bevy_symbios_shape 0.4.0

//! Procedural mesh generation for CGA shape terminals.
//!
//! Two entry points:
//!
//! - [`build_profiled_mesh`] dispatches on a terminal's [`FaceProfile`] and is
//!   what [`crate::spawner::SpawnShapeExt::spawn_shape`] calls when a terminal
//!   has no registered scene in [`crate::registry::ShapeRegistry`].
//! - [`build_tapered_cuboid`] is the lower-level 3-D builder used for the
//!   `Rectangle` and `Taper` profiles; the flat-face builders for
//!   `Triangle`, `Trapezoid` and `Polygon` are private but exercised through
//!   `build_profiled_mesh`.
//!
//! All meshes are emitted in *unit-mesh space* (`[-0.5, 0.5]` on each axis
//! with non-zero extents) and rely on the caller's `Transform::scale` —
//! provided by [`crate::transform::scope_to_transform`] — to stretch them to
//! the actual scope extents.

use bevy::asset::RenderAssetUsages;
use bevy::math::{DVec2, Vec2, Vec3};
use bevy::mesh::{Indices, Mesh, PrimitiveTopology, VertexAttributeValues};
use symbios_shape::FaceProfile;

/// Builds a tapered prism mesh from a unit cube that fits in a `(1, 1, 1)` bounding box.
///
/// The mesh is centered at the origin so that it can be placed via [`scope_to_transform`].
/// The caller's `Transform::scale` (from `scope_to_transform`) stretches the unit
/// geometry to fill the actual scope extents.
///
/// `taper` ∈ `[0.0, 1.0]`:
/// - `0.0`: box (all 4 top vertices at full width).
/// - `1.0`: pyramid (top 4 vertices collapse to a single apex).
/// - Intermediate: a frustum / tapered prism.
///
/// `size` is the world-space extent of the scope (`scope.size` cast to `f32`).
/// It is used **only** to scale UV coordinates so that one UV unit equals one world unit,
/// enabling texture tiling at a consistent texel density regardless of face dimensions.
///
/// Uses 24 vertices (4 per face) to allow per-face flat normals, with 36 indices.
///
/// ## UV layout
///
/// U always runs from the viewer's left edge (U=0) to the viewer's right edge (U=size),
/// where "viewer" means the camera looking at each face from outside.
///
/// | Face | U axis (viewer-left → viewer-right) | V axis |
/// |---|---|---|
/// | Bottom / Top | +X (0 … `size.x`) | +Z (0 … `size.z`) |
/// | Front (−Z) | +X→−X  (0 … `size.x`) | −Y (0=top … `size.y`=bottom) |
/// | Back (+Z) | −X→+X  (0 … `size.x`) | −Y (0=top … `size.y`=bottom) |
/// | Left (−X) | −Z→+Z  (0 … `size.z`) | −Y (0=top … `size.y`=bottom) |
/// | Right (+X) | +Z→−Z  (0 … `size.z`) | −Y (0=top … `size.y`=bottom) |
///
/// For tapered side faces the top-edge U range narrows proportionally to the taper factor.
///
/// [`scope_to_transform`]: crate::transform::scope_to_transform
pub fn build_tapered_cuboid(taper: f32, size: Vec3, stretch_uvs: bool) -> Mesh {
    let taper = taper.clamp(0.0, 1.0);

    // Unit box corners: X ∈ [-0.5, 0.5], Y ∈ [-0.5, 0.5], Z ∈ [-0.5, 0.5].
    // Taper shrinks the top four corners toward the Y-axis center (x=0, z=0).
    let shrink = taper * 0.5; // how much each top corner moves inward on X and Z

    // Bottom corners (y = -0.5), full width
    let b0 = Vec3::new(-0.5, -0.5, -0.5); // front-left
    let b1 = Vec3::new(0.5, -0.5, -0.5); // front-right
    let b2 = Vec3::new(0.5, -0.5, 0.5); // back-right
    let b3 = Vec3::new(-0.5, -0.5, 0.5); // back-left

    // Top corners (y = 0.5), tapered inward
    let t0 = Vec3::new(-0.5 + shrink, 0.5, -0.5 + shrink); // front-left
    let t1 = Vec3::new(0.5 - shrink, 0.5, -0.5 + shrink); // front-right
    let t2 = Vec3::new(0.5 - shrink, 0.5, 0.5 - shrink); // back-right
    let t3 = Vec3::new(-0.5 + shrink, 0.5, 0.5 - shrink); // back-left

    // Face normals (computed from face geometry, approximated as flat directions).
    // For a tapered prism the side normals tilt inward at the top; we compute them
    // from the actual face geometry using the cross product of two edge vectors.
    let normal_bottom = Vec3::NEG_Y;
    let normal_top = Vec3::Y;

    // Side face normals: computed from CCW vertex order (first triangle of each face).
    // face_normal(a, b, c, _) = (b-a) × (c-a), which points outward for CCW winding.
    let front_n = face_normal(b1, b0, t0, t1); // −Z (face at z = −0.5)
    let back_n = face_normal(b3, b2, t2, t3); // +Z (face at z = +0.5)
    let left_n = face_normal(b0, b3, t3, t0); // −X (face at x = −0.5)
    let right_n = face_normal(b2, b1, t1, t2); // +X (face at x = +0.5)

