ifc-lite-geometry 3.1.2

// 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/.

//! # Persistent, editable space topology (DCEL)
//!
//! This is the Rust core of the interactive space-sketch editor. It does
//! what the one-shot TS `auto-space-detect` pipeline does — turn a set of
//! 2D wall-axis segments into the enclosed room faces of a floor plate —
//! but instead of building a half-edge graph, walking it once, and
//! **throwing it away** (returning bare `Vec<[f64;2]>` outlines), it keeps
//! the [`SpacePlate`] alive as the authoritative topology and exposes
//! local, O(degree) edit operations on it:
//!
//! - [`SpacePlate::drag_vertex`] — move a vertex; every incident face
//!   follows in the same call because a shared wall is **one** edge whose
//!   endpoints are shared vertices. This is the whole "pull one room, the
//!   neighbour follows" trick, and it falls out of the structure for free.
//! - [`SpacePlate::split_face`] — insert a partition between two vertices
//!   of a face (subdivide a room). O(1) surgery; both children valid
//!   immediately.
//! - [`SpacePlate::merge_faces`] — remove a shared edge, unioning the two
//!   rooms it separated. O(1) surgery.
//!
//! Every edit returns the [`FaceId`]s it touched (with recomputed outline
//! + area) so the TS mirror can re-render only what changed.
//!
//! ## What this adds over the TS detector
//!
//! 1. **Persistence.** The DCEL survives across edits; ids are stable
//!    (tombstoned, never reused) so a TS-side hit-test mirror can reference
//!    a face/edge across frames.
//! 2. **Source-element provenance.** Each input segment carries the IFC
//!    element id it came from; that id is propagated through snapping,
//!    T-junction resolution and intersection-splitting onto every bounding
//!    half-edge. This is what makes `IfcRelSpaceBoundary` emission and the
//!    leak-repair affordance possible at bake — the TS detector drops it.
//! 3. **`prev` pointers + the outer face as a real face**, so split/merge
//!    are pure pointer surgery with no re-walk of the whole plate.
//!
//! ## Conventions
//!
//! - Interior (room) faces wind **CCW** (signed area > 0). The unbounded
//!   exterior winds **CW** and is flagged [`Face::is_outer`]. A plate with
//!   several disconnected wall clusters has one outer face per component.
//! - A half-edge's [`HalfEdge::face`] is the face on its **left**.
//! - Holes / nested faces (a room enclosing a courtyard) are out of scope
//!   for this prototype: every CW cycle is treated as exterior, matching
//!   the TS detector. See the module TODO at the bottom.

use rustc_hash::FxHashMap;

/// Stable handle to a vertex. Survives edits; never reused after tombstoning.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct VertexId(pub u32);

/// Stable handle to a directed half-edge.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct HalfEdgeId(pub u32);

/// Stable handle to a face (a room, or the unbounded exterior).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct FaceId(pub u32);

/// An input wall-axis segment tagged with the IFC element it came from.
///
/// `source_element` is the express id of the originating wall / divider.
/// `None` marks a synthetic segment with no single source (rare; e.g. a
/// user-drawn guide). It is propagated to every half-edge derived from this
/// segment so the bake step can recover `IfcRelSpaceBoundary` links.
#[derive(Debug, Clone, Copy)]
pub struct InputSegment {
    pub a: [f64; 2],
    pub b: [f64; 2],
    pub source_element: Option<u32>,
}

impl InputSegment {
    pub fn new(a: [f64; 2], b: [f64; 2], source_element: Option<u32>) -> Self {
        Self { a, b, source_element }
    }
}

/// Tuning for the arrangement pass. Distances are in the same units as the
/// input segments (the caller is expected to pre-scale to metres, exactly
/// as `extract-walls` does on the TS side).
#[derive(Debug, Clone, Copy)]
pub struct BuildOptions {
    /// Endpoints closer than this merge to one vertex. Also the radius for
    /// snapping a dangling wall-end onto a neighbour's interior (T-junction).
    pub snap_tolerance: f64,
    /// Faces below this absolute area are discarded as slivers.
    pub min_area: f64,
}

impl Default for BuildOptions {
    fn default() -> Self {
        // Mirrors the TS `generate-spaces` defaults.
        Self { snap_tolerance: 0.1, min_area: 0.5 }
    }
}

#[derive(Debug, Clone)]
struct Vertex {
    pos: [f64; 2],
    /// One outgoing half-edge, or `None` once isolated/tombstoned.
    outgoing: Option<HalfEdgeId>,
    alive: bool,
}

#[derive(Debug, Clone)]
struct HalfEdge {
    origin: VertexId,
    twin: HalfEdgeId,
    next: HalfEdgeId,
    prev: HalfEdgeId,
    face: FaceId,
    /// The IFC element this edge bounds, propagated from the input segment.
    /// `None` for the exterior side of a boundary-less cut or a user split.
    source_element: Option<u32>,
    alive: bool,
}

#[derive(Debug, Clone)]
struct Face {
    /// One boundary half-edge, or `None` once tombstoned.
    half_edge: Option<HalfEdgeId>,
    is_outer: bool,
    floor_z: f64,
    ceiling_z: f64,
    /// Set when the ceiling plane is inferred (e.g. pitched roof underside);
    /// carried through edits so the sketch UI can telegraph "approximate".
    non_planar_ceiling: bool,
    alive: bool,
}

/// A face touched by an edit, with enough geometry for the caller to
/// re-render it without re-querying the whole plate.
#[derive(Debug, Clone, PartialEq)]
pub struct FacePatch {
    pub face: FaceId,
    /// CCW outline; the first vertex is **not** repeated.
    pub outline: Vec<[f64; 2]>,
    pub area: f64,
    /// `false` when the edit left the face self-intersecting or degenerate.
    /// The op still applies (fluid dragging shouldn't fight the user); the
    /// caller decides whether to telegraph the invalid state.
    pub simple: bool,
}

/// Errors an edit op can reject with. Build never fails — a malformed input
/// just yields fewer (or zero) interior faces, matching the TS detector.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum EditError {
    /// A referenced id is tombstoned or out of range.
    StaleHandle,
    /// `split_face`: the two vertices aren't both on the target face.
    VerticesNotOnFace,
    /// `split_face`: the two endpoints are the same, or already adjacent on
    /// the face (the cut would have zero area on one side).
    DegenerateCut,
    /// `merge_faces`: the edge borders the exterior, so there's no second
    /// room to merge with (that's a delete, not a merge).
    BordersExterior,
    /// `merge_faces`: both sides are the same face — the edge is a bridge,
    /// and removing it would change connectivity rather than union two rooms.
    BridgeEdge,
}

/// The persistent floor-plate topology. See the module docs.
#[derive(Debug, Clone)]
pub struct SpacePlate {
    vertices: Vec<Vertex>,
    half_edges: Vec<HalfEdge>,
    faces: Vec<Face>,
}

const EPS: f64 = 1e-9;

impl SpacePlate {
    // ───────────────────────── construction ─────────────────────────

