kurbo/

stroke.rs

// Copyright 2023 the Kurbo Authors
// SPDX-License-Identifier: Apache-2.0 OR MIT

use core::{borrow::Borrow, f64::consts::PI};

use alloc::vec::Vec;

use smallvec::SmallVec;

#[cfg(not(feature = "std"))]
use crate::common::FloatFuncs;

use crate::{
    common::solve_quadratic, fit_to_bezpath, fit_to_bezpath_opt, offset::CubicOffset, Affine, Arc,
    BezPath, CubicBez, Line, ParamCurve, ParamCurveArclen, PathEl, PathSeg, Point, QuadBez, Vec2,
};

/// Defines the connection between two segments of a stroke.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub enum Join {
    /// A straight line connecting the segments.
    Bevel,
    /// The segments are extended to their natural intersection point.
    Miter,
    /// An arc between the segments.
    Round,
}

/// Defines the shape to be drawn at the ends of a stroke.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub enum Cap {
    /// Flat cap.
    Butt,
    /// Square cap with dimensions equal to half the stroke width.
    Square,
    /// Rounded cap with radius equal to half the stroke width.
    Round,
}

/// Describes the visual style of a stroke.
#[derive(Clone, Debug, PartialEq)]
#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub struct Stroke {
    /// Width of the stroke.
    pub width: f64,
    /// Style for connecting segments of the stroke.
    pub join: Join,
    /// Limit for miter joins.
    pub miter_limit: f64,
    /// Style for capping the beginning of an open subpath.
    pub start_cap: Cap,
    /// Style for capping the end of an open subpath.
    pub end_cap: Cap,
    /// Lengths of dashes in alternating on/off order.
    pub dash_pattern: Dashes,
    /// Offset of the first dash.
    pub dash_offset: f64,
}

/// Options for path stroking.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct StrokeOpts {
    opt_level: StrokeOptLevel,
}

/// Optimization level for computing
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum StrokeOptLevel {
    /// Adaptively subdivide segments in half.
    Subdivide,
    /// Compute optimized subdivision points to minimize error.
    Optimized,
}

impl Default for StrokeOpts {
    fn default() -> Self {
        let opt_level = StrokeOptLevel::Subdivide;
        StrokeOpts { opt_level }
    }
}

impl Default for Stroke {
    fn default() -> Self {
        Self {
            width: 1.0,
            join: Join::Round,
            miter_limit: 4.0,
            start_cap: Cap::Round,
            end_cap: Cap::Round,
            dash_pattern: SmallVec::default(),
            dash_offset: 0.0,
        }
    }
}

impl Stroke {
    /// Creates a new stroke with the specified width.
    pub fn new(width: f64) -> Self {
        Self {
            width,
            ..Stroke::default()
        }
    }

    /// Builder method for setting the join style.
    pub fn with_join(mut self, join: Join) -> Self {
        self.join = join;
        self
    }

    /// Builder method for setting the limit for miter joins.
    pub fn with_miter_limit(mut self, limit: f64) -> Self {
        self.miter_limit = limit;
        self
    }

    /// Builder method for setting the cap style for the start of the stroke.
    pub fn with_start_cap(mut self, cap: Cap) -> Self {
        self.start_cap = cap;
        self
    }

    /// Builder method for setting the cap style for the end of the stroke.
    pub fn with_end_cap(mut self, cap: Cap) -> Self {
        self.end_cap = cap;
        self
    }

    /// Builder method for setting the cap style.
    pub fn with_caps(mut self, cap: Cap) -> Self {
        self.start_cap = cap;
        self.end_cap = cap;
        self
    }

    /// Builder method for setting the dashing parameters.
    pub fn with_dashes<P>(mut self, offset: f64, pattern: P) -> Self
    where
        P: IntoIterator,
        P::Item: Borrow<f64>,
    {
        self.dash_offset = offset;
        self.dash_pattern.clear();
        self.dash_pattern
            .extend(pattern.into_iter().map(|dash| *dash.borrow()));
        self
    }
}

impl StrokeOpts {
    /// Set optimization level for computing stroke outlines.
    pub fn opt_level(mut self, opt_level: StrokeOptLevel) -> Self {
        self.opt_level = opt_level;
        self
    }
}

