graph-explorer-style 0.2.0

//! Scene model + animator: a target `Scene` (dense, index-keyed) is handed to
//! an `Animator`, which eases the displayed scene toward it each frame and
//! fills a caller-owned render-ready `Frame`. Pure — no GPU, no wasm — so it's
//! host-unit-testable, benchmarkable, and allocation-auditable.
//!
//! Everything here is keyed by `NodeIndex`/`LabelId` from `crate::interner`;
//! all element types are Copy and all per-frame containers are refilled in
//! place, so the steady state (graph at rest OR mid-transition) allocates
//! nothing once warm. `tests/zero_alloc.rs` enforces that with a counting
//! allocator — if you add an allocation to `set_target` (warm path),
//! `advance`, `interpolate_into`, `frame_into`, `tick_into`, or
//! `tick_positioned_into`, that test fails.

use crate::anim::{Easing, HaloEmphasis, lerp, lerp2, lerp4};
use crate::interner::{LabelId, NodeIndex};

#[derive(Debug, Clone, Copy, PartialEq)]
pub struct SceneNode {
    pub pos: [f32; 2],
    pub radius: f32,
    pub color: [f32; 4],
    pub opacity: f32,
    pub shape: u32,
    pub label: LabelId,
}

#[derive(Debug, Clone, Copy, PartialEq)]
pub struct SceneEdge {
    pub a: NodeIndex,
    pub b: NodeIndex,
    pub color: [f32; 4],
    pub width: f32,
    pub opacity: f32,
    /// Interned label text, or `EMPTY_LABEL` for an unlabelled edge. A
    /// `LabelId` and not a `String`: `SceneEdge` must stay `Copy` with no heap
    /// field, or the zero-alloc guarantee on the per-frame path (see
    /// `tests/zero_alloc.rs`) breaks.
    pub label: LabelId,
}

#[derive(Debug, Clone, Copy, PartialEq)]
pub struct SceneHalo {
    pub node: NodeIndex,
    pub emphasis: HaloEmphasis,
}

/// A fully-built target scene. `nodes` is dense over the interner's index
/// space; `None` means "not in this scene". Determinism comes from index
/// order (intern order), which is stable per session. (The old BTreeMap gave
/// lexicographic-id order; nothing may depend on that.)
#[derive(Debug, Clone, Default, PartialEq)]
pub struct Scene {
    pub nodes: Vec<Option<SceneNode>>,
    pub edges: Vec<SceneEdge>,
    pub halos: Vec<SceneHalo>,
}

impl Scene {
    /// Place `node` at `ix`, growing the dense vec as the interner grows.
    pub fn set(&mut self, ix: NodeIndex, node: SceneNode) {
        let i = ix as usize;
        if i >= self.nodes.len() { self.nodes.resize(i + 1, None); }
        self.nodes[i] = Some(node);
    }
    pub fn get(&self, ix: NodeIndex) -> Option<&SceneNode> {
        self.nodes.get(ix as usize).and_then(|s| s.as_ref())
    }
    pub fn clear(&mut self) {
        self.nodes.clear();
        self.edges.clear();
        self.halos.clear();
    }
}

// Render-ready outputs. `FrameNode.index` carries the node identity so the
// single wasm upload pass can fill hit targets and screen positions without
// a resolve; Strings appear nowhere in a Frame.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct FrameNode {
    pub index: NodeIndex,
    pub pos: [f32; 2],
    pub radius: f32,
    pub color: [f32; 4],
    pub opacity: f32,
    pub shape: u32,
    pub label: LabelId,
}
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct FrameEdge { pub a: [f32; 2], pub b: [f32; 2], pub color: [f32; 4], pub width: f32, pub opacity: f32, pub label: LabelId }
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct FrameHalo { pub node: NodeIndex, pub pos: [f32; 2], pub radius: f32, pub emphasis: HaloEmphasis }

