kui-core 0.1.0-alpha.45

//! Strokes: what a `line` node draws.
//!
//! A line is a run of points and a [`Stroke`]; the core flattens a curve
//! into a polyline here, boxes the run, and emits one
//! [`crate::QuadKind::Segment`] per straight piece. The points live in a
//! per-frame list beside the tree — rebuilt every frame, capacities kept —
//! and a node refers to its run by [`LineId`] the way a text node refers to
//! the frame's text list. The previous frame's list is kept by the same
//! gated buffer swap the text list uses, so a departing line can copy its
//! points out for its ghost.

use crate::color::Color;
use crate::geom::{Rect, Vec2};
use crate::retain::Kept;

/// Index into the frame's line list.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct LineId(pub u32);

/// How a `line` is drawn: a width, a colour, and whether the points are
/// the corners of a polyline or the knots of a curve through them.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct Stroke {
    /// Stroke width in logical px. Round caps at both ends of every
    /// segment, which is also the join between two of them.
    pub width: f32,
    /// The stroke colour. Inside the core it rides in the node's `bg`
    /// slot, so a `transition` eases it and `enter` / `exit` can start
    /// or end it there.
    pub color: Color,
    /// Draw a smooth curve *through* the points (a centripetal Catmull-Rom
    /// spline, flattened by [`flatten_curve`]) instead of the polyline. Two
    /// points are a straight segment either way.
    pub curve: bool,
    /// The marks and gaps the stroke is cut into; [`Dash::SOLID`], the
    /// default, is none.
    pub dash: Dash,
}

impl Stroke {
    pub fn new(width: f32, color: Color) -> Self {
        Self {
            width,
            color,
            curve: false,
            dash: Dash::SOLID,
        }
    }

    pub fn curve(mut self) -> Self {
        self.curve = true;
        self
    }

    /// Cuts the stroke into marks `on` long with gaps `off` long between
    /// them; see [`Dash`].
    pub fn dash(mut self, on: f32, off: f32) -> Self {
        self.dash = Dash {
            offset: self.dash.offset,
            ..Dash::new(on, off)
        };
        self
    }

    /// The whole pattern at once, for a dash-dot or one read from data.
    pub fn dashed(mut self, dash: Dash) -> Self {
        self.dash = dash;
        self
    }

    /// How far into the pattern the stroke starts; see [`Dash::offset`].
    pub fn dash_offset(mut self, offset: f32) -> Self {
        self.dash.offset = offset;
        self
    }
}

/// A stroke's dash pattern (backlog V2): a mark, a gap, a second mark and
/// a second gap, repeated along the stroke's length from its first point.
///
/// The lengths are the ones **seen**, in logical px. Every mark is
/// round-capped, as the stroke itself is, so a mark `on` long is drawn as
/// a capsule whose centre line is `on − width` long, and a mark no longer
/// than the stroke is wide is a dot. (SVG's `stroke-dasharray` measures
/// the centre line instead, which with a round cap makes `4 4` at a width
/// of 4 a solid line; this pattern at any width is SVG's
/// `on − width, off + width`.) The first mark's cap sits where a solid
/// stroke's would, half the width before the first point.
///
/// A dot is as wide as the stroke whatever its mark says, so where a
/// mark and its gap together come to no more than the width the dots
/// meet or overlap and the gap is not seen. Such a gap closes: the marks
/// either side of it are one mark, and a pattern with no gap left draws
/// solid (backlog RG118) — where `{2, 2}` at a width of 8 was 8 px dots
/// every 4 px, a lumpy solid line at a quad a dot. Every gap that is seen
/// is the length given.
///
/// The pattern runs along the arc length of the whole stroke, so it keeps
/// its phase across the corners of a polyline and the pieces of a curve;
/// on a `path` it restarts at every subpath, as SVG's does.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct Dash {
    /// Mark, gap, mark, gap. A two-length pattern is the same pair twice.
    pub pattern: [f32; 4],
    /// How far into the pattern the stroke starts, in logical px: growing
    /// it moves the marks towards the stroke's first point, which is what
    /// a marquee's marching ants are. Any number; it wraps.
    pub offset: f32,
}