/// Collection of values representing lengths in a dash pattern.
pub type Dashes = SmallVec<[f64; 4]>;

/// Internal structure used for creating strokes.
#[derive(Default)]
struct StrokeCtx {
    // As a possible future optimization, we might not need separate storage
    // for forward and backward paths, we can add forward to the output in-place.
    // However, this structure is clearer and the cost fairly modest.
    output: BezPath,
    forward_path: BezPath,
    backward_path: BezPath,
    start_pt: Point,
    start_norm: Vec2,
    start_tan: Vec2,
    last_pt: Point,
    last_tan: Vec2,
    // Precomputation of the join threshold, to optimize per-join logic.
    // If hypot < (hypot + dot) * join_thresh, omit join altogether.
    join_thresh: f64,
}

/// Expand a stroke into a fill.
///
/// The `tolerance` parameter controls the accuracy of the result. In general,
/// the number of subdivisions in the output scales to the -1/6 power of the
/// parameter, for example making it 1/64 as big generates twice as many
/// segments. The appropriate value depends on the application; if the result
/// of the stroke will be scaled up, a smaller value is needed.
///
/// This method attempts a fairly high degree of correctness, but ultimately
/// is based on computing parallel curves and adding joins and caps, rather than
/// computing the rigorously correct parallel sweep (which requires evolutes in
/// the general case). See [Nehab 2020] for more discussion.
///
/// [Nehab 2020]: https://dl.acm.org/doi/10.1145/3386569.3392392
pub fn stroke(
    path: impl IntoIterator<Item = PathEl>,
    style: &Stroke,
    opts: &StrokeOpts,
    tolerance: f64,
) -> BezPath {
    if style.dash_pattern.is_empty() {
        stroke_undashed(path, style, tolerance, *opts)
    } else {
        let dashed = dash(path.into_iter(), style.dash_offset, &style.dash_pattern);
        stroke_undashed(dashed, style, tolerance, *opts)
    }
}

/// Version of stroke expansion for styles with no dashes.
fn stroke_undashed(
    path: impl IntoIterator<Item = PathEl>,
    style: &Stroke,
    tolerance: f64,
    opts: StrokeOpts,
) -> BezPath {
    let mut ctx = StrokeCtx {
        join_thresh: 2.0 * tolerance / style.width,
        ..StrokeCtx::default()
    };
    for el in path {
        let p0 = ctx.last_pt;
        match el {
            PathEl::MoveTo(p) => {
                ctx.finish(style);
                ctx.start_pt = p;
                ctx.last_pt = p;
            }
            PathEl::LineTo(p1) => {
                if p1 != p0 {
                    let tangent = p1 - p0;
                    ctx.do_join(style, tangent);
                    ctx.last_tan = tangent;
                    ctx.do_line(style, tangent, p1);
                }
            }
            PathEl::QuadTo(p1, p2) => {
                if p1 != p0 || p2 != p0 {
                    let q = QuadBez::new(p0, p1, p2);
                    let (tan0, tan1) = PathSeg::Quad(q).tangents();
                    ctx.do_join(style, tan0);
                    ctx.do_cubic(style, q.raise(), tolerance, opts);
                    ctx.last_tan = tan1;
                }
            }
            PathEl::CurveTo(p1, p2, p3) => {
                if p1 != p0 || p2 != p0 || p3 != p0 {
                    let c = CubicBez::new(p0, p1, p2, p3);
                    let (tan0, tan1) = PathSeg::Cubic(c).tangents();
                    ctx.do_join(style, tan0);
                    ctx.do_cubic(style, c, tolerance, opts);
                    ctx.last_tan = tan1;
                }
            }
            PathEl::ClosePath => {
                if p0 != ctx.start_pt {
                    let tangent = ctx.start_pt - p0;
                    ctx.do_join(style, tangent);
                    ctx.last_tan = tangent;
                    ctx.do_line(style, tangent, ctx.start_pt);
                }
                ctx.finish_closed(style);
            }
        }
    }
    ctx.finish(style);
    ctx.output
}

