// Eldenwood sky dome: three-stop vertical gradient with sun disc and
// a mist band near the horizon. Drawn as a fullscreen triangle; the
// view ray is reconstructed from the inverse view-projection matrix.

struct SkyUniforms {
    inv_view_proj: mat4x4<f32>,
    sun_dir: vec3<f32>,        // normalized, world space
    time_of_day: f32,          // 0.0 = midnight, 0.5 = noon
    zenith_color: vec3<f32>,
    mist_amount: f32,          // 0..1, from region weather
    horizon_color: vec3<f32>,
    _pad: f32,
};

@group(0) @binding(0) var<uniform> sky: SkyUniforms;

struct VsOut {
    @builtin(position) clip_pos: vec4<f32>,
    @location(0) ndc: vec2<f32>,
};

@vertex
fn vs_main(@builtin(vertex_index) idx: u32) -> VsOut {
    // Fullscreen triangle without a vertex buffer.
    var pos = array<vec2<f32>, 3>(
        vec2(-1.0, -1.0), vec2(3.0, -1.0), vec2(-1.0, 3.0));
    var out: VsOut;
    out.clip_pos = vec4(pos[idx], 1.0, 1.0);   // z = far plane
    out.ndc = pos[idx];
    return out;
}

@fragment
fn fs_main(in: VsOut) -> @location(0) vec4<f32> {
    // Reconstruct the world-space view ray for this pixel.
    let far = sky.inv_view_proj * vec4(in.ndc, 1.0, 1.0);
    let ray = normalize(far.xyz / far.w);

    // Three-stop gradient keyed on ray elevation.
    let elev = clamp(ray.y, -0.05, 1.0);
    let ground = vec3(0.24, 0.26, 0.22);
    let low = mix(ground, sky.horizon_color, smoothstep(-0.05, 0.02, elev));
    let color = mix(low, sky.zenith_color, pow(smoothstep(0.0, 0.9, elev), 0.6));

    // Sun disc with a soft halo, faded at night.
    let sun_amount = pow(max(dot(ray, sky.sun_dir), 0.0), 900.0);
    let halo = pow(max(dot(ray, sky.sun_dir), 0.0), 12.0) * 0.15;
    let daylight = smoothstep(0.22, 0.30, sky.time_of_day)
                 * (1.0 - smoothstep(0.70, 0.78, sky.time_of_day));
    let sun = (sun_amount + halo) * daylight * vec3(1.0, 0.93, 0.78);

    // Eldenwood mist: a pale green band hugging the horizon.
    let mist_band = exp(-abs(elev) * 9.0) * sky.mist_amount;
    let misted = mix(color, vec3(0.66, 0.77, 0.63), mist_band);

    return vec4(misted + sun, 1.0);
}