alice-physics 2.2.0

//! Triangle Mesh Collision
//!
//! Static triangle mesh collider with BVH acceleration.
//! Supports ray queries and closest-point queries against arbitrary meshes.

use crate::bvh::{BvhPrimitive, LinearBvh};
use crate::collider::{Contact, AABB};
use crate::math::{Fix128, Vec3Fix};
use crate::raycast::{Ray, RayHit};

#[cfg(not(feature = "std"))]
use alloc::vec::Vec;

/// A single triangle
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Triangle {
    /// First vertex
    pub v0: Vec3Fix,
    /// Second vertex
    pub v1: Vec3Fix,
    /// Third vertex
    pub v2: Vec3Fix,
}

impl Triangle {
    /// Create a new triangle from three vertices
    #[inline]
    #[must_use]
    pub const fn new(v0: Vec3Fix, v1: Vec3Fix, v2: Vec3Fix) -> Self {
        Self { v0, v1, v2 }
    }

    /// Compute face normal (not normalized)
    #[inline]
    #[must_use]
    pub fn normal(&self) -> Vec3Fix {
        let e1 = self.v1 - self.v0;
        let e2 = self.v2 - self.v0;
        e1.cross(e2)
    }

    /// Compute normalized face normal
    #[inline]
    #[must_use]
    pub fn unit_normal(&self) -> Vec3Fix {
        self.normal().normalize()
    }

    /// Compute AABB enclosing this triangle
    #[must_use]
    pub fn aabb(&self) -> AABB {
        let min = Vec3Fix::new(
            min3(self.v0.x, self.v1.x, self.v2.x),
            min3(self.v0.y, self.v1.y, self.v2.y),
            min3(self.v0.z, self.v1.z, self.v2.z),
        );
        let max = Vec3Fix::new(
            max3(self.v0.x, self.v1.x, self.v2.x),
            max3(self.v0.y, self.v1.y, self.v2.y),
            max3(self.v0.z, self.v1.z, self.v2.z),
        );
        AABB::new(min, max)
    }

    /// Closest point on triangle to a given point
    #[must_use]
    pub fn closest_point(&self, p: Vec3Fix) -> Vec3Fix {
        let ab = self.v1 - self.v0;
        let ac = self.v2 - self.v0;
        let ap = p - self.v0;

        let d1 = ab.dot(ap);
        let d2 = ac.dot(ap);
        if d1 <= Fix128::ZERO && d2 <= Fix128::ZERO {
            return self.v0; // Vertex region A
        }

        let bp = p - self.v1;
        let d3 = ab.dot(bp);
        let d4 = ac.dot(bp);
        if d3 >= Fix128::ZERO && d4 <= d3 {
            return self.v1; // Vertex region B
        }

        let vc = d1 * d4 - d3 * d2;
        if vc <= Fix128::ZERO && d1 >= Fix128::ZERO && d3 <= Fix128::ZERO {
            let v = d1 / (d1 - d3);
            return self.v0 + ab * v; // Edge AB
        }

        let cp = p - self.v2;
        let d5 = ab.dot(cp);
        let d6 = ac.dot(cp);
        if d6 >= Fix128::ZERO && d5 <= d6 {
            return self.v2; // Vertex region C
        }

        let vb = d5 * d2 - d1 * d6;
        if vb <= Fix128::ZERO && d2 >= Fix128::ZERO && d6 <= Fix128::ZERO {
            let w = d2 / (d2 - d6);
            return self.v0 + ac * w; // Edge AC
        }

        let va = d3 * d6 - d5 * d4;
        if va <= Fix128::ZERO {
            let d4_d3 = d4 - d3;
            let d5_d6 = d5 - d6;
            if d4_d3 >= Fix128::ZERO && d5_d6 >= Fix128::ZERO {
                let w = d4_d3 / (d4_d3 + d5_d6);
                return self.v1 + (self.v2 - self.v1) * w; // Edge BC
            }
        }

        // Inside triangle
        let denom = va + vb + vc;
        if denom.is_zero() {
            return self.v0;
        }
        let v = vb / denom;
        let w = vc / denom;
        self.v0 + ab * v + ac * w
    }
}

/// Triangle mesh with BVH acceleration
pub struct TriMesh {
    /// Triangles
    pub triangles: Vec<Triangle>,
    /// BVH for acceleration
    pub(crate) bvh: LinearBvh,
    /// Overall AABB
    pub bounds: AABB,
}

impl TriMesh {
    /// Build from vertices and triangle indices
    #[must_use]
    pub fn from_indexed(vertices: &[Vec3Fix], indices: &[u32]) -> Self {
        let mut triangles = Vec::with_capacity(indices.len() / 3);
        let mut bvh_prims = Vec::with_capacity(indices.len() / 3);

        for i in (0..indices.len()).step_by(3) {
            if i + 2 >= indices.len() {
                break;
            }
            let v0 = vertices[indices[i] as usize];
            let v1 = vertices[indices[i + 1] as usize];
            let v2 = vertices[indices[i + 2] as usize];

            let tri = Triangle::new(v0, v1, v2);
            let aabb = tri.aabb();
            let tri_idx = triangles.len() as u32;

            triangles.push(tri);
            bvh_prims.push(BvhPrimitive {
                aabb,
                index: tri_idx,
                morton: 0,
            });
        }

        let bvh = LinearBvh::build(bvh_prims);
        let bounds = bvh.bounds;

        Self {
            triangles,
            bvh,
            bounds,
        }
    }

    /// Build from raw triangles
    #[must_use]
    pub fn from_triangles(triangles: Vec<Triangle>) -> Self {
        let bvh_prims: Vec<BvhPrimitive> = triangles
            .iter()
            .enumerate()
            .map(|(i, tri)| BvhPrimitive {
                aabb: tri.aabb(),
                index: i as u32,
                morton: 0,
            })
            .collect();

        let bvh = LinearBvh::build(bvh_prims);
        let bounds = bvh.bounds;

        Self {
            triangles,
            bvh,
            bounds,
        }
    }

    /// Ray query: find closest triangle intersection
    #[must_use]
    pub fn raycast(&self, ray: &Ray, max_t: Fix128) -> Option<RayHit> {
        let ray_aabb = ray_to_aabb(ray, max_t);
        let candidates = self.bvh.query(&ray_aabb);

        let mut best: Option<RayHit> = None;
        let mut best_t = max_t;

        for tri_idx in candidates {
            let tri = &self.triangles[tri_idx as usize];
            if let Some(mut hit) = ray_triangle(ray, tri, best_t) {
                hit.body_index = tri_idx as usize;
                best_t = hit.t;
                best = Some(hit);
            }
        }

        best
    }

    /// Closest point on mesh to a given point
    ///
    /// An empty mesh (no triangles) has no closest point to report: returns
    /// the query point itself with index 0.
    #[must_use]
    pub fn closest_point(&self, point: Vec3Fix) -> (Vec3Fix, usize) {
        if self.triangles.is_empty() {
            return (point, 0);
        }