    // 24 vertices: 4 per face, 6 faces, CCW winding from outside.
    // Indices always use pattern [b, b+1, b+2, b, b+2, b+3] (two CCW triangles per quad).
    // Vertex order verified via cross product: (v1−v0) × (v2−v0) points outward for all faces.
    //   Bottom: (b1−b0)×(b2−b0) = (1,0,0)×(1,0,1) = (0,−1,0) = −Y ✓
    //   Top:    (t3−t0)×(t2−t0) = (0,0,1)×(1,0,1) = (0,+1,0) = +Y ✓
    //   Front:  (b0−b1)×(t0−b1) = (−1,0,0)×(−1,1,0) = (0,0,−1) = −Z ✓
    //   Back:   (b2−b3)×(t2−b3) = (1,0,0)×(1,1,0) = (0,0,+1) = +Z ✓
    //   Left:   (b3−b0)×(t3−b0) = (0,0,1)×(0,1,1) = (−1,0,0) = −X ✓
    //   Right:  (b1−b2)×(t1−b2) = (0,0,−1)×(0,1,−1) = (+1,0,0) = +X ✓
    #[rustfmt::skip]
    let positions: Vec<[f32; 3]> = vec![
        // Bottom (normal = −Y): CCW from below
        b0.to_array(), b1.to_array(), b2.to_array(), b3.to_array(),
        // Top (normal = +Y): CCW from above
        t0.to_array(), t3.to_array(), t2.to_array(), t1.to_array(),
        // Front (normal = −Z): CCW from outside (−Z side)
        b1.to_array(), b0.to_array(), t0.to_array(), t1.to_array(),
        // Back (normal = +Z): CCW from outside (+Z side)
        b3.to_array(), b2.to_array(), t2.to_array(), t3.to_array(),
        // Left (normal = −X): CCW from outside (−X side)
        b0.to_array(), b3.to_array(), t3.to_array(), t0.to_array(),
        // Right (normal = +X): CCW from outside (+X side)
        b2.to_array(), b1.to_array(), t1.to_array(), t2.to_array(),
    ];

    #[rustfmt::skip]
    let normals: Vec<[f32; 3]> = vec![
        // Bottom
        normal_bottom.to_array(), normal_bottom.to_array(),
        normal_bottom.to_array(), normal_bottom.to_array(),
        // Top
        normal_top.to_array(), normal_top.to_array(),
        normal_top.to_array(), normal_top.to_array(),
        // Front
        front_n.to_array(), front_n.to_array(), front_n.to_array(), front_n.to_array(),
        // Back
        back_n.to_array(), back_n.to_array(), back_n.to_array(), back_n.to_array(),
        // Left
        left_n.to_array(), left_n.to_array(), left_n.to_array(), left_n.to_array(),
        // Right
        right_n.to_array(), right_n.to_array(), right_n.to_array(), right_n.to_array(),
    ];

    // UV mapping: world-space texel density so 1 UV unit = 1 world unit.
    // U always runs viewer-left (U=0) → viewer-right (U=size) for each face.
    //   Bottom / Top  → U: +X (0…w),  V: +Z (0…d)
    //   Front (−Z)    → U: +X→−X (0…w), V: −Y (0=top … h=bottom)
    //   Back  (+Z)    → U: −X→+X (0…w), V: −Y (0=top … h=bottom)
    //   Left  (−X)    → U: −Z→+Z (0…d), V: −Y (0=top … h=bottom)
    //   Right (+X)    → U: +Z→−Z (0…d), V: −Y (0=top … h=bottom)
    // Tapered side faces: the top-edge U range narrows proportionally (by factor s).
    // UV mapping logic:
    let (w, h, d) = if stretch_uvs {
        (1.0, 1.0, 1.0)
    } else {
        (size.x, size.y, size.z)
    };
    let s = shrink;

    #[rustfmt::skip]
    let uvs: Vec<[f32; 2]> = vec![
        // Bottom  (b0, b1, b2, b3): U→X, V→Z
        [0.0, 0.0], [w,   0.0], [w,   d  ], [0.0, d  ],
        // Top     (t0, t3, t2, t1): U→X, V→Z
        [0.0, 0.0], [0.0, d  ], [w,   d  ], [w,   0.0],
        // Front   (b1, b0, t0, t1): U→viewer-left(+X)…viewer-right(−X); top narrows with taper
        // b1 is at +X (viewer-left), b0 at −X (viewer-right): U runs 0…w left-to-right.
        [0.0, h  ], [w,   h  ], [(1.0-s)*w, 0.0], [s*w, 0.0],
        // Back    (b3, b2, t2, t3): U→viewer-left(−X)…viewer-right(+X)
        // b3 at −X (viewer-left), b2 at +X (viewer-right): already correct, U runs 0…w.
        [0.0, h  ], [w,   h  ], [(1.0-s)*w, 0.0], [s*w, 0.0],
        // Left    (b0, b3, t3, t0): U→viewer-left(−Z)…viewer-right(+Z)
        // b0 at −Z (viewer-left), b3 at +Z (viewer-right): correct, U runs 0…d.
        [0.0, h  ], [d,   h  ], [(1.0-s)*d, 0.0], [s*d, 0.0],
        // Right   (b2, b1, t1, t2): U→viewer-left(+Z)…viewer-right(−Z)
        // b2 at +Z (viewer-left), b1 at −Z (viewer-right): U runs 0…d left-to-right.
        [0.0, h  ], [d,   h  ], [(1.0-s)*d, 0.0], [s*d, 0.0],
    ];

