ifc-lite-geometry 4.1.3

// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at https://mozilla.org/MPL/2.0/.

//! Analytic fast path for axis-aligned rectangular openings cut through an
//! axis-aligned box host (the dominant case: rectangular windows/doors in a
//! straight wall). This sidesteps the exact mesh-arrangement CSG kernel — which
//! is at its single-threaded, memory-bandwidth-bound floor — for openings that
//! need no exact arithmetic at all.
//!
//! WATERTIGHTNESS RECIPE (the make-or-break the prior `subtract_box_fast`
//! attempt got wrong by recomputing rim coords on an incommensurate grid):
//!   - Build the cut on ONE canonical grid whose lines are the host edges plus
//!     every opening edge, each value SNAPPED to the kernel's own power-of-two
//!     `SNAP_GRID = 1/65536` (the grid the host operand already lives on).
//!   - CONFORMING-split every face by the crossing grid lines (no T-junctions):
//!     the front/back faces become a grid of cells with hole cells omitted; the
//!     side faces are split at the hole grid lines so their shared edge with the
//!     annulus matches sub-edge for sub-edge.
//!   - Every vertex reads its position from the SAME snapped grid value, so two
//!     faces meeting at a shared line emit BIT-IDENTICAL f32 → watertight by
//!     construction (value identity, not a numeric hash-match).
//!   - Per-face flat normals; vertices are NEVER welded across creases (#846).
//!
//! GATING: this is a PURE OPTIMIZATION. It returns `None` (→ caller falls back to
//! the exact kernel) the moment any precondition fails — non-box host, non-
//! through opening, opening off the face, or a NEAR-EDGE feature whose grid lines
//! would collapse into each other at the host's f32 magnitude (the robustness
//! gate that replaces a hard dependency on the per-element local frame).

use crate::mesh::Mesh;

/// The kernel's canonical reconcile grid. Power of two ⇒ the snap is an exact
/// f64 op ⇒ bit-deterministic native==wasm.
use crate::kernel::mesh_bridge::SNAP_GRID;

#[inline]
fn snap(c: f64) -> f64 {
    (c / SNAP_GRID).round() * SNAP_GRID
}

/// Telemetry: why the fast path fired or deferred, so the real fire-rate on a
/// void-heavy model is measurable (the prior attempt shipped at fired=0).
#[derive(Default, Clone, Copy, Debug, PartialEq, Eq)]
pub struct RectFastStats {
    pub fired: u64,
    pub openings_cut: u64,
    pub defer_host_not_box: u64,
    pub defer_not_through: u64,
    pub defer_off_face: u64,
    pub defer_near_edge: u64,
    pub defer_no_openings: u64,
    pub defer_too_many_openings: u64,
}

/// Escape hatch: `IFC_LITE_RECT_FAST=0` is the GLOBAL rect-fast kill switch.
/// It forces every opening back through the exact kernel (parity debugging /
/// bisection) by disabling BOTH the legacy world-axis-aligned path gated here
/// AND the parametric placement-frame path ([`param_enabled`] honours it), so
/// setting this single flag is sufficient to route all rectangular openings
/// through CSG. Default ON: the path is a pure optimization that defers on
/// any precondition miss. To disable only the parametric path, set
/// `IFC_LITE_RECT_PARAM=0` instead.
pub fn enabled() -> bool {
    use std::sync::OnceLock;
    static ON: OnceLock<bool> = OnceLock::new();
    *ON.get_or_init(|| std::env::var("IFC_LITE_RECT_FAST").as_deref() != Ok("0"))
}

static PARAM_OVERRIDE: std::sync::atomic::AtomicI8 = std::sync::atomic::AtomicI8::new(-1);

/// PARAMETRIC rectangular-opening fast path (placement-frame, ground-truth-exact cut).
/// DEFAULT ON -- opt out with `IFC_LITE_RECT_PARAM=0` (parametric path only),
/// `IFC_LITE_RECT_FAST=0` (the GLOBAL rect-fast kill switch: disables this path AND
/// the legacy [`enabled`] path, preserving the documented single-flag parity/bisection
/// hatch), or `param_set_enabled_override`. Gated separately from [`enabled`] because
/// it is a deliberate behaviour change: it emits the analytic box-minus-boxes solid,
/// which is MORE correct than the exact kernel on engulfing-opening walls.
/// Corpus-validated ON (tests/rect_param_validate.rs A/B over 5 real models): every
/// non-firing element byte-identical ON vs OFF, every firing host watertight and
/// matching the analytic ground truth. wasm has no env, so `std::env::var` errs there
/// and the default is ON on both targets -- native==wasm stays in lockstep; the
/// wasm-side `setRectParamFastPath` override remains the programmatic escape hatch
/// (and, being an explicit per-process call, takes precedence over both env flags).
pub fn param_enabled() -> bool {
    match PARAM_OVERRIDE.load(std::sync::atomic::Ordering::Relaxed) {
        0 => return false,
        1 => return true,
        _ => {}
    }
    use std::sync::OnceLock;
    static ON: OnceLock<bool> = OnceLock::new();
    *ON.get_or_init(|| {
        param_env_default(enabled(), std::env::var("IFC_LITE_RECT_PARAM").as_deref().ok())
    })
}