fn round_cap(out: &mut BezPath, tolerance: f64, center: Point, norm: Vec2) {
    round_join(out, tolerance, center, norm, PI);
}

fn round_join(out: &mut BezPath, tolerance: f64, center: Point, norm: Vec2, angle: f64) {
    let a = Affine::new([norm.x, norm.y, -norm.y, norm.x, center.x, center.y]);
    let arc = Arc::new(Point::ORIGIN, (1.0, 1.0), PI - angle, angle, 0.0);
    arc.to_cubic_beziers(tolerance, |p1, p2, p3| out.curve_to(a * p1, a * p2, a * p3));
}

fn round_join_rev(out: &mut BezPath, tolerance: f64, center: Point, norm: Vec2, angle: f64) {
    let a = Affine::new([norm.x, norm.y, norm.y, -norm.x, center.x, center.y]);
    let arc = Arc::new(Point::ORIGIN, (1.0, 1.0), PI - angle, angle, 0.0);
    arc.to_cubic_beziers(tolerance, |p1, p2, p3| out.curve_to(a * p1, a * p2, a * p3));
}

fn square_cap(out: &mut BezPath, close: bool, center: Point, norm: Vec2) {
    let a = Affine::new([norm.x, norm.y, -norm.y, norm.x, center.x, center.y]);
    out.line_to(a * Point::new(1.0, 1.0));
    out.line_to(a * Point::new(-1.0, 1.0));
    if close {
        out.close_path();
    } else {
        out.line_to(a * Point::new(-1.0, 0.0));
    }
}

fn extend_reversed(out: &mut BezPath, elements: &[PathEl]) {
    for i in (1..elements.len()).rev() {
        let end = elements[i - 1].end_point().unwrap();
        match elements[i] {
            PathEl::LineTo(_) => out.line_to(end),
            PathEl::QuadTo(p1, _) => out.quad_to(p1, end),
            PathEl::CurveTo(p1, p2, _) => out.curve_to(p2, p1, end),
            _ => unreachable!(),
        }
    }
}

fn fit_with_opts(co: &CubicOffset, tolerance: f64, opts: StrokeOpts) -> BezPath {
    match opts.opt_level {
        StrokeOptLevel::Subdivide => fit_to_bezpath(co, tolerance),
        StrokeOptLevel::Optimized => fit_to_bezpath_opt(co, tolerance),
    }
}

impl StrokeCtx {
    /// Append forward and backward paths to output.
    fn finish(&mut self, style: &Stroke) {
        // TODO: scale
        let tolerance = 1e-3;
        if self.forward_path.is_empty() {
            return;
        }
        self.output.extend(&self.forward_path);
        let back_els = self.backward_path.elements();
        let return_p = back_els[back_els.len() - 1].end_point().unwrap();
        let d = self.last_pt - return_p;
        match style.end_cap {
            Cap::Butt => self.output.line_to(return_p),
            Cap::Round => round_cap(&mut self.output, tolerance, self.last_pt, d),
            Cap::Square => square_cap(&mut self.output, false, self.last_pt, d),
        }
        extend_reversed(&mut self.output, back_els);
        match style.start_cap {
            Cap::Butt => self.output.close_path(),
            Cap::Round => round_cap(&mut self.output, tolerance, self.start_pt, self.start_norm),
            Cap::Square => square_cap(&mut self.output, true, self.start_pt, self.start_norm),
        }

        self.forward_path.truncate(0);
        self.backward_path.truncate(0);
    }

    /// Finish a closed path
    fn finish_closed(&mut self, style: &Stroke) {
        if self.forward_path.is_empty() {
            return;
        }
        self.do_join(style, self.start_tan);
        self.output.extend(&self.forward_path);
        self.output.close_path();
        let back_els = self.backward_path.elements();
        let last_pt = back_els[back_els.len() - 1].end_point().unwrap();
        self.output.move_to(last_pt);
        extend_reversed(&mut self.output, back_els);
        self.output.close_path();
        self.forward_path.truncate(0);
        self.backward_path.truncate(0);
    }