        // Grow a box around the point until the best candidate is no farther
        // than the box's half-size: every triangle nearer than that intersects
        // the box, so the answer is exact at any distance (a fixed 1000 box
        // found nothing past it and answered triangles[0].v0). Ties go to the
        // smallest triangle index, whatever order the BVH visits them in.
        let span =
            (self.bounds.max - self.bounds.min).length() + (point - self.bounds.min).length();
        let mut half = Fix128::ONE;
        loop {
            let query = AABB::new(
                point - Vec3Fix::new(half, half, half),
                point + Vec3Fix::new(half, half, half),
            );
            let mut best: Option<(Fix128, usize, Vec3Fix)> = None;
            for tri_idx in self.bvh.query(&query) {
                let idx = tri_idx as usize;
                let cp = self.triangles[idx].closest_point(point);
                let dist_sq = (cp - point).length_squared();
                let better = match best {
                    None => true,
                    Some((d, i, _)) => dist_sq < d || (dist_sq == d && idx < i),
                };
                if better {
                    best = Some((dist_sq, idx, cp));
                }
            }
            if let Some((dist_sq, idx, cp)) = best {
                if dist_sq <= half * half || half > span {
                    return (cp, idx);
                }
            }
            if half > span {
                // the box holds the whole mesh and still found nothing: an empty BVH
                return (self.triangles[0].v0, 0);
            }
            half = half.double();
        }
    }

    /// Sphere vs `TriMesh` collision: find deepest penetrating contact
    #[must_use]
    pub fn collide_sphere(&self, center: Vec3Fix, radius: Fix128) -> Option<Contact> {
        let query = AABB::new(
            center - Vec3Fix::new(radius, radius, radius),
            center + Vec3Fix::new(radius, radius, radius),
        );
        let mut deepest: Option<Contact> = None;
        let mut max_depth = Fix128::ZERO;

        // Candidates are visited in the same order `LinearBvh::query` would
        // return them, without collecting them into a `Vec` first.
        self.bvh.query_callback(&query, |tri_idx| {
            let tri = &self.triangles[tri_idx as usize];
            let cp = tri.closest_point(center);
            let delta = center - cp;
            let dist_sq = delta.length_squared();
            let r_sq = radius * radius;

            if dist_sq < r_sq {
                let dist = dist_sq.sqrt();
                let depth = radius - dist;

                if depth > max_depth {
                    max_depth = depth;
                    let normal = if dist.is_zero() {
                        tri.unit_normal()
                    } else {
                        delta / dist
                    };
                    deepest = Some(Contact {
                        depth,
                        normal,
                        point_a: center - normal * radius,
                        point_b: cp,
                    });
                }
            }
        });

        deepest
    }

    /// Capsule vs `TriMesh` collision: find deepest contact
    #[must_use]
    pub fn collide_capsule(&self, a: Vec3Fix, b: Vec3Fix, radius: Fix128) -> Option<Contact> {
        let cap_min = Vec3Fix::new(
            if a.x < b.x { a.x } else { b.x } - radius,
            if a.y < b.y { a.y } else { b.y } - radius,
            if a.z < b.z { a.z } else { b.z } - radius,
        );
        let cap_max = Vec3Fix::new(
            if a.x > b.x { a.x } else { b.x } + radius,
            if a.y > b.y { a.y } else { b.y } + radius,
            if a.z > b.z { a.z } else { b.z } + radius,
        );
        let query = AABB::new(cap_min, cap_max);
        let candidates = self.bvh.query(&query);

        let mut deepest: Option<Contact> = None;
        let mut max_depth = Fix128::ZERO;

        for tri_idx in candidates {
            let tri = &self.triangles[tri_idx as usize];
            // the exact closest pair between the capsule's segment and the triangle
            let (seg_cp, tri_cp) = closest_points_segment_triangle(a, b, tri);
            let delta = seg_cp - tri_cp;
            let dist_sq = delta.length_squared();
            let r_sq = radius * radius;

            if dist_sq < r_sq {
                let dist = dist_sq.sqrt();
                let depth = radius - dist;
                if depth > max_depth {
                    max_depth = depth;
                    let normal = if dist.is_zero() {
                        tri.unit_normal()
                    } else {
                        delta / dist
                    };
                    deepest = Some(Contact {
                        depth,
                        normal,
                        point_a: seg_cp - normal * radius,
                        point_b: tri_cp,
                    });
                }
            }
        }
        deepest
    }

    /// AABB (box) vs `TriMesh` collision: the deepest contact over the
    /// triangles that overlap the box.
    ///
    /// Each candidate triangle is tested against the box with the separating
    /// axis theorem on the 13 axes of a triangle-box pair (the 3 box faces,
    /// the triangle normal and the 9 cross products of box and triangle
    /// edges): a triangle overlaps the box exactly when no axis separates them.
    /// The contact of an overlapping triangle is taken along its face normal
    /// (not the axis of least overlap, which on a flat mesh can be an internal
    /// edge): `depth` is the box's overlap along the face normal, the smaller
    /// of the two ways out, and `normal` that way (from the mesh to the box).
    /// A box hanging over a convex edge (a step or a ledge) therefore overlaps
    /// the riser's face by more than its minimum translation and can be
    /// pushed sideways; `point_b` is the point of the triangle closest to the box's
    /// deepest point along `-normal` (on a tie in depth, the triangle whose
    /// point is nearest it), and `point_a = point_b - normal * depth`
    /// (the invariant every `Contact` of this module keeps).
    #[must_use]
    pub fn collide_aabb(&self, aabb: &AABB) -> Option<Contact> {
        let candidates = self.bvh.query(aabb);
        let center = (aabb.min + aabb.max) * Fix128::from_ratio(1, 2);
        let half = (aabb.max - aabb.min) * Fix128::from_ratio(1, 2);

        // the deepest contact; on equal depth, the triangle whose point is nearest
        // the box's support point (two coplanar triangles of a floor tie)
        let mut deepest: Option<(Contact, Fix128)> = None;
        for tri_idx in candidates {
            let tri = &self.triangles[tri_idx as usize];
            let Some((depth, normal)) = triangle_box_overlap(tri, center, half) else {
                continue;
            };
            if depth <= Fix128::ZERO {
                continue;
            }
            // the box's deepest point along -normal (the face / edge centre on the
            // axes the normal does not lean along), and the triangle point nearest it
            let lean = |n: Fix128, h: Fix128| {
                if n > Fix128::ZERO {
                    h
                } else if n < Fix128::ZERO {
                    -h
                } else {
                    Fix128::ZERO
                }
            };
            let support = center
                - Vec3Fix::new(
                    lean(normal.x, half.x),
                    lean(normal.y, half.y),
                    lean(normal.z, half.z),
                );
            let point_b = tri.closest_point(support);
            let gap = (support - point_b).length_squared();
            let better = match &deepest {
                None => true,
                Some((best, best_gap)) => {
                    depth > best.depth || (depth == best.depth && gap < *best_gap)
                }
            };
            if better {
                deepest = Some((
                    Contact {
                        depth,
                        normal,
                        point_a: point_b - normal * depth,
                        point_b,
                    },
                    gap,
                ));
            }
        }
        deepest.map(|(c, _)| c)
    }