/// Caller-owned, refilled in place every tick. Compact (only present nodes).
#[derive(Debug, Clone, Default, PartialEq)]
pub struct Frame {
    pub nodes: Vec<FrameNode>,
    pub edges: Vec<FrameEdge>,
    pub halos: Vec<FrameHalo>,
}

/// Eases a displayed `Scene` toward a target `Scene`, filling `Frame`s.
pub struct Animator {
    displayed: Scene,
    /// Snapshot of `displayed` at transition start. Kept allocated across
    /// transitions (`clone_from`) so retargeting doesn't allocate once warm.
    from_buf: Scene,
    to: Scene,
    start_ms: f64,
    dur_ms: f32,
    easing: Easing,
    animating: bool,
    /// Scratch for `interpolate_into` (nodes only; edges/halos come from
    /// `to`). Persistent for the same reason as `from_buf`.
    scratch: Vec<Option<SceneNode>>,
}

impl Default for Animator { fn default() -> Self { Self::new() } }

impl Animator {
    pub fn new() -> Self {
        Self {
            displayed: Scene::default(),
            from_buf: Scene::default(),
            to: Scene::default(),
            start_ms: 0.0,
            dur_ms: 0.0,
            easing: Easing::Linear,
            animating: false,
            scratch: Vec::new(),
        }
    }

    /// Point the animator at a new target. Tweens from the currently-displayed
    /// scene when `tween` is on, `dur_ms > 0`, and there's something to move
    /// from; otherwise snaps. (Same contract as before the index rewrite.)
    pub fn set_target(&mut self, target: Scene, now_ms: f64, dur_ms: f32, easing: Easing, tween: bool) {
        let has_displayed = self.displayed.nodes.iter().any(|n| n.is_some());
        if tween && dur_ms > 0.0 && has_displayed {
            self.from_buf.nodes.clone_from(&self.displayed.nodes);
            // from_buf carries nodes only; edges/halos always come from `to`.
            self.from_buf.edges.clear();
            self.from_buf.halos.clear();
            // Edges/halos switch to the new target's immediately (one-time
            // copy here, not per animating tick in `advance`: `to` is
            // immutable for the transition's duration).
            self.displayed.edges.clone_from(&target.edges);
            self.displayed.halos.clone_from(&target.halos);
            self.to = target;
            self.start_ms = now_ms;
            self.dur_ms = dur_ms;
            self.easing = easing;
            self.animating = true;
        } else {
            self.displayed.nodes.clone_from(&target.nodes);
            self.displayed.edges.clone_from(&target.edges);
            self.displayed.halos.clone_from(&target.halos);
            self.to = target;
            self.animating = false;
        }
    }

    /// Advance to `now_ms` and fill `out`. Commits the target when the
    /// transition completes.
    pub fn tick_into(&mut self, now_ms: f64, out: &mut Frame) {
        self.advance(now_ms);
        frame_into(&self.displayed, out);
    }

    /// Advance, then override node positions from `pos` (dense, indexed by
    /// `NodeIndex`; `None` = animator keeps its interpolated position) before
    /// filling `out`.
    ///
    /// This is the boundary between the two things that want to move a node:
    /// the **simulation owns position**, the **animator owns appearance**.
    /// The override is written into `self.displayed` IN PLACE — not into a
    /// throwaway copy — so `displayed` stays truthful about what's on screen;
    /// the next `set_target` snapshots `displayed` as the transition's `from`,
    /// and a stale snapshot makes any node that later leaves the simulation
    /// teleport (the Slice A bug; its regression test lives below).
    ///
    /// Caller contract (unchanged from Slice A): `pos` must cover every node
    /// the simulation owns, on every call — an owned-but-omitted index falls
    /// back to plain position-tweening for that call, which reads as the node
    /// fighting the simulation.
    pub fn tick_positioned_into(&mut self, now_ms: f64, pos: &[Option<[f32; 2]>], out: &mut Frame) {
        self.advance(now_ms);
        for (i, slot) in self.displayed.nodes.iter_mut().enumerate() {
            if let (Some(n), Some(Some(p))) = (slot.as_mut(), pos.get(i)) {
                n.pos = *p;
            }
        }
        frame_into(&self.displayed, out);
    }