    fn do_join(&mut self, style: &Stroke, tan0: Vec2) {
        // TODO: scale
        let tolerance = 1e-3;
        let scale = 0.5 * style.width / tan0.hypot();
        let norm = scale * Vec2::new(-tan0.y, tan0.x);
        let p0 = self.last_pt;
        if self.forward_path.elements().is_empty() {
            self.forward_path.move_to(p0 - norm);
            self.backward_path.move_to(p0 + norm);
            self.start_tan = tan0;
            self.start_norm = norm;
        } else {
            let ab = self.last_tan;
            let cd = tan0;
            let cross = ab.cross(cd);
            let dot = ab.dot(cd);
            let hypot = cross.hypot(dot);
            // possible TODO: a minor speedup could be squaring both sides
            if dot <= 0.0 || cross.abs() >= hypot * self.join_thresh {
                match style.join {
                    Join::Bevel => {
                        self.forward_path.line_to(p0 - norm);
                        self.backward_path.line_to(p0 + norm);
                    }
                    Join::Miter => {
                        if 2.0 * hypot < (hypot + dot) * style.miter_limit.powi(2) {
                            // TODO: maybe better to store last_norm or derive from path?
                            let last_scale = 0.5 * style.width / ab.hypot();
                            let last_norm = last_scale * Vec2::new(-ab.y, ab.x);
                            if cross > 0.0 {
                                let fp_last = p0 - last_norm;
                                let fp_this = p0 - norm;
                                let h = ab.cross(fp_this - fp_last) / cross;
                                let miter_pt = fp_this - cd * h;
                                self.forward_path.line_to(miter_pt);
                            } else if cross < 0.0 {
                                let fp_last = p0 + last_norm;
                                let fp_this = p0 + norm;
                                let h = ab.cross(fp_this - fp_last) / cross;
                                let miter_pt = fp_this - cd * h;
                                self.backward_path.line_to(miter_pt);
                            }
                        }
                        self.forward_path.line_to(p0 - norm);
                        self.backward_path.line_to(p0 + norm);
                    }
                    Join::Round => {
                        let angle = cross.atan2(dot);
                        if angle > 0.0 {
                            self.backward_path.line_to(p0 + norm);
                            round_join(&mut self.forward_path, tolerance, p0, norm, angle);
                        } else {
                            self.forward_path.line_to(p0 - norm);
                            round_join_rev(&mut self.backward_path, tolerance, p0, -norm, -angle);
                        }
                    }
                }
            }
        }
    }

    fn do_line(&mut self, style: &Stroke, tangent: Vec2, p1: Point) {
        let scale = 0.5 * style.width / tangent.hypot();
        let norm = scale * Vec2::new(-tangent.y, tangent.x);
        self.forward_path.line_to(p1 - norm);
        self.backward_path.line_to(p1 + norm);
        self.last_pt = p1;
    }

    fn do_cubic(&mut self, style: &Stroke, c: CubicBez, tolerance: f64, opts: StrokeOpts) {
        // First, detect degenerate linear case

        // Ordinarily, this is the direction of the chord, but if the chord is very
        // short, we take the longer control arm.
        let chord = c.p3 - c.p0;
        let mut chord_ref = chord;
        let mut chord_ref_hypot2 = chord_ref.hypot2();
        let d01 = c.p1 - c.p0;
        if d01.hypot2() > chord_ref_hypot2 {
            chord_ref = d01;
            chord_ref_hypot2 = chord_ref.hypot2();
        }
        let d23 = c.p3 - c.p2;
        if d23.hypot2() > chord_ref_hypot2 {
            chord_ref = d23;
            chord_ref_hypot2 = chord_ref.hypot2();
        }
        // Project Bézier onto chord
        let p0 = c.p0.to_vec2().dot(chord_ref);
        let p1 = c.p1.to_vec2().dot(chord_ref);
        let p2 = c.p2.to_vec2().dot(chord_ref);
        let p3 = c.p3.to_vec2().dot(chord_ref);
        const ENDPOINT_D: f64 = 0.01;
        if p3 <= p0
            || p1 > p2
            || p1 < p0 + ENDPOINT_D * (p3 - p0)
            || p2 > p3 - ENDPOINT_D * (p3 - p0)
        {
            // potentially a cusp inside
            let x01 = d01.cross(chord_ref);
            let x23 = d23.cross(chord_ref);
            let x03 = chord.cross(chord_ref);
            let thresh = tolerance.powi(2) * chord_ref_hypot2;
            if x01 * x01 < thresh && x23 * x23 < thresh && x03 * x03 < thresh {
                // control points are nearly co-linear
                let midpoint = c.p0.midpoint(c.p3);
                // Mapping back from projection of reference chord
                let ref_vec = chord_ref / chord_ref_hypot2;
                let ref_pt = midpoint - 0.5 * (p0 + p3) * ref_vec;
                self.do_linear(style, c, [p0, p1, p2, p3], ref_pt, ref_vec);
                return;
            }
        }