impl Default for Dash {
    fn default() -> Self {
        Dash::SOLID
    }
}

/// What a dashed stroke is cut by: the centre-line lengths of the
/// pattern's four entries — mark, gap, mark, gap — and where in it the
/// stroke starts, `0 <= offset < period`. Logical px.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct Cut {
    pub lens: [f32; 4],
    pub offset: f32,
}

impl Cut {
    pub fn period(&self) -> f32 {
        self.lens.iter().sum()
    }

    /// Whether a mark and its gap come to less than `min` together, on
    /// average: a pattern that fine reads as a tint, and is drawn solid.
    pub fn finer_than(&self, min: f32) -> bool {
        let pair = self.period() * 0.5;
        pair.is_nan() || pair < min
    }

    /// The pattern as one number, for a mask's key and for telling a
    /// pattern that moved from one that held.
    pub(crate) fn hash(&self) -> u64 {
        let [a, b, c, d] = self.lens.map(|l| u64::from(l.to_bits()));
        let mut h = a ^ b.rotate_left(16) ^ c.rotate_left(32) ^ d.rotate_left(48);
        h = h.wrapping_mul(0x0000_0100_0000_01b3) ^ u64::from(self.offset.to_bits());
        h.wrapping_mul(0x0000_0100_0000_01b3)
    }
}

/// The most marks one stroke is cut into; a pattern that would make more
/// — a period of a pixel along a line no screen is long enough for —
/// draws solid, which is what it would read as.
pub const MAX_MARKS: usize = 16384;

impl Dash {
    /// No dashes: the stroke unbroken.
    pub const SOLID: Dash = Dash {
        pattern: [0.0; 4],
        offset: 0.0,
    };

    /// Marks `on` long with gaps `off` long between them.
    pub fn new(on: f32, off: f32) -> Self {
        Dash {
            pattern: [on, off, on, off],
            offset: 0.0,
        }
    }

    /// The pattern as the bindings spell it: one length (marks and gaps
    /// alike), two (a mark and a gap) or four (a mark, a gap, a second
    /// mark, a second gap — a dash-dot). None for any other count.
    pub fn of(lengths: &[f32]) -> Option<Self> {
        let pattern = match *lengths {
            [a] => [a, a, a, a],
            [a, b] => [a, b, a, b],
            [a, b, c, d] => [a, b, c, d],
            _ => return None,
        };
        Some(Dash {
            pattern,
            offset: 0.0,
        })
    }

    /// [`Self::offset`] set.
    pub fn offset(mut self, offset: f32) -> Self {
        self.offset = offset;
        self
    }

    /// Whether the pattern leaves the stroke unbroken: no gap in it, or a
    /// length that is not a number.
    pub fn is_solid(&self) -> bool {
        self.cut(0.0).is_none()
    }

    /// The pattern as centre-line lengths for a stroke `width` wide, or
    /// None for one that draws solid: a length that is not finite, or no
    /// gap anywhere that the width leaves seen. Negative lengths are zero,
    /// a pair that is zero altogether is the other pair, and a gap the
    /// width closes joins the marks either side of it.
    #[inline]
    pub fn cut(&self, width: f32) -> Option<Cut> {
        // The solid stroke, which is nearly every stroke, is told by its
        // zeroes and pays for nothing below (backlog C52).
        if self.pattern == Dash::SOLID.pattern {
            return None;
        }
        self.cut_pattern(width)
    }