    /// Build the plate from tagged wall-axis segments.
    ///
    /// Pipeline (ported from `auto-space-detect.ts`, provenance added):
    /// 1. snap endpoints onto a spatial-hash grid,
    /// 2. snap dangling endpoints onto nearby edge interiors (T-junctions),
    /// 3. resolve interior crossings, splitting both hosts,
    /// 4. dedupe undirected edges,
    /// 5. build the half-edge graph with angle-sorted vertex fans,
    /// 6. assign `next`/`prev` by the leftmost-turn rule and walk faces,
    /// 7. flag CW cycles as exterior and drop sub-`min_area` rooms.
    pub fn build(segments: &[InputSegment], options: BuildOptions) -> Self {
        let arr = Arrangement::resolve(segments, options.snap_tolerance);
        Self::from_arrangement(arr, options.min_area)
    }

    fn from_arrangement(arr: Arrangement, min_area: f64) -> Self {
        let mut plate = SpacePlate {
            vertices: arr
                .vertices
                .iter()
                .map(|&pos| Vertex { pos, outgoing: None, alive: true })
                .collect(),
            half_edges: Vec::with_capacity(arr.edges.len() * 2),
            faces: Vec::new(),
        };

        // Two half-edges per undirected edge, twinned. Record outgoing fans.
        let mut fans: Vec<Vec<(HalfEdgeId, f64)>> = vec![Vec::new(); arr.vertices.len()];
        for e in &arr.edges {
            let (a, b, src) = (e.a, e.b, e.source);
            let pa = arr.vertices[a];
            let pb = arr.vertices[b];
            let fwd = HalfEdgeId(plate.half_edges.len() as u32);
            let bwd = HalfEdgeId(plate.half_edges.len() as u32 + 1);
            // Placeholder next/prev/face — patched after fans are sorted.
            plate.half_edges.push(HalfEdge {
                origin: VertexId(a as u32),
                twin: bwd,
                next: fwd,
                prev: fwd,
                face: FaceId(0),
                source_element: src,
                alive: true,
            });
            plate.half_edges.push(HalfEdge {
                origin: VertexId(b as u32),
                twin: fwd,
                next: bwd,
                prev: bwd,
                face: FaceId(0),
                source_element: src,
                alive: true,
            });
            fans[a].push((fwd, (pb[1] - pa[1]).atan2(pb[0] - pa[0])));
            fans[b].push((bwd, (pa[1] - pb[1]).atan2(pa[0] - pb[0])));
        }

        for (v, fan) in fans.iter_mut().enumerate() {
            // total_cmp (not partial_cmp/unwrap) so any NaN angle from a
            // degenerate/coincident segment sorts deterministically rather
            // than corrupting the next/prev wiring nondeterministically.
            fan.sort_by(|p, q| p.1.total_cmp(&q.1));
            plate.vertices[v].outgoing = fan.first().map(|(h, _)| *h);
        }

        // `next`: after arriving along `e` at vertex v = dest(e), leave on the
        // clockwise neighbour of `e.twin` in v's angular fan. That yields a
        // CCW interior walk. `prev` is the inverse.
        for he in 0..plate.half_edges.len() {
            let h = HalfEdgeId(he as u32);
            let dest = plate.dest(h);
            let twin = plate.half_edges[he].twin;
            let fan = &fans[dest.0 as usize];
            let idx = fan.iter().position(|(e, _)| *e == twin);
            let Some(idx) = idx else { continue }; // structurally impossible
            let nxt = fan[(idx + fan.len() - 1) % fan.len()].0;
            plate.half_edges[he].next = nxt;
            plate.half_edges[nxt.0 as usize].prev = h;
        }

        // Walk cycles → faces. CCW (signed > 0) = room; CW = exterior.
        let mut visited = vec![false; plate.half_edges.len()];
        for start in 0..plate.half_edges.len() {
            if visited[start] {
                continue;
            }
            let mut cycle = Vec::new();
            let mut cur = HalfEdgeId(start as u32);
            loop {
                if visited[cur.0 as usize] {
                    break;
                }
                visited[cur.0 as usize] = true;
                cycle.push(cur);
                cur = plate.half_edges[cur.0 as usize].next;
                if cur.0 as usize == start {
                    break;
                }
            }
            let signed = plate.signed_area_of_cycle(&cycle);
            let is_outer = signed <= 0.0;
            // Drop sub-min-area rooms by folding them into no face: we still
            // need a face record (half-edges must point somewhere), so we
            // keep the cycle but flag tiny CCW faces as outer so they're not
            // surfaced as rooms. Exterior cycles are always kept as outer.
            let too_small = !is_outer && signed.abs() < min_area;
            let face = FaceId(plate.faces.len() as u32);
            plate.faces.push(Face {
                half_edge: cycle.first().copied(),
                is_outer: is_outer || too_small,
                floor_z: 0.0,
                ceiling_z: 0.0,
                non_planar_ceiling: false,
                alive: true,
            });
            for h in cycle {
                plate.half_edges[h.0 as usize].face = face;
            }
        }

        plate
    }

    // ───────────────────────────── edits ─────────────────────────────

    /// Move `v` to `(x, y)`. Topology is untouched; every incident face is
    /// updated by construction (they all reference this one vertex). Returns
    /// a patch per incident *room* face — the same call updates a room and
    /// its neighbour across a shared wall.
    pub fn drag_vertex(&mut self, v: VertexId, x: f64, y: f64) -> Result<Vec<FacePatch>, EditError> {
        let idx = v.0 as usize;
        if idx >= self.vertices.len() || !self.vertices[idx].alive {
            return Err(EditError::StaleHandle);
        }
        self.vertices[idx].pos = [x, y];
        let mut faces: Vec<FaceId> = self
            .outgoing_half_edges(v)
            .map(|h| self.half_edges[h.0 as usize].face)
            .collect();
        faces.sort();
        faces.dedup();
        Ok(faces
            .into_iter()
            .filter(|f| !self.faces[f.0 as usize].is_outer)
            .map(|f| self.face_patch(f))
            .collect())
    }

    /// Subdivide `face` by inserting a partition edge between two of its
    /// vertices. Returns patches for the kept face and the new face.
    ///
    /// The new edge carries `source_element` (`None` = a brand-new partition
    /// the user drew, which the bake step materialises as a fresh wall or a
    /// virtual boundary).
    pub fn split_face(
        &mut self,
        face: FaceId,
        va: VertexId,
        vb: VertexId,
        source_element: Option<u32>,
    ) -> Result<Vec<FacePatch>, EditError> {
        self.check_face(face)?;
        if va == vb {
            return Err(EditError::DegenerateCut);
        }
        // Find the boundary half-edges of `face` leaving va and vb.
        let mut ha = None;
        let mut hb = None;
        for h in self.face_half_edges(face) {
            let o = self.half_edges[h.0 as usize].origin;
            if o == va {
                ha = Some(h);
            }
            if o == vb {
                hb = Some(h);
            }
        }
        let (ha, hb) = match (ha, hb) {
            (Some(a), Some(b)) => (a, b),
            _ => return Err(EditError::VerticesNotOnFace),
        };
        // Adjacent on the face → one side has zero area. Reject.
        if self.half_edges[ha.0 as usize].next == hb
            || self.half_edges[hb.0 as usize].next == ha
        {
            return Err(EditError::DegenerateCut);
        }

        let pa_prev = self.half_edges[ha.0 as usize].prev;
        let pb_prev = self.half_edges[hb.0 as usize].prev;

