//! TRACES: FR-DEV-8
//! The instance binding: a detail pass whose work is a list, not a kernel.
//!
//! Spot removal needs a detail pass to read a variable number of records —
//! sixty-four repairs and one repair are the same shader with a different
//! buffer behind it. That is binding 3, and this file proves the three things
//! about it that a picture would not tell you clearly:
//!
//! - the data uploaded is the data the shader reads, in order;
//! - a pass that declares no list still runs, bound to the placeholder;
//! - the same shader with a *different* list does not recompile, which is what
//!   keeps placing a spot as cheap as moving a slider.
//!
//! The passes here are synthetic on purpose. `spot_removal.rs` asserts the
//! repair; this asserts the plumbing, so a failure in one does not have to be
//! read to work out which of the two broke.

use dr_gpu::{AdjustPass, DemosaicedImage, GpuContext};
use dr_pipeline::detail::{ComposedDetail, ComposedDetailPass};
use dr_pipeline::{Affects, EditGraph};
use dr_types::ColourSpace;

const SIZE: u32 = 8;

fn ctx() -> Option<GpuContext> {
    match pollster::block_on(GpuContext::new_headless()) {
        Ok(c) => Some(c),
        Err(e) => {
            eprintln!("skipping: no GPU adapter ({e})");
            None
        }
    }
}

/// A flat mid-grey frame, so anything the pass adds is the whole answer.
fn grey(ctx: &GpuContext) -> DemosaicedImage {
    let data: Vec<u8> = (0..SIZE * SIZE).flat_map(|_| [0u8, 0, 0, 255]).collect();
    DemosaicedImage::from_rgba8(ctx, &data, SIZE, SIZE).expect("upload")
}

/// A pass that sums the instance list into the red channel and writes the
/// output. Deliberately trivial: the value on screen is then a direct readout
/// of what arrived in the buffer.
fn summing_pass(storage: Vec<[f32; 4]>, structure: u64) -> ComposedDetailPass {
    let source = "
@group(0) @binding(0) var source: texture_2d<f32>;
struct Params { detail_base: vec4<f32> }
@group(0) @binding(1) var<uniform> u: Params;
@group(0) @binding(2) var output: texture_storage_2d<rgba8unorm, write>;
@group(0) @binding(3) var<storage, read> instances: array<vec4<f32>>;

@compute @workgroup_size(8, 8, 1)
fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
    let dims = textureDimensions(output);
    if (gid.x >= dims.x || gid.y >= dims.y) { return; }

    // Weighted by index, so a buffer read back to front fails this rather
    // than passing by symmetry.
    var total = 0.0;
    let n = arrayLength(&instances);
    for (var i = 0u; i < n; i = i + 1u) {
        total = total + instances[i].x * f32(i + 1u);
    }
    textureStore(output, vec2<i32>(gid.xy), vec4<f32>(total, f32(n) / 255.0, 0.0, 1.0));
}
"
    .to_string();

    ComposedDetailPass {
        output_scale: 1,
        label: "test/instances".to_string(),
        source,
        uniforms: vec![SIZE as f32, SIZE as f32, 1.0, 0.0],
        storage,
        radius: 0,
        writes_output: true,
        // Any distinct number: the hash is a cache key, and these tests are
        // what decide whether two chains share a pipeline.
        structure_hash: structure,
    }
}

fn render(pass: &mut AdjustPass, source: &DemosaicedImage, chain: &ComposedDetail) -> Vec<u8> {
    // The fused half has to be composed knowing a detail stage follows it, or
    // it encodes its own output and the chain would quantise twice — a mismatch
    // `render_detailed` refuses outright. The probe is the graph that says so;
    // its own passes are not used, since the chain here is hand-built.
    let mut graph = EditGraph::with_detail_probe();
    graph.set_param(
        dr_pipeline::descriptor::OpId("detail_probe"),
        dr_pipeline::descriptor::ParamId("radius"),
        0.05,
    );
    let shader = graph.compose_for(ColourSpace::Srgb);
    let key = graph.invalidation().through(Affects::Colour);
    pass.render_detailed(source, &shader, SIZE, SIZE, None, chain, key)
        .expect("render");
    pass.export_pixels().expect("readback").0
}

/// The list arrives whole, in order, and the shader can tell how long it is.
#[test]
fn a_pass_reads_the_list_it_was_given() {
    let Some(ctx) = ctx() else { return };
    let source = grey(&ctx);
    let mut pass = AdjustPass::new(&ctx);

    // 0.1·1 + 0.2·2 + 0.3·3 = 1.4, which clips to 1.0 — so instead: values
    // chosen to land at a quarter, unambiguously distinguishable from both the
    // "read nothing" answer of 0 and the "read them unweighted" answer of 0.15.
    let chain = ComposedDetail {
        passes: vec![summing_pass(
            vec![[0.05, 0.0, 0.0, 0.0], [0.1, 0.0, 0.0, 0.0]],
            1,
        )],
    };

    let pixels = render(&mut pass, &source, &chain);
    let (red, green) = (pixels[0], pixels[1]);

    // 0.05·1 + 0.1·2 = 0.25, written straight to an rgba8 target.
    assert!(
        red.abs_diff((0.25 * 255.0) as u8) <= 1,
        "the shader summed {red}, not the list it was handed"
    );
    assert_eq!(green, 2, "arrayLength saw both entries");
}

/// A convolution declares no list and must still run: it is bound to the
/// placeholder rather than to nothing, because a zero-length storage buffer
/// cannot be bound at all and a second bind group layout for the difference
/// would be two layouts to keep in step.
#[test]
fn a_pass_with_no_list_still_runs() {
    let Some(ctx) = ctx() else { return };
    let source = grey(&ctx);
    let mut pass = AdjustPass::new(&ctx);

    let chain = ComposedDetail {
        passes: vec![summing_pass(Vec::new(), 2)],
    };

    let pixels = render(&mut pass, &source, &chain);
    assert_eq!(pixels[0], 0, "the placeholder is zeroed");
    assert_eq!(pixels[1], 1, "and is exactly one element long");
}

/// The property that makes placing the tenth spot as cheap as moving a slider:
/// the list is in the buffer, not in the source, so the pipeline is compiled
/// once however many entries arrive.
#[test]
fn changing_the_list_does_not_recompile() {
    let Some(ctx) = ctx() else { return };
    let source = grey(&ctx);
    let mut pass = AdjustPass::new(&ctx);

    for count in 1..=6 {
        let list = (0..count).map(|_| [0.01, 0.0, 0.0, 0.0]).collect();
        let chain = ComposedDetail {
            passes: vec![summing_pass(list, 3)],
        };
        render(&mut pass, &source, &chain);
    }

    assert_eq!(
        pass.cached_detail_pipelines(),
        1,
        "six different lists, one compiled pipeline"
    );
}