facett-graphview 0.1.12

//! **GPU-compute force-directed layout** — the elite engine's must-land core.
//!
//! A wgpu **compute-shader** n-body layout: per-node weights + CSR adjacency are
//! streamed from an Arrow edge batch straight into GPU **storage buffers**, and the
//! node positions then **iterate in VRAM** (ping-pong `src`→`dst`, no readback per
//! step) until they settle. The host reads the converged positions back as plain
//! `[f32; 2]` **DATA** — exactly the contract a headless test asserts on.
//!
//! This is the answer to the *layout-perf-at-scale* curse: the all-pairs repulsion
//! is the same O(n²) compute pattern as the boids in
//! [`facett_core::render::gpu::particles`] (reused verbatim — storage ping-pong, a
//! `@workgroup_size(64)` kernel, the headless `request_adapter` device probe), so a
//! 10k–100k-node re-layout runs in real time on the GPU instead of the CPU.
//!
//! # Force model (Fruchterman-Reingold, velocity-integrated)
//! For node `i` with mass `wᵢ` (its streamed weight):
//!   * **repulsion** from every other node `j`: a Fruchterman-Reingold `k/d` push,
//!     `rep · wᵢ · wⱼ · (pᵢ − pⱼ) / dist²` — the `(pᵢ−pⱼ)` numerator carries one
//!     `dist`, so the *magnitude* is `rep·wᵢ·wⱼ / dist` (heavier nodes shove harder
//!     → important nodes claim space),
//!   * **attraction** along each CSR neighbour `t`: a Hooke spring
//!     `attr · (dist − ideal) · (pₜ − pᵢ)/dist` (pulls connected nodes to `ideal_len`),
//!   * **gravity** `−grav · pᵢ` keeps the component centred,
//!   * integrate `v ← (v + F/wᵢ·dt)·damp`, `p ← p + v·dt` (heavy nodes move slower).
//!
//! It is a **pure, deterministic** function of the seed + params + step count (FC-7):
//! no RNG (the seed is a golden-angle spiral), fixed loop order. The CPU
//! [`step_cpu`] is the **no-GPU fallback** *and* the parity reference the GPU
//! readback proof checks against — the WGSL `cs_layout` mirrors it line-for-line.
//!
//! # Roadmap (see `.nornir/elite-graph-engine.md`)
//! The O(n²) repulsion is the foundation; a GPU **Barnes-Hut** quadtree (or a Morton
//! /  grid binning) drops it to O(n log n) for the 100k case — documented there, not
//! built here, because the foundation must land + test clean first.

use std::collections::BTreeSet;

/// CSR (compressed-sparse-row) adjacency + per-node weight — the **GPU-streamable**
/// graph. `offsets[i]..offsets[i+1]` indexes into `targets` for node `i`'s
/// neighbours; `weights[i]` is node `i`'s mass. The three `Vec`s map 1:1 onto the
/// three GPU storage buffers (`offsets`/`targets` read-only, `weights` packed into
/// the node buffer), so this *is* the upload format — no repack at the seam.
#[derive(Clone, Debug, PartialEq)]
pub struct GraphCsr {
    pub n: usize,
    /// Length `n + 1`. `offsets[i]..offsets[i+1]` = node `i`'s slice of `targets`.
    pub offsets: Vec<u32>,
    /// Neighbour node indices (undirected: each input edge contributes both ways).
    pub targets: Vec<u32>,
    /// Per-node mass / importance (default `1.0`). Streamed from an Arrow weight column.
    pub weights: Vec<f32>,
}