    // Build indices, omitting degenerate triangles for the fully-tapered (pyramid) case.
    //
    // At taper ≈ 1.0 the four top corners all collapse to a single apex point:
    //   - Top face (face 1): all four verts are identical → skip all 6 indices.
    //   - Side faces (2–5): verts 2 and 3 are both the apex → the second triangle
    //     [b, b+2, b+3] is degenerate (zero area). Emitting it causes mikktspace to
    //     produce NaN tangents on the *valid* triangle's shared apex vertex, which
    //     makes those faces invisible in the PBR shader. Skip the degenerate half.
    let is_pyramid = taper >= 1.0 - 1e-5;
    let mut indices: Vec<u32> = Vec::with_capacity(if is_pyramid { 18 } else { 36 });
    for face in 0u32..6 {
        let b = face * 4;
        match (is_pyramid, face) {
            (true, 1) => {}                                     // top face: completely degenerate
            (true, 2..=5) => indices.extend([b, b + 1, b + 2]), // side: only valid triangle
            _ => indices.extend([b, b + 1, b + 2, b, b + 2, b + 3]), // normal quad
        }
    }

    let mut mesh = Mesh::new(
        PrimitiveTopology::TriangleList,
        RenderAssetUsages::default(),
    );
    mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
    mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
    mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
    mesh.insert_indices(Indices::U32(indices));
    // Generate tangents; fall back to a default if the generator fails or produces
    // NaN values (which can occur if any remaining triangle is near-degenerate).
    let tangents_ok = mesh.generate_tangents().is_ok_and(|_| {
        !matches!(
            mesh.attribute(Mesh::ATTRIBUTE_TANGENT),
            Some(VertexAttributeValues::Float32x4(t))
                if t.iter().any(|v| v.iter().any(|f| f.is_nan()))
        )
    });
    if !tangents_ok {
        mesh.insert_attribute(Mesh::ATTRIBUTE_TANGENT, vec![[1.0_f32, 0.0, 0.0, 1.0]; 24]);
    }
    mesh
}

/// Computes the outward-facing normal for a quad with vertices in CCW order
/// (when viewed from outside). Uses the cross product of two diagonals.
fn face_normal(a: Vec3, b: Vec3, c: Vec3, d: Vec3) -> Vec3 {
    let _ = d; // fourth vertex unused; normal from first triangle suffices
    (b - a).cross(c - a).normalize()
}

// ── Profiled-face builders ────────────────────────────────────────────────────

/// Selects between `build_tapered_cuboid` and the flat-face builders based on
/// the [`FaceProfile`] carried by a [`Terminal`].
///
/// Roof panel scopes have `size.z ≈ 0`; [`crate::transform::scope_to_transform`]
/// clamps Z scale to `1e-3`, so the "depth" of the generated mesh is negligible.
/// The meaningful geometry is the XY cross-section described by the profile.
///
/// [`Terminal`]: symbios_shape::Terminal
pub fn build_profiled_mesh(profile: &FaceProfile, size: Vec3, stretch_uvs: bool) -> Mesh {
    match profile {
        FaceProfile::Rectangle => build_tapered_cuboid(0.0, size, stretch_uvs),
        FaceProfile::Taper(t) => build_tapered_cuboid(*t as f32, size, stretch_uvs),
        FaceProfile::Triangle { peak_offset } => {
            build_triangle_face(*peak_offset as f32, size, stretch_uvs)
        }
        FaceProfile::Trapezoid {
            top_width,
            offset_x,
        } => build_trapezoid_face(*top_width as f32, *offset_x as f32, size, stretch_uvs),
        FaceProfile::Polygon(pts) => build_polygon_face(pts, size, stretch_uvs),
    }
}

/// Builds a flat triangular face for a scope lying in the XY plane (`size.z ≈ 0`).
///
/// ## Coordinate convention
///
/// Unit-mesh space `[-0.5, 0.5]²`, matching `build_tapered_cuboid`:
/// - V0 = (−0.5, −0.5) — left eave corner
/// - V1 = (+0.5, −0.5) — right eave corner
/// - V2 = (`peak_offset` − 0.5, +0.5) — apex
///
/// ## UV mapping
///
/// Consistent with `build_tapered_cuboid` side faces:
/// U = 0…`size.x` left-to-right, V = 0 at the apex, V = `size.y` at the eave.
///
/// Both a front (+Z normal) and back (−Z normal) face are emitted.
fn build_triangle_face(peak_offset: f32, size: Vec3, stretch_uvs: bool) -> Mesh {
    let peak_offset = peak_offset.clamp(0.0, 1.0);
    let (w, h) = if stretch_uvs {
        (1.0, 1.0)
    } else {
        (size.x, size.y)
    };

    let v0 = [-0.5_f32, -0.5, 0.0];
    let v1 = [0.5_f32, -0.5, 0.0];
    let v2 = [peak_offset - 0.5, 0.5, 0.0];

    // Front (CCW from +Z, normal = +Z), Back (CCW from −Z, normal = −Z)
    let positions: Vec<[f32; 3]> = vec![v0, v1, v2, v0, v1, v2];

    let nf = [0.0_f32, 0.0, 1.0];
    let nb = [0.0_f32, 0.0, -1.0];
    let normals: Vec<[f32; 3]> = vec![nf, nf, nf, nb, nb, nb];

    let u0 = [0.0, h];
    let u1 = [w, h];
    let u2 = [peak_offset * w, 0.0_f32];
    let uvs: Vec<[f32; 2]> = vec![u0, u1, u2, u0, u1, u2];