    #[cold]
    #[inline(never)]
    fn cut_pattern(&self, width: f32) -> Option<Cut> {
        if !self.pattern.iter().all(|l| l.is_finite()) || !self.offset.is_finite() {
            return None;
        }
        let [a, b, c, d] = self.pattern.map(|l| l.max(0.0));
        if b <= 0.0 && d <= 0.0 {
            return None;
        }
        let (first, second) = match (a + b > 0.0, c + d > 0.0) {
            (true, true) => ([a, b], [c, d]),
            (true, false) => ([a, b], [a, b]),
            (false, _) => ([c, d], [c, d]),
        };
        let w = width.max(0.0);
        // The mark's caps are part of what is seen, so they come out of
        // its centre line and go into the gap's.
        let centre = |[on, off]: [f32; 2]| {
            let mark = (on - w).max(0.0);
            [mark, on + off - mark]
        };
        let ([m0, g0], [m1, g1]) = (centre(first), centre(second));
        let period = m0 + g0 + m1 + g1;
        // A gap is seen as its centre line less the caps of the marks
        // either side, half the width each; one that leaves nothing closes
        // and its marks are one (RG118). The merged pair is written twice,
        // so the walk is unchanged, and where the merged mark starts at the
        // second mark the offset moves with it.
        let (lens, from) = match (g0 > w, g1 > w) {
            (true, true) => ([m0, g0, m1, g1], 0.0),
            (false, true) => {
                let m = m0 + g0 + m1;
                ([m, g1, m, g1], 0.0)
            }
            (true, false) => {
                let m = m1 + g1 + m0;
                ([m, g0, m, g0], m0 + g0)
            }
            (false, false) => return None,
        };
        Some(Cut {
            lens,
            offset: (self.offset - from).rem_euclid(period),
        })
    }
}

impl Cut {
    /// Calls `mark(from, to)` for every mark along the polyline `points`,
    /// in order — a mark that turns a corner is one call per piece it
    /// lies on, meeting at the corner, and a dot is a call with both ends
    /// the same. False, with nothing called, for a stroke that draws
    /// solid instead: a pattern [`Self::finer_than`] `min`, more than
    /// [`MAX_MARKS`] marks, or a length that is zero or not finite.
    pub fn marks(&self, points: &[Vec2], min: f32, mut mark: impl FnMut(Vec2, Vec2)) -> bool {
        let period = self.period();
        let piece = |pair: &[Vec2]| (pair[1].x - pair[0].x).hypot(pair[1].y - pair[0].y);
        let total: f32 = points.windows(2).map(piece).sum();
        // Two marks a period; `!(..)` so a NaN draws solid too.
        let marks = total / period * 2.0;
        // A stroke of no length has no mark to lie on and is the dot its
        // solid one is (RG116).
        if self.finer_than(min) || marks.is_nan() || marks > MAX_MARKS as f32 || total <= 0.0 {
            return false;
        }
        // Where the stroke starts in the pattern: the entry and what is
        // left of it.
        let (mut i, mut left) = (0, self.lens[0]);
        let mut skip = self.offset;
        while skip >= left && skip > 0.0 {
            skip -= left;
            i = (i + 1) & 3;
            left = self.lens[i];
        }
        left -= skip;
        // A mark that ended exactly on a corner has been drawn up to it;
        // the entry after it starts on the next piece.
        for pair in points.windows(2) {
            let len = piece(pair);
            if len <= 0.0 {
                continue;
            }
            let (a, b) = (pair[0], pair[1]);
            let at = |s: f32| {
                let t = s / len;
                Vec2::new(a.x + (b.x - a.x) * t, a.y + (b.y - a.y) * t)
            };
            let mut pos = 0.0;
            loop {
                let take = left.min(len - pos);
                if i & 1 == 0 {
                    mark(at(pos), at(pos + take));
                }
                pos += take;
                left -= take;
                if left > 0.0 {
                    break;
                }
                i = (i + 1) & 3;
                left = self.lens[i];
                // The piece is used up: what starts here starts on the
                // next one, except a dot, which has nowhere else to be
                // when this is the last.
                if pos >= len && left > 0.0 {
                    break;
                }
            }
        }
        true
    }
}

