turtle/turtle-lib/src/tessellation.rs

//! Lyon tessellation utilities for turtle graphics
//!
//! This module provides helper functions to tessellate paths using Lyon,
//! which replaces the manual triangulation with GPU-optimized tessellation.

use lyon::math::{point, Point};
use lyon::path::{LineCap, LineJoin, Path};
use lyon::tessellation::{
    BuffersBuilder, FillOptions, FillRule, FillTessellator, FillVertex, StrokeOptions,
    StrokeTessellator, StrokeVertex, VertexBuffers,
};
use macroquad::prelude::*;

/// Convert macroquad Vec2 to Lyon Point
#[must_use]
pub(crate) fn to_lyon_point(v: Vec2) -> Point {
    point(v.x, v.y)
}

/// Convert Lyon Point to macroquad Vec2
#[allow(dead_code)]
#[must_use]
pub(crate) fn to_macroquad_vec2(p: Point) -> Vec2 {
    vec2(p.x, p.y)
}

/// Simple vertex type for Lyon tessellation
#[derive(Copy, Clone, Debug)]
pub(crate) struct SimpleVertex {
    pub(crate) position: [f32; 2],
}

/// Build mesh from Lyon tessellation
#[must_use]
pub(crate) fn build_mesh(
    vertices: &[SimpleVertex],
    indices: &[u16],
    color: Color,
) -> Mesh {
    let verts: Vec<Vertex> = vertices
        .iter()
        .map(|v| Vertex {
            position: Vec3::new(v.position[0], v.position[1], 0.0),
            uv: Vec2::ZERO,
            color: [
                (color.r * 255.0) as u8,
                (color.g * 255.0) as u8,
                (color.b * 255.0) as u8,
                (color.a * 255.0) as u8,
            ],
            normal: Vec4::ZERO,
        })
        .collect();

    Mesh {
        vertices: verts,
        indices: indices.to_vec(),
        texture: None,
    }
}

/// Tessellate a polygon and return mesh
///
/// This automatically handles holes when the path crosses itself.
///
/// # Errors
///
/// Returns an error if no vertices are provided or if tessellation fails.
pub(crate) fn tessellate_polygon(
    vertices: &[Vec2],
    color: Color,
) -> Result<Mesh, Box<dyn std::error::Error>> {
    if vertices.is_empty() {
        return Err("No vertices provided".into());
    }

    // Build path
    let mut builder = Path::builder();
    builder.begin(to_lyon_point(vertices[0]));
    for v in &vertices[1..] {
        builder.line_to(to_lyon_point(*v));
    }
    builder.end(true); // Close the path

    let path = builder.build();

    // Tessellate with EvenOdd fill rule (automatic hole detection)
    let mut geometry: VertexBuffers<SimpleVertex, u16> = VertexBuffers::new();
    let mut tessellator = FillTessellator::new();

    tessellator.tessellate_path(
        &path,
        &FillOptions::default().with_fill_rule(FillRule::EvenOdd),
        &mut BuffersBuilder::new(&mut geometry, |vertex: FillVertex| SimpleVertex {
            position: vertex.position().to_array(),
        }),
    )?;

    Ok(build_mesh(
        &geometry.vertices,
        &geometry.indices,
        color,
    ))
}

/// Tessellate multiple contours (outer boundary + holes) and return mesh
///
/// The first contour is the outer boundary, subsequent contours are holes.
/// Lyon's `EvenOdd` fill rule automatically creates holes where contours overlap.
///
/// # Errors
///
/// Returns an error if no contours are provided or if tessellation fails.
pub(crate) fn tessellate_multi_contour(
    contours: &[Vec<Vec2>],
    color: Color,
) -> Result<Mesh, Box<dyn std::error::Error>> {
    if contours.is_empty() {
        return Err("No contours provided".into());
    }

    let span = tracing::debug_span!("tessellate_multi_contour", contours = contours.len());
    let _enter = span.enter();

    tracing::debug!("Starting multi-contour tessellation");

    // Build path with multiple sub-paths (contours)
    let mut builder = Path::builder();

    for (idx, contour) in contours.iter().enumerate() {
        if contour.is_empty() {
            tracing::warn!(contour_idx = idx, "Contour is empty, skipping");
            continue;
        }

        tracing::trace!(
            contour_idx = idx,
            vertices = contour.len(),
            first_x = contour[0].x,
            first_y = contour[0].y,
            "Processing contour"
        );
        if contour.len() > 1 {
            tracing::trace!(
                last_x = contour[contour.len() - 1].x,
                last_y = contour[contour.len() - 1].y,
                "Contour end vertex"
            );
        }

        // Each contour is a separate closed sub-path
        builder.begin(to_lyon_point(contour[0]));
        for (i, v) in contour[1..].iter().enumerate() {
            builder.line_to(to_lyon_point(*v));
            if i < 3 || i >= contour.len() - 4 {
                tracing::trace!(vertex_idx = i + 1, x = v.x, y = v.y, "Contour vertex");
            } else if i == 3 {
                tracing::trace!(
                    omitted = contour.len() - 7,
                    "Additional vertices omitted from trace"
                );
            }
        }
        builder.end(true); // Close this contour
        tracing::trace!(contour_idx = idx, "Contour closed");
    }

    tracing::debug!("Building Lyon path");
    let path = builder.build();
    tracing::debug!("Path built successfully");