        // A tuning parameter for regularization. A value too large may distort the curve,
        // while a value too small may fail to generate smooth curves. This is a somewhat
        // arbitrary value, and should be revisited.
        const DIM_TUNE: f64 = 0.25;
        let dimension = tolerance * DIM_TUNE;
        let co = CubicOffset::new_regularized(c, -0.5 * style.width, dimension);
        let forward = fit_with_opts(&co, tolerance, opts);
        self.forward_path.extend(forward.into_iter().skip(1));
        let co = CubicOffset::new_regularized(c, 0.5 * style.width, dimension);
        let backward = fit_with_opts(&co, tolerance, opts);
        self.backward_path.extend(backward.into_iter().skip(1));
        self.last_pt = c.p3;
    }

    /// Do a cubic which is actually linear.
    ///
    /// The `p` argument is the control points projected to the reference chord.
    /// The ref arguments are the inverse map of a projection back to the client
    /// coordinate space.
    fn do_linear(
        &mut self,
        style: &Stroke,
        c: CubicBez,
        p: [f64; 4],
        ref_pt: Point,
        ref_vec: Vec2,
    ) {
        // Always do round join, to model cusp as limit of finite curvature (see Nehab).
        let style = Stroke::new(style.width).with_join(Join::Round);
        // Tangents of endpoints (for connecting to joins)
        let (tan0, tan1) = PathSeg::Cubic(c).tangents();
        self.last_tan = tan0;
        // find cusps
        let c0 = p[1] - p[0];
        let c1 = 2.0 * p[2] - 4.0 * p[1] + 2.0 * p[0];
        let c2 = p[3] - 3.0 * p[2] + 3.0 * p[1] - p[0];
        let roots = solve_quadratic(c0, c1, c2);
        // discard cusps right at endpoints
        const EPSILON: f64 = 1e-6;
        for t in roots {
            if t > EPSILON && t < 1.0 - EPSILON {
                let mt = 1.0 - t;
                let z = mt * (mt * mt * p[0] + 3.0 * t * (mt * p[1] + t * p[2])) + t * t * t * p[3];
                let p = ref_pt + z * ref_vec;
                let tan = p - self.last_pt;
                self.do_join(&style, tan);
                self.do_line(&style, tan, p);
                self.last_tan = tan;
            }
        }
        let tan = c.p3 - self.last_pt;
        self.do_join(&style, tan);
        self.do_line(&style, tan, c.p3);
        self.last_tan = tan;
        self.do_join(&style, tan1);
    }
}

/// An implementation of dashing as an iterator-to-iterator transformation.
#[doc(hidden)]
pub struct DashIterator<'a, T> {
    inner: T,
    input_done: bool,
    closepath_pending: bool,
    dashes: &'a [f64],
    dash_ix: usize,
    init_dash_ix: usize,
    init_dash_remaining: f64,
    init_is_active: bool,
    is_active: bool,
    state: DashState,
    current_seg: PathSeg,
    t: f64,
    dash_remaining: f64,
    seg_remaining: f64,
    start_pt: Point,
    last_pt: Point,
    stash: Vec<PathEl>,
    stash_ix: usize,
}