    /// Shared transition bookkeeping for both tick entry points.
    fn advance(&mut self, now_ms: f64) {
        if self.animating {
            let raw = ((now_ms - self.start_ms) as f32 / self.dur_ms).clamp(0.0, 1.0);
            let t = self.easing.apply(raw);
            interpolate_into(&self.from_buf, &self.to, t, &mut self.scratch);
            // keep visible node state current for a mid-flight retarget
            // (edges/halos were copied from `to` once, in `set_target`)
            self.displayed.nodes.clone_from(&self.scratch);
            if raw >= 1.0 {
                self.displayed.nodes.clone_from(&self.to.nodes);
                self.animating = false;
            }
        }
    }

    /// Positions of the current *target* scene's nodes (post-transition).
    pub fn target_positions(&self) -> impl Iterator<Item = [f32; 2]> + '_ {
        self.to.nodes.iter().flatten().map(|n| n.pos)
    }

    /// Target position of one node (post-transition), if present.
    pub fn target_position(&self, ix: NodeIndex) -> Option<[f32; 2]> {
        self.to.get(ix).map(|n| n.pos)
    }
}

/// Interpolate two node sets at eased `t` into `out` (cleared and refilled).
/// Nodes present in both lerp; entering (to-only) fade in; exiting
/// (from-only) fade out and are kept at reduced alpha until `t == 1`.
fn interpolate_into(from: &Scene, to: &Scene, t: f32, out: &mut Vec<Option<SceneNode>>) {
    let len = from.nodes.len().max(to.nodes.len());
    out.clear();
    out.resize(len, None);
    for (i, slot) in out.iter_mut().enumerate() {
        let f = from.nodes.get(i).and_then(|s| s.as_ref());
        let g = to.nodes.get(i).and_then(|s| s.as_ref());
        *slot = match (f, g) {
            (Some(fnode), Some(tn)) => Some(SceneNode {
                pos: lerp2(fnode.pos, tn.pos, t),
                radius: lerp(fnode.radius, tn.radius, t),
                color: lerp4(fnode.color, tn.color, t),
                opacity: lerp(fnode.opacity, tn.opacity, t),
                shape: tn.shape,
                label: tn.label,
            }),
            (None, Some(tn)) => Some(SceneNode { opacity: tn.opacity * t, ..*tn }),
            (Some(fnode), None) if t < 1.0 => {
                Some(SceneNode { opacity: fnode.opacity * (1.0 - t), ..*fnode })
            }
            _ => None,
        };
    }
}

/// Resolve a scene into `out` (cleared and refilled): edge endpoints and halo
/// positions/radii looked up by index; references to an absent node are
/// skipped. O(1) per lookup, no hashing, no Strings.
fn frame_into(s: &Scene, out: &mut Frame) {
    out.nodes.clear();
    out.edges.clear();
    out.halos.clear();
    for (i, n) in s.nodes.iter().enumerate() {
        if let Some(n) = n {
            out.nodes.push(FrameNode {
                index: i as NodeIndex,
                pos: n.pos, radius: n.radius, color: n.color,
                opacity: n.opacity, shape: n.shape, label: n.label,
            });
        }
    }
    for e in &s.edges {
        let (Some(a), Some(b)) = (s.get(e.a), s.get(e.b)) else { continue };
        out.edges.push(FrameEdge { a: a.pos, b: b.pos, color: e.color, width: e.width, opacity: e.opacity, label: e.label });
    }
    for h in &s.halos {
        let Some(n) = s.get(h.node) else { continue };
        out.halos.push(FrameHalo { node: h.node, pos: n.pos, radius: n.radius, emphasis: h.emphasis });
    }
}