        // New twin pair: e_ab (va→vb) keeps `face`; e_ba (vb→va) gets a new face.
        let e_ab = HalfEdgeId(self.half_edges.len() as u32);
        let e_ba = HalfEdgeId(self.half_edges.len() as u32 + 1);
        let new_face = FaceId(self.faces.len() as u32);

        self.half_edges.push(HalfEdge {
            origin: va,
            twin: e_ba,
            next: hb,
            prev: pa_prev,
            face,
            source_element,
            alive: true,
        });
        self.half_edges.push(HalfEdge {
            origin: vb,
            twin: e_ab,
            next: ha,
            prev: pb_prev,
            face: new_face,
            source_element,
            alive: true,
        });

        // Rewire the four neighbours around the new diagonal.
        self.half_edges[pa_prev.0 as usize].next = e_ab;
        self.half_edges[hb.0 as usize].prev = e_ab;
        self.half_edges[pb_prev.0 as usize].next = e_ba;
        self.half_edges[ha.0 as usize].prev = e_ba;

        // Kept face anchors on e_ab; new face owns e_ba's cycle.
        self.faces[face.0 as usize].half_edge = Some(e_ab);
        let parent = self.faces[face.0 as usize].clone();
        self.faces.push(Face {
            half_edge: Some(e_ba),
            is_outer: false,
            floor_z: parent.floor_z,
            ceiling_z: parent.ceiling_z,
            non_planar_ceiling: parent.non_planar_ceiling,
            alive: true,
        });
        // Re-home the new face's cycle (collect first — the walk borrows
        // `self`, the assignment mutates it).
        let new_cycle: Vec<HalfEdgeId> = self.face_half_edges(new_face).collect();
        for h in new_cycle {
            self.half_edges[h.0 as usize].face = new_face;
        }

        Ok(vec![self.face_patch(face), self.face_patch(new_face)])
    }

    /// Insert a new vertex at `(x, y)` on the undirected edge `edge` (and its
    /// twin), subdividing it. No face is created — both incident faces simply
    /// gain a boundary vertex. Returns the new vertex so the caller can use it
    /// as a `split_face` endpoint. Pass a point on the edge segment to keep
    /// areas unchanged (the caller projects the click onto the edge).
    ///
    /// This is what lets the user add a node where there wasn't a corner, so a
    /// partition can start/end mid-wall rather than only at existing vertices.
    pub fn split_edge(&mut self, edge: HalfEdgeId, x: f64, y: f64) -> Result<VertexId, EditError> {
        let h = edge;
        if h.0 as usize >= self.half_edges.len() || !self.half_edges[h.0 as usize].alive {
            return Err(EditError::StaleHandle);
        }
        let t = self.half_edges[h.0 as usize].twin;
        let f1 = self.half_edges[h.0 as usize].face;
        let f2 = self.half_edges[t.0 as usize].face;
        let h_next = self.half_edges[h.0 as usize].next;
        let t_next = self.half_edges[t.0 as usize].next;
        let h_src = self.half_edges[h.0 as usize].source_element;
        let t_src = self.half_edges[t.0 as usize].source_element;

        let n = VertexId(self.vertices.len() as u32);
        let e1 = HalfEdgeId(self.half_edges.len() as u32); // N → B (was dest of h)
        let e2 = HalfEdgeId(self.half_edges.len() as u32 + 1); // N → A (was dest of t)

        self.vertices.push(Vertex { pos: [x, y], outgoing: Some(e1), alive: true });
        self.half_edges.push(HalfEdge {
            origin: n, twin: t, next: h_next, prev: h, face: f1, source_element: h_src, alive: true,
        });
        self.half_edges.push(HalfEdge {
            origin: n, twin: h, next: t_next, prev: t, face: f2, source_element: t_src, alive: true,
        });

        // h becomes A→N; t becomes B→N. Their twins/nexts re-point through N.
        self.half_edges[h.0 as usize].twin = e2;
        self.half_edges[h.0 as usize].next = e1;
        self.half_edges[t.0 as usize].twin = e1;
        self.half_edges[t.0 as usize].next = e2;
        self.half_edges[h_next.0 as usize].prev = e1;
        self.half_edges[t_next.0 as usize].prev = e2;

        Ok(n)
    }

    /// Remove the shared edge `edge` (and its twin), unioning the two rooms
    /// it separated into one. Returns a patch for the surviving face.
    pub fn merge_faces(&mut self, edge: HalfEdgeId) -> Result<Vec<FacePatch>, EditError> {
        let h = edge;
        if h.0 as usize >= self.half_edges.len() || !self.half_edges[h.0 as usize].alive {
            return Err(EditError::StaleHandle);
        }
        let t = self.half_edges[h.0 as usize].twin;
        let f_keep = self.half_edges[h.0 as usize].face;
        let f_drop = self.half_edges[t.0 as usize].face;
        if self.faces[f_keep.0 as usize].is_outer || self.faces[f_drop.0 as usize].is_outer {
            return Err(EditError::BordersExterior);
        }
        if f_keep == f_drop {
            return Err(EditError::BridgeEdge);
        }

        let (hn, hp) = {
            let he = &self.half_edges[h.0 as usize];
            (he.next, he.prev)
        };
        let (tn, tp) = {
            let te = &self.half_edges[t.0 as usize];
            (te.next, te.prev)
        };

        // Bypass both half-edges of the shared wall.
        self.half_edges[hp.0 as usize].next = tn;
        self.half_edges[tn.0 as usize].prev = hp;
        self.half_edges[tp.0 as usize].next = hn;
        self.half_edges[hn.0 as usize].prev = tp;

        // Re-home f_drop's loop onto f_keep, then tombstone f_drop + the edge.
        self.faces[f_keep.0 as usize].half_edge = Some(hp);
        let merged_cycle: Vec<HalfEdgeId> = self.face_half_edges(f_keep).collect();
        for he in merged_cycle {
            self.half_edges[he.0 as usize].face = f_keep;
        }
        self.faces[f_drop.0 as usize].alive = false;
        self.faces[f_drop.0 as usize].half_edge = None;

        // Detach the removed half-edges from their endpoints' outgoing slot,
        // tombstone them, and drop any vertex left isolated.
        for (he, origin) in [(h, self.half_edges[h.0 as usize].origin), (t, self.half_edges[t.0 as usize].origin)] {
            self.half_edges[he.0 as usize].alive = false;
            self.repair_vertex_outgoing(origin, he);
        }

        Ok(vec![self.face_patch(f_keep)])
    }

    // ─────────────────────────── queries ────────────────────────────

    /// The face on the far side of `edge` — its twin's face. O(1). This is
    /// the "who's my neighbour across this wall" query the UX leans on.
    pub fn neighbor_across(&self, edge: HalfEdgeId) -> Option<FaceId> {
        let he = self.half_edges.get(edge.0 as usize)?;
        if !he.alive {
            return None;
        }
        Some(self.half_edges[he.twin.0 as usize].face)
    }

    /// Every live interior (room) face.
    pub fn rooms(&self) -> impl Iterator<Item = FaceId> + '_ {
        (0..self.faces.len())
            .map(|i| FaceId(i as u32))
            .filter(move |f| {
                let face = &self.faces[f.0 as usize];
                face.alive && !face.is_outer
            })
    }

    /// CCW outline of a face (no repeated closing vertex).
    pub fn face_outline(&self, face: FaceId) -> Vec<[f64; 2]> {
        self.face_half_edges(face)
            .map(|h| self.vertices[self.half_edges[h.0 as usize].origin.0 as usize].pos)
            .collect()
    }