    /// Number of triangles
    #[inline]
    #[must_use]
    pub fn triangle_count(&self) -> usize {
        self.triangles.len()
    }
}

/// Separating-axis test of a triangle against the box `center ± half`.
/// `None` when one of the 13 axes separates them (or the triangle has no
/// area); otherwise the overlap along the triangle's own normal and that unit
/// normal, signed to push the box away from the triangle. The contact uses the
/// face axis rather than the axis of least overlap: on a mesh the least-overlap
/// axis of one triangle is often an edge shared with its neighbour (an
/// internal edge), which would push the box sideways off a flat floor.
fn triangle_box_overlap(
    tri: &Triangle,
    center: Vec3Fix,
    half: Vec3Fix,
) -> Option<(Fix128, Vec3Fix)> {
    let v = [tri.v0 - center, tri.v1 - center, tri.v2 - center];
    let edges = [v[1] - v[0], v[2] - v[1], v[0] - v[2]];
    let face = edges[0].cross(edges[1]);
    let face_len = face.length();
    if face_len.is_zero() {
        return None;
    }
    // overlap of the box and the triangle along a unit axis: (push along +l, along -l)
    let pushes = |l: Vec3Fix| {
        let r = half.x * l.x.abs() + half.y * l.y.abs() + half.z * l.z.abs();
        let p = [v[0].dot(l), v[1].dot(l), v[2].dot(l)];
        let (lo, hi) = (p[0].min(p[1]).min(p[2]), p[0].max(p[1]).max(p[2]));
        if lo > r || hi < -r {
            None
        } else {
            Some((hi + r, r - lo))
        }
    };
    let axes_box = [Vec3Fix::UNIT_X, Vec3Fix::UNIT_Y, Vec3Fix::UNIT_Z];
    for b in axes_box {
        pushes(b)?;
        for e in edges {
            let axis = b.cross(e);
            let len = axis.length();
            if !len.is_zero() {
                pushes(axis / len)?;
            }
        }
    }
    let n = face / face_len;
    let (up, down) = pushes(n)?;
    Some(if up <= down { (up, n) } else { (down, -n) })
}

/// Moller-Trumbore parallel-check epsilon (2^-48), relative: the ray counts
/// as parallel to the triangle when `|det| < MT_EPSILON * |e1 × e2| * |d|`,
/// i.e. when the sine of the angle between the ray and the triangle's plane is
/// below it, for any shape of triangle and any length of `d` (`det = −d · (e1 ×
/// e2)`; a threshold on `|e1| |e2|` also scaled with the sine of the corner
/// angle at `v0`, and dropped slivers seen head-on when `v0` was their sharp
/// corner). It used to be 2^-24, which dropped real grazing hits: a sphere
/// resting on a mesh floor and cast at a slope of 2^-31 found nothing, and a
/// slope of 1e-8 ahead of it was skipped. A `det` of at most [`MT_DET_FLOOR`]
/// is rounding whatever the triangle and also counts as parallel. The size is
/// not free: for edges of about `1e-6` m and below the barycentric numerators
/// are quantised at `2^-64`, so a grazing ray can still miss.
const MT_EPSILON: Fix128 = Fix128 {
    hi: 0,
    lo: 0x0000_0000_0001_0000,
};

/// A `det` this small (8 steps of `2^-64`) is the rounding of the cross and dot
/// products, not a direction: the ray is treated as parallel.
const MT_DET_FLOOR: Fix128 = Fix128 { hi: 0, lo: 8 };

/// Ray-Triangle intersection (Moller-Trumbore algorithm)
#[must_use]
pub fn ray_triangle(ray: &Ray, tri: &Triangle, max_t: Fix128) -> Option<RayHit> {
    let e1 = tri.v1 - tri.v0;
    let e2 = tri.v2 - tri.v0;
    let h = ray.direction.cross(e2);
    let det = e1.dot(h);
    // relative to the triangle's area and the ray's length (an absolute
    // threshold made every triangle below about 2.4e-4 m on a side invisible)
    if det.abs() <= MT_DET_FLOOR
        || det.abs() < MT_EPSILON * e1.cross(e2).length() * ray.direction.length()
    {
        return None;
    }

    // Division-free: compare the barycentric and distance numerators with det
    // (sign folded into det > 0) and divide once, after t <= max_t is known.
    // 1/det does not fit Fix128 once |det| <= 2^-63 (a triangle 2^-20 m on a
    // side met at a grazing angle), and its truncation to 0 used to put every
    // such hit at t = 0, the ray origin
    let flip = det.is_negative();
    let signed = |x: Fix128| if flip { -x } else { x };
    let det = det.abs();
    let s = ray.origin - tri.v0;
    let u = signed(s.dot(h));
    if u < Fix128::ZERO || u > det {
        return None;
    }

    let q = s.cross(e1);
    let v = signed(ray.direction.dot(q));
    if v < Fix128::ZERO || u + v > det {
        return None;
    }

    let t_num = signed(e2.dot(q));
    // t_num <= max_t * det; when the product overflows it exceeds every t_num,
    // and t_num / det < max_t still fits
    let within = match max_t.checked_mul(det) {
        Some(limit) => t_num <= limit,
        None => true,
    };
    if t_num < Fix128::ZERO || !within {
        return None;
    }
    let t = t_num / det;

    if t >= Fix128::ZERO && t <= max_t {
        let point = ray.at(t);
        let normal = tri.unit_normal();
        // Ensure normal faces the ray
        let normal = if normal.dot(ray.direction) > Fix128::ZERO {
            -normal
        } else {
            normal
        };
        Some(RayHit {
            t,
            point,
            normal,
            body_index: 0,
        })
    } else {
        None
    }
}

/// Create an AABB enclosing a ray segment
fn ray_to_aabb(ray: &Ray, max_t: Fix128) -> AABB {
    let end = ray.at(max_t);
    let one = Fix128::ONE;
    AABB::new(
        Vec3Fix::new(
            if ray.origin.x < end.x {
                ray.origin.x - one
            } else {
                end.x - one
            },
            if ray.origin.y < end.y {
                ray.origin.y - one
            } else {
                end.y - one
            },
            if ray.origin.z < end.z {
                ray.origin.z - one
            } else {
                end.z - one
            },
        ),
        Vec3Fix::new(
            if ray.origin.x > end.x {
                ray.origin.x + one
            } else {
                end.x + one
            },
            if ray.origin.y > end.y {
                ray.origin.y + one
            } else {
                end.y + one
            },
            if ray.origin.z > end.z {
                ray.origin.z + one
            } else {
                end.z + one
            },
        ),
    )
}