impl GraphCsr {
    /// Build CSR from a **directed** edge list (`from`,`to` node indices) over `n`
    /// nodes. Layout forces are undirected, so each edge attracts **both** endpoints
    /// (the `targets` row holds both directions); self-loops + out-of-range ids are
    /// dropped, and parallel edges are de-duplicated. `weights` (per node) default to
    /// `1.0` when `None` or short.
    ///
    /// The `edges` are exactly the `(src, dst)` pairs an Arrow edge `RecordBatch`
    /// exposes (`Int64` columns cast to `u32`); [`GraphCsr::from_arrow`] is the
    /// zero-glue Arrow front door over this.
    #[must_use]
    pub fn from_edges(n: usize, edges: &[(u32, u32)], weights: Option<&[f32]>) -> Self {
        let mut adj: Vec<BTreeSet<u32>> = vec![BTreeSet::new(); n];
        for &(a, b) in edges {
            let (a, b) = (a as usize, b as usize);
            if a == b || a >= n || b >= n {
                continue;
            }
            adj[a].insert(b as u32);
            adj[b].insert(a as u32);
        }
        let mut offsets = Vec::with_capacity(n + 1);
        let mut targets = Vec::new();
        offsets.push(0u32);
        for nbrs in &adj {
            targets.extend(nbrs.iter().copied());
            offsets.push(targets.len() as u32);
        }
        let weights = (0..n)
            .map(|i| weights.and_then(|w| w.get(i)).copied().filter(|w| *w > 0.0).unwrap_or(1.0))
            .collect();
        Self { n, offsets, targets, weights }
    }

    /// The number of undirected adjacency entries (`= 2 × distinct edges`).
    #[must_use]
    pub fn degree_sum(&self) -> usize {
        self.targets.len()
    }

    /// Node `i`'s neighbour slice.
    #[must_use]
    pub fn neighbours(&self, i: usize) -> &[u32] {
        let (s, e) = (self.offsets[i] as usize, self.offsets[i + 1] as usize);
        &self.targets[s..e]
    }
}

#[cfg(feature = "arrow")]
impl GraphCsr {
    /// **Stream a CSR straight from an Arrow edge batch** — `src`/`dst` are `Int64`
    /// node-id columns, the optional `weight` a `Float64` per-node mass (indexed by
    /// node id). Rows with a null endpoint, a self-loop, or an out-of-range id are
    /// skipped. This is the "adjacency straight from Arrow into a GPU storage buffer"
    /// path: the returned `Vec`s upload unchanged.
    pub fn from_arrow(
        n: usize,
        src: &arrow_array::Int64Array,
        dst: &arrow_array::Int64Array,
        weight: Option<&arrow_array::Float64Array>,
    ) -> Self {
        use arrow_array::Array;
        let mut edges = Vec::with_capacity(src.len());
        for i in 0..src.len().min(dst.len()) {
            if src.is_null(i) || dst.is_null(i) {
                continue;
            }
            let (a, b) = (src.value(i), dst.value(i));
            if a < 0 || b < 0 {
                continue;
            }
            edges.push((a as u32, b as u32));
        }
        let w: Option<Vec<f32>> = weight.map(|w| {
            (0..n).map(|i| if i < w.len() && !w.is_null(i) { w.value(i) as f32 } else { 1.0 }).collect()
        });
        Self::from_edges(n, &edges, w.as_deref())
    }
}

/// Force-layout tuning. [`Default`] is a calm, convergent spring system.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct LayoutParams {
    /// Inverse-square repulsion strength (node spreading).
    pub repulsion: f32,
    /// Spring (edge attraction) stiffness.
    pub attraction: f32,
    /// Spring rest length — the distance a single edge settles to.
    pub ideal_len: f32,
    /// Pull toward the origin (keeps a component from drifting / exploding).
    pub gravity: f32,
    /// Per-step velocity retention `∈ (0,1)` — `< 1` bleeds energy so it settles.
    pub damping: f32,
    /// Integration timestep.
    pub dt: f32,
    /// Distance floor (clamps the repulsion singularity when two nodes coincide).
    pub min_dist: f32,
}

impl Default for LayoutParams {
    fn default() -> Self {
        Self {
            repulsion: 1.0,
            attraction: 2.0,
            ideal_len: 1.0,
            gravity: 0.05,
            damping: 0.9,
            dt: 0.05,
            min_dist: 0.05,
        }
    }
}