    /// Absolute area of a face.
    pub fn face_area(&self, face: FaceId) -> f64 {
        self.signed_area_of_cycle(&self.face_half_edges(face).collect::<Vec<_>>()).abs()
    }

    /// The bounding half-edges of a face paired with the IFC element each
    /// came from — the raw material for `IfcRelSpaceBoundary` at bake.
    pub fn bounding_elements(&self, face: FaceId) -> Vec<(HalfEdgeId, Option<u32>)> {
        self.face_half_edges(face)
            .map(|h| (h, self.half_edges[h.0 as usize].source_element))
            .collect()
    }

    /// Set the floor / ceiling planes of a face (the vertical dimension that
    /// turns a 2D face into a prismatic space at bake).
    pub fn set_face_height(&mut self, face: FaceId, floor_z: f64, ceiling_z: f64, non_planar_ceiling: bool) {
        if let Some(f) = self.faces.get_mut(face.0 as usize) {
            f.floor_z = floor_z;
            f.ceiling_z = ceiling_z;
            f.non_planar_ceiling = non_planar_ceiling;
        }
    }

    pub fn vertex_position(&self, v: VertexId) -> Option<[f64; 2]> {
        self.vertices.get(v.0 as usize).filter(|v| v.alive).map(|v| v.pos)
    }

    /// Count of live rooms — handy for tests / UI summaries.
    pub fn room_count(&self) -> usize {
        self.rooms().count()
    }

    /// Nearest live vertex to `(x, y)` within `tol`, for hit-testing a drag
    /// target from the TS side. Linear scan — a floor plate is small.
    pub fn find_vertex_near(&self, x: f64, y: f64, tol: f64) -> Option<VertexId> {
        let tol2 = tol * tol;
        let mut best: Option<(VertexId, f64)> = None;
        for (i, vtx) in self.vertices.iter().enumerate() {
            if !vtx.alive {
                continue;
            }
            let (dx, dy) = (vtx.pos[0] - x, vtx.pos[1] - y);
            let d2 = dx * dx + dy * dy;
            if d2 <= tol2 && best.map(|(_, b)| d2 < b).unwrap_or(true) {
                best = Some((VertexId(i as u32), d2));
            }
        }
        best.map(|(v, _)| v)
    }

    /// Snapshot every live room as a patch (outline + area + simple flag) —
    /// for bulk render or seeding a fresh TS mirror after a rebuild.
    pub fn room_patches(&self) -> Vec<FacePatch> {
        self.rooms().map(|f| self.face_patch(f)).collect()
    }

    // ─────────────────────────── internals ──────────────────────────

    fn dest(&self, h: HalfEdgeId) -> VertexId {
        self.half_edges[self.half_edges[h.0 as usize].twin.0 as usize].origin
    }

    /// Walk the face cycle starting at its anchor half-edge.
    fn face_half_edges(&self, face: FaceId) -> FaceWalk<'_> {
        // Tolerate an out-of-range or tombstoned face id (a stale handle from
        // JS) — yield an empty walk instead of panicking, so every public
        // query (`face_outline`/`face_area`/`bounding_elements`) is safe at the
        // wasm boundary.
        let start = self
            .faces
            .get(face.0 as usize)
            .filter(|f| f.alive)
            .and_then(|f| f.half_edge);
        FaceWalk { plate: self, start, cur: None }
    }

    /// Outgoing half-edges around a vertex (via twin/next), live only.
    fn outgoing_half_edges(&self, v: VertexId) -> impl Iterator<Item = HalfEdgeId> + '_ {
        let start = self.vertices[v.0 as usize].outgoing;
        VertexFan { plate: self, start, cur: None }
    }

    fn signed_area_of_cycle(&self, cycle: &[HalfEdgeId]) -> f64 {
        let mut acc = 0.0;
        for &h in cycle {
            let p = self.vertices[self.half_edges[h.0 as usize].origin.0 as usize].pos;
            let q = self.vertices[self.dest(h).0 as usize].pos;
            acc += p[0] * q[1] - q[0] * p[1];
        }
        acc * 0.5
    }

    fn check_face(&self, face: FaceId) -> Result<(), EditError> {
        match self.faces.get(face.0 as usize) {
            Some(f) if f.alive && !f.is_outer => Ok(()),
            Some(_) => Err(EditError::StaleHandle),
            None => Err(EditError::StaleHandle),
        }
    }

    /// After an edit, ensure `v.outgoing` doesn't point at the now-dead
    /// half-edge `dead`; pick any surviving outgoing edge, else isolate. We
    /// can't walk the vertex fan here (its start may be the dead edge), so we
    /// scan — merges are rare and this keeps the rotation system honest.
    fn repair_vertex_outgoing(&mut self, v: VertexId, dead: HalfEdgeId) {
        if self.vertices[v.0 as usize].outgoing != Some(dead) {
            return;
        }
        let replacement = (0..self.half_edges.len())
            .map(|i| HalfEdgeId(i as u32))
            .find(|h| {
                let he = &self.half_edges[h.0 as usize];
                he.alive && he.origin == v && *h != dead
            });
        self.vertices[v.0 as usize].outgoing = replacement;
        if replacement.is_none() {
            self.vertices[v.0 as usize].alive = false;
        }
    }

    fn face_patch(&self, face: FaceId) -> FacePatch {
        let outline = self.face_outline(face);
        let area = polygon_area(&outline).abs();
        let simple = is_simple_polygon(&outline);
        FacePatch { face, outline, area, simple }
    }
}

/// Iterator over the half-edges of one face cycle.
struct FaceWalk<'a> {
    plate: &'a SpacePlate,
    start: Option<HalfEdgeId>,
    cur: Option<HalfEdgeId>,
}

impl Iterator for FaceWalk<'_> {
    type Item = HalfEdgeId;
    fn next(&mut self) -> Option<HalfEdgeId> {
        let start = self.start?;
        let cur = match self.cur {
            None => start,
            Some(c) => {
                let n = self.plate.half_edges[c.0 as usize].next;
                if n == start {
                    return None;
                }
                n
            }
        };
        self.cur = Some(cur);
        Some(cur)
    }
}

/// Iterator over the outgoing half-edges around a vertex (twin → next).
struct VertexFan<'a> {
    plate: &'a SpacePlate,
    start: Option<HalfEdgeId>,
    cur: Option<HalfEdgeId>,
}

impl Iterator for VertexFan<'_> {
    type Item = HalfEdgeId;
    fn next(&mut self) -> Option<HalfEdgeId> {
        let start = self.start?;
        loop {
            let cur = match self.cur {
                None => start,
                Some(c) => {
                    // Around a vertex: twin (incoming) then its next (outgoing).
                    let twin = self.plate.half_edges[c.0 as usize].twin;
                    let n = self.plate.half_edges[twin.0 as usize].next;
                    if n == start {
                        return None;
                    }
                    n
                }
            };
            self.cur = Some(cur);
            if self.plate.half_edges[cur.0 as usize].alive {
                return Some(cur);
            }
            if cur == start {
                return None;
            }
        }
    }
}

// ───────────────────── arrangement (the 2D sweep) ─────────────────────

/// An undirected edge of the resolved arrangement.
struct ArrEdge {
    a: usize,
    b: usize,
    source: Option<u32>,
}