/// Env-flag decision core for [`param_enabled`], pure so the semantics are
/// unit-testable without mutating process env: the parametric path is ON unless
/// its own flag opts out (`IFC_LITE_RECT_PARAM=0`) or the global rect-fast kill
/// switch is thrown (`IFC_LITE_RECT_FAST=0`, surfaced here as `rect_fast_on ==
/// false` via [`enabled`]). `IFC_LITE_RECT_PARAM=1` cannot resurrect the path
/// past the kill switch: the kill switch means "exact kernel for everything".
fn param_env_default(rect_fast_on: bool, rect_param: Option<&str>) -> bool {
    rect_fast_on && rect_param != Some("0")
}

/// Test-only: force `param_enabled()` on/off (or `None` for the env default).
pub fn param_set_enabled_override(v: Option<bool>) {
    PARAM_OVERRIDE.store(
        match v {
            None => -1,
            Some(false) => 0,
            Some(true) => 1,
        },
        std::sync::atomic::Ordering::Relaxed,
    );
}

static PARAM_FIRES: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);

/// Count one parametric fast-path fire (the analytic cut was emitted).
pub fn param_record_fire() {
    PARAM_FIRES.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
}
/// Read + reset the parametric fast-path fire counter.
pub fn take_param_fires() -> u64 {
    PARAM_FIRES.swap(0, std::sync::atomic::Ordering::Relaxed)
}

// rect_fast engagement counters are now REQUEST-LOCAL: each cut records into its
// router via `GeometryRouter::record_rect_fast` (drained by `take_rect_fast_stats`),
// so concurrent native geometry passes get isolated per-load `rectFast` diagnostics.
// The previous process-global atomic sink (`record_global` / `take_global_stats`)
// was removed because it cross-contaminated concurrent loads.

/// An axis-aligned box AABB extracted from a host mesh, plus a check that every
/// face is axis-aligned (the precondition for the world-coord cut).
struct AlignedBox {
    min: [f64; 3],
    max: [f64; 3],
}

/// Verify `mesh` is an axis-aligned box (every triangle normal ≈ ±X/±Y/±Z, and
/// the surface spans exactly the AABB on all 6 sides) and return its AABB.
/// Conservative: any deviation ⇒ `None` ⇒ defer to the exact kernel.
fn aligned_box(mesh: &Mesh) -> Option<AlignedBox> {
    if mesh.indices.len() < 36 {
        // a box is ≥ 12 triangles; fewer can't be a closed box
        return None;
    }
    let p = |i: u32| -> [f64; 3] {
        let b = i as usize * 3;
        [
            mesh.positions[b] as f64,
            mesh.positions[b + 1] as f64,
            mesh.positions[b + 2] as f64,
        ]
    };
    let (mn_f, mx_f) = mesh.bounds();
    let min = [mn_f.x as f64, mn_f.y as f64, mn_f.z as f64];
    let max = [mx_f.x as f64, mx_f.y as f64, mx_f.z as f64];
    // Degenerate extent on any axis ⇒ not a 3D box.
    for k in 0..3 {
        if max[k] - min[k] <= SNAP_GRID {
            return None;
        }
    }
    // Every triangle must be axis-aligned (normal along one axis) AND lie on the
    // corresponding min or max face plane — i.e. the mesh is exactly the AABB
    // shell, nothing protruding or interior.
    const PLANE_TOL: f64 = 1e-4;
    let mut seen_face = [false; 6]; // -x,+x,-y,+y,-z,+z
    for tri in mesh.indices.chunks_exact(3) {
        let (a, b, c) = (p(tri[0]), p(tri[1]), p(tri[2]));
        let e1 = [b[0] - a[0], b[1] - a[1], b[2] - a[2]];
        let e2 = [c[0] - a[0], c[1] - a[1], c[2] - a[2]];
        let n = [
            e1[1] * e2[2] - e1[2] * e2[1],
            e1[2] * e2[0] - e1[0] * e2[2],
            e1[0] * e2[1] - e1[1] * e2[0],
        ];
        let len = (n[0] * n[0] + n[1] * n[1] + n[2] * n[2]).sqrt();
        if len == 0.0 {
            continue; // degenerate tri — ignore (hygiene drops it anyway)
        }
        // which axis is the normal along?
        let mut axis = usize::MAX;
        for k in 0..3 {
            if (n[k].abs() / len) > 0.999 {
                axis = k;
            }
        }
        if axis == usize::MAX {
            return None; // a non-axis-aligned face ⇒ not an aligned box
        }
        // all 3 verts must sit on the min or max plane of that axis
        let on_min = a[axis] <= min[axis] + PLANE_TOL
            && b[axis] <= min[axis] + PLANE_TOL
            && c[axis] <= min[axis] + PLANE_TOL;
        let on_max = a[axis] >= max[axis] - PLANE_TOL
            && b[axis] >= max[axis] - PLANE_TOL
            && c[axis] >= max[axis] - PLANE_TOL;
        if on_min {
            seen_face[axis * 2] = true;
        } else if on_max {
            seen_face[axis * 2 + 1] = true;
        } else {
            return None; // an interior / protruding triangle ⇒ not a clean box
        }
    }
    if seen_face.iter().all(|&s| s) {
        Some(AlignedBox { min, max })
    } else {
        None
    }
}