/// The mutable layout state: one position + velocity per node (the ping-pong the GPU
/// iterates in VRAM; here, the CPU mirror). Positions are the **DATA** a test reads.
#[derive(Clone, Debug, PartialEq)]
pub struct LayoutState {
    pub pos: Vec<[f32; 2]>,
    pub vel: Vec<[f32; 2]>,
}

impl LayoutState {
    /// The **deterministic seed**: a golden-angle (sunflower) spiral of radius `scale`,
    /// zero velocity. No RNG ⇒ the whole sim is reproducible (FC-7).
    #[must_use]
    pub fn seed(n: usize, scale: f32) -> Self {
        const GOLDEN: f32 = 2.399_963_2; // π·(3−√5)
        let pos = (0..n)
            .map(|i| {
                let r = scale * ((i as f32 + 0.5) / n.max(1) as f32).sqrt();
                let a = i as f32 * GOLDEN;
                [r * a.cos(), r * a.sin()]
            })
            .collect();
        Self { pos, vel: vec![[0.0, 0.0]; n] }
    }

    /// Total kinetic energy `Σ |vᵢ|²` — the **convergence metric**: a settling layout's
    /// energy decays toward zero (read it as DATA to prove it converged).
    #[must_use]
    pub fn kinetic_energy(&self) -> f32 {
        self.vel.iter().map(|v| v[0] * v[0] + v[1] * v[1]).sum()
    }

    /// Total edge length over `g` — the **layout-quality metric** the FDEB / structure
    /// tests read (connected nodes pulled together shrink this).
    #[must_use]
    pub fn total_edge_len(&self, g: &GraphCsr) -> f32 {
        let mut sum = 0.0;
        for i in 0..g.n {
            for &t in g.neighbours(i) {
                if (t as usize) > i {
                    let d = [self.pos[t as usize][0] - self.pos[i][0], self.pos[t as usize][1] - self.pos[i][1]];
                    sum += (d[0] * d[0] + d[1] * d[1]).sqrt();
                }
            }
        }
        sum
    }
}

/// One **CPU** force step — the no-GPU fallback *and* the parity reference the WGSL
/// `cs_layout` mirrors exactly. Pure: `step_cpu(s)` depends only on `s`, `g`, `p`.
#[must_use]
pub fn step_cpu(s: &LayoutState, g: &GraphCsr, p: &LayoutParams) -> LayoutState {
    let n = g.n;
    let mut out = LayoutState { pos: Vec::with_capacity(n), vel: Vec::with_capacity(n) };
    let min2 = p.min_dist * p.min_dist;
    for i in 0..n {
        let pi = s.pos[i];
        let wi = g.weights[i];
        let mut force = [0.0f32, 0.0f32];
        // ── all-pairs inverse-square repulsion (the O(n²) GPU mirror) ──
        for (j, pj) in s.pos.iter().enumerate() {
            if j == i {
                continue;
            }
            let d = [pi[0] - pj[0], pi[1] - pj[1]];
            let dist2 = (d[0] * d[0] + d[1] * d[1]).max(min2);
            let f = p.repulsion * wi * g.weights[j] / dist2;
            force[0] += d[0] * f;
            force[1] += d[1] * f;
        }
        // ── spring attraction along CSR neighbours ──
        for &t in g.neighbours(i) {
            let pt = s.pos[t as usize];
            let d = [pt[0] - pi[0], pt[1] - pi[1]];
            let dist = (d[0] * d[0] + d[1] * d[1]).sqrt().max(1e-4);
            let f = p.attraction * (dist - p.ideal_len) / dist;
            force[0] += d[0] * f;
            force[1] += d[1] * f;
        }
        // ── centring gravity ──
        force[0] -= pi[0] * p.gravity;
        force[1] -= pi[1] * p.gravity;
        // ── integrate (mass = wi: heavy nodes move slower) ──
        let inv_m = 1.0 / wi.max(1e-4);
        let vx = (s.vel[i][0] + force[0] * inv_m * p.dt) * p.damping;
        let vy = (s.vel[i][1] + force[1] * inv_m * p.dt) * p.damping;
        out.vel.push([vx, vy]);
        out.pos.push([pi[0] + vx * p.dt, pi[1] + vy * p.dt]);
    }
    out
}

