OneCAD-Draft/src/modules/draw/modify/spline_ops.rs

// Shared B-spline utilities for modify commands (TRIM, BREAK, OFFSET, LENGTHEN).
//
// The evaluation lives in cadkernel; this file converts a drawing's SPLINE
// entity to and from it and keeps the knot-domain call shapes the commands
// were written against.
//
// Weights are carried through now. The previous path evaluated splines with a
// non-rational B-spline and cleared the weights on every split, which turns a
// piece of a NURBS circle into a piece of a parabola — a change that looks
// fine until it is measured.

use acadrust::entities::Spline;
use acadrust::kernel::geom2d::{NurbsCurve, Parameterization};
use acadrust::kernel::space::Plane;
use acadrust::types::Vector3;
use acadrust::Handle;

/// The drawing plane a spline sits on.
///
/// Read from whichever point list the spline actually stores: a fit-point
/// spline carries no control points, and defaulting its elevation to zero
/// would drop every such spline to the ground plane on the first edit.
fn elevation(spl: &Spline) -> f64 {
    spl.control_points
        .first()
        .or_else(|| spl.fit_points.first())
        .map(|v| v.z)
        .unwrap_or(0.0)
}

// ── Conversion ─────────────────────────────────────────────────────────────

/// Convert an acadrust `Spline` to a kernel `NurbsCurve`, weights included.
///
/// A SPLINE is stored one of two ways and both turn up: a control polygon the
/// curve approaches, or fit points it must pass through with optional end
/// tangents. Control points win when they describe a curve; otherwise the fit
/// points are interpolated into one, so a fit-point spline is as editable as
/// any other instead of failing every command that touches it.
///
/// `None` only when neither form has enough points to describe a curve.
///
/// Reads the stored points as XY directly, which is right for the splines the
/// editing commands work on — they run in the world frame. A spline on an
/// extruded plane needs [`spline_to_nurbs_on`] instead.
pub fn spline_to_nurbs(spl: &Spline) -> Option<NurbsCurve> {
    spline_to_nurbs_with(spl, |p| [p.x, p.y], |v| [v.x, v.y])
}

/// [`spline_to_nurbs`] with the points expressed in `plane`'s coordinates.
///
/// The two differ only for a spline whose extrusion normal is not +Z. Where
/// it is, the projection reduces to reading X and Y, so this is the same
/// conversion with the frame made explicit.
///
/// `None` additionally when the plane is degenerate, since there is then no
/// coordinate to express the points in.
pub fn spline_to_nurbs_on(spl: &Spline, plane: &Plane) -> Option<NurbsCurve> {
    let point = |p: &Vector3| plane.project([p.x, p.y, p.z]).unwrap_or([p.x, p.y]);
    let vector = |v: &Vector3| plane.project_vector([v.x, v.y, v.z]).unwrap_or([v.x, v.y]);
    plane.normal()?;
    spline_to_nurbs_with(spl, point, vector)
}

fn spline_to_nurbs_with(
    spl: &Spline,
    point: impl Fn(&Vector3) -> [f64; 2],
    vector: impl Fn(&Vector3) -> [f64; 2],
) -> Option<NurbsCurve> {
    let degree = (spl.degree.max(1)) as usize;
    let control_points: Vec<[f64; 2]> = spl.control_points.iter().map(&point).collect();
    let weights = (!spl.weights.is_empty()).then(|| spl.weights.clone());

    if let Some(curve) =
        NurbsCurve::new(degree, control_points, spl.knots.clone(), weights)
    {
        return Some(curve);
    }

    // No usable control polygon, so this is a fit-point spline.
    let mut fit: Vec<[f64; 2]> = spl.fit_points.iter().map(&point).collect();
    if spl.flags.closed || spl.flags.periodic {
        // The interpolation is a clamped solve and does not model a wrap, so
        // a closed spline came back as an open curve that never returned to
        // its start — and a TRIM against it then cut nothing along the seam.
        // Repeating the first point closes it. The seam is C¹ rather than the
        // C² the rest of the curve has, which is the honest limit of a
        // clamped solve; the alternative was a curve with a gap in it.
        if let (Some(&first), Some(&last)) = (fit.first(), fit.last()) {
            if first != last {
                fit.push(first);
            }
        }
    }
    let tangent = |v: &Vector3| {
        let flat = vector(v);
        (flat[0] * flat[0] + flat[1] * flat[1] > 1e-18).then_some(flat)
    };
    NurbsCurve::interpolate(
        &fit,
        tangent(&spl.begin_tangent),
        tangent(&spl.end_tangent),
        match spl.knot_parameterization {
            2 => Parameterization::Uniform,
            1 => Parameterization::Centripetal,
            _ => Parameterization::Chord,
        },
    )
}