/// The planar arrangement: snapped vertices + split, deduped edges. This is
/// the faithful port of the `auto-space-detect.ts` geometry, with a
/// `source` tag threaded through every stage.
struct Arrangement {
    vertices: Vec<[f64; 2]>,
    edges: Vec<ArrEdge>,
}

impl Arrangement {
    fn resolve(segments: &[InputSegment], snap: f64) -> Arrangement {
        // Corner cleanup first: real wall centrelines miss each corner by ~half
        // a wall thickness (one overshoots, the neighbour undershoots), so a
        // plain endpoint snap closes them at a skewed position → trapezoids.
        // Pull each wall-end onto the true intersection of its line with the
        // nearest crossing wall so orthogonal walls form clean rectangles.
        let mut owned: Vec<InputSegment> = segments.to_vec();
        snap_corners(&mut owned, snap);
        let segments: &[InputSegment] = &owned;
        let cell = snap.max(EPS);
        let mut vertices: Vec<[f64; 2]> = Vec::new();
        let mut grid: FxHashMap<(i64, i64), Vec<usize>> = FxHashMap::default();
        let snap_sq = snap * snap;

        let mut lookup = |pt: [f64; 2], vertices: &mut Vec<[f64; 2]>| -> usize {
            let cx = (pt[0] / cell).floor() as i64;
            let cy = (pt[1] / cell).floor() as i64;
            for dx in -1..=1 {
                for dy in -1..=1 {
                    if let Some(bucket) = grid.get(&(cx + dx, cy + dy)) {
                        for &id in bucket {
                            let ddx = vertices[id][0] - pt[0];
                            let ddy = vertices[id][1] - pt[1];
                            if ddx * ddx + ddy * ddy <= snap_sq {
                                return id;
                            }
                        }
                    }
                }
            }
            let id = vertices.len();
            vertices.push(pt);
            grid.entry((cx, cy)).or_default().push(id);
            id
        };

        // 1. Snap endpoints. `Seg` carries its source through every stage.
        let mut segs: Vec<Seg> = Vec::with_capacity(segments.len());
        for s in segments {
            let ai = lookup(s.a, &mut vertices);
            let bi = lookup(s.b, &mut vertices);
            if ai != bi {
                segs.push(Seg { a: ai, b: bi, source: s.source_element });
            }
        }

        // 2. T-junction snap: a dangling endpoint that lands inside another
        // segment splits that host at the projection.
        let mut guard = 0usize;
        let limit = (segments.len() * 5).max(50);
        loop {
            let mut applied = false;
            let endpoints: Vec<usize> = {
                let mut s: Vec<usize> = segs.iter().flat_map(|s| [s.a, s.b]).collect();
                s.sort_unstable();
                s.dedup();
                s
            };
            'outer: for vid in endpoints {
                let p = vertices[vid];
                for si in 0..segs.len() {
                    let Seg { a, b, source } = segs[si];
                    if a == vid || b == vid {
                        continue;
                    }
                    if let Some((proj, t)) = closest_point_on_segment(p, vertices[a], vertices[b]) {
                        let ddx = proj[0] - p[0];
                        let ddy = proj[1] - p[1];
                        if ddx * ddx + ddy * ddy > snap_sq {
                            continue;
                        }
                        if !(1e-6..=1.0 - 1e-6).contains(&t) {
                            continue; // endpoint, not interior
                        }
                        segs[si] = Seg { a, b: vid, source };
                        segs.push(Seg { a: vid, b, source });
                        applied = true;
                        break 'outer;
                    }
                }
            }
            guard += 1;
            if !applied || guard >= limit {
                break;
            }
        }

        // 3. Interior crossings: collect every split position per seed segment
        // in one O(N²) pass (bbox-pruned), then cut each segment once.
        let seeds: Vec<Seg> = segs.clone();
        let mut splits: Vec<Vec<(f64, usize)>> =
            seeds.iter().map(|s| vec![(0.0, s.a), (1.0, s.b)]).collect();
        let bboxes: Vec<[f64; 4]> = seeds
            .iter()
            .map(|s| {
                let (pa, pb) = (vertices[s.a], vertices[s.b]);
                [pa[0].min(pb[0]), pa[1].min(pb[1]), pa[0].max(pb[0]), pa[1].max(pb[1])]
            })
            .collect();
        for i in 0..seeds.len() {
            for j in (i + 1)..seeds.len() {
                let (si, sj) = (&seeds[i], &seeds[j]);
                if si.a == sj.a || si.a == sj.b || si.b == sj.a || si.b == sj.b {
                    continue;
                }
                let (bi, bj) = (bboxes[i], bboxes[j]);
                if bi[2] < bj[0] || bj[2] < bi[0] || bi[3] < bj[1] || bj[3] < bi[1] {
                    continue;
                }
                if let Some((point, t, u)) = segment_intersection_param(
                    vertices[si.a], vertices[si.b], vertices[sj.a], vertices[sj.b],
                ) {
                    let nv = lookup(point, &mut vertices);
                    if nv != si.a && nv != si.b {
                        splits[i].push((t, nv));
                    }
                    if nv != sj.a && nv != sj.b {
                        splits[j].push((u, nv));
                    }
                }
            }
        }

        // 4. Emit split pieces, deduped on the undirected pair.
        let mut edges: Vec<ArrEdge> = Vec::new();
        let mut seen: std::collections::HashSet<(usize, usize)> = std::collections::HashSet::new();
        for (i, mut cuts) in splits.into_iter().enumerate() {
            let source = seeds[i].source;
            if cuts.len() <= 2 {
                push_edge(&mut edges, &mut seen, cuts[0].1, cuts[1].1, source);
                continue;
            }
            cuts.sort_by(|p, q| p.0.partial_cmp(&q.0).unwrap_or(std::cmp::Ordering::Equal));
            for w in cuts.windows(2) {
                push_edge(&mut edges, &mut seen, w[0].1, w[1].1, source);
            }
        }

        Arrangement { vertices, edges }
    }
}

#[derive(Clone, Copy)]
struct Seg {
    a: usize,
    b: usize,
    source: Option<u32>,
}

fn push_edge(
    edges: &mut Vec<ArrEdge>,
    seen: &mut std::collections::HashSet<(usize, usize)>,
    a: usize,
    b: usize,
    source: Option<u32>,
) {
    if a == b {
        return;
    }
    let key = if a < b { (a, b) } else { (b, a) };
    if seen.insert(key) {
        edges.push(ArrEdge { a, b, source });
    }
}

/// Intersection of the two infinite lines through `a1→b1` and `a2→b2`.
/// `None` when (near-)parallel.
fn line_intersection(a1: [f64; 2], b1: [f64; 2], a2: [f64; 2], b2: [f64; 2]) -> Option<[f64; 2]> {
    let d1 = [b1[0] - a1[0], b1[1] - a1[1]];
    let d2 = [b2[0] - a2[0], b2[1] - a2[1]];
    let denom = d1[0] * d2[1] - d1[1] * d2[0];
    if denom.abs() < EPS {
        return None;
    }
    let t = ((a2[0] - a1[0]) * d2[1] - (a2[1] - a1[1]) * d2[0]) / denom;
    Some([a1[0] + t * d1[0], a1[1] + t * d1[1]])
}

