// WebGPU counterpart of the GPU heightfield raymarcher (the WGSL port of
// webgl_terrain_frag.glsl / terrain.frag). The Vulkan push constants become a
// uniform buffer of five vec4s, laid out exactly as `ra3::wasmgl::detail::terrain_uniforms`.
//
// Bind group 0: uniform data[5], the R16 heightmap (u32), the RGBA16 cell record
// (vec4<u32>), the RGBA8 tile atlas (array), and an atlas sampler.

struct TerrainUniforms {
    data: array<vec4<f32>, 5>,
};

@group(0) @binding(0) var<uniform> u: TerrainUniforms;
@group(0) @binding(1) var u_heightmap: texture_2d<u32>;    // R16Uint heights
@group(0) @binding(2) var u_celldata: texture_2d<u32>;     // RGBA16Uint blend record (texel is vec4<u32>)
@group(0) @binding(3) var u_atlas: texture_2d_array<f32>;  // RGBA8 tile materials
@group(0) @binding(4) var u_atlas_samp: sampler;

const CELL: f32 = 10.0; // must match ra3::terrain::cell_size

fn cam_uniform() -> vec4<f32> { return u.data[0]; }    // x=target_x, y=target_y, z=yaw, w=height
fn params_uniform() -> vec4<f32> { return u.data[1]; } // x=pitch, y=fov, z=water_z, w=has_water
fn sun_uniform() -> vec4<f32> { return u.data[2]; }    // xyz=sun dir, w=ambient
fn mapinfo_uniform() -> vec4<f32> { return u.data[3]; }// x=W, y=H, z=unused, w=z_scale
fn misc_uniform() -> vec4<f32> { return u.data[4]; }   // x=time, y=unused, z=cells per repeat, w=aspect

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

@vertex
fn vs_main(@builtin(vertex_index) vertex_index: u32) -> TerrainOut {
    let p = vec2<f32>(f32((vertex_index << 1u) & 2u), f32(vertex_index & 2u));
    var out: TerrainOut;
    out.uv = p;
    out.position = vec4<f32>(p.x * 2.0 - 1.0, 1.0 - p.y * 2.0, 0.0, 1.0);
    return out;
}

fn height_at(cell: vec2<i32>) -> f32 {
    let limit = vec2<i32>(mapinfo_uniform().xy) - vec2<i32>(1);
    let c = clamp(cell, vec2<i32>(0), limit);
    return f32(textureLoad(u_heightmap, c, 0).r) * mapinfo_uniform().w;
}

fn world_height(wx: f32, wy: f32) -> f32 {
    let world_w = mapinfo_uniform().x * CELL;
    let world_h = mapinfo_uniform().y * CELL;
    if (wx < 0.0 || wy < 0.0 || wx >= world_w || wy >= world_h) {
        return -1.0e9;
    }
    let c = vec2<i32>(i32(wx / CELL), i32((world_h - wy) / CELL));
    return height_at(c);
}

fn sky_color(dir: vec3<f32>) -> vec3<f32> {
    let d = normalize(dir);
    let sun_dir = normalize(sun_uniform().xyz);
    let t = clamp(d.z, 0.0, 1.0);
    let horizon = vec3<f32>(0.70, 0.78, 0.85);
    let zenith = vec3<f32>(0.28, 0.48, 0.80);
    var col = mix(horizon, zenith, pow(t, 0.6));
    let sun = max(dot(d, sun_dir), 0.0);
    col += vec3<f32>(1.0, 0.95, 0.82) * pow(sun, 300.0) * 1.6;
    col += vec3<f32>(1.0, 0.90, 0.72) * pow(sun, 8.0) * 0.18;
    return col;
}

// The retail SAGE blend ramp (see terrain.frag).
fn blend_factor(direction: u32, flags: u32, f_in: vec2<f32>) -> f32 {
    var f = f_in;
    let flipped = (flags & 1u) != 0u;
    let two_sided = (flags & 2u) != 0u;
    if (flipped) {
        if (direction == 1u) {
            f.x = 1.0 - f.x;
        } else if (direction == 2u || direction == 4u || direction == 8u) {
            f.y = 1.0 - f.y;
        }
    }
    if (direction == 1u) { return f.x; }
    if (direction == 2u) { return f.y; }
    if (direction == 4u) {
        let s = (1.0 - f.x) + (1.0 - f.y);
        return select(clamp(1.0 - s, 0.0, 1.0), 1.0 - clamp(s - 1.0, 0.0, 1.0), two_sided);
    }
    if (direction == 8u) {
        let s = f.x + (1.0 - f.y);
        return select(clamp(1.0 - s, 0.0, 1.0), 1.0 - clamp(s - 1.0, 0.0, 1.0), two_sided);
    }
    return 0.0;
}

fn sample_layer(layer: u32, wx: f32, wy: f32) -> vec3<f32> {
    let span = max(misc_uniform().z, 1.0);
    let layer_count = textureNumLayers(u_atlas);
    let l = f32(min(layer, layer_count - 1u));
    // The atlas is 0xAARRGGBB (BGRA in memory), uploaded as RGBA8.
    let c = textureSampleLevel(u_atlas, u_atlas_samp, vec2<f32>(wx, wy) / span, i32(l), 0.0);
    return vec3<f32>(c.b, c.g, c.r);
}