#[derive(PartialEq, Eq)]
enum DashState {
    NeedInput,
    ToStash,
    Working,
    FromStash,
}

impl<T: Iterator<Item = PathEl>> Iterator for DashIterator<'_, T> {
    type Item = PathEl;

    fn next(&mut self) -> Option<PathEl> {
        loop {
            match self.state {
                DashState::NeedInput => {
                    if self.input_done {
                        return None;
                    }
                    self.get_input();
                    if self.input_done {
                        return None;
                    }
                    self.state = DashState::ToStash;
                }
                DashState::ToStash => {
                    if let Some(el) = self.step() {
                        self.stash.push(el);
                    }
                }
                DashState::Working => {
                    if let Some(el) = self.step() {
                        return Some(el);
                    }
                }
                DashState::FromStash => {
                    if let Some(el) = self.stash.get(self.stash_ix) {
                        self.stash_ix += 1;
                        return Some(*el);
                    } else {
                        self.stash.clear();
                        self.stash_ix = 0;
                        if self.input_done {
                            return None;
                        }
                        if self.closepath_pending {
                            self.closepath_pending = false;
                            self.state = DashState::NeedInput;
                        } else {
                            self.state = DashState::ToStash;
                        }
                    }
                }
            }
        }
    }
}

fn seg_to_el(el: &PathSeg) -> PathEl {
    match el {
        PathSeg::Line(l) => PathEl::LineTo(l.p1),
        PathSeg::Quad(q) => PathEl::QuadTo(q.p1, q.p2),
        PathSeg::Cubic(c) => PathEl::CurveTo(c.p1, c.p2, c.p3),
    }
}

const DASH_ACCURACY: f64 = 1e-6;

/// Create a new dashing iterator.
///
/// Handling of dashes is fairly orthogonal to stroke expansion. This iterator
/// is an internal detail of the stroke expansion logic, but is also available
/// separately, and is expected to be useful when doing stroke expansion on
/// GPU.
///
/// It is implemented as an iterator-to-iterator transform. Because it consumes
/// the input sequentially and produces consistent output with correct joins,
/// it requires internal state and may allocate.
///
/// Accuracy is currently hard-coded to 1e-6. This is better than generally
/// expected, and care is taken to get cusps correct, among other things.
pub fn dash<'a>(
    inner: impl Iterator<Item = PathEl> + 'a,
    dash_offset: f64,
    dashes: &'a [f64],
) -> impl Iterator<Item = PathEl> + 'a {
    dash_impl(inner, dash_offset, dashes)
}

// This is only a separate function to make `DashIterator::new()` typecheck.
fn dash_impl<T: Iterator<Item = PathEl>>(
    inner: T,
    dash_offset: f64,
    dashes: &[f64],
) -> DashIterator<'_, T> {
    // ensure that offset is positive and minimal by normalization using period
    let period = dashes.iter().sum();
    let dash_offset = dash_offset.rem_euclid(period);

    let mut dash_ix = 0;
    let mut dash_remaining = dashes[dash_ix] - dash_offset;
    let mut is_active = true;
    // Find place in dashes array for initial offset.
    while dash_remaining < 0.0 {
        dash_ix = (dash_ix + 1) % dashes.len();
        dash_remaining += dashes[dash_ix];
        is_active = !is_active;
    }
    DashIterator {
        inner,
        input_done: false,
        closepath_pending: false,
        dashes,
        dash_ix,
        init_dash_ix: dash_ix,
        init_dash_remaining: dash_remaining,
        init_is_active: is_active,
        is_active,
        state: DashState::NeedInput,
        current_seg: PathSeg::Line(Line::new(Point::ORIGIN, Point::ORIGIN)),
        t: 0.0,
        dash_remaining,
        seg_remaining: 0.0,
        start_pt: Point::ORIGIN,
        last_pt: Point::ORIGIN,
        stash: Vec::new(),
        stash_ix: 0,
    }
}

impl<'a, T: Iterator<Item = PathEl>> DashIterator<'a, T> {
    #[doc(hidden)]
    #[deprecated(since = "0.10.4", note = "use dash() instead")]
    pub fn new(inner: T, dash_offset: f64, dashes: &'a [f64]) -> Self {
        dash_impl(inner, dash_offset, dashes)
    }