/// Pull each wall-end onto the true line-intersection with the nearest crossing
/// wall (within `tol`), so offset centrelines — whose ends miss the corner by
/// ~half a wall thickness — close into clean (e.g. rectangular) rooms instead
/// of trapezoids. Intersections are computed against the original geometry;
/// ends with no crossing wall within `tol` are left untouched (leaks stay
/// leaks). T-junctions fall out for free: an end near where its line crosses
/// another wall snaps onto that crossing.
fn snap_corners(segs: &mut [InputSegment], tol: f64) {
    let lines: Vec<([f64; 2], [f64; 2])> = segs.iter().map(|s| (s.a, s.b)).collect();
    let n = lines.len();
    let tol2 = tol * tol;
    for i in 0..n {
        for slot in 0..2 {
            let e = if slot == 0 { segs[i].a } else { segs[i].b };
            let mut best: Option<[f64; 2]> = None;
            let mut best_d2 = tol2;
            for j in 0..n {
                if i == j {
                    continue;
                }
                let Some(p) = line_intersection(lines[i].0, lines[i].1, lines[j].0, lines[j].1) else {
                    continue;
                };
                // The intersection must lie near segment j's FINITE extent, not
                // just its infinite line — else a distant aligned wall could
                // pull this end onto a phantom corner and fabricate a room.
                match closest_point_on_segment(p, lines[j].0, lines[j].1) {
                    Some((host, _)) if (host[0] - p[0]).powi(2) + (host[1] - p[1]).powi(2) <= tol2 => {}
                    _ => continue,
                }
                let d2 = (p[0] - e[0]).powi(2) + (p[1] - e[1]).powi(2);
                if d2 < best_d2 {
                    best_d2 = d2;
                    best = Some(p);
                }
            }
            if let Some(p) = best {
                if slot == 0 {
                    segs[i].a = p;
                } else {
                    segs[i].b = p;
                }
            }
        }
    }
}

fn closest_point_on_segment(q: [f64; 2], a: [f64; 2], b: [f64; 2]) -> Option<([f64; 2], f64)> {
    let dx = b[0] - a[0];
    let dy = b[1] - a[1];
    let len2 = dx * dx + dy * dy;
    if len2 < 1e-12 {
        return None;
    }
    let mut t = ((q[0] - a[0]) * dx + (q[1] - a[1]) * dy) / len2;
    t = t.clamp(0.0, 1.0);
    Some(([a[0] + t * dx, a[1] + t * dy], t))
}

/// Proper-crossing test. Returns the point + both parametric positions when
/// the segments cross strictly inside at least one of them (a shared endpoint
/// alone produces no new vertex). Naive f64 — see the robustness TODO.
fn segment_intersection_param(
    p1: [f64; 2], p2: [f64; 2], p3: [f64; 2], p4: [f64; 2],
) -> Option<([f64; 2], f64, f64)> {
    let denom = (p1[0] - p2[0]) * (p3[1] - p4[1]) - (p1[1] - p2[1]) * (p3[0] - p4[0]);
    if denom.abs() < EPS {
        return None;
    }
    let t = ((p1[0] - p3[0]) * (p3[1] - p4[1]) - (p1[1] - p3[1]) * (p3[0] - p4[0])) / denom;
    let u = -((p1[0] - p2[0]) * (p1[1] - p3[1]) - (p1[1] - p2[1]) * (p1[0] - p3[0])) / denom;
    let tol = 1e-7;
    if !(-tol..=1.0 + tol).contains(&t) || !(-tol..=1.0 + tol).contains(&u) {
        return None;
    }
    if (t < tol || t > 1.0 - tol) && (u < tol || u > 1.0 - tol) {
        return None;
    }
    Some(([p1[0] + t * (p2[0] - p1[0]), p1[1] + t * (p2[1] - p1[1])], t, u))
}

fn polygon_area(pts: &[[f64; 2]]) -> f64 {
    if pts.len() < 3 {
        return 0.0;
    }
    let mut acc = 0.0;
    for i in 0..pts.len() {
        let p = pts[i];
        let q = pts[(i + 1) % pts.len()];
        acc += p[0] * q[1] - q[0] * p[1];
    }
    acc * 0.5
}

/// Cheap self-intersection screen for edit feedback: O(n²) edge-pair test on a
/// single face boundary (faces are small). Not a robustness guarantee.
fn is_simple_polygon(pts: &[[f64; 2]]) -> bool {
    let n = pts.len();
    if n < 4 {
        return n == 3;
    }
    for i in 0..n {
        let a1 = pts[i];
        let a2 = pts[(i + 1) % n];
        for j in (i + 1)..n {
            // Skip shared-endpoint neighbours.
            if j == i || (i + 1) % n == j || (j + 1) % n == i {
                continue;
            }
            let b1 = pts[j];
            let b2 = pts[(j + 1) % n];
            if segment_intersection_param(a1, a2, b1, b2).is_some() {
                return false;
            }
        }
    }
    true
}

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

    /// Build a closed loop of `corners` (CCW), each edge tagged with one
    /// source element id (so we can assert provenance).
    fn loop_segments(corners: &[[f64; 2]], source_base: u32) -> Vec<InputSegment> {
        let n = corners.len();
        (0..n)
            .map(|i| InputSegment::new(corners[i], corners[(i + 1) % n], Some(source_base + i as u32)))
            .collect()
    }

    fn rect(x0: f64, y0: f64, x1: f64, y1: f64) -> Vec<[f64; 2]> {
        vec![[x0, y0], [x1, y0], [x1, y1], [x0, y1]]
    }

    #[test]
    fn single_room_area_matches() {
        let segs = loop_segments(&rect(0.0, 0.0, 4.0, 3.0), 100);
        let plate = SpacePlate::build(&segs, BuildOptions::default());
        assert_eq!(plate.room_count(), 1);
        let room = plate.rooms().next().unwrap();
        assert!((plate.face_area(room) - 12.0).abs() < 1e-6, "area {}", plate.face_area(room));
    }

    /// Two rooms sharing a central wall — the canonical "shared edge"
    /// fixture. A vertical wall at x=4 splits a 8×3 box into two 4×3 rooms.
    fn two_room_plate() -> SpacePlate {
        let segs = vec![
            // outer box
            InputSegment::new([0.0, 0.0], [8.0, 0.0], Some(1)),
            InputSegment::new([8.0, 0.0], [8.0, 3.0], Some(2)),
            InputSegment::new([8.0, 3.0], [0.0, 3.0], Some(3)),
            InputSegment::new([0.0, 3.0], [0.0, 0.0], Some(4)),
            // shared central wall
            InputSegment::new([4.0, 0.0], [4.0, 3.0], Some(99)),
        ];
        SpacePlate::build(&segs, BuildOptions::default())
    }

    #[test]
    fn shared_wall_yields_two_rooms() {
        let plate = two_room_plate();
        assert_eq!(plate.room_count(), 2, "central wall splits the box into two rooms");
        let total: f64 = plate.rooms().map(|f| plate.face_area(f)).sum();
        assert!((total - 24.0).abs() < 1e-6, "areas sum to the box: {total}");
    }