/// Result of the cut: a watertight `Mesh`, or `None` to defer to the exact
/// kernel. `openings` are world AABBs (min,max) of axis-aligned rectangular
/// cutters. Handles WINDOWS, DOORS (flush to an edge), recesses, and overlapping
/// openings uniformly via a 3D CELLULAR decomposition: split the host box by
/// every opening plane on all three axes, mark each cell solid (in the host,
/// outside every opening) or void, and emit each cell face that borders a void
/// cell or the grid boundary. Watertight by construction — adjacent cells share
/// snapped grid vertices bit-identically, so no T-junctions and no cracks.
pub fn subtract_rect_openings(
    host: &Mesh,
    openings: &[([f64; 3], [f64; 3])],
    stats: &mut RectFastStats,
) -> Option<Mesh> {
    if openings.is_empty() {
        stats.defer_no_openings += 1;
        return None;
    }
    // build_cellular is O(cells * openings) and cells grow with the opening count,
    // so a crafted host with thousands of tiny openings is O(N^3) -> unbounded
    // hang. 128 is far above any real wall (1-20 openings); defer the pathological
    // case to the exact kernel (bounded per-element, #1121) rather than hang.
    const MAX_FAST_OPENINGS: usize = 128;
    if openings.len() > MAX_FAST_OPENINGS {
        stats.defer_too_many_openings += 1;
        return None;
    }
    let bx = match aligned_box(host) {
        Some(b) => b,
        None => {
            stats.defer_host_not_box += 1;
            return None;
        }
    };

    // Scale-aware near-edge epsilon: two grid lines closer than this would
    // collapse into one f32 at the host's world magnitude, cracking the cut.
    // max(|coord|)·2^-21 keeps >= 4 f32 ULP between distinct lines; floored at the
    // snap grid so origin-scale hosts stay permissive.
    let mag = (0..3)
        .map(|k| bx.min[k].abs().max(bx.max[k].abs()))
        .fold(0.0f64, f64::max);
    let near_eps = (mag * (1.0 / 2_097_152.0)).max(SNAP_GRID);