/// Run the CPU fallback to convergence: seed, then `iters` [`step_cpu`]s. Returns the
/// settled state (positions = DATA). Deterministic in `(g, p, iters)`.
#[must_use]
pub fn relax_cpu(g: &GraphCsr, p: &LayoutParams, iters: usize) -> LayoutState {
    let mut s = LayoutState::seed(g.n, p.ideal_len * (g.n as f32).sqrt());
    for _ in 0..iters {
        s = step_cpu(&s, g, p);
    }
    s
}

// ─────────────────────────── GPU compute path (feature `gpu`) ───────────────────────────
#[cfg(feature = "gpu")]
mod gpu {
    use super::{GraphCsr, LayoutParams, LayoutState};
    use wgpu::util::DeviceExt;

    /// Packed node for the storage buffer: `a = (pos.x, pos.y, vel.x, vel.y)`,
    /// `b = (weight, 0, 0, 0)`. 32 bytes, 16-byte aligned — matches `LNode` in
    /// `layout.wgsl`. Two storage buffers of this ping-pong (`src`→`dst`).
    #[repr(C)]
    #[derive(Clone, Copy, Debug, Default, PartialEq, bytemuck::Pod, bytemuck::Zeroable)]
    pub struct GpuNode {
        pub a: [f32; 4],
        pub b: [f32; 4],
    }

    #[repr(C)]
    #[derive(Clone, Copy, bytemuck::Pod, bytemuck::Zeroable)]
    struct Uniforms {
        a: [f32; 4], // n, repulsion, attraction, ideal_len
        b: [f32; 4], // gravity, damping, dt, min_dist
    }

    /// The layout compute shader source (`cs_layout`).
    pub const LAYOUT_WGSL: &str = include_str!("layout.wgsl");

    /// Pack a [`LayoutState`] + per-node weights into the GPU node array.
    pub fn pack(s: &LayoutState, weights: &[f32]) -> Vec<GpuNode> {
        (0..s.pos.len())
            .map(|i| GpuNode { a: [s.pos[i][0], s.pos[i][1], s.vel[i][0], s.vel[i][1]], b: [weights[i], 0.0, 0.0, 0.0] })
            .collect()
    }

    /// The compiled layout pipeline + the streamed CSR buffers. Positions iterate in
    /// VRAM via the [`GpuLayout::step`] ping-pong; [`GpuLayout::read_positions`] copies
    /// the settled positions back as `[f32; 2]` DATA.
    pub struct GpuLayout {
        pipeline: wgpu::ComputePipeline,
        bgl: wgpu::BindGroupLayout,
        uniforms: wgpu::Buffer,
        // streamed-once, read-only adjacency
        offsets: wgpu::Buffer,
        targets: wgpu::Buffer,
        // ping-pong node buffers
        src: wgpu::Buffer,
        dst: wgpu::Buffer,
        count: u32,
    }

    impl GpuLayout {
        /// Compile `cs_layout` and **stream** the CSR (`offsets`/`targets`) + the seeded
        /// nodes into VRAM. Requires a device with compute + ≥4 storage buffers/stage.
        pub fn new(device: &wgpu::Device, g: &GraphCsr, seed: &LayoutState) -> Self {
            let shader = device.create_shader_module(wgpu::ShaderModuleDescriptor {
                label: Some("graphview_layout"),
                source: wgpu::ShaderSource::Wgsl(LAYOUT_WGSL.into()),
            });
            let storage = |ro: bool| wgpu::BindGroupLayoutEntry {
                binding: 0,
                visibility: wgpu::ShaderStages::COMPUTE,
                ty: wgpu::BindingType::Buffer { ty: wgpu::BufferBindingType::Storage { read_only: ro }, has_dynamic_offset: false, min_binding_size: None },
                count: None,
            };
            let bgl = device.create_bind_group_layout(&wgpu::BindGroupLayoutDescriptor {
                label: Some("graphview_layout_bgl"),
                entries: &[
                    wgpu::BindGroupLayoutEntry {
                        binding: 0,
                        visibility: wgpu::ShaderStages::COMPUTE,
                        ty: wgpu::BindingType::Buffer { ty: wgpu::BufferBindingType::Uniform, has_dynamic_offset: false, min_binding_size: None },
                        count: None,
                    },
                    wgpu::BindGroupLayoutEntry { binding: 1, ..storage(true) },
                    wgpu::BindGroupLayoutEntry { binding: 2, ..storage(false) },
                    wgpu::BindGroupLayoutEntry { binding: 3, ..storage(true) },
                    wgpu::BindGroupLayoutEntry { binding: 4, ..storage(true) },
                ],
            });
            let pipeline = device.create_compute_pipeline(&wgpu::ComputePipelineDescriptor {
                label: Some("graphview_layout_step"),
                layout: Some(&device.create_pipeline_layout(&wgpu::PipelineLayoutDescriptor {
                    label: Some("graphview_layout_pll"),
                    bind_group_layouts: &[Some(&bgl)],
                    immediate_size: 0,
                })),
                module: &shader,
                entry_point: Some("cs_layout"),
                compilation_options: Default::default(),
                cache: None,
            });