    #[test]
    fn shared_wall_is_one_edge_with_a_twin_in_the_neighbour() {
        let plate = two_room_plate();
        let rooms: Vec<FaceId> = plate.rooms().collect();
        // Find the half-edge of room[0] whose twin sits in a different room.
        let mut found = false;
        for h in plate.face_half_edges(rooms[0]) {
            if let Some(nbr) = plate.neighbor_across(h) {
                if nbr != rooms[0] && !plate.faces[nbr.0 as usize].is_outer {
                    found = true;
                    // The shared wall carries source element 99 on BOTH sides.
                    assert_eq!(plate.half_edges[h.0 as usize].source_element, Some(99));
                    let twin = plate.half_edges[h.0 as usize].twin;
                    assert_eq!(plate.half_edges[twin.0 as usize].source_element, Some(99));
                }
            }
        }
        assert!(found, "the two rooms must share exactly one wall edge via twin()");
    }

    #[test]
    fn drag_shared_vertex_updates_both_rooms_in_one_call() {
        let mut plate = two_room_plate();
        // The shared wall's bottom endpoint is the snapped vertex at (4,0).
        // Find it, then slide it to x=5 — room A grows, room B shrinks, both
        // returned by the single drag call.
        let v = (0..plate.vertices.len())
            .map(|i| VertexId(i as u32))
            .find(|v| {
                let p = plate.vertices[v.0 as usize].pos;
                (p[0] - 4.0).abs() < 1e-9 && (p[1] - 0.0).abs() < 1e-9
            })
            .expect("shared bottom vertex at (4,0)");
        let patches = plate.drag_vertex(v, 5.0, 0.0).expect("drag");
        assert_eq!(patches.len(), 2, "both incident rooms come back from one drag");
        let total: f64 = patches.iter().map(|p| p.area).sum();
        // Trapezoids, but the plate area is conserved.
        assert!((total - 24.0).abs() < 1e-6, "total area conserved under drag: {total}");
        assert!(patches.iter().all(|p| p.simple), "both faces stay simple");
        // The areas actually diverged from 12/12.
        let areas: Vec<f64> = patches.iter().map(|p| p.area).collect();
        assert!((areas[0] - areas[1]).abs() > 1.0, "drag must make the rooms unequal: {areas:?}");
    }

    #[test]
    fn split_then_areas_sum_to_parent() {
        let segs = loop_segments(&rect(0.0, 0.0, 4.0, 4.0), 200);
        let mut plate = SpacePlate::build(&segs, BuildOptions::default());
        let room = plate.rooms().next().unwrap();
        let before = plate.face_area(room);
        // Split corner (0,0)→(4,4) diagonal.
        let v00 = plate.find_vertex([0.0, 0.0]);
        let v44 = plate.find_vertex([4.0, 4.0]);
        let patches = plate.split_face(room, v00, v44, None).expect("split");
        assert_eq!(patches.len(), 2);
        assert_eq!(plate.room_count(), 2, "one room became two");
        let after: f64 = patches.iter().map(|p| p.area).sum();
        assert!((after - before).abs() < 1e-6, "split conserves area: {before} vs {after}");
        // Each child is ~half the 16 m² square = 8 m².
        for p in &patches {
            assert!((p.area - 8.0).abs() < 1e-6, "each half is 8 m²: {}", p.area);
            assert!(p.simple);
        }
    }

    #[test]
    fn split_rejects_degenerate_cuts() {
        let segs = loop_segments(&rect(0.0, 0.0, 4.0, 4.0), 300);
        let mut plate = SpacePlate::build(&segs, BuildOptions::default());
        let room = plate.rooms().next().unwrap();
        let v00 = plate.find_vertex([0.0, 0.0]);
        let v40 = plate.find_vertex([4.0, 0.0]);
        // Adjacent corners → zero-area sliver. Must reject.
        assert_eq!(plate.split_face(room, v00, v40, None), Err(EditError::DegenerateCut));
        // Same vertex twice.
        assert_eq!(plate.split_face(room, v00, v00, None), Err(EditError::DegenerateCut));
    }

    #[test]
    fn split_edge_adds_a_shared_node_without_changing_area() {
        let mut plate = two_room_plate();
        let rooms: Vec<FaceId> = plate.rooms().collect();
        let areas_before: Vec<f64> = rooms.iter().map(|f| plate.face_area(*f)).collect();
        let verts_before = plate.face_outline(rooms[0]).len();

        // The shared interior wall edge of room 0, and its midpoint.
        let shared = plate
            .face_half_edges(rooms[0])
            .find(|h| {
                plate
                    .neighbor_across(*h)
                    .map(|n| n != rooms[0] && !plate.faces[n.0 as usize].is_outer)
                    .unwrap_or(false)
            })
            .expect("shared edge");
        let a = plate.vertices[plate.half_edges[shared.0 as usize].origin.0 as usize].pos;
        let bvid = plate.half_edges[plate.half_edges[shared.0 as usize].twin.0 as usize].origin;
        let b = plate.vertices[bvid.0 as usize].pos;
        let mid = [(a[0] + b[0]) / 2.0, (a[1] + b[1]) / 2.0];

        let n = plate.split_edge(shared, mid[0], mid[1]).expect("split_edge");
        assert_eq!(plate.vertex_position(n), Some(mid), "node sits at the requested point");
        assert_eq!(plate.room_count(), 2, "edge split creates no new face");
        let areas_after: Vec<f64> = rooms.iter().map(|f| plate.face_area(*f)).collect();
        for (a0, a1) in areas_before.iter().zip(&areas_after) {
            assert!((a0 - a1).abs() < 1e-6, "areas unchanged on-segment: {a0} vs {a1}");
        }
        assert_eq!(plate.face_outline(rooms[0]).len(), verts_before + 1, "room 0 gained the node");
        assert!(
            plate.face_outline(rooms[1]).iter().any(|p| (p[0] - mid[0]).abs() < 1e-9 && (p[1] - mid[1]).abs() < 1e-9),
            "the neighbour room shares the new node",
        );
    }

    #[test]
    fn merge_undoes_a_shared_wall() {
        let mut plate = two_room_plate();
        assert_eq!(plate.room_count(), 2);
        // Find the interior shared edge.
        let rooms: Vec<FaceId> = plate.rooms().collect();
        let shared = plate
            .face_half_edges(rooms[0])
            .find(|h| {
                plate
                    .neighbor_across(*h)
                    .map(|n| n != rooms[0] && !plate.faces[n.0 as usize].is_outer)
                    .unwrap_or(false)
            })
            .expect("shared edge");
        let patches = plate.merge_faces(shared).expect("merge");
        assert_eq!(patches.len(), 1, "merge returns the surviving room");
        assert_eq!(plate.room_count(), 1, "two rooms became one");
        assert!((patches[0].area - 24.0).abs() < 1e-6, "merged area = full box: {}", patches[0].area);
        assert!(patches[0].simple, "merged room is a clean rectangle");
    }

    #[test]
    fn merge_rejects_exterior_walls() {
        let mut plate = two_room_plate();
        let rooms: Vec<FaceId> = plate.rooms().collect();
        // An edge whose twin is the exterior.
        let exterior_edge = plate
            .face_half_edges(rooms[0])
            .find(|h| {
                plate
                    .neighbor_across(*h)
                    .map(|n| plate.faces[n.0 as usize].is_outer)
                    .unwrap_or(false)
            })
            .expect("an outer wall");
        assert_eq!(plate.merge_faces(exterior_edge), Err(EditError::BordersExterior));
    }