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

    fn node(pos: [f32; 2], radius: f32, opacity: f32) -> SceneNode {
        SceneNode { pos, radius, color: [1.0, 0.0, 0.0, 1.0], opacity, shape: 0, label: 0 }
    }

    fn scene_with(nodes: &[(NodeIndex, SceneNode)]) -> Scene {
        let mut s = Scene::default();
        for (ix, n) in nodes { s.set(*ix, *n); }
        s
    }

    #[test]
    fn no_transition_emits_target_immediately() {
        let mut a = Animator::new();
        let s = scene_with(&[(0, node([1.0, 2.0], 10.0, 1.0))]);
        a.set_target(s, 0.0, 200.0, Easing::Linear, true); // empty displayed => snaps
        let mut f = Frame::default();
        a.tick_into(0.0, &mut f);
        assert_eq!(f.nodes.len(), 1);
        assert_eq!(f.nodes[0].pos, [1.0, 2.0]);
    }

    #[test]
    fn moved_node_interpolates_position() {
        let mut b = Animator::new();
        let mut f = Frame::default();
        b.set_target(scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true);
        b.tick_into(0.0, &mut f); // commits displayed = from (empty before => snaps to this)
        b.set_target(scene_with(&[(0, node([10.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true);
        b.tick_into(0.0, &mut f);
        assert!((f.nodes[0].pos[0] - 0.0).abs() < 1e-4);
        b.tick_into(100.0, &mut f);
        assert!((f.nodes[0].pos[0] - 5.0).abs() < 1e-4);
        b.tick_into(200.0, &mut f);
        assert!((f.nodes[0].pos[0] - 10.0).abs() < 1e-4);
    }

    #[test]
    fn entering_node_fades_in_exiting_fades_out() {
        let mut a = Animator::new();
        let mut f = Frame::default();
        a.set_target(scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true);
        a.tick_into(0.0, &mut f);
        a.set_target(scene_with(&[(1, node([5.0, 5.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true);
        a.tick_into(100.0, &mut f); // midpoint
        let get = |f: &Frame, ix: NodeIndex| f.nodes.iter().find(|n| n.index == ix).map(|n| n.opacity);
        assert!((get(&f, 0).unwrap() - 0.5).abs() < 1e-4, "exiting alpha");
        assert!((get(&f, 1).unwrap() - 0.5).abs() < 1e-4, "entering alpha");
        a.tick_into(200.0, &mut f);
        assert_eq!(f.nodes.len(), 1);
        assert_eq!(f.nodes[0].index, 1);
    }

    #[test]
    fn edges_resolve_endpoints_from_interpolated_positions() {
        let mut a = Animator::new();
        let mut f = Frame::default();
        let mut s0 = scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0)), (1, node([0.0, 0.0], 10.0, 1.0))]);
        s0.edges.push(SceneEdge { a: 0, b: 1, color: [0.0; 4], width: 1.0, opacity: 1.0, label: EMPTY_LABEL });
        a.set_target(s0, 0.0, 200.0, Easing::Linear, true);
        a.tick_into(0.0, &mut f);
        let mut s1 = scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0)), (1, node([10.0, 0.0], 10.0, 1.0))]);
        s1.edges.push(SceneEdge { a: 0, b: 1, color: [0.0; 4], width: 1.0, opacity: 1.0, label: EMPTY_LABEL });
        a.set_target(s1, 0.0, 200.0, Easing::Linear, true);
        a.tick_into(100.0, &mut f);
        assert_eq!(f.edges.len(), 1);
        assert!((f.edges[0].b[0] - 5.0).abs() < 1e-4); // b endpoint mid-glide
    }