/// Closest pair `(point on segment, point on triangle)` between the segment
/// `seg_a`–`seg_b` and `tri`, exact: zero distance where the segment crosses
/// the triangle, otherwise the nearest of the two endpoints against the
/// triangle and the segment against each of the three edges (the distance
/// between a segment and a triangle that do not cross is attained at one of
/// these). Ties keep the first candidate in that order.
fn closest_points_segment_triangle(
    seg_a: Vec3Fix,
    seg_b: Vec3Fix,
    tri: &Triangle,
) -> (Vec3Fix, Vec3Fix) {
    if let Some(p) = segment_crosses_triangle(seg_a, seg_b, tri) {
        return (p, p);
    }
    let mut best = (seg_a, tri.closest_point(seg_a));
    let mut best_d = (best.0 - best.1).length_squared();
    let mut consider = |pair: (Vec3Fix, Vec3Fix)| {
        let d = (pair.0 - pair.1).length_squared();
        if d < best_d {
            best_d = d;
            best = pair;
        }
    };
    consider((seg_b, tri.closest_point(seg_b)));
    for (e0, e1) in [(tri.v0, tri.v1), (tri.v1, tri.v2), (tri.v2, tri.v0)] {
        consider(closest_points_segment_segment(seg_a, seg_b, e0, e1));
    }
    best
}

/// The point where the segment `a`–`b` crosses the triangle, if it does
/// (Moller-Trumbore with the unnormalised direction, parameter in `[0, 1]`).
fn segment_crosses_triangle(a: Vec3Fix, b: Vec3Fix, tri: &Triangle) -> Option<Vec3Fix> {
    let d = b - a;
    let e1 = tri.v1 - tri.v0;
    let e2 = tri.v2 - tri.v0;
    let h = d.cross(e2);
    let det = e1.dot(h);
    if det.is_zero() {
        return None;
    }
    let inv = Fix128::ONE / det;
    let s = a - tri.v0;
    let u = s.dot(h) * inv;
    if u < Fix128::ZERO || u > Fix128::ONE {
        return None;
    }
    let q = s.cross(e1);
    let v = d.dot(q) * inv;
    if v < Fix128::ZERO || u + v > Fix128::ONE {
        return None;
    }
    let t = e2.dot(q) * inv;
    if t < Fix128::ZERO || t > Fix128::ONE {
        return None;
    }
    Some(a + d * t)
}

/// Closest points between the segments `p1`–`q1` and `p2`–`q2` (Ericson,
/// Real-Time Collision Detection 5.1.9), each parameter clamped to `[0, 1]`.
fn closest_points_segment_segment(
    p1: Vec3Fix,
    q1: Vec3Fix,
    p2: Vec3Fix,
    q2: Vec3Fix,
) -> (Vec3Fix, Vec3Fix) {
    let clamp01 = |x: Fix128| x.max(Fix128::ZERO).min(Fix128::ONE);
    let d1 = q1 - p1;
    let d2 = q2 - p2;
    let r = p1 - p2;
    let a = d1.dot(d1);
    let e = d2.dot(d2);
    let f = d2.dot(r);
    let (s, t) = if a.is_zero() && e.is_zero() {
        (Fix128::ZERO, Fix128::ZERO)
    } else if a.is_zero() {
        (Fix128::ZERO, clamp01(f / e))
    } else {
        let c = d1.dot(r);
        if e.is_zero() {
            (clamp01(-c / a), Fix128::ZERO)
        } else {
            let b = d1.dot(d2);
            let denom = a * e - b * b;
            let mut s = if denom.is_zero() {
                Fix128::ZERO
            } else {
                clamp01((b * f - c * e) / denom)
            };
            let mut t = (b * s + f) / e;
            if t < Fix128::ZERO {
                t = Fix128::ZERO;
                s = clamp01(-c / a);
            } else if t > Fix128::ONE {
                t = Fix128::ONE;
                s = clamp01((b - c) / a);
            }
            (s, t)
        }
    };
    (p1 + d1 * s, p2 + d2 * t)
}

#[inline]
fn min3(a: Fix128, b: Fix128, c: Fix128) -> Fix128 {
    let ab = if a < b { a } else { b };
    if ab < c {
        ab
    } else {
        c
    }
}

#[inline]
fn max3(a: Fix128, b: Fix128, c: Fix128) -> Fix128 {
    let ab = if a > b { a } else { b };
    if ab > c {
        ab
    } else {
        c
    }
}

#[cfg(all(test, feature = "std"))]
mod tests {
    use super::*;

    /// A ray lying in the plane of a triangle 1e-9 m across is parallel to it;
    /// the rounding of `det` (a few steps of `2^-64`) must not read as a
    /// direction. Without [`MT_DET_FLOOR`] 1 in 6 of such rays was a hit.
    #[test]
    fn a_ray_in_the_plane_of_a_tiny_triangle_is_parallel() {
        let mut seed: u64 = 0x9E37_79B9_7F4A_7C15;
        let mut rnd = || {
            seed ^= seed << 13;
            seed ^= seed >> 7;
            seed ^= seed << 17;
            Fix128::from_raw(0, seed) - Fix128::from_ratio(1, 2)
        };
        let tiny = Fix128::from_f64(1e-9);
        let mut hits = 0;
        for _ in 0..2000 {
            let v0 = Vec3Fix::new(rnd(), rnd(), rnd());
            let a = Vec3Fix::new(rnd() * tiny, rnd() * tiny, rnd() * tiny);
            let b = Vec3Fix::new(rnd() * tiny, rnd() * tiny, rnd() * tiny);
            let tri = Triangle {
                v0,
                v1: v0 + a,
                v2: v0 + b,
            };
            let direction = (a + b * Fix128::from_ratio(3, 10)).normalize();
            let ray = Ray {
                origin: v0 + (a + b) * Fix128::from_ratio(1, 4) - direction,
                direction,
            };
            if ray_triangle(&ray, &tri, Fix128::from_int(10)).is_some() {
                hits += 1;
            }
        }
        assert_eq!(
            hits, 0,
            "{hits} of 2000 rays in a tiny triangle's plane hit it"
        );
    }