    // Front: 0→1→2 (CCW from +Z).  Back: 3→5→4 (CCW from −Z).
    let indices: Vec<u32> = vec![0, 1, 2, 3, 5, 4];

    // For a flat XY face with U→+X and V decreasing toward +Y:
    //   front tangent (N=+Z): T=(1,0,0), B=cross(N,T)=(0,1,0), sign=+1 so B=(0,-1,0)
    //   handedness sign: dot(cross(N,T), B_tex) = dot((0,1,0),(0,-1,0)) = -1 → w=-1
    //   back  tangent  (N=−Z): cross(N,T)=(0,-1,0), B_tex=(0,-1,0) → w=+1
    let tangents: Vec<[f32; 4]> = vec![
        [1.0, 0.0, 0.0, -1.0],
        [1.0, 0.0, 0.0, -1.0],
        [1.0, 0.0, 0.0, -1.0],
        [1.0, 0.0, 0.0, 1.0],
        [1.0, 0.0, 0.0, 1.0],
        [1.0, 0.0, 0.0, 1.0],
    ];

    let mut mesh = Mesh::new(
        PrimitiveTopology::TriangleList,
        RenderAssetUsages::default(),
    );
    mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
    mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
    mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
    mesh.insert_attribute(Mesh::ATTRIBUTE_TANGENT, tangents);
    mesh.insert_indices(Indices::U32(indices));
    mesh
}

/// Builds a flat trapezoidal face for a scope lying in the XY plane (`size.z ≈ 0`).
///
/// Used for Hip-roof front/back slopes where the ridge is shorter than the eave.
///
/// ## Parameters (normalised to `[0, 1]` of scope width)
///
/// - `top_width`: fraction of the bottom edge width occupied by the top edge.
/// - `offset_x`: left-side indent of the top edge (0 = flush-left).
///
/// ## Vertex layout in unit `[-0.5, 0.5]²` space
///
/// ```text
///  V3─────────────V2          ← Y = +0.5 (ridge)
///  V0─────────────────────V1  ← Y = −0.5 (eave)
/// ```
fn build_trapezoid_face(top_width: f32, offset_x: f32, size: Vec3, stretch_uvs: bool) -> Mesh {
    let top_width = top_width.clamp(0.0, 1.0);
    let offset_x = offset_x.clamp(0.0, (1.0 - top_width).max(0.0));

    let (w, h) = if stretch_uvs {
        (1.0, 1.0)
    } else {
        (size.x, size.y)
    };

    let v0 = [-0.5_f32, -0.5, 0.0]; // bottom-left eave
    let v1 = [0.5_f32, -0.5, 0.0]; // bottom-right eave
    let v2 = [offset_x + top_width - 0.5, 0.5, 0.0]; // top-right ridge
    let v3 = [offset_x - 0.5, 0.5, 0.0]; // top-left ridge

    let positions: Vec<[f32; 3]> = vec![v0, v1, v2, v3, v0, v1, v2, v3];

    let nf = [0.0_f32, 0.0, 1.0];
    let nb = [0.0_f32, 0.0, -1.0];
    let normals: Vec<[f32; 3]> = vec![nf, nf, nf, nf, nb, nb, nb, nb];

    let uv0 = [0.0, h];
    let uv1 = [w, h];
    let uv2 = [(offset_x + top_width) * w, 0.0_f32];
    let uv3 = [offset_x * w, 0.0_f32];
    let uvs: Vec<[f32; 2]> = vec![uv0, uv1, uv2, uv3, uv0, uv1, uv2, uv3];

    // Front: [0,1,2, 0,2,3]  Back (reversed): [4,6,5, 4,7,6]
    let indices: Vec<u32> = vec![0, 1, 2, 0, 2, 3, 4, 6, 5, 4, 7, 6];

    let tangents: Vec<[f32; 4]> = vec![[1.0, 0.0, 0.0, -1.0]; 4]
        .into_iter()
        .chain(vec![[1.0_f32, 0.0, 0.0, 1.0]; 4])
        .collect();

    let mut mesh = Mesh::new(
        PrimitiveTopology::TriangleList,
        RenderAssetUsages::default(),
    );
    mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
    mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
    mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
    mesh.insert_attribute(Mesh::ATTRIBUTE_TANGENT, tangents);
    mesh.insert_indices(Indices::U32(indices));
    mesh
}

/// Builds a flat polygon face from a list of 2-D points in normalised scope
/// coordinates (`[0,1]²`) using ear-clipping triangulation.
///
/// Input `pts` uses double-precision scope coordinates:
/// - X: 0 = left eave edge, 1 = right eave edge
/// - Y: 0 = bottom (eave), 1 = top (ridge)
///
/// The polygon must be simple (no self-intersections). Both a front (+Z) and
/// back (−Z) face are generated. Falls back to `build_tapered_cuboid(0.0, …)`
/// if the polygon has fewer than 3 points or ear-clipping produces no triangles.
fn build_polygon_face(pts: &[DVec2], size: Vec3, stretch_uvs: bool) -> Mesh {
    if pts.len() < 3 {
        return build_tapered_cuboid(0.0, size, stretch_uvs);
    }

    let (w, h) = if stretch_uvs {
        (1.0, 1.0)
    } else {
        (size.x, size.y)
    };

    // Convert to f32 scope coords and unit-mesh coords.
    let scope_pts: Vec<Vec2> = pts
        .iter()
        .map(|p| Vec2::new(p.x as f32, p.y as f32))
        .collect();