/// One stroke's run in the frame's point list. The points are stored
/// relative to the node's box, so a node that eases or slides carries
/// them along.
#[derive(Clone, Copy, Debug, PartialEq)]
pub(crate) struct Run {
    pub first: u32,
    pub len: u32,
    /// Logical px.
    pub width: f32,
    /// What cuts the stroke into marks; None for a solid one.
    pub dash: Option<Cut>,
}

/// The frame's strokes, and the previous frame's while an `exit` needs it.
#[derive(Default)]
pub struct LineStore {
    runs: Kept<Run>,
    points: Kept<Vec2>,
}

/// How far a stroke's box extends past its points: half the width, plus
/// two logical px so a backend's edge ramp (`AA` = 0.75 physical px each
/// side, sampled at pixel centres) is never cut by the quad, at scale 1
/// included.
pub(crate) fn pad(width: f32) -> f32 {
    width.max(0.0) * 0.5 + 2.0
}

/// A curve span is flattened into one piece per this many logical px of
/// chord, at least one and at most [`CURVE_MAX_PIECES`]. Fixed rather than
/// tolerance-driven so the piece count is a function of the declared
/// geometry alone — every binding gets the same segments, and the
/// conformance corpus can pin them.
pub const CURVE_STEP: f32 = 6.0;
pub const CURVE_MAX_PIECES: usize = 32;

impl LineStore {
    /// Starts a frame. `keep_prev` retains the list just finished so a
    /// departing line's ghost can copy its points out of it.
    pub(crate) fn begin_frame(&mut self, keep_prev: bool) {
        self.runs.begin(keep_prev);
        self.points.begin(keep_prev);
    }

    /// Adds a stroke through `points` (parent-box coordinates): flattens a
    /// curve, boxes the run, and stores the points relative to the box.
    /// Returns the id and the box, or None for fewer than two points,
    /// which draw nothing.
    pub(crate) fn push(&mut self, points: &[Vec2], stroke: &Stroke) -> Option<(LineId, Rect)> {
        if points.len() < 2 {
            return None;
        }
        let first = self.points.len();
        if stroke.curve && points.len() > 2 {
            flatten_curve(points, &mut self.points);
        } else {
            self.points.extend_from_slice(points);
        }
        let run = &mut self.points[first..];
        let (mut min, mut max) = (run[0], run[0]);
        for p in run.iter() {
            min.x = min.x.min(p.x);
            min.y = min.y.min(p.y);
            max.x = max.x.max(p.x);
            max.y = max.y.max(p.y);
        }
        let pad = pad(stroke.width);
        let origin = Vec2::new(min.x - pad, min.y - pad);
        for p in run.iter_mut() {
            p.x -= origin.x;
            p.y -= origin.y;
        }
        let rect = Rect::new(
            origin.x,
            origin.y,
            max.x - min.x + 2.0 * pad,
            max.y - min.y + 2.0 * pad,
        );
        let id = LineId(self.runs.len() as u32);
        self.runs.push(Run {
            first: first as u32,
            len: (self.points.len() - first) as u32,
            width: stroke.width,
            dash: stroke.dash.cut(stroke.width),
        });
        Some((id, rect))
    }

    /// This frame's run: its width and its points, relative to the node.
    pub(crate) fn run(&self, id: LineId) -> (Run, &[Vec2]) {
        let run = self.runs[id.0 as usize];
        (
            run,
            &self.points[run.first as usize..(run.first + run.len) as usize],
        )
    }

    /// The same, read from the previous frame's list (a departing line's
    /// id indexes that list, not this frame's). Empty for an id the kept
    /// frame does not have, which cannot happen while `begin_frame` keeps
    /// the two in step.
    pub(crate) fn prev_run(&self, id: LineId) -> (Run, &[Vec2]) {
        match self.runs.prev().get(id.0 as usize) {
            Some(run) => (
                *run,
                &self.points.prev()[run.first as usize..(run.first + run.len) as usize],
            ),
            None => (
                Run {
                    first: 0,
                    len: 0,
                    width: 0.0,
                    dash: None,
                },
                &[],
            ),
        }
    }