    /// Whether a grazing ray counts as parallel depends on the angle alone: the
    /// same ray with its direction scaled by 4 or 1/4 (as a `Ray` built without
    /// `Ray::new` may carry) gets the same answer on both sides of `2^-48`.
    #[test]
    fn the_parallel_threshold_does_not_depend_on_the_direction_length() {
        let tri = Triangle {
            v0: Vec3Fix::ZERO,
            v1: Vec3Fix::UNIT_X,
            v2: Vec3Fix::UNIT_Y,
        };
        let target = Vec3Fix::new(
            Fix128::from_ratio(1, 4),
            Fix128::from_ratio(1, 4),
            Fix128::ZERO,
        );
        let hits = |sine: Fix128, scale: Fix128| {
            let direction = Vec3Fix::new(Fix128::ONE, Fix128::ZERO, sine);
            let ray = Ray {
                origin: target - direction,
                direction: direction * scale,
            };
            ray_triangle(&ray, &tri, Fix128::from_int(100)).is_some()
        };
        let below = Fix128::ONE / Fix128::from_int(1i64 << 49);
        let above = Fix128::ONE / Fix128::from_int(1i64 << 47);
        for scale in [Fix128::ONE, Fix128::from_int(4), Fix128::from_ratio(1, 4)] {
            assert!(
                !hits(below, scale),
                "sine 2^-25 at direction scale {scale:?} hit"
            );
            assert!(
                hits(above, scale),
                "sine 2^-23 at direction scale {scale:?} missed"
            );
        }
    }

    #[test]
    fn segment_segment_closest_points_clamp_both_ends() {
        let v = |x: i64, y: i64, z: i64| Vec3Fix::from_int(x, y, z);
        // the lines meet at (4, 0, 0), past the second segment's end: t clamps to
        // 1 at (3, 1, 0) and s is re-solved for that point, (3, 0, 0)
        let (a, b) = closest_points_segment_segment(v(0, 0, 0), v(4, 0, 0), v(1, 3, 0), v(3, 1, 0));
        assert_eq!((a, b), (v(3, 0, 0), v(3, 1, 0)));
        // crossing skew segments: the common perpendicular, (1, 0, 0) and (1, 0, 2)
        let (a, b) =
            closest_points_segment_segment(v(0, 0, 0), v(2, 0, 0), v(1, -1, 2), v(1, 1, 2));
        assert_eq!((a, b), (v(1, 0, 0), v(1, 0, 2)));
        // the first segment ends before the foot (s clamps to 1), then t follows
        let (a, b) =
            closest_points_segment_segment(v(0, 0, 0), v(1, 0, 0), v(3, -1, 0), v(3, 1, 0));
        assert_eq!((a, b), (v(1, 0, 0), v(3, 0, 0)));
    }

    fn make_ground_mesh() -> TriMesh {
        // Simple ground plane as two triangles
        let vertices = vec![
            Vec3Fix::from_int(-10, 0, -10),
            Vec3Fix::from_int(10, 0, -10),
            Vec3Fix::from_int(10, 0, 10),
            Vec3Fix::from_int(-10, 0, 10),
        ];
        let indices = vec![0, 1, 2, 0, 2, 3];
        TriMesh::from_indexed(&vertices, &indices)
    }

    #[test]
    fn test_build_trimesh() {
        let mesh = make_ground_mesh();
        assert_eq!(mesh.triangle_count(), 2);
    }

    #[test]
    fn test_ray_triangle_hit() {
        let tri = Triangle::new(
            Vec3Fix::from_int(-1, 0, -1),
            Vec3Fix::from_int(1, 0, -1),
            Vec3Fix::from_int(0, 0, 1),
        );
        let ray = Ray::new(
            Vec3Fix::from_int(0, 5, 0),
            Vec3Fix::new(Fix128::ZERO, Fix128::NEG_ONE, Fix128::ZERO),
        );

        let hit = ray_triangle(&ray, &tri, Fix128::from_int(100));
        assert!(hit.is_some(), "Ray should hit triangle");
        let t = hit.unwrap().t;
        let error = (t - Fix128::from_int(5)).abs();
        assert!(error < Fix128::ONE, "t should be ~5");
    }

    #[test]
    fn test_ray_triangle_miss() {
        let tri = Triangle::new(
            Vec3Fix::from_int(-1, 0, -1),
            Vec3Fix::from_int(1, 0, -1),
            Vec3Fix::from_int(0, 0, 1),
        );
        let ray = Ray::new(
            Vec3Fix::from_int(5, 5, 0),
            Vec3Fix::new(Fix128::ZERO, Fix128::NEG_ONE, Fix128::ZERO),
        );

        assert!(ray_triangle(&ray, &tri, Fix128::from_int(100)).is_none());
    }

    #[test]
    fn test_mesh_raycast() {
        let mesh = make_ground_mesh();
        let ray = Ray::new(
            Vec3Fix::from_int(0, 10, 0),
            Vec3Fix::new(Fix128::ZERO, Fix128::NEG_ONE, Fix128::ZERO),
        );

        let hit = mesh.raycast(&ray, Fix128::from_int(100));
        assert!(hit.is_some(), "Should hit ground mesh");
    }

    #[test]
    fn test_sphere_trimesh_collision() {
        let mesh = make_ground_mesh();
        // Sphere overlapping with ground plane
        let contact = mesh.collide_sphere(
            Vec3Fix::from_int(0, 0, 0), // Center at ground level
            Fix128::ONE,                // Radius 1 => penetrating
        );
        assert!(contact.is_some(), "Sphere should collide with ground mesh");
    }

    #[test]
    fn test_closest_point_on_triangle() {
        let tri = Triangle::new(
            Vec3Fix::from_int(0, 0, 0),
            Vec3Fix::from_int(10, 0, 0),
            Vec3Fix::from_int(0, 0, 10),
        );

        // Point above triangle center
        let cp = tri.closest_point(Vec3Fix::from_int(3, 5, 3));
        assert_eq!(
            cp.y.hi, 0,
            "Closest point should be on triangle plane (y=0)"
        );
    }

    #[test]
    fn test_capsule_trimesh_collision() {
        let mesh = make_ground_mesh();
        // Capsule straddling the ground plane
        let contact = mesh.collide_capsule(
            Vec3Fix::from_int(0, -1, 0),
            Vec3Fix::from_int(0, 1, 0),
            Fix128::from_ratio(5, 10), // radius 0.5
        );
        assert!(contact.is_some(), "Capsule should collide with ground mesh");
    }

    #[test]
    fn test_aabb_trimesh_collision() {
        let mesh = make_ground_mesh();
        // AABB overlapping with ground plane
        let aabb = AABB::new(Vec3Fix::from_int(-1, -1, -1), Vec3Fix::from_int(1, 1, 1));
        let contact = mesh.collide_aabb(&aabb);
        assert!(contact.is_some(), "AABB should collide with ground mesh");
    }