    // Ensure CCW winding (required by ear-clipper).
    let area: f32 = (0..scope_pts.len())
        .map(|i| {
            let j = (i + 1) % scope_pts.len();
            scope_pts[i].x * scope_pts[j].y - scope_pts[j].x * scope_pts[i].y
        })
        .sum::<f32>()
        * 0.5;

    let pts_ccw: Vec<Vec2> = if area < 0.0 {
        scope_pts.iter().rev().cloned().collect()
    } else {
        scope_pts.clone()
    };

    let tris = ear_clip(&pts_ccw);
    if tris.is_empty() {
        return build_tapered_cuboid(0.0, size, stretch_uvs);
    }

    let n = pts_ccw.len() as u32;

    // Front positions (Z=0, normal=+Z) then back (same XY, normal=−Z).
    let positions: Vec<[f32; 3]> = pts_ccw
        .iter()
        .chain(pts_ccw.iter())
        .map(|p| [p.x - 0.5, p.y - 0.5, 0.0])
        .collect();

    let nf = [0.0_f32, 0.0, 1.0];
    let nb = [0.0_f32, 0.0, -1.0];
    let normals: Vec<[f32; 3]> = (0..n as usize)
        .map(|_| nf)
        .chain((0..n as usize).map(|_| nb))
        .collect();

    // UV: U = scope_x * w, V = (1 - scope_y) * h  (V=0 at ridge, V=h at eave)
    let uvs: Vec<[f32; 2]> = pts_ccw
        .iter()
        .chain(pts_ccw.iter())
        .map(|p| [p.x * w, (1.0 - p.y) * h])
        .collect();

    let tangents: Vec<[f32; 4]> = (0..n as usize)
        .map(|_| [1.0_f32, 0.0, 0.0, -1.0])
        .chain((0..n as usize).map(|_| [1.0_f32, 0.0, 0.0, 1.0]))
        .collect();

    // Front indices, then back indices (vertices offset by n, winding reversed).
    let indices: Vec<u32> = tris
        .iter()
        .flat_map(|&[a, b, c]| [a, b, c])
        .chain(tris.iter().flat_map(|&[a, b, c]| [a + n, c + n, b + n]))
        .collect();

    let mut mesh = Mesh::new(
        PrimitiveTopology::TriangleList,
        RenderAssetUsages::default(),
    );
    mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
    mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
    mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
    mesh.insert_attribute(Mesh::ATTRIBUTE_TANGENT, tangents);
    mesh.insert_indices(Indices::U32(indices));
    mesh
}

// ── Ear-clipping triangulator ─────────────────────────────────────────────────

/// O(n²) ear-clipping triangulator for simple (non-self-intersecting) polygons.
///
/// Expects vertices in **CCW** winding order. Returns triangles as `[i0, i1, i2]`
/// index triples into the original vertex slice, in CCW order.
fn ear_clip(verts: &[Vec2]) -> Vec<[u32; 3]> {
    let n = verts.len();
    if n < 3 {
        return vec![];
    }
    if n == 3 {
        return vec![[0, 1, 2]];
    }

    let mut active: Vec<usize> = (0..n).collect();
    let mut tris: Vec<[u32; 3]> = Vec::with_capacity(n - 2);
    let mut safety = n * n + n; // prevent infinite loop on degenerate input

    while active.len() > 3 && safety > 0 {
        safety -= 1;
        let m = active.len();
        let mut clipped = false;
        for i in 0..m {
            let (ia, ib, ic) = (active[(i + m - 1) % m], active[i], active[(i + 1) % m]);
            let (a, b, c) = (verts[ia], verts[ib], verts[ic]);
            // Skip reflex (right-turning) vertices — they cannot be ears.
            if cross2(b - a, c - b) <= 0.0 {
                continue;
            }
            // An ear is valid only if no other active vertex lies strictly inside abc.
            let is_ear = !active
                .iter()
                .any(|&j| j != ia && j != ib && j != ic && point_in_tri(verts[j], a, b, c));
            if !is_ear {
                continue;
            }
            tris.push([ia as u32, ib as u32, ic as u32]);
            active.remove(i);
            clipped = true;
            break;
        }
        if !clipped {
            break; // degenerate polygon — stop rather than loop forever
        }
    }

    if active.len() == 3 {
        tris.push([active[0] as u32, active[1] as u32, active[2] as u32]);
    }
    tris
}

/// 2-D cross product (signed area of parallelogram).
#[inline]
fn cross2(a: Vec2, b: Vec2) -> f32 {
    a.x * b.y - a.y * b.x
}

/// Returns `true` if `p` is strictly inside triangle `(a, b, c)` (CCW winding).
#[inline]
fn point_in_tri(p: Vec2, a: Vec2, b: Vec2, c: Vec2) -> bool {
    cross2(b - a, p - a) > 0.0 && cross2(c - b, p - b) > 0.0 && cross2(a - c, p - c) > 0.0
}

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

    #[test]
    fn box_has_correct_vertex_count() {
        let mesh = build_tapered_cuboid(0.0, Vec3::ONE, false);
        let positions = mesh.attribute(Mesh::ATTRIBUTE_POSITION).unwrap();
        assert_eq!(positions.len(), 24);
    }

