// cf: https://thebookofshaders.com/10/
fn random(uv: vec2<f32>) -> f32 {
    let c = vec2<f32>(12.9898, 78.233) * 43758.5453123;
    return fract(sin(dot(uv, c)));
}

struct VertexOutput {
    @builtin(position) position: vec4<f32>,
    @location(0) uv: vec2<f32>,
}

@vertex
fn vs_main(@location(0) idx: u32) -> VertexOutput {
    var vertex: array<vec2<f32>, 4>;
    vertex[0] = vec2<f32>(-1.0, -1.0);
    vertex[1] = vec2<f32>(1.0, -1.0);
    vertex[2] = vec2<f32>(-1.0, 1.0);
    vertex[3] = vec2<f32>(1.0, 1.0);

    return VertexOutput(vec4<f32>(vertex[idx], 0.0, 1.0), vertex[idx]);
}

@fragment
fn unicolor(@location(0) _uv: vec2<f32>) -> @location(0) vec4<f32> {
    return vec4<f32>(0.2, 0.4, 0.6, 0.8);
}

@fragment
fn random_texture(@location(0) uv: vec2<f32>) -> @location(0) vec4<f32> {
    let r = random(uv);
    let g = random(uv.yx);
    let b = random(vec2<f32>(r, g));
    return vec4<f32>(r, g, b, 1.0);
}

@fragment
fn gradation_texture(@location(0) uv: vec2<f32>) -> @location(0) vec4<f32> {
    let r = length(uv) / sqrt(2.0);
    let l = 1.0 - r;
    let col0 = vec3<f32>(r, r * r, r * r * r);
    let col1 = vec3<f32>(l * l * l, l, l * l);
    return vec4<f32>(clamp(col0 + col1, vec3<f32>(0.0), vec3<f32>(1.0)), 1.0);
}