    #[test]
    fn from_triangles_matches_from_indexed_and_covers_every_triangle() {
        // 地面 2 枚 + 離れた高台 1 枚 (BVH が全 primitive を包むことを確認)
        let tris = vec![
            Triangle::new(
                Vec3Fix::from_int(-10, 0, -10),
                Vec3Fix::from_int(10, 0, -10),
                Vec3Fix::from_int(10, 0, 10),
            ),
            Triangle::new(
                Vec3Fix::from_int(-10, 0, -10),
                Vec3Fix::from_int(10, 0, 10),
                Vec3Fix::from_int(-10, 0, 10),
            ),
            Triangle::new(
                Vec3Fix::from_int(99, 5, 99),
                Vec3Fix::from_int(101, 5, 99),
                Vec3Fix::from_int(100, 5, 101),
            ),
        ];
        let mesh = TriMesh::from_triangles(tris.clone());
        assert_eq!(mesh.triangle_count(), 3);
        assert_eq!(mesh.triangles, tris);
        // bounds = 全 triangle AABB の union (独立計算)
        let mut want = tris[0].aabb();
        for t in &tris[1..] {
            want = want.union(&t.aabb());
        }
        assert_eq!(mesh.bounds, want);
        assert_eq!(mesh.bounds.min, Vec3Fix::from_int(-10, 0, -10));
        assert_eq!(mesh.bounds.max, Vec3Fix::from_int(101, 5, 101));

        // 同じ geometry を from_indexed で組んだ mesh と query 結果が一致する
        let vertices = vec![
            Vec3Fix::from_int(-10, 0, -10),
            Vec3Fix::from_int(10, 0, -10),
            Vec3Fix::from_int(10, 0, 10),
            Vec3Fix::from_int(-10, 0, 10),
            Vec3Fix::from_int(99, 5, 99),
            Vec3Fix::from_int(101, 5, 99),
            Vec3Fix::from_int(100, 5, 101),
        ];
        let indexed = TriMesh::from_indexed(&vertices, &[0, 1, 2, 0, 2, 3, 4, 5, 6]);
        assert_eq!(indexed.triangles, mesh.triangles);
        assert_eq!(indexed.bounds, mesh.bounds);

        let down = Vec3Fix::new(Fix128::ZERO, Fix128::NEG_ONE, Fix128::ZERO);
        // (5, 5, -5) の真下は triangle 0、(-5, 5, 5) は triangle 1、(100, 10, 100) は triangle 2
        let cases = [
            (Vec3Fix::from_int(5, 5, -5), 0usize, 5i64),
            (Vec3Fix::from_int(-5, 5, 5), 1, 5),
            (Vec3Fix::from_int(100, 10, 100), 2, 5),
        ];
        for (origin, tri_idx, t_want) in cases {
            let ray = Ray::new(origin, down);
            let hit = mesh.raycast(&ray, Fix128::from_int(100));
            let hit_indexed = indexed.raycast(&ray, Fix128::from_int(100));
            match (hit, hit_indexed) {
                (Some(h), Some(hi)) => {
                    assert_eq!(h.body_index, tri_idx, "origin {origin:?}");
                    // Möller–Trumbore の除算で数 ulp 丸まる
                    let err = (h.t - Fix128::from_int(t_want)).abs();
                    assert!(
                        err < Fix128 { hi: 0, lo: 1 << 24 },
                        "origin {origin:?}: t {:?}",
                        h.t
                    );
                    assert_eq!(
                        (h.t, h.body_index, h.point),
                        (hi.t, hi.body_index, hi.point)
                    );
                }
                other => panic!("both meshes must hit for {origin:?}: {other:?}"),
            }
        }
        // 何もない場所は miss
        let miss = Ray::new(Vec3Fix::from_int(50, 5, 50), down);
        assert!(mesh.raycast(&miss, Fix128::from_int(100)).is_none());
        // closest_point も一致し、高台の真上では triangle 2 の頂点面 (y = 5) に落ちる
        let q = Vec3Fix::from_int(100, 8, 100);
        assert_eq!(mesh.closest_point(q), indexed.closest_point(q));
        let (cp, idx) = mesh.closest_point(q);
        assert_eq!(idx, 2);
        assert_eq!(cp, Vec3Fix::from_int(100, 5, 100));

        // 空 mesh: triangle 0、bounds は退化 AABB、raycast は None
        let empty = TriMesh::from_triangles(Vec::new());
        assert_eq!(empty.triangle_count(), 0);
        assert_eq!(empty.bounds, AABB::new(Vec3Fix::ZERO, Vec3Fix::ZERO));
        assert!(empty
            .raycast(
                &Ray::new(Vec3Fix::from_int(0, 5, 0), down),
                Fix128::from_int(100)
            )
            .is_none());
    }

    /// `collide_sphere` as it was written before it switched to
    /// `LinearBvh::query_callback`: collect the candidates with `query`, then
    /// walk them in that order. Kept here as the reference the allocation-free
    /// version must reproduce to the bit.
    fn collide_sphere_via_query(
        mesh: &TriMesh,
        center: Vec3Fix,
        radius: Fix128,
    ) -> Option<Contact> {
        let query = AABB::new(
            center - Vec3Fix::new(radius, radius, radius),
            center + Vec3Fix::new(radius, radius, radius),
        );
        let mut deepest: Option<Contact> = None;
        let mut max_depth = Fix128::ZERO;
        for tri_idx in mesh.bvh.query(&query) {
            let tri = &mesh.triangles[tri_idx as usize];
            let cp = tri.closest_point(center);
            let delta = center - cp;
            let dist_sq = delta.length_squared();
            if dist_sq < radius * radius {
                let dist = dist_sq.sqrt();
                let depth = radius - dist;
                if depth > max_depth {
                    max_depth = depth;
                    let normal = if dist.is_zero() {
                        tri.unit_normal()
                    } else {
                        delta / dist
                    };
                    deepest = Some(Contact {
                        depth,
                        normal,
                        point_a: center - normal * radius,
                        point_b: cp,
                    });
                }
            }
        }
        deepest
    }

    fn contacts_bit_equal(a: Option<Contact>, b: Option<Contact>) -> bool {
        match (a, b) {
            (None, None) => true,
            (Some(a), Some(b)) => {
                a.depth == b.depth
                    && a.normal == b.normal
                    && a.point_a == b.point_a
                    && a.point_b == b.point_b
            }
            _ => false,
        }
    }