    #[test]
    fn halos_resolve_position_and_radius_from_their_node() {
        let mut a = Animator::new();
        let mut f = Frame::default();
        let mut s = scene_with(&[(0, node([3.0, 4.0], 12.0, 1.0))]);
        s.halos.push(SceneHalo { node: 0, emphasis: HaloEmphasis::Primary });
        a.set_target(s, 0.0, 0.0, Easing::Linear, false); // snap
        a.tick_into(0.0, &mut f);
        assert_eq!(f.halos.len(), 1);
        assert_eq!(f.halos[0].pos, [3.0, 4.0]);
        assert_eq!(f.halos[0].radius, 12.0);
        assert_eq!(f.halos[0].node, 0);
    }

    #[test]
    fn edges_and_halos_referencing_absent_nodes_are_skipped() {
        let mut a = Animator::new();
        let mut f = Frame::default();
        let mut s = scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0))]);
        // index 7 is absent from the scene: the edge and halo must be dropped,
        // not emitted with garbage endpoints.
        s.edges.push(SceneEdge { a: 0, b: 7, color: [0.0; 4], width: 1.0, opacity: 1.0, label: EMPTY_LABEL });
        s.halos.push(SceneHalo { node: 7, emphasis: HaloEmphasis::Primary });
        a.set_target(s, 0.0, 0.0, Easing::Linear, false);
        a.tick_into(0.0, &mut f);
        assert_eq!(f.nodes.len(), 1);
        assert!(f.edges.is_empty());
        assert!(f.halos.is_empty());
    }

    #[test]
    fn target_position_looks_up_one_node_by_index() {
        let mut a = Animator::new();
        a.set_target(scene_with(&[(0, node([3.0, 4.0], 10.0, 1.0))]), 0.0, 0.0, Easing::Linear, false);
        assert_eq!(a.target_position(0), Some([3.0, 4.0]));
        assert_eq!(a.target_position(9), None); // missing index
    }

    #[test]
    fn target_positions_reflect_destination_even_mid_transition() {
        let mut a = Animator::new();
        let mut f = Frame::default();
        a.set_target(scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true);
        a.tick_into(0.0, &mut f);
        a.set_target(scene_with(&[(0, node([100.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true);
        // the emitted frame at t=0 still shows the OLD position...
        a.tick_into(0.0, &mut f);
        assert!((f.nodes[0].pos[0] - 0.0).abs() < 1e-4);
        // ...but target_positions already reports the destination.
        let tgt: Vec<[f32; 2]> = a.target_positions().collect();
        assert_eq!(tgt, vec![[100.0, 0.0]]);
    }

    #[test]
    fn tween_disabled_snaps_positions() {
        let mut a = Animator::new();
        let mut f = Frame::default();
        a.set_target(scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, false);
        a.tick_into(0.0, &mut f);
        a.set_target(scene_with(&[(0, node([10.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, false);
        a.tick_into(0.0, &mut f);
        assert_eq!(f.nodes[0].pos[0], 10.0); // no interpolation
    }

    #[test]
    fn tick_positioned_overrides_node_positions() {
        let mut a = Animator::new();
        let mut f = Frame::default();
        let s = scene_with(&[(0, node([0.0, 0.0], 5.0, 1.0))]);
        a.set_target(s, 0.0, 0.0, Easing::Linear, false);

        let pos = vec![Some([77.0, -3.0])];
        a.tick_positioned_into(0.0, &pos, &mut f);
        assert_eq!(f.nodes[0].pos, [77.0, -3.0], "the simulation owns position");
        assert_eq!(f.nodes[0].radius, 5.0, "the animator still owns appearance");
    }

