OneCAD/crates/kernel/src/meshbody.rs

//! Mesh bodies: triangles without a B-rep (imported 3MF/STL). They render, measure, move and
//! export; B-rep features refuse them.

use std::collections::HashMap;
use std::sync::Arc;

use solvecraft_geom::{Mesh, Vec3};

use crate::body::Body;
use crate::{KernelError, Result};

/// Most triangles in one mesh body.
pub const MAX_TRIANGLES: usize = 20_000_000;

/// Indexed triangles with shared vertices, wound counter-clockwise seen from outside.
#[derive(Clone, Debug, Default)]
pub struct TriMesh {
    pub positions: Vec<Vec3>,
    pub triangles: Vec<[u32; 3]>,
}

impl TriMesh {
    /// Display/measure mesh: corners split per triangle so normals stay flat.
    pub(crate) fn to_mesh(&self) -> Mesh {
        let mut out = Mesh::default();
        out.positions.reserve(self.triangles.len() * 3);
        for t in &self.triangles {
            let (Some(a), Some(b), Some(c)) =
                (self.positions.get(t[0] as usize), self.positions.get(t[1] as usize), self.positions.get(t[2] as usize))
            else {
                continue;
            };
            let n = (*b - *a).cross(*c - *a).normalized().unwrap_or(Vec3::Z);
            let base = u32::try_from(out.positions.len()).unwrap_or(u32::MAX);
            out.positions.extend([*a, *b, *c]);
            out.normals.extend([n, n, n]);
            out.triangles.push([base, base.saturating_add(1), base.saturating_add(2)]);
            out.tri_face.push(0);
        }
        out
    }

    /// Distinct undirected triangle sides.
    pub(crate) fn edge_count(&self) -> usize {
        let mut set = std::collections::HashSet::new();
        for t in &self.triangles {
            for (a, b) in [(t[0], t[1]), (t[1], t[2]), (t[2], t[0])] {
                set.insert((a.min(b), a.max(b)));
            }
        }
        set.len()
    }

    /// Signed volume ×6.
    fn volume6(&self) -> f64 {
        self.triangles
            .iter()
            .filter_map(|t| Some((self.positions.get(t[0] as usize)?, self.positions.get(t[1] as usize)?, self.positions.get(t[2] as usize)?)))
            .map(|(a, b, c)| a.dot(b.cross(*c)))
            .sum()
    }
}

/// A mesh body from triangles (`positions` in mm). Coincident vertices are merged, degenerate
/// triangles dropped, and a closed mesh with inward winding is turned outward.
pub fn mesh_body(positions: &[Vec3], triangles: &[[u32; 3]]) -> Result<Body> {
    if triangles.len() > MAX_TRIANGLES {
        return Err(KernelError::Invalid(format!("mesh with {} triangles (limit {MAX_TRIANGLES})", triangles.len())));
    }
    if positions.iter().any(|p| !(p.is_finite() && p.x.abs() < 1e9 && p.y.abs() < 1e9 && p.z.abs() < 1e9)) {
        return Err(KernelError::Invalid("mesh with non-finite or huge coordinates".into()));
    }
    // Merge vertices that are exactly equal (bit pattern of the coordinates).
    let mut map: HashMap<[u64; 3], u32> = HashMap::new();
    let mut remap: Vec<u32> = Vec::with_capacity(positions.len());
    let mut out = TriMesh::default();
    for p in positions {
        let key = [(p.x + 0.0).to_bits(), (p.y + 0.0).to_bits(), (p.z + 0.0).to_bits()];
        let next = u32::try_from(out.positions.len()).map_err(|_| KernelError::Invalid("too many vertices".into()))?;
        let i = *map.entry(key).or_insert_with(|| {
            out.positions.push(*p);
            next
        });
        remap.push(i);
    }
    for t in triangles {
        let (Some(&a), Some(&b), Some(&c)) = (remap.get(t[0] as usize), remap.get(t[1] as usize), remap.get(t[2] as usize)) else {
            return Err(KernelError::Invalid("mesh triangle refers to a missing vertex".into()));
        };
        if a == b || b == c || c == a {
            continue;
        }
        out.triangles.push([a, b, c]);
    }
    if out.triangles.is_empty() {
        return Err(KernelError::Invalid("mesh has no triangles".into()));
    }
    if out.volume6() < 0.0 && is_closed(&out) {
        for t in &mut out.triangles {
            t.swap(1, 2);
        }
    }
    Ok(Body { solid: Arc::new(crate::body::Solid::new_unchecked(Vec::new())), mesh: Some(Arc::new(out)), color: None, paint: None })
}

/// Every triangle side is shared by exactly two triangles in opposite directions.
fn is_closed(m: &TriMesh) -> bool {
    let mut count: HashMap<(u32, u32), i32> = HashMap::new();
    for t in &m.triangles {
        for (a, b) in [(t[0], t[1]), (t[1], t[2]), (t[2], t[0])] {
            *count.entry((a, b)).or_default() += 1;
        }
    }
    count.iter().all(|(&(a, b), &n)| n == 1 && count.get(&(b, a)) == Some(&1))
}

impl Body {
    /// Shared-vertex triangles of a mesh body.
    pub fn triangle_mesh(&self) -> Option<(&[Vec3], &[[u32; 3]])> {
        self.mesh.as_ref().map(|m| (m.positions.as_slice(), m.triangles.as_slice()))
    }
    /// Does a mesh body enclose a volume (every side shared by two triangles)?
    pub fn is_closed_mesh(&self) -> bool {
        self.mesh.as_ref().is_some_and(|m| is_closed(m))
    }

    /// A mesh body moved by a column-major affine matrix (reflections keep outward winding).
    pub(crate) fn mesh_transformed(&self, m: &[[f64; 4]; 4]) -> Option<Body> {
        let tm = self.mesh.as_ref()?;
        let f = |p: Vec3| {
            Vec3::new(
                m[0][0] * p.x + m[1][0] * p.y + m[2][0] * p.z + m[3][0],
                m[0][1] * p.x + m[1][1] * p.y + m[2][1] * p.z + m[3][1],
                m[0][2] * p.x + m[1][2] * p.y + m[2][2] * p.z + m[3][2],
            )
        };
        let det = m[0][0] * (m[1][1] * m[2][2] - m[2][1] * m[1][2]) - m[1][0] * (m[0][1] * m[2][2] - m[2][1] * m[0][2])
            + m[2][0] * (m[0][1] * m[1][2] - m[1][1] * m[0][2]);
        let mut out = TriMesh { positions: tm.positions.iter().map(|p| f(*p)).collect(), triangles: tm.triangles.clone() };
        if det < 0.0 {
            for t in &mut out.triangles {
                t.swap(1, 2);
            }
        }
        Some(Body { solid: self.solid.clone(), mesh: Some(Arc::new(out)), color: self.color, paint: self.paint.clone() })
    }
}