    /// Runs this frame.
    pub fn len(&self) -> usize {
        self.runs.len()
    }

    pub fn is_empty(&self) -> bool {
        self.runs.is_empty()
    }
}

/// Flattens a **centripetal** Catmull-Rom spline through `knots` (at least
/// three) into `out`, starting at the first knot and ending at the last.
/// Each span is cut into `ceil(chord / CURVE_STEP)` pieces, clamped to
/// `1..=CURVE_MAX_PIECES`.
///
/// Centripetal means the spline's knot parameter advances by the square
/// root of each chord instead of by 1. A *uniform*
/// spline — the textbook one, and what this was until it drew a mind
/// map — ignores how far apart its knots are, so knots that are close
/// together get as much parameter as knots that are far apart, and the
/// curve has to move fast through the tight ones. On an elbow (a long run,
/// then a sharp turn) that shows up as a bow out of the wrong side of the
/// corner; on knots spaced unevenly enough it becomes a cusp or a loop,
/// which a centripetal span provably never has. It bows less
/// too, though it is not overshoot-free: a spline that must pass *through*
/// a corner has to lean into it. A shape that should only be *pulled*
/// towards its middle points is a Bézier, and the caller samples one
/// (`examples/rust/widgets/line.rs`).
///
/// The end knots are not doubled — a repeated knot is a zero-length chord,
/// which centripetal has no parameter for. Each end gets a mirrored
/// phantom instead (`2·p1 − p2`), which spaces evenly and leaves the first
/// span's tangent along its own chord. A coincident pair of *real* knots
/// takes the same branch, so a duplicated point in a caller's list stays a
/// harmless kink rather than a division by zero.
///
/// The chords are rolled forward rather than recomputed, so a span costs
/// two square roots and not six; `frame_1k_curves` is the bench that
/// watches the rest of it.
pub fn flatten_curve(knots: &[Vec2], out: &mut Vec<Vec2>) {
    let n = knots.len();
    if n < 2 {
        return;
    }
    out.push(knots[0]);
    // This span's chord and its knot step (√chord), and the span before's:
    // a zero chord means there is no neighbour on that side, either
    // because the run ends there or because the two knots coincide.
    let (mut prev, mut prev_step) = (0.0, 0.0);
    let (mut chord, mut step) = chord_and_step(knots[0], knots[1]);
    for i in 0..n - 1 {
        let (p1, p2) = (knots[i], knots[i + 1]);
        let (next, next_step) = match knots.get(i + 2) {
            Some(&p) => chord_and_step(p2, p),
            None => (0.0, 0.0),
        };
        let pieces = ((chord / CURVE_STEP).ceil() as usize).clamp(1, CURVE_MAX_PIECES);
        if chord == 0.0 {
            // Nothing to parameterize: the span is a point.
            out.push(p2);
        } else {
            let mirror = |p: Vec2, q: Vec2| Vec2::new(2.0 * p.x - q.x, 2.0 * p.y - q.y);
            let (p0, s0) = match prev > 0.0 {
                true => (knots[i - 1], prev_step),
                false => (mirror(p1, p2), step),
            };
            let (p3, s2) = match next > 0.0 {
                true => (knots[i + 2], next_step),
                false => (mirror(p2, p1), step),
            };
            // Knot times: 0, then one step per span.
            let span = Span::new([p0, p1, p2, p3], [0.0, s0, s0 + step, s0 + step + s2]);
            for k in 1..pieces {
                out.push(span.at(s0 + step * (k as f32 / pieces as f32)));
            }
            // The knot itself rather than the evaluation at its time, so
            // the run passes through it bit for bit whatever the
            // arithmetic rounds to.
            out.push(p2);
        }
        (prev, prev_step) = (chord, step);
        (chord, step) = (next, next_step);
    }
}