    #[test]
    fn offset_centrelines_close_into_a_clean_rectangle() {
        // An 8×8 room whose wall centrelines miss each corner by ~0.1 m (one
        // wall overshoots, the neighbour undershoots) — the real-world case
        // that produced trapezoids. Corner-snap must recover the exact
        // rectangle (area 64), not a skewed quad.
        let segs = vec![
            InputSegment::new([-0.1, 8.0], [8.1, 8.0], Some(1)), // top, overshoots both ends
            InputSegment::new([8.0, 7.9], [8.0, -0.1], Some(2)), // right, undershoots top
            InputSegment::new([8.1, 0.0], [-0.1, 0.0], Some(3)), // bottom
            InputSegment::new([0.0, 8.1], [0.0, 0.1], Some(4)),  // left, undershoots bottom
        ];
        let plate = SpacePlate::build(&segs, BuildOptions { snap_tolerance: 0.25, min_area: 0.5 });
        assert_eq!(plate.room_count(), 1, "the four offset walls close into one room");
        let room = plate.rooms().next().unwrap();
        assert!(
            (plate.face_area(room) - 64.0).abs() < 1e-6,
            "corners must snap to the line intersections → exact 8×8; got {}",
            plate.face_area(room),
        );
    }

    #[test]
    fn corner_snap_skips_distant_wall_extensions() {
        // A short wall's end (1.9, 0) sits near where a FAR wall's line (x=2,
        // y 3..8) would cross, but that wall is nowhere near — the corner-snap
        // must NOT pull the end onto the phantom (2, 0).
        let mut segs = vec![
            InputSegment::new([0.0, 0.0], [1.9, 0.0], None),
            InputSegment::new([2.0, 3.0], [2.0, 8.0], None),
        ];
        super::snap_corners(&mut segs, 0.25);
        let e = segs[0].b;
        assert!(
            (e[0] - 1.9).abs() < 1e-9 && e[1].abs() < 1e-9,
            "end must stay put (no phantom snap to 2,0): {e:?}",
        );
    }

    #[test]
    fn t_junction_closes_a_room_without_shared_corners() {
        // Two rooms where the central wall's axis ends ON the outer walls'
        // interiors (no shared corner vertex) — the messy-IFC case the TS
        // detector's T-junction pass exists for. Central wall runs y=0..3 at
        // x=4 but its endpoints are at (4, 0.02) and (4, 2.98), i.e. they
        // don't coincide with the box corners; T-junction snap must still
        // close two rooms.
        let segs = vec![
            InputSegment::new([0.0, 0.0], [8.0, 0.0], Some(1)),
            InputSegment::new([8.0, 0.0], [8.0, 3.0], Some(2)),
            InputSegment::new([8.0, 3.0], [0.0, 3.0], Some(3)),
            InputSegment::new([0.0, 3.0], [0.0, 0.0], Some(4)),
            InputSegment::new([4.0, 0.02], [4.0, 2.98], Some(99)),
        ];
        let plate = SpacePlate::build(&segs, BuildOptions { snap_tolerance: 0.1, min_area: 0.5 });
        assert_eq!(plate.room_count(), 2, "T-junction snap should still yield two rooms");
    }

    #[test]
    fn queries_tolerate_invalid_face_ids() {
        // A stale/out-of-range face id from JS must not panic the wasm module.
        let plate = two_room_plate();
        let bogus = FaceId(99_999);
        assert_eq!(plate.face_area(bogus), 0.0);
        assert!(plate.face_outline(bogus).is_empty());
        assert!(plate.bounding_elements(bogus).is_empty());
    }

    #[test]
    fn clone_is_independent_for_undo_snapshots() {
        // The wasm handle's `duplicate()` clones the plate for undo/redo.
        // Editing the clone must NOT touch the original (and vice-versa) —
        // that independence is what makes the undo stack correct.
        let plate = two_room_plate();
        let snapshot = plate.clone();

        let mut edited = plate;
        // Collapse a room by merging across the shared wall on the edited copy.
        let rooms: Vec<FaceId> = edited.rooms().collect();
        let shared = edited
            .face_half_edges(rooms[0])
            .find(|h| {
                edited
                    .neighbor_across(*h)
                    .map(|n| n != rooms[0] && !edited.faces[n.0 as usize].is_outer)
                    .unwrap_or(false)
            })
            .expect("shared edge");
        edited.merge_faces(shared).expect("merge");

        assert_eq!(edited.room_count(), 1, "edited copy merged to one room");
        assert_eq!(snapshot.room_count(), 2, "snapshot is untouched by the edit");
        let snap_total: f64 = snapshot.rooms().map(|f| snapshot.face_area(f)).sum();
        assert!((snap_total - 24.0).abs() < 1e-6, "snapshot geometry intact: {snap_total}");
    }

    #[test]
    fn provenance_distinguishes_walls_from_user_splits() {
        let segs = loop_segments(&rect(0.0, 0.0, 4.0, 4.0), 500);
        let mut plate = SpacePlate::build(&segs, BuildOptions::default());
        let room = plate.rooms().next().unwrap();
        // Every boundary edge of the derived room came from a real wall.
        for (_h, src) in plate.bounding_elements(room) {
            assert!(src.is_some(), "derived walls carry their source element");
        }
        // After a user split with source None, the new partition is unsourced.
        let v00 = plate.find_vertex([0.0, 0.0]);
        let v44 = plate.find_vertex([4.0, 4.0]);
        plate.split_face(room, v00, v44, None).expect("split");
        let child = plate.rooms().next().unwrap();
        let unsourced = plate
            .bounding_elements(child)
            .iter()
            .filter(|(_, s)| s.is_none())
            .count();
        assert!(unsourced >= 1, "the user-drawn partition has no source element");
    }

    // Test-only helper.
    impl SpacePlate {
        fn find_vertex(&self, pt: [f64; 2]) -> VertexId {
            (0..self.vertices.len())
                .map(|i| VertexId(i as u32))
                .find(|v| {
                    let p = self.vertices[v.0 as usize].pos;
                    self.vertices[v.0 as usize].alive
                        && (p[0] - pt[0]).abs() < 1e-6
                        && (p[1] - pt[1]).abs() < 1e-6
                })
                .unwrap_or_else(|| panic!("no live vertex near {pt:?}"))
        }
    }
}

// TODO(space-dcel, follow-ups for the real feature):
//  - Robust predicates: `segment_intersection_param` is naive f64. Share the
//    adaptive-orientation floor being built for the pure-Rust CSG kernel
//    (csg-predicate-floor worktree) so dense national-grid wall sets don't
//    accumulate snap error.
//  - Holes / nested faces: a CW cycle is treated as exterior, so a room
//    enclosing a courtyard is mishandled. Add containment nesting.
//  - Net vs gross area: faces are centreline. Net area = inset each bounding
//    edge by half its source wall's thickness at quantity time; thickness must
//    ride `InputSegment` (extend with a `half_thickness` field).
//  - Leak diagnostics: detect open half-edges (a boundary that fails to close)
//    and surface them as per-face repair markers (§2.4 of the RFC).
//  - WASM seam: wrap `SpacePlate` in a stateful handle on `IfcAPI` with
//    explicit create/free — long-lived handles share the dlmalloc-GC hazard
//    from the cache-load crash fix; do NOT rely on JS GC to drop it.