            let nodes = pack(seed, &g.weights);
            let node_usage = wgpu::BufferUsages::STORAGE | wgpu::BufferUsages::COPY_SRC | wgpu::BufferUsages::COPY_DST;
            let src = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("graphview_layout_src"),
                contents: bytemuck::cast_slice(&nodes),
                usage: node_usage,
            });
            let dst = device.create_buffer(&wgpu::BufferDescriptor {
                label: Some("graphview_layout_dst"),
                size: (std::mem::size_of::<GpuNode>() * nodes.len().max(1)) as u64,
                usage: node_usage,
                mapped_at_creation: false,
            });
            // CSR streamed read-only (pad to ≥1 elem so empty graphs still bind).
            let off_data: &[u32] = if g.offsets.is_empty() { &[0] } else { &g.offsets };
            let tgt_data: Vec<u32> = if g.targets.is_empty() { vec![0] } else { g.targets.clone() };
            let offsets = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("graphview_layout_offsets"),
                contents: bytemuck::cast_slice(off_data),
                usage: wgpu::BufferUsages::STORAGE,
            });
            let targets = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
                label: Some("graphview_layout_targets"),
                contents: bytemuck::cast_slice(&tgt_data),
                usage: wgpu::BufferUsages::STORAGE,
            });
            let uniforms = device.create_buffer(&wgpu::BufferDescriptor {
                label: Some("graphview_layout_uniforms"),
                size: std::mem::size_of::<Uniforms>() as u64,
                usage: wgpu::BufferUsages::UNIFORM | wgpu::BufferUsages::COPY_DST,
                mapped_at_creation: false,
            });
            Self { pipeline, bgl, uniforms, offsets, targets, src, dst, count: nodes.len() as u32 }
        }

        /// Iterate `iters` layout steps **entirely in VRAM** (ping-pong, no per-step
        /// readback). After this the settled nodes are in `self.src`.
        pub fn iterate(&mut self, device: &wgpu::Device, queue: &wgpu::Queue, g: &GraphCsr, p: &LayoutParams, iters: usize) {
            let u = Uniforms {
                a: [self.count as f32, p.repulsion, p.attraction, p.ideal_len],
                b: [p.gravity, p.damping, p.dt, p.min_dist],
            };
            queue.write_buffer(&self.uniforms, 0, bytemuck::bytes_of(&u));
            let _ = g; // (offsets/targets already streamed in `new`)
            let mut enc = device.create_command_encoder(&Default::default());
            for _ in 0..iters {
                let bind = device.create_bind_group(&wgpu::BindGroupDescriptor {
                    label: Some("graphview_layout_bind"),
                    layout: &self.bgl,
                    entries: &[
                        wgpu::BindGroupEntry { binding: 0, resource: self.uniforms.as_entire_binding() },
                        wgpu::BindGroupEntry { binding: 1, resource: self.src.as_entire_binding() },
                        wgpu::BindGroupEntry { binding: 2, resource: self.dst.as_entire_binding() },
                        wgpu::BindGroupEntry { binding: 3, resource: self.offsets.as_entire_binding() },
                        wgpu::BindGroupEntry { binding: 4, resource: self.targets.as_entire_binding() },
                    ],
                });
                {
                    let mut cp = enc.begin_compute_pass(&wgpu::ComputePassDescriptor { label: Some("graphview_layout_pass"), timestamp_writes: None });
                    cp.set_pipeline(&self.pipeline);
                    cp.set_bind_group(0, &bind, &[]);
                    cp.dispatch_workgroups(self.count.div_ceil(64), 1, 1);
                }
                std::mem::swap(&mut self.src, &mut self.dst);
            }
            queue.submit(Some(enc.finish()));
            device.poll(wgpu::PollType::wait_indefinitely()).ok();
        }