    // Tessellate with EvenOdd fill rule - overlapping areas become holes
    let mut geometry: VertexBuffers<SimpleVertex, u16> = VertexBuffers::new();
    let mut tessellator = FillTessellator::new();

    tracing::debug!("Starting tessellation with EvenOdd fill rule");
    match tessellator.tessellate_path(
        &path,
        &FillOptions::default().with_fill_rule(FillRule::EvenOdd),
        &mut BuffersBuilder::new(&mut geometry, |vertex: FillVertex| SimpleVertex {
            position: vertex.position().to_array(),
        }),
    ) {
        Ok(()) => {
            tracing::debug!(
                vertices = geometry.vertices.len(),
                indices = geometry.indices.len(),
                triangles = geometry.indices.len() / 3,
                "Tessellation successful"
            );
        }
        Err(e) => {
            tracing::error!(error = %e, "Tessellation failed");
            return Err(Box::new(e));
        }
    }

    Ok(build_mesh(
        &geometry.vertices,
        &geometry.indices,
        color,
    ))
}

/// Tessellate a stroked path and return mesh
///
/// # Errors
///
/// Returns an error if no vertices are provided or if tessellation fails.
pub(crate) fn tessellate_stroke(
    vertices: &[Vec2],
    color: Color,
    width: f32,
    closed: bool,
) -> Result<Mesh, Box<dyn std::error::Error>> {
    if vertices.is_empty() {
        return Err("No vertices provided".into());
    }

    // Build path
    let mut builder = Path::builder();
    builder.begin(to_lyon_point(vertices[0]));
    for v in &vertices[1..] {
        builder.line_to(to_lyon_point(*v));
    }
    builder.end(closed);
    let path = builder.build();

    // Tessellate with round caps and joins for smooth lines
    let mut geometry: VertexBuffers<SimpleVertex, u16> = VertexBuffers::new();
    let mut tessellator = StrokeTessellator::new();

    tessellator.tessellate_path(
        &path,
        &StrokeOptions::default()
            .with_line_width(width)
            .with_line_cap(LineCap::Round)
            .with_line_join(LineJoin::Round),
        &mut BuffersBuilder::new(&mut geometry, |vertex: StrokeVertex| SimpleVertex {
            position: vertex.position().to_array(),
        }),
    )?;

    Ok(build_mesh(
        &geometry.vertices,
        &geometry.indices,
        color,
    ))
}

/// Tessellate a circle and return mesh
///
/// # Errors
///
/// Returns an error if tessellation fails.
pub(crate) fn tessellate_circle(
    center: Vec2,
    radius: f32,
    color: Color,
    filled: bool,
    stroke_width: f32,
) -> Result<Mesh, Box<dyn std::error::Error>> {
    let mut builder = Path::builder();
    builder.add_circle(to_lyon_point(center), radius, lyon::path::Winding::Positive);
    let path = builder.build();

    let mut geometry: VertexBuffers<SimpleVertex, u16> = VertexBuffers::new();

    if filled {
        let mut tessellator = FillTessellator::new();
        tessellator.tessellate_path(
            &path,
            &FillOptions::default(),
            &mut BuffersBuilder::new(&mut geometry, |vertex: FillVertex| SimpleVertex {
                position: vertex.position().to_array(),
            }),
        )?;
    } else {
        let mut tessellator = StrokeTessellator::new();
        tessellator.tessellate_path(
            &path,
            &StrokeOptions::default().with_line_width(stroke_width),
            &mut BuffersBuilder::new(&mut geometry, |vertex: StrokeVertex| SimpleVertex {
                position: vertex.position().to_array(),
            }),
        )?;
    }

    Ok(build_mesh(
        &geometry.vertices,
        &geometry.indices,
        color,
    ))
}

/// Tessellate an arc (partial circle) and return mesh
///
/// # Errors
///
/// Returns an error if tessellation fails.
#[allow(clippy::too_many_arguments)]
pub(crate) fn tessellate_arc(
    center: Vec2,
    radius: f32,
    start_angle_degrees: f32,
    arc_angle_degrees: f32,
    color: Color,
    stroke_width: f32,
    segments: usize,
    direction: crate::circle_geometry::CircleDirection,
) -> Result<Mesh, Box<dyn std::error::Error>> {
    use crate::circle_geometry::arc_points;

    let start_angle = start_angle_degrees.to_radians();
    let sweep_angle = arc_angle_degrees.to_radians();

    let mut builder = Path::builder();

    // Start point of the arc (arc_points returns everything *after* this)
    builder.begin(point(
        center.x + radius * start_angle.cos(),
        center.y + radius * start_angle.sin(),
    ));

    // Remaining points — single source of truth for arc sampling
    for pt in arc_points(
        center,
        radius,
        start_angle,
        sweep_angle,
        segments,
        direction,
    ) {
        builder.line_to(point(pt.x, pt.y));
    }

    builder.end(false); // open arc, not a closed polygon
    let path = builder.build();

    // Tessellate stroke
    let mut geometry: VertexBuffers<SimpleVertex, u16> = VertexBuffers::new();
    let mut tessellator = StrokeTessellator::new();

    tessellator.tessellate_path(
        &path,
        &StrokeOptions::default()
            .with_line_width(stroke_width)
            .with_line_cap(lyon::tessellation::LineCap::Round)
            .with_line_join(lyon::tessellation::LineJoin::Round),
        &mut BuffersBuilder::new(&mut geometry, |vertex: StrokeVertex| SimpleVertex {
            position: vertex.position().to_array(),
        }),
    )?;

    Ok(build_mesh(
        &geometry.vertices,
        &geometry.indices,
        color,
    ))
}