    #[test]
    fn box_has_correct_index_count() {
        let mesh = build_tapered_cuboid(0.0, Vec3::ONE, false);
        if let Some(Indices::U32(idx)) = mesh.indices() {
            assert_eq!(idx.len(), 36);
        } else {
            panic!("expected U32 indices");
        }
    }

    #[test]
    fn pyramid_has_same_structure() {
        let mesh = build_tapered_cuboid(1.0, Vec3::ONE, false);
        let positions = mesh.attribute(Mesh::ATTRIBUTE_POSITION).unwrap();
        assert_eq!(positions.len(), 24);
    }

    /// Verify every triangle's winding produces an outward-facing normal by checking
    /// the sign of its dot product with the stored per-face normal attribute.
    #[test]
    fn winding_is_ccw_from_outside() {
        for taper in [0.0f32, 0.5, 1.0] {
            let mesh = build_tapered_cuboid(taper, Vec3::ONE, false);

            let positions = match mesh.attribute(Mesh::ATTRIBUTE_POSITION) {
                Some(bevy::mesh::VertexAttributeValues::Float32x3(p)) => p,
                _ => panic!("expected Float32x3 positions"),
            };
            let normals = match mesh.attribute(Mesh::ATTRIBUTE_NORMAL) {
                Some(bevy::mesh::VertexAttributeValues::Float32x3(n)) => n,
                _ => panic!("expected Float32x3 normals"),
            };
            let indices = match mesh.indices() {
                Some(Indices::U32(idx)) => idx.clone(),
                _ => panic!("expected U32 indices"),
            };

            for tri in indices.chunks(3) {
                let (i0, i1, i2) = (tri[0] as usize, tri[1] as usize, tri[2] as usize);
                let p0 = Vec3::from(positions[i0]);
                let p1 = Vec3::from(positions[i1]);
                let p2 = Vec3::from(positions[i2]);
                let face_n = Vec3::from(normals[i0]); // all 4 verts of a face share the same normal
                let computed = (p1 - p0).cross(p2 - p0);
                // Skip degenerate triangles (e.g. taper=1 collapses the top face to a point).
                if computed.length_squared() < 1e-10 {
                    continue;
                }
                assert!(
                    computed.dot(face_n) > 0.0,
                    "CW winding at taper={taper}: tri ({i0},{i1},{i2}), \
                     computed={computed:?}, stored={face_n:?}"
                );
            }
        }
    }

    /// Verify that side-face UVs use U=0 at one edge and U=size at the other,
    /// and that V=0 is at the top and V=size.y at the bottom.
    /// Also checks that the top face maps U→X and V→Z (not the old rotated mapping).
    #[test]
    fn uvs_have_correct_orientation_and_scale() {
        let size = Vec3::new(4.0, 3.0, 2.0); // w=4, h=3, d=2
        let mesh = build_tapered_cuboid(0.0, size, false);

        let uvs = match mesh.attribute(Mesh::ATTRIBUTE_UV_0) {
            Some(bevy::mesh::VertexAttributeValues::Float32x2(u)) => u.clone(),
            _ => panic!("expected Float32x2 UVs"),
        };

        // Face vertex layout (4 verts per face, 6 faces):
        //   0..4  bottom, 4..8 top, 8..12 front, 12..16 back, 16..20 left, 20..24 right

        // Bottom face (b0,b1,b2,b3): U→X (0…w), V→Z (0…d)
        assert_eq!(uvs[0], [0.0, 0.0], "bottom b0");
        assert_eq!(uvs[1], [4.0, 0.0], "bottom b1");
        assert_eq!(uvs[2], [4.0, 2.0], "bottom b2");
        assert_eq!(uvs[3], [0.0, 2.0], "bottom b3");

        // Top face (t0,t3,t2,t1): U→X (0…w), V→Z (0…d)
        assert_eq!(uvs[4], [0.0, 0.0], "top t0 front-left");
        assert_eq!(uvs[5], [0.0, 2.0], "top t3 back-left");
        assert_eq!(uvs[6], [4.0, 2.0], "top t2 back-right");
        assert_eq!(uvs[7], [4.0, 0.0], "top t1 front-right");

        // Front face (b1,b0,t0,t1): camera looks in +Z so right=−X.
        // Viewer-left=b1(+X), viewer-right=b0(−X): U runs 0…w left-to-right.
        assert_eq!(uvs[8], [0.0, 3.0], "front b1 viewer-left-bottom");
        assert_eq!(uvs[9], [4.0, 3.0], "front b0 viewer-right-bottom");
        assert_eq!(uvs[10], [4.0, 0.0], "front t0 viewer-right-top (taper=0)");
        assert_eq!(uvs[11], [0.0, 0.0], "front t1 viewer-left-top (taper=0)");

        // Left face (b0,b3,t3,t0): U→Z, V: h=bottom, 0=top
        assert_eq!(uvs[16], [0.0, 3.0], "left b0 front-bottom");
        assert_eq!(uvs[17], [2.0, 3.0], "left b3 back-bottom");
        assert_eq!(uvs[18], [2.0, 0.0], "left t3 back-top (taper=0)");
        assert_eq!(uvs[19], [0.0, 0.0], "left t0 front-top (taper=0)");
    }

    #[test]
    fn tapered_side_face_uvs_narrow_at_top() {
        let size = Vec3::new(4.0, 3.0, 2.0);
        let mesh = build_tapered_cuboid(1.0, size, false); // full pyramid: shrink=0.5

        let uvs = match mesh.attribute(Mesh::ATTRIBUTE_UV_0) {
            Some(bevy::mesh::VertexAttributeValues::Float32x2(u)) => u.clone(),
            _ => panic!("expected Float32x2 UVs"),
        };

