// Copyright 2020 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// vertex shader

[[stage(vertex)]]
fn vert_main([[location(0)]] a_particlePos : vec2<f32>,
             [[location(1)]] a_particleVel : vec2<f32>,
             [[location(2)]] a_pos : vec2<f32>)
          -> [[builtin(position)]] vec4<f32> {
  var angle : f32 = -atan2(a_particleVel.x, a_particleVel.y);
  var pos : vec2<f32> = vec2<f32>(
      (a_pos.x * cos(angle)) - (a_pos.y * sin(angle)),
      (a_pos.x * sin(angle)) + (a_pos.y * cos(angle)));
  return vec4<f32>(pos + a_particlePos, 0.0, 1.0);
}

// fragment shader

[[stage(fragment)]]
fn frag_main() -> [[location(0)]] vec4<f32> {
  return vec4<f32>(1.0, 1.0, 1.0, 1.0);
}

// compute shader
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 : array<Particle, 5>;
};

[[binding(0), group(0)]] var<uniform> params : SimParams;
[[binding(1), group(0)]] var<storage, read_write> particlesA : Particles;
[[binding(2), group(0)]] var<storage, read_write> particlesB : Particles;

// https://github.com/austinEng/Project6-Vulkan-Flocking/blob/master/data/shaders/computeparticles/particle.comp
[[stage(compute), workgroup_size(1)]]
fn comp_main(
  [[builtin(global_invocation_id)]] gl_GlobalInvocationID : vec3<u32>) {
  var index : u32 = gl_GlobalInvocationID.x;
  if (index >= 5u) {
    return;
  }

  var vPos : vec2<f32> = particlesA.particles[index].pos;
  var vVel : vec2<f32> = particlesA.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>;
  for(var i : u32 = 0u; i < 5u; i = i + 1u) {
    if (i == index) {
      continue;
    }

    pos = particlesA.particles[i].pos.xy;
    vel = particlesA.particles[i].vel.xy;

    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;
    }
  }
  if (cMassCount > 0) {
    cMass =
      (cMass / vec2<f32>(f32(cMassCount), f32(cMassCount))) - vPos;
  }
  if (cVelCount > 0) {
    cVel = cVel / vec2<f32>(f32(cVelCount), 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
  particlesB.particles[index].pos = vPos;
  particlesB.particles[index].vel = vVel;
}