/// A span's length and the parameter it is worth: `√chord` is Lee's
/// α = ½, the centripetal one. α = 0 would be `1.0` (uniform) and α = 1
/// the chord itself (chordal).
fn chord_and_step(a: Vec2, b: Vec2) -> (f32, f32) {
    let chord = ((b.x - a.x).powi(2) + (b.y - a.y).powi(2)).sqrt();
    (chord, chord.sqrt())
}

/// One span of the spline, evaluated between `p[1]` and `p[2]` by Barry
/// and Goldman's pyramid: three interpolations of the knots in their own
/// times, then two of those, then one. Written this way rather than as a
/// cubic in `t` because the times are no longer evenly spaced.
///
/// A span is built once and asked for every piece, because the pyramid's
/// five distinct denominators are the same five numbers each time. They
/// are reciprocals, so `at(t[2])` is only *nearly* `p[2]` — the caller
/// pushes the knot itself instead of asking for it.
struct Span {
    p: [Vec2; 4],
    t: [f32; 4],
    /// `1/(t[1]-t[0])`, `1/(t[2]-t[1])`, `1/(t[3]-t[2])`, `1/(t[2]-t[0])`,
    /// `1/(t[3]-t[1])`, in the order the pyramid needs them.
    inv: [f32; 5],
}

impl Span {
    fn new(p: [Vec2; 4], t: [f32; 4]) -> Self {
        let inv = [
            1.0 / (t[1] - t[0]),
            1.0 / (t[2] - t[1]),
            1.0 / (t[3] - t[2]),
            1.0 / (t[2] - t[0]),
            1.0 / (t[3] - t[1]),
        ];
        Span { p, t, inv }
    }

    fn at(&self, u: f32) -> Vec2 {
        let (p, t) = (&self.p, &self.t);
        let blend = |a: Vec2, b: Vec2, ta: f32, inv: f32| {
            let w = (u - ta) * inv;
            Vec2::new(a.x + (b.x - a.x) * w, a.y + (b.y - a.y) * w)
        };
        let a1 = blend(p[0], p[1], t[0], self.inv[0]);
        let a2 = blend(p[1], p[2], t[1], self.inv[1]);
        let a3 = blend(p[2], p[3], t[2], self.inv[2]);
        let b1 = blend(a1, a2, t[0], self.inv[3]);
        let b2 = blend(a2, a3, t[1], self.inv[4]);
        blend(b1, b2, t[1], self.inv[1])
    }
}

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

    #[test]
    fn a_curve_passes_through_its_knots_and_ends_where_they_end() {
        let knots = [
            Vec2::new(0.0, 0.0),
            Vec2::new(30.0, 40.0),
            Vec2::new(60.0, 0.0),
        ];
        let mut out = Vec::new();
        flatten_curve(&knots, &mut out);
        assert_eq!(out[0], knots[0]);
        assert_eq!(*out.last().unwrap(), knots[2]);
        // Chord 50 → 9 pieces per span, 1 + 9 + 9 points.
        assert_eq!(out.len(), 19);
        assert_eq!(out[9], knots[1]);
    }

    /// How sharply the run doubles back, worst piece, in degrees. A
    /// smoothly flattened curve turns a few degrees a piece; a cusp turns
    /// most of the way round.
    fn worst_turn(pts: &[Vec2]) -> f32 {
        pts.windows(3).fold(0.0f32, |worst, w| {
            let (a, b) = (
                Vec2::new(w[1].x - w[0].x, w[1].y - w[0].y),
                Vec2::new(w[2].x - w[1].x, w[2].y - w[1].y),
            );
            let len = |v: Vec2| (v.x * v.x + v.y * v.y).sqrt();
            let (la, lb) = (len(a), len(b));
            if la < 1e-6 || lb < 1e-6 {
                return worst;
            }
            let cos = ((a.x * b.x + a.y * b.y) / (la * lb)).clamp(-1.0, 1.0);
            worst.max(cos.acos().to_degrees())
        })
    }

