struct VertexOutput {
  @builtin(position) position : vec4f,
  @location(4) color : vec4f,
}

@vertex
fn vert_main(
  @location(0) a_particlePos : vec3f,
  @location(1) a_particleVel : vec3f,
  @builtin(vertex_index) index: u32,
) -> VertexOutput {
  const square_vertices = array(
    vec2(0.0, 0.0), vec2(0.0, 1.0), vec2(1.0, 1.0),
    vec2(1.0, 1.0), vec2(1.0, 0.0), vec2(0.0, 0.0),
  );
  let square_vertex = square_vertices[index];
  let angle = -atan2(a_particleVel.x, a_particleVel.y);
  let pos = vec2(
    (a_pos.x * cos(angle)) - (a_pos.y * sin(angle)),
    (a_pos.x * sin(angle)) + (a_pos.y * cos(angle))
  );

  var output : VertexOutput;
  output.position = vec4(pos + a_particlePos, 0.0, 1.0);
  output.color = vec4(
    1.0 - sin(angle + 1.0) - a_particleVel.y,
    pos.x * 100.0 - a_particleVel.y + 0.1,
    a_particleVel.x + cos(angle + 0.5),
    1.0);
  return output;
}

@fragment
fn frag_main(@location(4) color : vec4f) -> @location(0) vec4f {
  return color;
}