//! Arc-length parameterised polylines ("tracks") that brushes lay art along.
//!
//! A track is one flattened subpath. `frame(s)` gives the point and unit tangent at arc length
//! `s`; tangents are blended across the gentle vertices of flattened curves (so warped art bends
//! smoothly) but stay discontinuous at real corners (so art folds there, like Illustrator).

use vectorcraft_geom::{BezPath, PathEl, Point, Vec2};

/// Turn angle (degrees) above which a vertex is a corner.
pub const CORNER_DEG: f64 = 30.0;

#[derive(Clone, Debug)]
pub struct Track {
    /// Vertices; closed tracks repeat the first vertex at the end.
    pub verts: Vec<Point>,
    /// Cumulative arc length at each vertex.
    pub cum: Vec<f64>,
    pub closed: bool,
    /// Unit direction of each segment.
    pub dirs: Vec<Vec2>,
    /// Smoothed unit tangent at each vertex.
    pub tangents: Vec<Vec2>,
    /// Is the vertex a corner (turn > [`CORNER_DEG`])? Open ends are never corners.
    pub corner: Vec<bool>,
}

fn unit(v: Vec2) -> Vec2 {
    let l = v.hypot();
    if l < 1e-12 { Vec2::new(1.0, 0.0) } else { v / l }
}

/// Left-hand normal in y-down document space (for a path running +x it points +y).
pub fn normal(t: Vec2) -> Vec2 {
    Vec2::new(-t.y, t.x)
}

impl Track {
    fn new(mut pts: Vec<Point>, closed: bool) -> Option<Track> {
        pts.dedup_by(|a, b| a.distance(*b) < 1e-9);
        if closed && pts.len() > 2 && pts[0].distance(pts[pts.len() - 1]) < 1e-9 {
            pts.pop();
        }
        if pts.len() < 2 {
            return None;
        }
        let closed = closed && pts.len() > 2;
        if closed {
            pts.push(pts[0]);
        }
        let nseg = pts.len() - 1;
        let dirs: Vec<Vec2> = (0..nseg).map(|i| unit(pts[i + 1] - pts[i])).collect();
        let mut cum = vec![0.0];
        for i in 0..nseg {
            cum.push(cum[i] + pts[i].distance(pts[i + 1]));
        }
        let mut tangents = Vec::with_capacity(nseg + 1);
        let mut corner = Vec::with_capacity(nseg + 1);
        let cos_limit = CORNER_DEG.to_radians().cos();
        for i in 0..=nseg {
            let incoming = if i > 0 {
                Some(dirs[i - 1])
            } else if closed {
                Some(dirs[nseg - 1])
            } else {
                None
            };
            let outgoing = if i < nseg {
                Some(dirs[i])
            } else if closed {
                Some(dirs[0])
            } else {
                None
            };
            match (incoming, outgoing) {
                (Some(a), Some(b)) => {
                    corner.push(a.dot(b) < cos_limit);
                    tangents.push(unit(a + b));
                }
                (a, b) => {
                    // An end of an open track (both `None` only for a track without segments).
                    corner.push(false);
                    tangents.push(a.or(b).unwrap_or(Vec2::new(1.0, 0.0)));
                }
            }
        }
        Some(Track { verts: pts, cum, closed, dirs, tangents, corner })
    }

    pub fn len(&self) -> f64 {
        *self.cum.last().unwrap_or(&0.0)
    }
    pub fn is_empty(&self) -> bool {
        self.len() <= 1e-9
    }
    pub fn segments(&self) -> usize {
        self.dirs.len()
    }

    /// Incoming and outgoing unit directions at vertex `i`.
    pub fn in_out(&self, i: usize) -> (Vec2, Vec2) {
        let n = self.segments();
        let inc = if i > 0 {
            self.dirs[i - 1]
        } else if self.closed {
            self.dirs[n - 1]
        } else {
            self.dirs[0]
        };
        let out = if i < n {
            self.dirs[i]
        } else if self.closed {
            self.dirs[0]
        } else {
            self.dirs[n - 1]
        };
        (inc, out)
    }

    /// Arc-length positions of the corner vertices (closed tracks list vertex 0 once).
    pub fn corner_positions(&self) -> Vec<(usize, f64)> {
        let last = if self.closed { self.segments() } else { self.segments() + 1 };
        (0..last).filter(|i| self.corner[*i]).map(|i| (i, self.cum[i])).collect()
    }

    /// Point and unit tangent at arc length `s`. Open tracks extrapolate beyond their ends along
    /// the end tangents; closed tracks wrap.
    pub fn frame(&self, s: f64) -> (Point, Vec2) {
        let l = self.len();
        let n = self.segments();
        let s = if self.closed && l > 0.0 { s.rem_euclid(l) } else { s };
        if s <= 0.0 && !self.closed {
            let t = self.dirs[0];
            return (self.verts[0] + t * s, t);
        }
        if s >= l && !self.closed {
            let t = self.dirs[n - 1];
            return (self.verts[n] + t * (s - l), t);
        }
        // Binary search the segment.
        let i = match self.cum.binary_search_by(|c| c.partial_cmp(&s).unwrap_or(std::cmp::Ordering::Less)) {
            Ok(i) => i.min(n - 1),
            Err(i) => i.saturating_sub(1).min(n - 1),
        };
        let seg = self.cum[i + 1] - self.cum[i];
        let u = if seg > 1e-12 { ((s - self.cum[i]) / seg).clamp(0.0, 1.0) } else { 0.0 };
        let p = self.verts[i].lerp(self.verts[i + 1], u);
        let t0 = if self.corner[i] { self.dirs[i] } else { self.tangents[i] };
        let t1 = if self.corner[i + 1] { self.dirs[i] } else { self.tangents[i + 1] };
        (p, unit(t0 + (t1 - t0) * u))
    }