    /// Whether any two non-adjacent pieces of the run cross.
    fn ties_a_loop(pts: &[Vec2]) -> bool {
        let side =
            |a: Vec2, b: Vec2, c: Vec2| (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);
        (0..pts.len() - 1).any(|i| {
            (i + 2..pts.len() - 1).any(|j| {
                let (a, b, c, d) = (pts[i], pts[i + 1], pts[j], pts[j + 1]);
                side(a, b, c) * side(a, b, d) < 0.0 && side(c, d, a) * side(c, d, b) < 0.0
            })
        })
    }

    /// The reason the parameterization is centripetal and not uniform.
    /// Chords 671, 36 and 328: a knot pair nineteen times tighter than its
    /// neighbours. A uniform parameter hands that 36px chord as much curve
    /// as the 671px one, and the middle span has to tie a loop to spend
    /// it — at `CURVE_STEP`'s own sampling, one crossing and a piece that
    /// turns 95°. Centripetal spends parameter by `√chord`, so the tight
    /// pair is just a corner.
    #[test]
    fn a_tight_knot_between_two_long_ones_neither_loops_nor_cusps() {
        let knots = [
            Vec2::new(0.0, 400.0),
            Vec2::new(600.0, 100.0),
            Vec2::new(630.0, 80.0),
            Vec2::new(560.0, 400.0),
        ];
        let mut out = Vec::new();
        flatten_curve(&knots, &mut out);
        assert!(!ties_a_loop(&out), "the run crosses itself");
        assert!(worst_turn(&out) < 45.0, "cusp: {:.1}°", worst_turn(&out));
    }

    /// A caller's list may repeat a point — a mind map with two cards at
    /// the same place, a path snapped to a grid. The mirrored phantom
    /// covers it: a zero chord has no `√chord` to divide by, and every
    /// point that comes back is a number.
    #[test]
    fn a_repeated_knot_is_a_kink_and_not_a_division_by_zero() {
        let knots = [
            Vec2::new(0.0, 0.0),
            Vec2::new(40.0, 0.0),
            Vec2::new(40.0, 0.0),
            Vec2::new(40.0, 40.0),
            Vec2::new(80.0, 40.0),
        ];
        let mut out = Vec::new();
        flatten_curve(&knots, &mut out);
        assert!(out.iter().all(|p| p.x.is_finite() && p.y.is_finite()));
        assert_eq!(out[0], knots[0]);
        assert_eq!(*out.last().unwrap(), knots[4]);
    }

    #[test]
    fn two_points_are_one_segment_even_as_a_curve() {
        let mut store = LineStore::default();
        let (id, rect) = store
            .push(
                &[Vec2::new(10.0, 10.0), Vec2::new(50.0, 40.0)],
                &Stroke::new(2.0, Color::WHITE).curve(),
            )
            .unwrap();
        let (run, pts) = store.run(id);
        assert_eq!(run.len, 2);
        assert_eq!(run.width, 2.0);
        // Padded by half the width plus two: the box starts at (7, 7).
        assert_eq!((rect.x, rect.y, rect.w, rect.h), (7.0, 7.0, 46.0, 36.0));
        assert_eq!(pts[0], Vec2::new(3.0, 3.0));
        assert_eq!(pts[1], Vec2::new(43.0, 33.0));
    }

    #[test]
    fn one_point_draws_nothing() {
        let mut store = LineStore::default();
        assert!(
            store
                .push(&[Vec2::new(1.0, 1.0)], &Stroke::new(1.0, Color::WHITE))
                .is_none()
        );
    }

    #[test]
    fn the_previous_frame_is_kept_only_when_asked() {
        let mut store = LineStore::default();
        let (id, _) = store
            .push(
                &[Vec2::new(0.0, 0.0), Vec2::new(4.0, 0.0)],
                &Stroke::new(1.0, Color::WHITE),
            )
            .unwrap();
        store.begin_frame(true);
        assert_eq!(store.prev_run(id).0.len, 2);
        assert!(store.is_empty());
        store.begin_frame(false);
        assert_eq!(store.prev_run(id).0.len, 0);
    }
}