    // Clamp every opening to the host shell (a cutter extended through the wall
    // pokes past) and keep only those that overlap on all 3 axes. A
    // non-overlapping opening is a no-op (the SILENT NO-OP case) — drop it.
    let smin = [snap(bx.min[0]), snap(bx.min[1]), snap(bx.min[2])];
    let smax = [snap(bx.max[0]), snap(bx.max[1]), snap(bx.max[2])];
    let mut clamped: Vec<[[f64; 3]; 2]> = Vec::with_capacity(openings.len());
    for (omn, omx) in openings {
        let mut cmn = [0.0; 3];
        let mut cmx = [0.0; 3];
        let mut overlaps = true;
        let mut covers_full = true;
        for k in 0..3 {
            cmn[k] = snap(omn[k].max(bx.min[k]));
            cmx[k] = snap(omx[k].min(bx.max[k]));
            if cmx[k] - cmn[k] <= near_eps {
                overlaps = false;
            }
            // Does the clamped opening reach the host edge-to-edge on this axis?
            if cmn[k] > smin[k] + near_eps || cmx[k] < smax[k] - near_eps {
                covers_full = false;
            }
        }
        // An opening that CONTAINS the whole host on all three axes is a
        // degenerate / double-encoded redundant void: the void is already baked
        // into the wall profile and re-added as an opening element (#964). The
        // exact kernel treats it as a no-op (coplanar cutter faces → host left
        // unchanged), so the fast path MUST NOT cut it — doing so erases the
        // entire wall and leaves the window floating in a giant void (#1167).
        // Defer the whole host to the exact path to match its behaviour.
        if covers_full {
            stats.defer_off_face += 1;
            return None;
        }
        if overlaps {
            clamped.push([cmn, cmx]);
        }
    }
    if clamped.is_empty() {
        // Nothing actually cuts — defer the no-op to the exact path (it owns the
        // SILENT NO-OP diagnostic) rather than returning an unchanged clone.
        stats.defer_off_face += 1;
        return None;
    }

    // Per-axis grid lines: host bounds + every clamped opening edge, snapped,
    // sorted, deduplicated; near-coincident-but-distinct lines (< near_eps) →
    // defer (f32-collapse risk).
    let mut grid: [Vec<f64>; 3] = [Vec::new(), Vec::new(), Vec::new()];
    for k in 0..3 {
        let mut edges = vec![snap(bx.min[k]), snap(bx.max[k])];
        for c in &clamped {
            edges.push(c[0][k]);
            edges.push(c[1][k]);
        }
        match dedup_axis(edges, near_eps) {
            Some(g) => grid[k] = g,
            None => {
                stats.defer_near_edge += 1;
                return None;
            }
        }
    }

    let cut = build_cellular(&grid, &clamped);
    stats.fired += 1;
    stats.openings_cut += clamped.len() as u64;
    Some(cut)
}

/// Sort + dedup grid lines on one axis; `None` if two DISTINCT lines are closer
/// than `near_eps` (an f32-collapse risk → defer). Identical snapped values
/// (e.g. a door edge coinciding with the host edge) collapse to one line.
fn dedup_axis(mut edges: Vec<f64>, near_eps: f64) -> Option<Vec<f64>> {
    // A non-finite coord (NaN/Inf from a corrupt mesh) would panic the sort under
    // `panic=abort` and crash the whole batch — defer to the exact kernel instead.
    if edges.iter().any(|v| !v.is_finite()) {
        return None;
    }
    edges.sort_by(|a, b| a.total_cmp(b));
    let mut out: Vec<f64> = Vec::with_capacity(edges.len());
    for c in edges {
        match out.last() {
            Some(&last) => {
                let d = (c - last).abs();
                if d == 0.0 {
                    // same line — keep one
                } else if d < near_eps {
                    return None; // distinct but too close → collapse risk
                } else {
                    out.push(c);
                }
            }
            None => out.push(c),
        }
    }
    if out.len() < 2 {
        return None;
    }
    Some(out)
}

struct Builder {
    positions: Vec<f32>,
    normals: Vec<f32>,
    indices: Vec<u32>,
}

impl Builder {
    fn vert(&mut self, p: [f64; 3], n: [f32; 3]) -> u32 {
        let idx = (self.positions.len() / 3) as u32;
        self.positions.push(p[0] as f32);
        self.positions.push(p[1] as f32);
        self.positions.push(p[2] as f32);
        self.normals.extend_from_slice(&n);
        idx
    }
    /// A planar quad (4 coplanar world corners in perimeter order) emitted with
    /// the winding that makes its front face point along `n` — robust to the
    /// handedness of the (u,v,w) axis assignment (e.g. a Y-through wall is a
    /// left-handed frame, which would otherwise invert every face).
    fn quad(&mut self, c: [[f64; 3]; 4], n: [f32; 3]) {
        let e1 = [c[1][0] - c[0][0], c[1][1] - c[0][1], c[1][2] - c[0][2]];
        let e2 = [c[2][0] - c[0][0], c[2][1] - c[0][1], c[2][2] - c[0][2]];
        let cr = [
            e1[1] * e2[2] - e1[2] * e2[1],
            e1[2] * e2[0] - e1[0] * e2[2],
            e1[0] * e2[1] - e1[1] * e2[0],
        ];
        let dot = cr[0] * n[0] as f64 + cr[1] * n[1] as f64 + cr[2] * n[2] as f64;
        let o = if dot >= 0.0 { [0, 1, 2, 3] } else { [0, 3, 2, 1] };
        let v0 = self.vert(c[o[0]], n);
        let v1 = self.vert(c[o[1]], n);
        let v2 = self.vert(c[o[2]], n);
        let v3 = self.vert(c[o[3]], n);
        self.indices.extend_from_slice(&[v0, v1, v2, v0, v2, v3]);
    }
}