    fn get_input(&mut self) {
        loop {
            if self.closepath_pending {
                self.handle_closepath();
                break;
            }
            let Some(next_el) = self.inner.next() else {
                self.input_done = true;
                self.state = DashState::FromStash;
                return;
            };
            let p0 = self.last_pt;
            match next_el {
                PathEl::MoveTo(p) => {
                    if !self.stash.is_empty() {
                        self.state = DashState::FromStash;
                    }
                    self.start_pt = p;
                    self.last_pt = p;
                    self.reset_phase();
                    continue;
                }
                PathEl::LineTo(p1) => {
                    let l = Line::new(p0, p1);
                    self.seg_remaining = l.arclen(DASH_ACCURACY);
                    self.current_seg = PathSeg::Line(l);
                    self.last_pt = p1;
                }
                PathEl::QuadTo(p1, p2) => {
                    let q = QuadBez::new(p0, p1, p2);
                    self.seg_remaining = q.arclen(DASH_ACCURACY);
                    self.current_seg = PathSeg::Quad(q);
                    self.last_pt = p2;
                }
                PathEl::CurveTo(p1, p2, p3) => {
                    let c = CubicBez::new(p0, p1, p2, p3);
                    self.seg_remaining = c.arclen(DASH_ACCURACY);
                    self.current_seg = PathSeg::Cubic(c);
                    self.last_pt = p3;
                }
                PathEl::ClosePath => {
                    self.closepath_pending = true;
                    if p0 != self.start_pt {
                        let l = Line::new(p0, self.start_pt);
                        self.seg_remaining = l.arclen(DASH_ACCURACY);
                        self.current_seg = PathSeg::Line(l);
                        self.last_pt = self.start_pt;
                    } else {
                        self.handle_closepath();
                    }
                }
            }
            break;
        }
        self.t = 0.0;
    }

    /// Move arc length forward to next event.
    fn step(&mut self) -> Option<PathEl> {
        let mut result = None;
        if self.state == DashState::ToStash && self.stash.is_empty() {
            if self.is_active {
                result = Some(PathEl::MoveTo(self.current_seg.start()));
            } else {
                self.state = DashState::Working;
            }
        } else if self.dash_remaining < self.seg_remaining {
            // next transition is a dash transition
            let seg = self.current_seg.subsegment(self.t..1.0);
            let t1 = seg.inv_arclen(self.dash_remaining, DASH_ACCURACY);
            if self.is_active {
                let subseg = seg.subsegment(0.0..t1);
                result = Some(seg_to_el(&subseg));
                self.state = DashState::Working;
            } else {
                let p = seg.eval(t1);
                result = Some(PathEl::MoveTo(p));
            }
            self.is_active = !self.is_active;
            self.t += t1 * (1.0 - self.t);
            self.seg_remaining -= self.dash_remaining;
            self.dash_ix += 1;
            if self.dash_ix == self.dashes.len() {
                self.dash_ix = 0;
            }
            self.dash_remaining = self.dashes[self.dash_ix];
        } else {
            if self.is_active {
                let seg = self.current_seg.subsegment(self.t..1.0);
                result = Some(seg_to_el(&seg));
            }
            self.dash_remaining -= self.seg_remaining;
            self.get_input();
        }
        result
    }

    fn handle_closepath(&mut self) {
        if self.state == DashState::ToStash {
            // Have looped back without breaking a dash, just play it back
            self.stash.push(PathEl::ClosePath);
        } else if self.is_active {
            // connect with path in stash, skip MoveTo.
            self.stash_ix = 1;
        }
        self.state = DashState::FromStash;
        self.reset_phase();
    }

    fn reset_phase(&mut self) {
        self.dash_ix = self.init_dash_ix;
        self.dash_remaining = self.init_dash_remaining;
        self.is_active = self.init_is_active;
    }
}

#[cfg(test)]
mod tests {
    use crate::{
        dash, segments, stroke, Cap::Butt, CubicBez, Join::Miter, Line, PathSeg, Shape, Stroke,
        StrokeOpts,
    };