        // Front face top verts collapse to centre (s=0.5): U ∈ [s*w, (1-s)*w] = [2, 2]
        let front_t0_u = uvs[10][0];
        let front_t1_u = uvs[11][0];
        assert!(
            (front_t0_u - 2.0).abs() < 1e-5,
            "front top-left U should be 2.0, got {front_t0_u}"
        );
        assert!(
            (front_t1_u - 2.0).abs() < 1e-5,
            "front top-right U should be 2.0, got {front_t1_u}"
        );
    }

    #[test]
    fn normals_are_unit_length() {
        let mesh = build_tapered_cuboid(0.5, Vec3::ONE, false);
        if let Some(bevy::mesh::VertexAttributeValues::Float32x3(normals)) =
            mesh.attribute(Mesh::ATTRIBUTE_NORMAL)
        {
            for n in normals {
                let len = (n[0] * n[0] + n[1] * n[1] + n[2] * n[2]).sqrt();
                assert!((len - 1.0).abs() < 1e-5, "non-unit normal: {:?}", n);
            }
        } else {
            panic!("expected Float32x3 normals");
        }
    }

    // ── build_triangle_face ──────────────────────────────────────────────────

    #[test]
    fn triangle_face_vertex_count() {
        let mesh = build_triangle_face(0.5, Vec3::new(4.0, 3.0, 0.0), false);
        let pos = mesh.attribute(Mesh::ATTRIBUTE_POSITION).unwrap();
        assert_eq!(pos.len(), 6, "expected 3 front + 3 back verts");
    }

    #[test]
    fn triangle_face_index_count() {
        let mesh = build_triangle_face(0.5, Vec3::new(4.0, 3.0, 0.0), false);
        if let Some(Indices::U32(idx)) = mesh.indices() {
            assert_eq!(idx.len(), 6, "expected 2 triangles × 3 indices");
        } else {
            panic!("expected U32 indices");
        }
    }

    #[test]
    fn triangle_face_front_winding_is_ccw() {
        let mesh = build_triangle_face(0.5, Vec3::ONE, false);
        let positions = match mesh.attribute(Mesh::ATTRIBUTE_POSITION) {
            Some(VertexAttributeValues::Float32x3(p)) => p,
            _ => panic!("expected Float32x3"),
        };
        // Front triangle is indices [0,1,2]; cross product must give +Z.
        let p0 = Vec3::from(positions[0]);
        let p1 = Vec3::from(positions[1]);
        let p2 = Vec3::from(positions[2]);
        let normal = (p1 - p0).cross(p2 - p0);
        assert!(
            normal.z > 0.0,
            "front face should have +Z normal, got {normal:?}"
        );
    }

    #[test]
    fn triangle_face_uvs_correct() {
        let size = Vec3::new(4.0, 3.0, 0.0);
        let mesh = build_triangle_face(0.5, size, false);
        let uvs = match mesh.attribute(Mesh::ATTRIBUTE_UV_0) {
            Some(VertexAttributeValues::Float32x2(u)) => u.clone(),
            _ => panic!("expected Float32x2 UVs"),
        };
        // V0 (bottom-left): U=0, V=h
        assert!((uvs[0][0]).abs() < 1e-5, "V0 U should be 0");
        assert!((uvs[0][1] - 3.0).abs() < 1e-5, "V0 V should be size.y=3");
        // V1 (bottom-right): U=w, V=h
        assert!((uvs[1][0] - 4.0).abs() < 1e-5, "V1 U should be size.x=4");
        // V2 (apex at peak=0.5): U=0.5*w=2, V=0
        assert!(
            (uvs[2][0] - 2.0).abs() < 1e-5,
            "V2 U should be 2.0 for peak=0.5"
        );
        assert!((uvs[2][1]).abs() < 1e-5, "V2 V should be 0 at ridge");
    }

    #[test]
    fn triangle_face_normals_are_unit() {
        let mesh = build_triangle_face(0.3, Vec3::new(5.0, 4.0, 0.0), false);
        if let Some(VertexAttributeValues::Float32x3(normals)) =
            mesh.attribute(Mesh::ATTRIBUTE_NORMAL)
        {
            for n in normals {
                let len = (n[0] * n[0] + n[1] * n[1] + n[2] * n[2]).sqrt();
                assert!((len - 1.0).abs() < 1e-5, "non-unit normal: {n:?}");
            }
        } else {
            panic!("expected Float32x3 normals");
        }
    }

    // ── build_trapezoid_face ─────────────────────────────────────────────────

    #[test]
    fn trapezoid_face_vertex_count() {
        let mesh = build_trapezoid_face(0.5, 0.25, Vec3::new(10.0, 5.0, 0.0), false);
        let pos = mesh.attribute(Mesh::ATTRIBUTE_POSITION).unwrap();
        assert_eq!(pos.len(), 8, "expected 4 front + 4 back verts");
    }

    #[test]
    fn trapezoid_face_index_count() {
        let mesh = build_trapezoid_face(0.5, 0.25, Vec3::new(10.0, 5.0, 0.0), false);
        if let Some(Indices::U32(idx)) = mesh.indices() {
            assert_eq!(idx.len(), 12, "expected 4 triangles × 3 indices");
        } else {
            panic!("expected U32 indices");
        }
    }