        /// Copy the settled positions out of VRAM as `[f32; 2]` **DATA**.
        pub fn read_positions(&self, device: &wgpu::Device, queue: &wgpu::Queue) -> Vec<[f32; 2]> {
            let size = (std::mem::size_of::<GpuNode>() as u32 * self.count) as u64;
            let readback = device.create_buffer(&wgpu::BufferDescriptor {
                label: Some("graphview_layout_readback"),
                size,
                usage: wgpu::BufferUsages::COPY_DST | wgpu::BufferUsages::MAP_READ,
                mapped_at_creation: false,
            });
            let mut enc = device.create_command_encoder(&Default::default());
            enc.copy_buffer_to_buffer(&self.src, 0, &readback, 0, size);
            queue.submit(Some(enc.finish()));
            let slice = readback.slice(..);
            let (tx, rx) = std::sync::mpsc::channel();
            slice.map_async(wgpu::MapMode::Read, move |r| { let _ = tx.send(r); });
            device.poll(wgpu::PollType::wait_indefinitely()).ok();
            rx.recv().unwrap().unwrap();
            let data = slice.get_mapped_range();
            let nodes: Vec<GpuNode> = bytemuck::cast_slice(&data).to_vec();
            drop(data);
            readback.unmap();
            nodes.iter().map(|nd| [nd.a[0], nd.a[1]]).collect()
        }
    }
}

#[cfg(feature = "gpu")]
pub use gpu::{GpuLayout, GpuNode, LAYOUT_WGSL};

#[cfg(test)]
mod tests {
    use super::*;

    /// A small fixture: two triangles (0-1-2, 3-4-5) bridged by a single edge 2-3.
    fn two_triangles() -> GraphCsr {
        let edges = [(0, 1), (1, 2), (2, 0), (3, 4), (4, 5), (5, 3), (2, 3)];
        GraphCsr::from_edges(6, &edges, None)
    }

    #[test]
    fn csr_is_symmetric_and_dedups() {
        // Parallel + reversed edges collapse; adjacency is undirected.
        let g = GraphCsr::from_edges(3, &[(0, 1), (1, 0), (0, 1), (1, 2)], None);
        assert_eq!(g.neighbours(0), &[1]);
        assert_eq!(g.neighbours(1), &[0, 2]);
        assert_eq!(g.neighbours(2), &[1]);
        assert_eq!(g.degree_sum(), 4, "two undirected edges → 4 directed entries");
        // self-loops + out-of-range dropped
        let g2 = GraphCsr::from_edges(2, &[(0, 0), (0, 5), (0, 1)], None);
        assert_eq!(g2.neighbours(0), &[1]);
    }

    #[test]
    fn seed_is_deterministic_and_bounded() {
        let a = LayoutState::seed(50, 3.0);
        let b = LayoutState::seed(50, 3.0);
        assert_eq!(a, b, "no RNG: the seed is reproducible");
        for q in &a.pos {
            assert!(q[0].is_finite() && q[1].is_finite());
            assert!((q[0] * q[0] + q[1] * q[1]).sqrt() <= 3.0 + 1e-3, "inside the seed disc");
        }
    }

    #[test]
    fn step_cpu_is_deterministic() {
        let g = two_triangles();
        let p = LayoutParams::default();
        let s = LayoutState::seed(g.n, 2.0);
        let a = step_cpu(&s, &g, &p);
        let b = step_cpu(&s, &g, &p);
        for (x, y) in a.pos.iter().zip(&b.pos) {
            assert_eq!(x[0].to_bits(), y[0].to_bits(), "bit-identical pos.x");
            assert_eq!(x[1].to_bits(), y[1].to_bits(), "bit-identical pos.y");
        }
    }