/// 3D cellular exposed-face extraction. `grid[axis]` is the sorted snapped grid
/// lines on that axis. A cell is solid iff its centre lies outside every opening
/// (it is always inside the host); emit every face of a solid cell that borders
/// a void cell or the grid boundary, with the outward normal — skipping faces
/// internal to the solid region. Watertight: a shared face between two solid
/// cells is emitted by neither; every boundary face is emitted exactly once on
/// the snapped grid, so reverse edges cancel bit-for-bit.
fn build_cellular(grid: &[Vec<f64>; 3], openings: &[[[f64; 3]; 2]]) -> Mesh {
    let nc = [grid[0].len() - 1, grid[1].len() - 1, grid[2].len() - 1];
    let center = |axis: usize, i: usize| (grid[axis][i] + grid[axis][i + 1]) * 0.5;
    let solid = |c: [usize; 3]| -> bool {
        let p = [center(0, c[0]), center(1, c[1]), center(2, c[2])];
        !openings
            .iter()
            .any(|o| (0..3).all(|a| p[a] > o[0][a] && p[a] < o[1][a]))
    };
    let mut b = Builder {
        positions: Vec::new(),
        normals: Vec::new(),
        indices: Vec::new(),
    };

    for i in 0..nc[0] {
        for j in 0..nc[1] {
            for k in 0..nc[2] {
                let cell = [i, j, k];
                if !solid(cell) {
                    continue;
                }
                for axis in 0..3 {
                    let (a1, a2) = match axis {
                        0 => (1usize, 2usize),
                        1 => (0usize, 2usize),
                        _ => (0usize, 1usize),
                    };
                    for &pos in &[false, true] {
                        let exposed = if pos {
                            cell[axis] == nc[axis] - 1 || !solid({
                                let mut n = cell;
                                n[axis] += 1;
                                n
                            })
                        } else {
                            cell[axis] == 0 || !solid({
                                let mut n = cell;
                                n[axis] -= 1;
                                n
                            })
                        };
                        if !exposed {
                            continue;
                        }
                        let coord = grid[axis][cell[axis] + usize::from(pos)];
                        let (lo1, hi1) = (grid[a1][cell[a1]], grid[a1][cell[a1] + 1]);
                        let (lo2, hi2) = (grid[a2][cell[a2]], grid[a2][cell[a2] + 1]);
                        let corner = |x1: f64, x2: f64| {
                            let mut q = [0.0; 3];
                            q[axis] = coord;
                            q[a1] = x1;
                            q[a2] = x2;
                            q
                        };
                        let mut n = [0.0f32; 3];
                        n[axis] = if pos { 1.0 } else { -1.0 };
                        b.quad(
                            [
                                corner(lo1, lo2),
                                corner(hi1, lo2),
                                corner(hi1, hi2),
                                corner(lo1, hi2),
                            ],
                            n,
                        );
                    }
                }
            }
        }
    }

    let mut m = Mesh::new();
    m.positions = b.positions;
    m.normals = b.normals;
    m.indices = b.indices;
    m
}

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

    /// Closed axis-aligned box, 12 outward triangles.
    fn box_mesh(min: [f64; 3], max: [f64; 3]) -> Mesh {
        let c = [
            [min[0], min[1], min[2]],
            [max[0], min[1], min[2]],
            [max[0], max[1], min[2]],
            [min[0], max[1], min[2]],
            [min[0], min[1], max[2]],
            [max[0], min[1], max[2]],
            [max[0], max[1], max[2]],
            [min[0], max[1], max[2]],
        ];
        let faces: [([usize; 4], [f32; 3]); 6] = [
            ([0, 3, 2, 1], [0.0, 0.0, -1.0]),
            ([4, 5, 6, 7], [0.0, 0.0, 1.0]),
            ([0, 1, 5, 4], [0.0, -1.0, 0.0]),
            ([2, 3, 7, 6], [0.0, 1.0, 0.0]),
            ([0, 4, 7, 3], [-1.0, 0.0, 0.0]),
            ([1, 2, 6, 5], [1.0, 0.0, 0.0]),
        ];
        let mut m = Mesh::new();
        for (idx, n) in faces {
            let b = (m.positions.len() / 3) as u32;
            for &i in &idx {
                m.positions.extend_from_slice(&[c[i][0] as f32, c[i][1] as f32, c[i][2] as f32]);
                m.normals.extend_from_slice(&n);
            }
            m.indices.extend_from_slice(&[b, b + 1, b + 2, b, b + 2, b + 3]);
        }
        m
    }