    #[test]
    fn trapezoid_face_front_winding_is_ccw() {
        let mesh = build_trapezoid_face(0.5, 0.25, Vec3::ONE, false);
        let positions = match mesh.attribute(Mesh::ATTRIBUTE_POSITION) {
            Some(VertexAttributeValues::Float32x3(p)) => p,
            _ => panic!("expected Float32x3"),
        };
        // First front triangle: [0,1,2]
        let p0 = Vec3::from(positions[0]);
        let p1 = Vec3::from(positions[1]);
        let p2 = Vec3::from(positions[2]);
        let n = (p1 - p0).cross(p2 - p0);
        assert!(
            n.z > 0.0,
            "front face first tri should have +Z normal, got {n:?}"
        );
    }

    #[test]
    fn trapezoid_face_uvs_correct() {
        // top_width=0.5, offset_x=0.25 → top edge X ∈ [0.25, 0.75] of scope width
        let size = Vec3::new(10.0, 5.0, 0.0);
        let mesh = build_trapezoid_face(0.5, 0.25, size, false);
        let uvs = match mesh.attribute(Mesh::ATTRIBUTE_UV_0) {
            Some(VertexAttributeValues::Float32x2(u)) => u.clone(),
            _ => panic!("expected Float32x2 UVs"),
        };
        // V0 bottom-left: U=0, V=h=5
        assert!((uvs[0][0]).abs() < 1e-5);
        assert!((uvs[0][1] - 5.0).abs() < 1e-5);
        // V1 bottom-right: U=w=10, V=5
        assert!((uvs[1][0] - 10.0).abs() < 1e-5);
        // V2 top-right: U=(0.25+0.5)*10=7.5, V=0
        assert!((uvs[2][0] - 7.5).abs() < 1e-5);
        assert!((uvs[2][1]).abs() < 1e-5);
        // V3 top-left: U=0.25*10=2.5, V=0
        assert!((uvs[3][0] - 2.5).abs() < 1e-5);
        assert!((uvs[3][1]).abs() < 1e-5);
    }

    // ── ear_clip ─────────────────────────────────────────────────────────────

    #[test]
    fn ear_clip_triangle() {
        let verts = vec![
            Vec2::new(0.0, 0.0),
            Vec2::new(1.0, 0.0),
            Vec2::new(0.5, 1.0),
        ];
        let tris = ear_clip(&verts);
        assert_eq!(tris, vec![[0, 1, 2]]);
    }

    #[test]
    fn ear_clip_square_gives_two_triangles() {
        // CCW square
        let verts = vec![
            Vec2::new(0.0, 0.0),
            Vec2::new(1.0, 0.0),
            Vec2::new(1.0, 1.0),
            Vec2::new(0.0, 1.0),
        ];
        let tris = ear_clip(&verts);
        assert_eq!(tris.len(), 2, "square → 2 triangles");
        // Verify no index out of bounds
        for tri in &tris {
            for &i in tri {
                assert!(i < 4);
            }
        }
    }

    #[test]
    fn ear_clip_pentagon_gives_three_triangles() {
        use std::f32::consts::TAU;
        // Regular pentagon, CCW
        let verts: Vec<Vec2> = (0..5)
            .map(|i| {
                let t = TAU * i as f32 / 5.0;
                Vec2::new(t.cos(), t.sin())
            })
            .collect();
        let tris = ear_clip(&verts);
        assert_eq!(tris.len(), 3, "pentagon → 3 triangles");
    }

    // ── build_profiled_mesh dispatch ─────────────────────────────────────────

    #[test]
    fn profiled_mesh_rectangle_matches_cuboid() {
        let size = Vec3::new(2.0, 3.0, 1.0);
        let a = build_profiled_mesh(&FaceProfile::Rectangle, size, false);
        let b = build_tapered_cuboid(0.0, size, false);
        assert_eq!(
            a.attribute(Mesh::ATTRIBUTE_POSITION).unwrap().len(),
            b.attribute(Mesh::ATTRIBUTE_POSITION).unwrap().len()
        );
    }

    #[test]
    fn profiled_mesh_triangle_has_six_verts() {
        let mesh = build_profiled_mesh(
            &FaceProfile::Triangle { peak_offset: 0.5 },
            Vec3::new(4.0, 3.0, 0.0),
            false,
        );
        assert_eq!(mesh.attribute(Mesh::ATTRIBUTE_POSITION).unwrap().len(), 6);
    }

    #[test]
    fn profiled_mesh_trapezoid_has_eight_verts() {
        let mesh = build_profiled_mesh(
            &FaceProfile::Trapezoid {
                top_width: 0.5,
                offset_x: 0.25,
            },
            Vec3::new(10.0, 5.0, 0.0),
            false,
        );
        assert_eq!(mesh.attribute(Mesh::ATTRIBUTE_POSITION).unwrap().len(), 8);
    }

    #[test]
    fn profiled_mesh_polygon_square_has_eight_verts() {
        use bevy::math::DVec2;
        let pts = vec![
            DVec2::new(0.0, 0.0),
            DVec2::new(1.0, 0.0),
            DVec2::new(1.0, 1.0),
            DVec2::new(0.0, 1.0),
        ];
        let mesh = build_profiled_mesh(&FaceProfile::Polygon(pts), Vec3::new(4.0, 4.0, 0.0), false);
        // 4 front + 4 back
        assert_eq!(mesh.attribute(Mesh::ATTRIBUTE_POSITION).unwrap().len(), 8);
    }
}