    #[test]
    fn layout_converges_energy_decays() {
        let g = two_triangles();
        let p = LayoutParams::default();
        let mut s = LayoutState::seed(g.n, p.ideal_len * (g.n as f32).sqrt());
        // skip the first few steps (the system spikes as springs engage), then watch
        // the kinetic energy fall as it settles — the convergence proof, as DATA.
        for _ in 0..15 {
            s = step_cpu(&s, &g, &p);
        }
        let e_early = s.kinetic_energy();
        for _ in 0..200 {
            s = step_cpu(&s, &g, &p);
        }
        let e_late = s.kinetic_energy();
        assert!(e_late < e_early * 0.1, "energy decayed an order of magnitude (converged): {e_early} → {e_late}");
        assert!(e_late < 0.05, "settled to near-rest: {e_late}");
        for q in &s.pos {
            assert!(q[0].is_finite() && q[1].is_finite(), "no NaN/inf blow-up");
        }
    }

    #[test]
    fn springs_settle_an_edge_at_the_analytic_equilibrium() {
        // A single edge 0—1 balances the FR repulsion (magnitude rep/d) against the
        // Hooke spring (attr·(d−ideal)): rep/d = attr·(d−ideal) ⇒ for the defaults
        // (rep=1, attr=2, ideal=1) ⇒ 2d²−2d−1=0 ⇒ d = (1+√3)/2 ≈ 1.366.
        let g = GraphCsr::from_edges(2, &[(0, 1)], None);
        let p = LayoutParams { gravity: 0.0, ..LayoutParams::default() };
        let s = relax_cpu(&g, &p, 600);
        let d = ((s.pos[1][0] - s.pos[0][0]).powi(2) + (s.pos[1][1] - s.pos[0][1]).powi(2)).sqrt();
        let expected = (1.0 + 3.0_f32.sqrt()) / 2.0;
        assert!((d - expected).abs() < 0.02, "edge rests at the force equilibrium {expected} (got {d})");
    }

    #[test]
    fn relax_is_deterministic_positions_are_data() {
        let g = two_triangles();
        let p = LayoutParams::default();
        let a = relax_cpu(&g, &p, 120);
        let b = relax_cpu(&g, &p, 120);
        assert_eq!(a.pos, b.pos, "same params → identical positions (reproducible DATA)");
        // The bridged triangles separate: the cross-bridge edge 2—3 is the graph's
        // longest single edge by construction (its endpoints anchor opposite clusters).
        let len = |i: usize, j: usize| ((a.pos[j][0] - a.pos[i][0]).powi(2) + (a.pos[j][1] - a.pos[i][1]).powi(2)).sqrt();
        let within = len(0, 1).max(len(1, 2)).max(len(3, 4)).max(len(4, 5));
        assert!(len(2, 3) > within * 0.9, "the two clusters spread apart along the bridge");
    }

    #[test]
    fn heavier_nodes_move_less_per_step() {
        // Same geometry, but node 0 is 50× heavier → its first-step displacement is
        // far smaller (mass in the integrator).
        let edges = [(0, 1), (0, 2)];
        let light = GraphCsr::from_edges(3, &edges, None);
        let heavy = GraphCsr::from_edges(3, &edges, Some(&[50.0, 1.0, 1.0]));
        let p = LayoutParams::default();
        let s = LayoutState::seed(3, 2.0);
        let dl = step_cpu(&s, &light, &p);
        let dh = step_cpu(&s, &heavy, &p);
        let disp = |st: &LayoutState| ((st.pos[0][0] - s.pos[0][0]).powi(2) + (st.pos[0][1] - s.pos[0][1]).powi(2)).sqrt();
        assert!(disp(&dh) < disp(&dl) * 0.5, "the heavy node barely moves ({} vs {})", disp(&dh), disp(&dl));
    }