@fragment
fn fs_main(in: TerrainOut) -> @location(0) vec4<f32> {
    let p = cam_uniform();
    let pitch = clamp(params_uniform().x, 0.15, 1.45);
    let fov = clamp(params_uniform().y, 0.3, 1.4);
    let world_w = mapinfo_uniform().x * CELL;
    let world_h = mapinfo_uniform().y * CELL;

    let cp = cos(pitch);
    let fwd = vec3<f32>(cp * sin(p.z), cp * cos(p.z), -sin(pitch));
    let right = normalize(cross(fwd, vec3<f32>(0.0, 0.0, 1.0)));
    let up = cross(right, fwd);

    var target_z = world_height(p.x, p.y);
    if (target_z < -1.0e8) { target_z = 0.0; }
    let dist = p.w / sin(pitch);
    let cam = vec3<f32>(p.x, p.y, target_z + p.w) - fwd * dist;

    let ndc = vec2<f32>(in.uv.x * 2.0 - 1.0, 1.0 - in.uv.y * 2.0);
    let aspect = misc_uniform().w;
    let th = tan(fov * 0.5);
    let dir = normalize(fwd + right * ndc.x * th * aspect + up * ndc.y * th);

    if (dir.z >= -1e-4) {
        return vec4<f32>(sky_color(dir), 1.0);
    }

    // March the heightfield. The step grows quickly: the map diagonal is only
    // ~9000 world units, so marching past ~20000 adds cost without detail.
    var t = CELL * 0.5;
    var dt = CELL * 0.5;
    var prev = t;
    var hit = false;
    var hit_t = 0.0;
    for (var i = 0; i < 256 && t < 20000.0; i = i + 1) {
        let w = cam + dir * t;
        if (w.x < 0.0 || w.y < 0.0 || w.x >= world_w || w.y >= world_h) {
            prev = t;
            dt *= 1.10;
            t += dt;
            continue;
        }
        if (params_uniform().w > 0.5 && w.z <= params_uniform().z) {
            hit = true;
            hit_t = t;
            break;
        }
        if (w.z <= world_height(w.x, w.y)) {
            hit = true;
            hit_t = t;
            break;
        }
        prev = t;
        dt *= 1.10;
        t += dt;
    }
    if (!hit) {
        return vec4<f32>(sky_color(dir), 1.0);
    }

    var lo = prev;
    var hi = hit_t;
    for (var i = 0; i < 6; i = i + 1) {
        let mid = 0.5 * (lo + hi);
        let w = cam + dir * mid;
        let water = params_uniform().w > 0.5 && w.z <= params_uniform().z;
        if (water || w.z <= world_height(w.x, w.y)) {
            hi = mid;
        } else {
            lo = mid;
        }
    }
    let hitpos = cam + dir * hi;

    let sun = normalize(sun_uniform().xyz);
    let ambient = sun_uniform().w;

    if (params_uniform().w > 0.5 && hitpos.z <= params_uniform().z + 0.01) {
        let time = misc_uniform().x;
        let q = hitpos.xy * 0.015;
        let nx = sin(q.x * 1.3 + time * 1.7) + 0.5 * sin(q.x * 3.1 - time * 2.3);
        let ny = sin(q.y * 1.1 - time * 1.3) + 0.5 * sin(q.y * 2.7 + time * 1.9);
        let n = normalize(vec3<f32>(nx * 0.06, ny * 0.06, 1.0));
        let fres = pow(1.0 - clamp(-dir.z, 0.0, 1.0), 3.0);
        let deep = vec3<f32>(0.03, 0.16, 0.28);
        let refl = sky_color(reflect(dir, n));
        let lam = max(0.0, dot(n, sun));
        var water = mix(deep, refl, clamp(0.25 + 0.55 * fres, 0.0, 0.9));
        water += vec3<f32>(1.0, 0.98, 0.9) * pow(lam, 64.0) * 0.6;
        let wfog = clamp(1.0 - exp(-hi * 0.00009), 0.0, 0.75);
        water = mix(water, sky_color(vec3<f32>(dir.x, dir.y, 0.0)), wfog);
        return vec4<f32>(water, 1.0);
    }

    let wx = hitpos.x / CELL;
    let wy = (world_h - hitpos.y) / CELL;
    let cx = clamp(i32(wx), 0, i32(mapinfo_uniform().x) - 1);
    let cy = clamp(i32(wy), 0, i32(mapinfo_uniform().y) - 1);
    let fx = wx - floor(wx);
    let fy = wy - floor(wy);

    let record = textureLoad(u_celldata, vec2<i32>(cx, cy), 0);
    let packed = record.w;
    let dir1 = packed & 0xFu;
    let flags1 = (packed >> 4u) & 0x3u;
    let dir2 = (packed >> 8u) & 0xFu;
    let flags2 = (packed >> 12u) & 0x3u;

    let frac_uv = vec2<f32>(fx, fy);
    let c0 = sample_layer(record.x, wx, wy);
    let c1 = sample_layer(record.y, wx, wy);
    let c2 = sample_layer(record.z, wx, wy);
    let f1 = blend_factor(dir1, flags1, frac_uv);
    let f2 = blend_factor(dir2, flags2, frac_uv);
    let albedo = mix(mix(c0, c1, f1), c2, f2);

    let hl = world_height(hitpos.x - CELL, hitpos.y);
    let hr = world_height(hitpos.x + CELL, hitpos.y);
    let hd = world_height(hitpos.x, hitpos.y - CELL);
    let hu = world_height(hitpos.x, hitpos.y + CELL);
    let n = normalize(vec3<f32>(hl - hr, hd - hu, 2.0 * CELL));
    let lambert = max(0.0, dot(n, sun));
    var lit = albedo * (ambient + (1.0 - ambient) * lambert);
    let fog = clamp(1.0 - exp(-hi * 0.00009), 0.0, 0.75);
    lit = mix(lit, sky_color(vec3<f32>(dir.x, dir.y, 0.0)), fog);
    return vec4<f32>(lit, 1.0);
}