struct Camera {
    scale: vec2f,
    centre: vec2f,
};

struct Instance {
    centre: vec2f,
    cos_sin: vec2f,
    half_size: vec2f,
    dims: vec2u,
    cell_offset: u32,
};

@group(0) @binding(0) var<uniform> camera: Camera;
@group(0) @binding(1) var<storage, read> palette: array<vec4f>;
@group(0) @binding(2) var<storage, read> instances: array<Instance>;
@group(0) @binding(3) var<storage, read> cells: array<u32>;

struct VertexOutput {
    @builtin(position) clip_position: vec4<f32>,
    @location(0) world: vec2<f32>,
    @location(1) @interpolate(flat) instance: u32,
};

@vertex
fn vs_main(
    @builtin(vertex_index) i: u32,
    @builtin(instance_index) n: u32,
) -> VertexOutput {
    let instance = instances[n];

    // (-1,-1),(1,-1),(-1,1),(1,1)
    let corner = vec2f(f32(i & 1u), f32(i >> 1u)) * 2.0 - 1.0;
    let local = corner * (instance.half_size + 2.0);
    let world = instance.centre + vec2f(
        local.x * instance.cos_sin.x - local.y * instance.cos_sin.y,
        local.x * instance.cos_sin.y + local.y * instance.cos_sin.x,
    );

    var out: VertexOutput;
    out.clip_position = vec4f((world - camera.centre) * camera.scale, 0.0, 1.0);
    out.world = world;
    out.instance = n;

    return out;
}

@fragment
fn fs_main(in: VertexOutput) -> @location(0) vec4<f32> {
    let instance = instances[in.instance];

    let d = in.world - instance.centre;
    let q = floor(d) + vec2f(0.5);
    let local = vec2f(
        q.x * instance.cos_sin.x + q.y * instance.cos_sin.y,
        -q.x * instance.cos_sin.y + q.y * instance.cos_sin.x,
    ) + instance.half_size;

    let cell = vec2i(floor(local));
    if any(cell < vec2i(0)) || any(cell >= vec2i(instance.dims)) {
        discard;
    }

    let idx = instance.cell_offset + u32(cell.y) * instance.dims.x + u32(cell.x);
    let material = (cells[idx >> 2u] >> ((idx & 3u) * 8u)) & 0xFFu;
    if material == 0u {
        discard;
    }

    return palette[material];
}