    // ─────────────────────────── GPU parity proof (feature `gpu`) ───────────────────────────
    /// Headless compute device with the adapter's real limits; `None` ⇒ self-skip.
    #[cfg(feature = "gpu")]
    fn compute_device() -> Option<(wgpu::Device, wgpu::Queue)> {
        let instance = wgpu::Instance::default();
        let adapter = pollster::block_on(instance.request_adapter(&wgpu::RequestAdapterOptions {
            power_preference: wgpu::PowerPreference::default(),
            force_fallback_adapter: false,
            compatible_surface: None,
        }))
        .ok()?;
        if !adapter.get_downlevel_capabilities().flags.contains(wgpu::DownlevelFlags::COMPUTE_SHADERS) {
            return None;
        }
        if adapter.limits().max_storage_buffers_per_shader_stage < 4 {
            return None;
        }
        pollster::block_on(adapter.request_device(&wgpu::DeviceDescriptor {
            label: Some("graphview-layout-proof"),
            required_features: wgpu::Features::empty(),
            required_limits: adapter.limits(),
            memory_hints: wgpu::MemoryHints::default(),
            experimental_features: wgpu::ExperimentalFeatures::disabled(),
            trace: wgpu::Trace::Off,
        }))
        .ok()
    }

    /// GPU↔CPU PARITY PROOF (self-skips without a compute device): the same CSR seeded
    /// the same way and stepped N times **in VRAM** lands on the same positions as the
    /// CPU fallback (within float tolerance) — proving `cs_layout` and `step_cpu` are
    /// one deterministic function, and that positions read back as real DATA.
    #[cfg(feature = "gpu")]
    #[test]
    fn gpu_layout_matches_cpu_reference() {
        let Some((device, queue)) = compute_device() else {
            eprintln!("[gpu_layout] no compute device — skipping GPU parity proof");
            return;
        };
        let g = two_triangles();
        let p = LayoutParams::default();
        let iters = 50;
        let seed = LayoutState::seed(g.n, p.ideal_len * (g.n as f32).sqrt());

        let mut gl = GpuLayout::new(&device, &g, &seed);
        gl.iterate(&device, &queue, &g, &p, iters);
        let gpu = gl.read_positions(&device, &queue);

        let mut cpu = seed.clone();
        for _ in 0..iters {
            cpu = step_cpu(&cpu, &g, &p);
        }
        assert_eq!(gpu.len(), cpu.pos.len());
        let mut moved = false;
        for (gp, cp) in gpu.iter().zip(&cpu.pos) {
            assert!((gp[0] - cp[0]).abs() < 1e-2, "pos.x parity: gpu {} vs cpu {}", gp[0], cp[0]);
            assert!((gp[1] - cp[1]).abs() < 1e-2, "pos.y parity: gpu {} vs cpu {}", gp[1], cp[1]);
            assert!(gp[0].is_finite() && gp[1].is_finite(), "readback is finite DATA");
            if (gp[0] - seed.pos[0][0]).abs() > 1e-3 {
                moved = true;
            }
        }
        assert!(moved, "the VRAM iteration actually advanced the layout");
    }

    /// INJECT-ASSERT: the WGSL exposes the entry point + workgroup size the pipeline
    /// names, and `GpuNode` is the 32-byte two-vec4 the shader's `LNode` expects.
    #[cfg(feature = "gpu")]
    #[test]
    fn layout_shader_entry_and_node_size() {
        assert!(LAYOUT_WGSL.contains("fn cs_layout"));
        assert!(LAYOUT_WGSL.contains("@workgroup_size(64)"));
        assert_eq!(std::mem::size_of::<GpuNode>(), 32);
    }

    #[cfg(feature = "arrow")]
    #[test]
    fn csr_streams_from_an_arrow_edge_batch() {
        use arrow_array::{Float64Array, Int64Array};
        let src = Int64Array::from(vec![0, 1, 2, 0]);
        let dst = Int64Array::from(vec![1, 2, 0, 2]);
        let w = Float64Array::from(vec![3.0, 1.0, 1.0]);
        let g = GraphCsr::from_arrow(3, &src, &dst, Some(&w));
        assert_eq!(g.neighbours(0), &[1, 2]);
        assert_eq!(g.weights[0], 3.0, "weight column streamed in");
        // It lays out to finite, deterministic DATA.
        let s = relax_cpu(&g, &LayoutParams::default(), 80);
        assert!(s.pos.iter().all(|q| q[0].is_finite() && q[1].is_finite()));
    }
}