    /// A bumpy 8x8 terrain (128 triangles, several BVH leaves) plus two
    /// horizontal triangles at y = +1/2 and y = -1/2 over the same footprint,
    /// so a sphere at the origin touches both at exactly the same depth and the
    /// winner is decided by visiting order alone.
    fn terrain_with_tied_pair() -> TriMesh {
        let n = 8i64;
        let mut vertices = Vec::new();
        for z in 0..=n {
            for x in 0..=n {
                let y = Fix128::from_ratio((x * 7 + z * 3) % 5, 8);
                vertices.push(Vec3Fix::new(
                    Fix128::from_int(x + 10),
                    y,
                    Fix128::from_int(z),
                ));
            }
        }
        let mut triangles = Vec::new();
        let w = (n + 1) as usize;
        for z in 0..n as usize {
            for x in 0..n as usize {
                let a = vertices[z * w + x];
                let b = vertices[z * w + x + 1];
                let c = vertices[(z + 1) * w + x];
                let d = vertices[(z + 1) * w + x + 1];
                triangles.push(Triangle::new(a, c, b));
                triangles.push(Triangle::new(b, c, d));
            }
        }
        let half = Fix128::from_ratio(1, 2);
        let two = Fix128::from_int(2);
        for y in [half, -half] {
            triangles.push(Triangle::new(
                Vec3Fix::new(-two, y, -two),
                Vec3Fix::new(-two, y, two),
                Vec3Fix::new(two, y, -two),
            ));
        }
        TriMesh::from_triangles(triangles)
    }

    #[test]
    fn collide_sphere_matches_the_collect_then_walk_reference_to_the_bit() {
        let mesh = terrain_with_tied_pair();
        // Every candidate order matters for the tie: both slabs are hit at
        // depth 1/4 by a sphere of radius 3/4 at the origin.
        let tie = mesh.collide_sphere(Vec3Fix::ZERO, Fix128::from_ratio(3, 4));
        let tie_ref = collide_sphere_via_query(&mesh, Vec3Fix::ZERO, Fix128::from_ratio(3, 4));
        assert!(tie.is_some());
        assert!(
            contacts_bit_equal(tie, tie_ref),
            "tie: {tie:?} vs {tie_ref:?}"
        );
        assert_eq!(tie.map(|c| c.depth), Some(Fix128::from_ratio(1, 4)));

        let mut hits = 0usize;
        for zi in -1..=18i64 {
            for xi in -6..=38i64 {
                for yi in [-1i64, 0, 1, 3] {
                    let center = Vec3Fix::new(
                        Fix128::from_ratio(xi, 2),
                        Fix128::from_ratio(yi, 4),
                        Fix128::from_ratio(zi, 2),
                    );
                    for radius in [Fix128::from_ratio(1, 4), Fix128::from_ratio(3, 4)] {
                        let got = mesh.collide_sphere(center, radius);
                        let want = collide_sphere_via_query(&mesh, center, radius);
                        assert!(
                            contacts_bit_equal(got, want),
                            "center {center:?} radius {radius:?}: {got:?} vs {want:?}"
                        );
                        hits += usize::from(got.is_some());
                    }
                }
            }
        }
        // The sweep must actually exercise contacts, not compare None to None.
        assert!(hits > 100, "only {hits} contacts in the sweep");
    }

    fn half() -> Fix128 {
        Fix128::from_ratio(1, 2)
    }

    fn quarter() -> Fix128 {
        Fix128::from_ratio(1, 4)
    }

    fn vq(x: Fix128, y: Fix128, z: Fix128) -> Vec3Fix {
        Vec3Fix::new(x, y, z)
    }

    /// within 2^-40 per component (the closest point on a triangle rounds in
    /// its last bits)
    fn assert_near(got: Vec3Fix, want: Vec3Fix) {
        let tol = Fix128 { hi: 0, lo: 1 << 24 };
        let d = got - want;
        assert!(
            d.x.abs() <= tol && d.y.abs() <= tol && d.z.abs() <= tol,
            "{got:?} vs {want:?}"
        );
    }

    #[test]
    fn trailing_indices_are_ignored_and_an_empty_mesh_answers_the_query_point() {
        let vertices = [
            Vec3Fix::from_int(0, 0, 0),
            Vec3Fix::from_int(1, 0, 0),
            Vec3Fix::from_int(0, 0, 1),
        ];
        let mesh = TriMesh::from_indexed(&vertices, &[0, 1, 2, 0, 1]);
        assert_eq!(mesh.triangle_count(), 1);
        assert_eq!(mesh.triangles[0].v2, Vec3Fix::from_int(0, 0, 1));

        let empty = TriMesh::from_indexed(&vertices, &[]);
        assert_eq!(empty.triangle_count(), 0);
        let p = Vec3Fix::from_int(3, -2, 7);
        assert_eq!(empty.closest_point(p), (p, 0));
    }

    #[test]
    fn capsule_parallel_above_the_floor_has_depth_radius_minus_gap() {
        // the segment runs along x a quarter above y = 0, parallel to two
        // floor edges: depth 1/2 - 1/4, normal +y, contact under the segment
        let mesh = make_ground_mesh();
        let c = mesh
            .collide_capsule(
                vq(Fix128::from_int(-1), quarter(), Fix128::ZERO),
                vq(Fix128::from_int(1), quarter(), Fix128::ZERO),
                half(),
            )
            .expect("a quarter gap under a half radius is a contact");
        assert_eq!(c.depth, quarter());
        assert_near(c.normal, Vec3Fix::UNIT_Y);
        assert_eq!(c.point_b.y, Fix128::ZERO);
        assert_eq!(c.point_a, c.point_b - c.normal * c.depth);
        // a gap equal to the radius is not a contact
        assert!(mesh
            .collide_capsule(
                vq(Fix128::from_int(-1), half(), Fix128::ZERO),
                vq(Fix128::from_int(1), half(), Fix128::ZERO),
                half(),
            )
            .is_none());
    }

    #[test]
    fn capsule_above_or_crossing_the_floor() {
        let mesh = make_ground_mesh();
        // vertical, bottom end a quarter above the floor: the plane lies
        // before the segment's start, depth 1/2 - 1/4 at (2, 0, -3)
        let c = mesh
            .collide_capsule(
                vq(Fix128::from_int(2), quarter(), Fix128::from_int(-3)),
                Vec3Fix::from_int(2, 3, -3),
                half(),
            )
            .unwrap();
        assert_eq!(c.depth, quarter());
        assert_near(c.normal, Vec3Fix::UNIT_Y);
        assert_near(c.point_b, Vec3Fix::from_int(2, 0, -3));

        // crossing the floor at (5, 0, -5) (inside one triangle, outside the
        // other): zero distance, depth = radius along the triangle's normal
        let c = mesh
            .collide_capsule(
                Vec3Fix::from_int(5, -1, -5),
                Vec3Fix::from_int(5, 1, -5),
                half(),
            )
            .unwrap();
        assert_eq!(c.depth, half());
        assert_near(c.point_b, Vec3Fix::from_int(5, 0, -5));
        assert_eq!(c.normal.x, Fix128::ZERO);
        assert_eq!(c.normal.y.abs(), Fix128::ONE);
        assert_eq!(c.normal.z, Fix128::ZERO);