    /// The point at arc length `s`, offset `d` along the left normal.
    pub fn map(&self, s: f64, d: f64) -> Point {
        let (p, t) = self.frame(s);
        p + normal(t) * d
    }
}

/// Flatten `bp` into tracks (one per subpath) with `tol` accuracy.
pub fn tracks(bp: &BezPath, tol: f64) -> Vec<Track> {
    let mut out = vec![];
    let mut cur: Vec<Point> = vec![];
    let flush = |cur: &mut Vec<Point>, closed: bool, out: &mut Vec<Track>| {
        if let Some(t) = Track::new(std::mem::take(cur), closed) {
            out.push(t);
        }
    };
    let mut els = vec![];
    kurbo::flatten(bp.iter(), tol.max(1e-4), |el| els.push(el));
    for el in els {
        match el {
            PathEl::MoveTo(p) => {
                flush(&mut cur, false, &mut out);
                cur.push(p);
            }
            PathEl::LineTo(p) => cur.push(p),
            PathEl::ClosePath => flush(&mut cur, true, &mut out),
            _ => {}
        }
    }
    flush(&mut cur, false, &mut out);
    out
}

/// Deterministic pseudo-random numbers (SplitMix64), seeded from the path so a brushed stroke
/// looks the same on every frame and in exports.
#[derive(Clone, Debug)]
pub struct Rng(u64);

impl Rng {
    pub fn new(seed: u64) -> Self {
        Rng(seed ^ 0x9e37_79b9_7f4a_7c15)
    }
    pub fn next_u64(&mut self) -> u64 {
        self.0 = self.0.wrapping_add(0x9e37_79b9_7f4a_7c15);
        let mut z = self.0;
        z = (z ^ (z >> 30)).wrapping_mul(0xbf58_476d_1ce4_e5b9);
        z = (z ^ (z >> 27)).wrapping_mul(0x94d0_49bb_1331_11eb);
        z ^ (z >> 31)
    }
    /// Uniform in [0, 1).
    pub fn unit(&mut self) -> f64 {
        (self.next_u64() >> 11) as f64 / (1u64 << 53) as f64
    }
    /// Uniform in [a, b] (a == b returns a without consuming randomness differently).
    pub fn range(&mut self, a: f64, b: f64) -> f64 {
        let u = self.unit();
        if (b - a).abs() < 1e-12 { a } else { a + (b - a) * u }
    }
}

/// A stable seed for a path (FNV-1a over its coordinates' bits, quantised to 1/64 pt).
pub fn seed_of(bp: &BezPath, salt: &str) -> u64 {
    let mut h: u64 = 0xcbf2_9ce4_8422_2325;
    let mut eat = |v: u64| {
        h ^= v;
        h = h.wrapping_mul(0x0000_0100_0000_01b3);
    };
    for b in salt.bytes() {
        eat(b as u64);
    }
    for el in bp.elements() {
        let pts: Vec<Point> = match *el {
            PathEl::MoveTo(p) | PathEl::LineTo(p) => vec![p],
            PathEl::QuadTo(a, b) => vec![a, b],
            PathEl::CurveTo(a, b, c) => vec![a, b, c],
            PathEl::ClosePath => vec![],
        };
        for p in pts {
            eat((p.x * 64.0).round() as i64 as u64);
            eat((p.y * 64.0).round() as i64 as u64);
        }
    }
    h
}

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

    #[test]
    fn straight_track_frames() {
        let mut bp = BezPath::new();
        bp.move_to((0.0, 0.0));
        bp.line_to((100.0, 0.0));
        let t = &tracks(&bp, 0.1)[0];
        assert_eq!(t.len(), 100.0);
        let (p, d) = t.frame(25.0);
        assert_eq!(p, Point::new(25.0, 0.0));
        assert_eq!(d, Vec2::new(1.0, 0.0));
        assert_eq!(t.map(50.0, 5.0), Point::new(50.0, 5.0));
        assert_eq!(t.map(110.0, 0.0), Point::new(110.0, 0.0));
    }

    #[test]
    fn closed_square_has_four_corners() {
        let mut bp = BezPath::new();
        bp.move_to((0.0, 0.0));
        bp.line_to((10.0, 0.0));
        bp.line_to((10.0, 10.0));
        bp.line_to((0.0, 10.0));
        bp.close_path();
        let t = &tracks(&bp, 0.1)[0];
        assert!(t.closed);
        assert_eq!(t.len(), 40.0);
        assert_eq!(t.corner_positions().len(), 4);
        assert_eq!(t.frame(45.0).0, Point::new(5.0, 0.0));
    }

    #[test]
    fn rng_is_deterministic() {
        let (mut a, mut b) = (Rng::new(7), Rng::new(7));
        for _ in 0..10 {
            assert_eq!(a.next_u64(), b.next_u64());
        }
        let v = Rng::new(1).range(2.0, 3.0);
        assert!((2.0..=3.0).contains(&v));
    }
}