    // A degenerate stroke with a cusp at the endpoint.
    #[test]
    fn pathological_stroke() {
        let curve = CubicBez::new(
            (602.469, 286.585),
            (641.975, 286.585),
            (562.963, 286.585),
            (562.963, 286.585),
        );
        let path = curve.into_path(0.1);
        let stroke_style = Stroke::new(1.);
        let stroked = stroke(path, &stroke_style, &StrokeOpts::default(), 0.001);
        assert!(stroked.is_finite());
    }

    // Test cases adapted from https://github.com/linebender/vello/pull/388
    #[test]
    fn broken_strokes() {
        let broken_cubics = [
            [
                (465.24423, 107.11105),
                (475.50754, 107.11105),
                (475.50754, 107.11105),
                (475.50754, 107.11105),
            ],
            [(0., -0.01), (128., 128.001), (128., -0.01), (0., 128.001)], // Near-cusp
            [(0., 0.), (0., -10.), (0., -10.), (0., 10.)],                // Flat line with 180
            [(10., 0.), (0., 0.), (20., 0.), (10., 0.)],                  // Flat line with 2 180s
            [(39., -39.), (40., -40.), (40., -40.), (0., 0.)],            // Flat diagonal with 180
            [(40., 40.), (0., 0.), (200., 200.), (0., 0.)],               // Diag w/ an internal 180
            [(0., 0.), (1e-2, 0.), (-1e-2, 0.), (0., 0.)],                // Circle
            // Flat line with no turns:
            [
                (400.75, 100.05),
                (400.75, 100.05),
                (100.05, 300.95),
                (100.05, 300.95),
            ],
            [(0.5, 0.), (0., 0.), (20., 0.), (10., 0.)], // Flat line with 2 180s
            [(10., 0.), (0., 0.), (10., 0.), (10., 0.)], // Flat line with a 180
        ];
        let stroke_style = Stroke::new(30.).with_caps(Butt).with_join(Miter);
        for cubic in &broken_cubics {
            let path = CubicBez::new(cubic[0], cubic[1], cubic[2], cubic[3]).into_path(0.1);
            let stroked = stroke(path, &stroke_style, &StrokeOpts::default(), 0.001);
            assert!(stroked.is_finite());
        }
    }

    #[test]
    fn dash_sequence() {
        let shape = Line::new((0.0, 0.0), (21.0, 0.0));
        let dashes = [1., 5., 2., 5.];
        let expansion = [
            PathSeg::Line(Line::new((6., 0.), (8., 0.))),
            PathSeg::Line(Line::new((13., 0.), (14., 0.))),
            PathSeg::Line(Line::new((19., 0.), (21., 0.))),
            PathSeg::Line(Line::new((0., 0.), (1., 0.))),
        ];
        let iter = segments(dash(shape.path_elements(0.), 0., &dashes));
        assert_eq!(iter.collect::<Vec<PathSeg>>(), expansion);
    }

    #[test]
    fn dash_sequence_offset() {
        // Same as dash_sequence, but with a dash offset
        // of 3, which skips the first dash and cuts into
        // the first gap.
        let shape = Line::new((0.0, 0.0), (21.0, 0.0));
        let dashes = [1., 5., 2., 5.];
        let expansion = [
            PathSeg::Line(Line::new((3., 0.), (5., 0.))),
            PathSeg::Line(Line::new((10., 0.), (11., 0.))),
            PathSeg::Line(Line::new((16., 0.), (18., 0.))),
        ];
        let iter = segments(dash(shape.path_elements(0.), 3., &dashes));
        assert_eq!(iter.collect::<Vec<PathSeg>>(), expansion);
    }

    #[test]
    fn dash_negative_offset() {
        let shape = Line::new((0.0, 0.0), (28.0, 0.0));
        let dashes = [4., 2.];
        let pos = segments(dash(shape.path_elements(0.), 60., &dashes)).collect::<Vec<PathSeg>>();
        let neg = segments(dash(shape.path_elements(0.), -60., &dashes)).collect::<Vec<PathSeg>>();
        assert_eq!(neg, pos);
    }
}