/// Rebuild an acadrust `Spline` from a kernel curve.
///
/// The `template` supplies entity common data and the Z elevation; splines
/// here are planar, so the kernel's 2D points are lifted back onto it.
pub fn nurbs_to_spline(curve: &NurbsCurve, template: &Spline) -> Spline {
    let z = elevation(template);
    let mut spl = template.clone();
    spl.common.handle = Handle::NULL;
    spl.degree = curve.degree() as i32;
    spl.knots = curve.knots().to_vec();
    spl.control_points = curve
        .control_points()
        .iter()
        .map(|p| Vector3::new(p[0], p[1], z))
        .collect();
    spl.weights = if curve.is_rational() {
        curve.weights().to_vec()
    } else {
        Vec::new()
    };
    // Fit points and their tangents describe the original interpolation, which
    // a piece of it no longer satisfies. What comes back is a control-point
    // spline.
    spl.fit_points.clear();
    spl.begin_tangent = Vector3::new(0.0, 0.0, 0.0);
    spl.end_tangent = Vector3::new(0.0, 0.0, 0.0);
    spl
}

// ── Domain and splitting ───────────────────────────────────────────────────

/// The knot values the spline is defined over, `(start, end)`.
pub fn spline_range(spl: &Spline) -> Option<(f64, f64)> {
    Some(spline_to_nurbs(spl)?.domain())
}

/// Split a spline at knot value `t`, returning `(left, right)`.
///
/// Both halves are exact — the kernel raises the cut to full knot
/// multiplicity rather than refitting — and a rational spline stays rational.
/// `None` when the cut falls at or outside an end.
pub fn spline_cut(spl: &Spline, t: f64) -> Option<(Spline, Spline)> {
    let curve = spline_to_nurbs(spl)?;
    let (start, end) = curve.domain();
    if (end - start).abs() < 1e-12 {
        return None;
    }
    let normalised = (t - start) / (end - start);
    let (left, right) = curve.split_at(normalised)?;
    Some((
        nurbs_to_spline(&left, spl),
        nurbs_to_spline(&right, spl),
    ))
}

// ── Sampling ───────────────────────────────────────────────────────────────

/// Sample the spline's XY projection at `n+1` evenly spaced knot values.
/// Returns `(knot_params, xy_points)`, both of length `n+1`.
pub fn spline_sample_xy(spl: &Spline, n: usize) -> (Vec<f64>, Vec<[f64; 2]>) {
    let Some(curve) = spline_to_nurbs(spl) else {
        return (vec![], vec![]);
    };
    let (t0, t1) = curve.domain();
    let ts: Vec<f64> = (0..=n)
        .map(|i| t0 + (t1 - t0) * (i as f64 / n as f64))
        .collect();
    let pts: Vec<[f64; 2]> = ts.iter().map(|t| curve.point_at_knot(*t)).collect();
    (ts, pts)
}

/// The knot parameter closest to the XY point `(x, y)`.
pub fn spline_nearest_t(spl: &Spline, x: f64, y: f64) -> Option<f64> {
    let curve = spline_to_nurbs(spl)?;
    let (t0, t1) = curve.domain();
    let normalised = curve.parameter_at([x, y]);
    Some(t0 + normalised * (t1 - t0))
}

/// Normalise an actual knot parameter into `0..=1` over `[t0, t1]`.
pub fn t_to_rel(t_actual: f64, t0: f64, t1: f64) -> f64 {
    if (t1 - t0).abs() < 1e-12 {
        return 0.0;
    }
    ((t_actual - t0) / (t1 - t0)).clamp(0.0, 1.0)
}

// ── Wire preview ───────────────────────────────────────────────────────────

/// World-space wire points for a Spline.
///
/// Sampled per knot span rather than uniformly, so a curve whose knots bunch
/// up is cut finely where its shape actually changes.
pub fn spline_pts_wire(spl: &Spline) -> Vec<[f32; 3]> {
    let Some(curve) = spline_to_nurbs(spl) else {
        return vec![];
    };
    let elev = elevation(spl) as f32;
    curve
        .tessellate(16)
        .iter()
        .map(|p| [p[0] as f32, p[1] as f32, elev])
        .collect()
}