.particles[index].vel;

  var cMass : vec2<f32> = vec2<f32>(0.0, 0.0);
  var cVel : vec2<f32> = vec2<f32>(0.0, 0.0);
  var colVel : vec2<f32> = vec2<f32>(0.0, 0.0);
  var cMassCount : i32 = 0;
  var cVelCount : i32 = 0;

  var pos : vec2<f32>;
  var vel : vec2<f32>;
  var i : u32 = 0u;
  loop {
    if (i >= NUM_PARTICLES) {
      break;
    }
    if (i == index) {
      continue;
    }

    pos = particlesSrc.particles[i].pos;
    vel = particlesSrc.particles[i].vel;

    if (distance(pos, vPos) < params.rule1Distance) {
      cMass = cMass + pos;
      cMassCount = cMassCount + 1;
    }
    if (distance(pos, vPos) < params.rule2Distance) {
      colVel = colVel - (pos - vPos);
    }
    if (distance(pos, vPos) < params.rule3Distance) {
      cVel = cVel + vel;
      cVelCount = cVelCount + 1;
    }

    continuing {
      i = i + 1u;
    }
  }
  if (cMassCount > 0) {
    cMass = cMass / f32(cMassCount) - vPos;
  }
  if (cVelCount > 0) {
    cVel = cVel / f32(cVelCount);
  }

  vVel = vVel + (cMass * params.rule1Scale) +
      (colVel * params.rule2Scale) +
      (cVel * params.rule3Scale);

  // clamp velocity for a more pleasing simulation
  vVel = normalize(vVel) * clamp(length(vVel), 0.0, 0.1);

  // kinematic update
  vPos = vPos + (vVel * params.deltaT);

  // Wrap around boundary
  if (vPos.x < -1.0) {
    vPos.x = 1.0;
  }
  if (vPos.x > 1.0) {
    vPos.x = -1.0;
  }
  if (vPos.y < -1.0) {
    vPos.y = 1.0;
  }
  if (vPos.y > 1.0) {
    vPos.y = -1.0;
  }

  // Write back
  particlesDst.particles[index].pos = vPos;
  particlesDst.particles[index].vel = vVel;
}