    /// DIRECTED exact-f32-bit edge audit (the production crack detector,
    /// mesh_bridge::exact_open_edges): a watertight oriented surface has every
    /// directed edge cancelled by its reverse — catches both cracks AND
    /// inconsistent winding.
    fn open_edges(m: &Mesh) -> usize {
        use std::collections::HashMap;
        let key = |i: u32| {
            let b = i as usize * 3;
            (m.positions[b].to_bits(), m.positions[b + 1].to_bits(), m.positions[b + 2].to_bits())
        };
        let mut edges: HashMap<_, i64> = HashMap::new();
        for t in m.indices.chunks_exact(3) {
            for (a, b) in [(t[0], t[1]), (t[1], t[2]), (t[2], t[0])] {
                *edges.entry((key(a), key(b))).or_insert(0) += 1;
                *edges.entry((key(b), key(a))).or_insert(0) -= 1;
            }
        }
        edges.values().filter(|&&c| c != 0).count()
    }

    fn degenerate(m: &Mesh) -> usize {
        let v = |i: u32| {
            let b = i as usize * 3;
            [m.positions[b], m.positions[b + 1], m.positions[b + 2]]
        };
        let mut n = 0;
        for t in m.indices.chunks_exact(3) {
            let (a, b, c) = (v(t[0]), v(t[1]), v(t[2]));
            let e1 = [b[0] - a[0], b[1] - a[1], b[2] - a[2]];
            let e2 = [c[0] - a[0], c[1] - a[1], c[2] - a[2]];
            let cr = [
                e1[1] * e2[2] - e1[2] * e2[1],
                e1[2] * e2[0] - e1[0] * e2[2],
                e1[0] * e2[1] - e1[1] * e2[0],
            ];
            if cr[0] * cr[0] + cr[1] * cr[1] + cr[2] * cr[2] == 0.0 {
                n += 1;
            }
        }
        n
    }

    /// Signed volume × 6 (divergence), about origin.
    fn vol6(m: &Mesh) -> f64 {
        let v = |i: u32| {
            let b = i as usize * 3;
            [m.positions[b] as f64, m.positions[b + 1] as f64, m.positions[b + 2] as f64]
        };
        let mut s = 0.0;
        for t in m.indices.chunks_exact(3) {
            let (a, b, c) = (v(t[0]), v(t[1]), v(t[2]));
            let cr = [
                b[1] * c[2] - b[2] * c[1],
                b[2] * c[0] - b[0] * c[2],
                b[0] * c[1] - b[1] * c[0],
            ];
            s += a[0] * cr[0] + a[1] * cr[1] + a[2] * cr[2];
        }
        s
    }

    // 4m × 0.2m × 3m wall (thin along Y). Opening boxes poke through Y.
    fn wall(base: [f64; 3]) -> Mesh {
        box_mesh(base, [base[0] + 4.0, base[1] + 0.2, base[2] + 3.0])
    }
    fn opening(base: [f64; 3], u0: f64, u1: f64, z0: f64, z1: f64) -> ([f64; 3], [f64; 3]) {
        (
            [base[0] + u0, base[1] - 0.1, base[2] + z0],
            [base[0] + u1, base[1] + 0.3, base[2] + z1],
        )
    }