        // a zero-length capsule is a sphere: depth radius - height
        let p = vq(Fix128::from_int(-3), quarter(), Fix128::from_int(4));
        let c = mesh.collide_capsule(p, p, half()).unwrap();
        assert_eq!(c.depth, quarter());
        assert_near(c.normal, Vec3Fix::UNIT_Y);
        assert_near(c.point_b, Vec3Fix::from_int(-3, 0, 4));
    }

    #[test]
    fn segment_segment_degenerate_and_parallel_cases() {
        let v = |x: i64, y: i64, z: i64| Vec3Fix::from_int(x, y, z);
        // both segments points
        assert_eq!(
            closest_points_segment_segment(v(1, 2, 3), v(1, 2, 3), v(4, 5, 6), v(4, 5, 6)),
            (v(1, 2, 3), v(4, 5, 6))
        );
        // first a point: its foot on the second, clamped to the end
        assert_eq!(
            closest_points_segment_segment(v(1, 5, 0), v(1, 5, 0), v(0, 0, 0), v(4, 0, 0)),
            (v(1, 5, 0), v(1, 0, 0))
        );
        assert_eq!(
            closest_points_segment_segment(v(9, 5, 0), v(9, 5, 0), v(0, 0, 0), v(4, 0, 0)),
            (v(9, 5, 0), v(4, 0, 0))
        );
        // second a point: its foot on the first
        assert_eq!(
            closest_points_segment_segment(v(0, 0, 0), v(4, 0, 0), v(3, 2, 0), v(3, 2, 0)),
            (v(3, 0, 0), v(3, 2, 0))
        );
        // parallel: s starts at 0, t = -1/2 clamps to 0 and s is re-solved
        // for the second segment's start (distance 1, the parallel gap)
        assert_eq!(
            closest_points_segment_segment(v(0, 0, 0), v(4, 0, 0), v(1, 1, 0), v(3, 1, 0)),
            (v(1, 0, 0), v(1, 1, 0))
        );
        // t below 0 clamps to the second segment's start and s is re-solved
        assert_eq!(
            closest_points_segment_segment(v(0, 0, 0), v(4, 0, 0), v(2, 1, 0), v(2, 3, 0)),
            (v(2, 0, 0), v(2, 1, 0))
        );
    }

    #[test]
    fn segment_crossing_rejects_parallel_and_outside_segments() {
        let tri = Triangle::new(
            Vec3Fix::from_int(0, 0, 0),
            Vec3Fix::from_int(2, 0, 0),
            Vec3Fix::from_int(0, 0, 2),
        );
        let v = |x: i64, y: i64, z: i64| Vec3Fix::from_int(x, y, z);
        // through the interior
        assert_eq!(
            segment_crosses_triangle(v(1, 1, 0), v(1, -1, 0), &tri),
            Some(v(1, 0, 0))
        );
        // parallel to the plane
        assert_eq!(segment_crosses_triangle(v(0, 1, 0), v(2, 1, 0), &tri), None);
        // through the plane beside the triangle on either barycentric side
        assert_eq!(
            segment_crosses_triangle(v(-1, 1, 1), v(-1, -1, 1), &tri),
            None
        );
        assert_eq!(
            segment_crosses_triangle(v(1, 1, -1), v(1, -1, -1), &tri),
            None
        );
        assert_eq!(
            segment_crosses_triangle(v(2, 1, 2), v(2, -1, 2), &tri),
            None
        );
        // short of the plane on either end
        assert_eq!(segment_crosses_triangle(v(1, 3, 0), v(1, 1, 0), &tri), None);
        assert_eq!(
            segment_crosses_triangle(v(1, -1, 0), v(1, -3, 0), &tri),
            None
        );
    }

    #[test]
    fn box_resting_mostly_above_the_floor_is_pushed_up() {
        // centre y = 1/2, half 1: the box reaches 1/2 below the floor
        let mesh = make_ground_mesh();
        let aabb = AABB::new(
            vq(Fix128::NEG_ONE, -half(), Fix128::NEG_ONE),
            vq(Fix128::ONE, Fix128::from_ratio(3, 2), Fix128::ONE),
        );
        let c = mesh.collide_aabb(&aabb).unwrap();
        assert_eq!(c.depth, half());
        assert_eq!(c.normal, Vec3Fix::UNIT_Y);
        assert_eq!(c.point_b, Vec3Fix::ZERO);
        assert_eq!(c.point_a, vq(Fix128::ZERO, -half(), Fix128::ZERO));
    }

    #[test]
    fn box_touching_separated_by_a_slanted_face_or_degenerate_has_no_contact() {
        // resting exactly on the floor: zero depth is not a contact
        let mesh = make_ground_mesh();
        let touching = AABB::new(Vec3Fix::from_int(-1, 0, -1), Vec3Fix::from_int(1, 2, 1));
        assert!(mesh.collide_aabb(&touching).is_none());

        // x + y + z = 2: the boxes overlap but the face normal separates them
        // (the triangle is 2/sqrt3 from the centre, the box reaches 1.5/sqrt3)
        let slanted = TriMesh::from_indexed(
            &[
                Vec3Fix::from_int(2, 0, 0),
                Vec3Fix::from_int(0, 2, 0),
                Vec3Fix::from_int(0, 0, 2),
            ],
            &[0, 1, 2],
        );
        let cube = AABB::new(vq(-half(), -half(), -half()), vq(half(), half(), half()));
        assert!(slanted.bounds.min.x < cube.max.x);
        assert!(slanted.collide_aabb(&cube).is_none());

        // a zero-area triangle through the box has no face to push along
        let sliver = TriMesh::from_indexed(
            &[
                Vec3Fix::from_int(-1, 0, 0),
                Vec3Fix::from_int(0, 0, 0),
                Vec3Fix::from_int(1, 0, 0),
            ],
            &[0, 1, 2],
        );
        assert!(sliver.collide_aabb(&cube).is_none());
    }

    #[test]
    fn ray_hit_with_a_max_t_whose_product_overflows() {
        // |det| = 400 for this floor triangle; max_t * 400 overflows Fix128
        let tri = Triangle::new(
            Vec3Fix::from_int(-10, 0, -10),
            Vec3Fix::from_int(10, 0, -10),
            Vec3Fix::from_int(10, 0, 10),
        );
        let ray = Ray::new(
            Vec3Fix::from_int(1, 5, -1),
            Vec3Fix::new(Fix128::ZERO, Fix128::NEG_ONE, Fix128::ZERO),
        );
        let max_t = Fix128::from_int(1_i64 << 60);
        assert!(max_t.checked_mul(Fix128::from_int(400)).is_none());
        let hit = ray_triangle(&ray, &tri, max_t).unwrap();
        assert_eq!(hit.t, Fix128::from_int(5));
        assert_eq!(hit.point, Vec3Fix::from_int(1, 0, -1));
        assert_eq!(hit.normal, Vec3Fix::UNIT_Y);
    }
}