struct VertexOutput {
    @builtin(position) clip_position: vec4<f32>,
    @location(0) grid_position: vec2<f32>,
    @location(1) grid_scale: f32,
};

struct CameraUniform {
    view: mat4x4<f32>,
    proj: mat4x4<f32>,
    scale: f32
};

@group(0) @binding(0)
var<uniform> camera: CameraUniform;

const QUAD_VERTICES = array<vec3<f32>,6>(
    vec3<f32>(-1.0, 0.0, -1.0),
    vec3<f32>(-1.0, 0.0, 1.0),
    vec3<f32>(1.0, 0.0, 1.0),
    vec3<f32>(1.0, 0.0, 1.0),
    vec3<f32>(1.0, 0.0, -1.0),
    vec3<f32>(-1.0, 0.0, -1.0),
);

@vertex
fn vs_main(
    @builtin(vertex_index) index: u32
) -> VertexOutput {
    var out: VertexOutput;

    let model_pos = QUAD_VERTICES[index] * 50.0;
    let world_pos = camera.proj * camera.view * vec4<f32>(model_pos.xyz, 1.0);

    out.clip_position = world_pos;
    out.grid_position = vec2<f32>(model_pos.x, model_pos.z);
    out.grid_scale = camera.scale * 1e6;
    return out;
}

fn modulus(a: f32, b: f32) -> f32
{ return a - b * floor(a / b); }

fn mod_vec2(a: vec2<f32>, b: vec2<f32>) -> vec2<f32>
{ return a - b * floor(a / b); }

const cell_line_thickness: f32 = 0.001;
const subcell_line_thickness: f32 = 0.0001;

const cell_line_color: vec4<f32> = vec4<f32>(0.25, 0.25, 0.25, 0.5);
const subcell_line_color: vec4<f32> = vec4<f32>(0.125, 0.125, 0.125, 0.5);

@fragment
fn fs_main(in: VertexOutput
) -> @location(0) vec4<f32> {

    let scale_mod = modulus(log(1.0 / in.grid_scale) * 10.0, 10.0) + 1.0;

    let cell_size = 1.0 / scale_mod;
    let half_cell_size = cell_size * 0.5;
    let subcell_size = 0.1 / scale_mod;
    let half_subcell_size = subcell_size * 0.5;

    //Position relative to the center of the quad.
    let frag_pos_rel = (in.grid_position / 2.0);

    let cell_pos =    mod_vec2(frag_pos_rel, vec2<f32>(cell_size));
    let subcell_pos = mod_vec2(frag_pos_rel, vec2<f32>(subcell_size));

    let cell_dist = abs(cell_pos);
    let subcell_dist = abs(subcell_pos);

    let d = fwidth(in.grid_position);
    let clt_real = 0.5 * (cell_line_thickness + d);
    let sclt_real = 0.5 * (subcell_line_thickness + d);

    let center_distance = length(in.grid_position);
    let falloff = smoothstep(1.0, 0.0, center_distance / 2.0);
    let subcell_falloff = smoothstep(1.0, 0.5, scale_mod / 11.0);

    var color = vec4<f32>(0.0);
    if subcell_dist.x < sclt_real.x || subcell_dist.y < sclt_real.y 
        { color = subcell_line_color * subcell_falloff; }
    if cell_dist.x < clt_real.x || cell_dist.y < clt_real.y 
        { color = cell_line_color; }

    return color * falloff;
}