    fn check_watertight(base: [f64; 3], openings: &[([f64; 3], [f64; 3])], label: &str) {
        let host = wall(base);
        let mut st = RectFastStats::default();
        let cut = subtract_rect_openings(&host, openings, &mut st)
            .unwrap_or_else(|| panic!("{label}: expected fast path to fire, deferred: {st:?}"));
        assert_eq!(open_edges(&cut), 0, "{label}: not watertight");
        assert_eq!(degenerate(&cut), 0, "{label}: degenerate triangles");
        // Removed volume ≈ Σ opening∩host volume.
        let removed = (vol6(&host) - vol6(&cut)) / 6.0;
        let mut expect = 0.0;
        let (hmn, hmx) = (base, [base[0] + 4.0, base[1] + 0.2, base[2] + 3.0]);
        for (omn, omx) in openings {
            let mut vv = 1.0;
            for k in 0..3 {
                vv *= (omx[k].min(hmx[k]) - omn[k].max(hmn[k])).max(0.0);
            }
            expect += vv;
        }
        // Volume tolerance scales with f32 ULP at the host's magnitude (a wall
        // 220 km from origin carries ~12 mm of inherent f32 error per coordinate
        // — true of ANY f32 mesh there, exact kernel included; the cut is still
        // watertight). Relative 2% OR absolute scale-aware, whichever is looser.
        let mag = base[0].abs().max(base[2].abs()).max(1.0);
        let vtol = (expect * 0.02).max(mag * 2f64.powi(-23) * 6.0);
        assert!(
            (removed - expect).abs() < vtol,
            "{label}: removed {removed} != expected {expect} (tol {vtol})"
        );
    }

    /// Flag semantics (pure core, no env mutation, safe under parallel tests):
    /// `IFC_LITE_RECT_FAST=0` is the GLOBAL kill switch and also disables the
    /// parametric path (even against an explicit `IFC_LITE_RECT_PARAM=1`);
    /// `IFC_LITE_RECT_PARAM=0` disables only the parametric path; both unset
    /// means the parametric path defaults ON.
    #[test]
    fn param_env_default_honours_global_kill_switch() {
        // Global kill switch thrown (IFC_LITE_RECT_FAST=0 => enabled() false).
        assert!(!param_env_default(false, None));
        assert!(!param_env_default(false, Some("1")));
        assert!(!param_env_default(false, Some("0")));
        // Parametric-only opt-out.
        assert!(!param_env_default(true, Some("0")));
        // Defaults ON; explicit "1" is also ON.
        assert!(param_env_default(true, None));
        assert!(param_env_default(true, Some("1")));
    }

    #[test]
    fn single_opening_watertight_origin() {
        check_watertight([0.0, 0.0, 0.0], &[opening([0.0, 0.0, 0.0], 1.5, 2.5, 0.5, 2.0)], "single-origin");
    }

    #[test]
    fn single_opening_watertight_building_scale() {
        check_watertight(
            [221_534.0, 98_210.0, 47_001.0],
            &[opening([221_534.0, 98_210.0, 47_001.0], 1.5, 2.5, 0.5, 2.0)],
            "single-building",
        );
    }

    #[test]
    fn multi_opening_watertight() {
        let base = [10.0, 5.0, 2.0];
        let ops = [
            opening(base, 0.4, 1.2, 0.5, 2.4),
            opening(base, 1.6, 2.4, 0.5, 2.4),
            opening(base, 2.8, 3.6, 0.5, 1.0),
        ];
        check_watertight(base, &ops, "multi-3");
    }

    #[test]
    fn defers_non_box_host() {
        // a tetrahedron-ish non-box host
        let mut host = Mesh::new();
        host.positions = vec![0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0];
        host.normals = vec![0.0; 12];
        host.indices = vec![0, 2, 1, 0, 1, 3, 0, 3, 2, 1, 2, 3];
        let mut st = RectFastStats::default();
        assert!(subtract_rect_openings(&host, &[opening([0.0, 0.0, 0.0], 1.5, 2.5, 0.5, 2.0)], &mut st).is_none());
        assert_eq!(st.defer_host_not_box, 1);
    }

    #[test]
    fn defers_pathological_opening_count() {
        // A crafted host with hundreds of tiny openings makes build_cellular
        // O(N^3); the fast path must defer to the exact kernel, not hang.
        let host = box_mesh([0.0, 0.0, 0.0], [10.0, 3.0, 0.3]);
        let openings: Vec<([f64; 3], [f64; 3])> = (0..200)
            .map(|i| {
                let x = i as f64 * 0.04;
                ([x, 0.5, -0.1], [x + 0.02, 1.0, 0.4])
            })
            .collect();
        let mut st = RectFastStats::default();
        assert!(subtract_rect_openings(&host, &openings, &mut st).is_none());
        assert_eq!(st.defer_too_many_openings, 1);
    }

    #[test]
    fn door_flush_to_floor_watertight() {
        // DOOR: full thickness, z from BELOW the floor up to 2.0 → touches the
        // wall's bottom edge (no sill reveal); the bottom face gets a hole. The
        // case the face-based v1 deferred (off_face); cellular cuts it watertight.
        let base = [0.0, 0.0, 0.0];
        let door = (
            [base[0] + 1.5, base[1] - 0.1, base[2] - 0.5],
            [base[0] + 2.5, base[1] + 0.3, base[2] + 2.0],
        );
        check_watertight(base, &[door], "door-flush-floor");
    }

    #[test]
    fn edge_notch_watertight() {
        // opening that exceeds the wall's X extent → a notch flush with the right
        // edge (no right jamb). Was an off_face defer; cellular handles it.
        let base = [0.0, 0.0, 0.0];
        let notch = (
            [base[0] + 3.8, base[1] - 0.1, base[2] + 0.5],
            [base[0] + 4.5, base[1] + 0.3, base[2] + 2.0],
        );
        check_watertight(base, &[notch], "edge-notch");
    }

    #[test]
    fn recess_cut_watertight() {
        // RECESS: does NOT span the full thickness (a niche/pocket). Was a
        // not_through defer; cellular cuts the pocket watertight.
        let base = [0.0, 0.0, 0.0];
        let recess = (
            [base[0] + 1.5, base[1] + 0.05, base[2] + 0.5],
            [base[0] + 2.5, base[1] + 0.15, base[2] + 2.0],
        );
        check_watertight(base, &[recess], "recess");
    }

    #[test]
    fn opening_fully_outside_defers() {
        // genuine no-op: opening entirely outside the host → no overlap → defer.
        let base = [0.0, 0.0, 0.0];
        let outside = (
            [base[0] + 10.0, base[1] - 0.1, base[2] + 0.5],
            [base[0] + 11.0, base[1] + 0.3, base[2] + 2.0],
        );
        let mut st = RectFastStats::default();
        assert!(subtract_rect_openings(&wall(base), &[outside], &mut st).is_none());
    }

    #[test]
    fn defers_near_edge_at_building_scale() {
        // 8 µm reveal at ~220 km: below f32 ULP → must defer (the robustness gate)
        let base = [221_534.0, 98_210.0, 47_001.0];
        let tight = opening(base, 8e-6, 4.0 - 8e-6, 8e-6, 3.0 - 8e-6);
        let mut st = RectFastStats::default();
        assert!(
            subtract_rect_openings(&wall(base), &[tight], &mut st).is_none(),
            "near-edge at building scale must defer, not crack"
        );
    }

    #[test]
    fn deterministic_output() {
        let base = [10.0, 5.0, 2.0];
        let ops = [opening(base, 1.5, 2.5, 0.5, 2.0)];
        let mut s1 = RectFastStats::default();
        let mut s2 = RectFastStats::default();
        let a = subtract_rect_openings(&wall(base), &ops, &mut s1).unwrap();
        let b = subtract_rect_openings(&wall(base), &ops, &mut s2).unwrap();
        assert_eq!(a.positions, b.positions);
        assert_eq!(a.indices, b.indices);
    }

    #[test]
    fn redundant_whole_wall_opening_defers() {
        // #1167: a double-encoded void (#964) whose box CONTAINS the whole wall
        // — full width × full height, through the thickness. The exact kernel
        // treats this as a no-op (coplanar cutter → host unchanged), so the fast
        // path MUST defer; cutting it would erase the entire wall and leave the
        // window floating in a giant void.
        let base = [10.0, 5.0, 2.0];
        let whole = opening(base, 0.0, 4.0, 0.0, 3.0); // spans the full 4 m × 3 m face
        let mut st = RectFastStats::default();
        assert!(
            subtract_rect_openings(&wall(base), &[whole], &mut st).is_none(),
            "opening that contains the whole host must defer, not erase the wall"
        );
        assert_eq!(st.defer_off_face, 1);

        // A whole-wall opening MIXED with a genuine window still defers the host
        // (the redundant void poisons the batch → hand the whole element to the
        // exact path, which cuts the real window and no-ops the redundant one).
        let mut st_mix = RectFastStats::default();
        assert!(
            subtract_rect_openings(
                &wall(base),
                &[whole, opening(base, 0.5, 1.5, 0.4, 2.6)],
                &mut st_mix
            )
            .is_none(),
            "a redundant whole-wall opening must defer the whole host"
        );

        // Sanity: a genuine interior window (margin on every in-face axis) still
        // fires the fast path — the guard is not over-broad.
        let mut st_ok = RectFastStats::default();
        assert!(
            subtract_rect_openings(&wall(base), &[opening(base, 0.5, 3.5, 0.4, 2.6)], &mut st_ok)
                .is_some(),
            "an interior window must still fire the fast path"
        );
    }
}