    #[test]
    fn edges_and_halos_follow_the_overridden_positions() {
        // This is the whole reason the override happens before frame assembly:
        // edges and halos resolve endpoints out of the node set, so they track
        // the simulation for free instead of needing their own path.
        let mut sc = scene_with(&[(0, node([0.0, 0.0], 5.0, 1.0)), (1, node([1.0, 1.0], 5.0, 1.0))]);
        sc.edges.push(SceneEdge { a: 0, b: 1, color: [1.0; 4], width: 1.0, opacity: 1.0, label: EMPTY_LABEL });
        sc.halos.push(SceneHalo { node: 0, emphasis: HaloEmphasis::Primary });

        let mut an = Animator::new();
        let mut f = Frame::default();
        an.set_target(sc, 0.0, 0.0, Easing::Linear, false);

        let pos = vec![Some([100.0, 0.0]), Some([200.0, 50.0])];
        an.tick_positioned_into(0.0, &pos, &mut f);

        assert_eq!(f.edges[0].a, [100.0, 0.0]);
        assert_eq!(f.edges[0].b, [200.0, 50.0]);
        assert_eq!(f.halos[0].pos, [100.0, 0.0]);
    }

    #[test]
    fn a_node_absent_from_the_overrides_keeps_its_interpolated_position() {
        // Synthetic radial affordances (`__more`, `__agg:…`) are not in the
        // simulation, so they must keep tweening normally. Both an empty
        // slice and an explicit None slot mean "animator keeps its position".
        let mut a = Animator::new();
        let mut f = Frame::default();
        a.set_target(scene_with(&[(0, node([9.0, 9.0], 5.0, 1.0))]), 0.0, 0.0, Easing::Linear, false);
        let empty: Vec<Option<[f32; 2]>> = Vec::new();
        a.tick_positioned_into(0.0, &empty, &mut f);
        assert_eq!(f.nodes[0].pos, [9.0, 9.0]);
        a.tick_positioned_into(0.0, &[None], &mut f);
        assert_eq!(f.nodes[0].pos, [9.0, 9.0]);
    }

    #[test]
    fn tick_positioned_still_advances_the_transition() {
        // The override must not bypass the tween bookkeeping, or a mid-flight
        // colour transition would freeze whenever the sim is running.
        let mut a = Animator::new();
        let mut f = Frame::default();
        a.set_target(scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0))]), 0.0, 0.0, Easing::Linear, false);
        a.set_target(scene_with(&[(0, node([0.0, 0.0], 20.0, 1.0))]), 0.0, 100.0, Easing::Linear, true);

        let pos: Vec<Option<[f32; 2]>> = Vec::new();
        a.tick_positioned_into(50.0, &pos, &mut f);
        assert!((f.nodes[0].radius - 15.0).abs() < 0.01, "radius should be halfway");
        a.tick_positioned_into(100.0, &pos, &mut f);
        assert_eq!(f.nodes[0].radius, 20.0);
    }

    #[test]
    fn exiting_node_keeps_its_simulated_position_while_it_fades() {
        // The more common trigger for the clone-and-discard bug: a node
        // removed from the graph is also removed from the simulation's
        // position set. `interpolate_into` keeps a source-only ("exiting")
        // node at `from`'s position while it fades out. If the override were
        // written into a throwaway copy instead of into `self.displayed`,
        // `displayed` would still hold the pre-simulation, scene-authored
        // position when `set_target` snapshots it as the new `from` — so the
        // exiting node would teleport back to that stale position and only
        // then fade out.
        let mut a = Animator::new();
        let mut f = Frame::default();
        let s0 = scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0)), (1, node([0.0, 0.0], 10.0, 1.0))]);
        a.set_target(s0, 0.0, 0.0, Easing::Linear, false); // snap

        let pos = vec![Some([500.0, 0.0]), Some([10.0, 10.0])];
        a.tick_positioned_into(0.0, &pos, &mut f); // displayed now holds simulated positions

        // node 1 drops out of the graph (and out of the simulation); 0 remains.
        let s1 = scene_with(&[(0, node([1.0, 1.0], 10.0, 1.0))]);
        a.set_target(s1, 0.0, 200.0, Easing::Linear, true); // node 1 becomes exiting

        let pos2 = vec![Some([501.0, 0.0]), None]; // node 1 no longer simulated
        a.tick_positioned_into(0.0, &pos2, &mut f); // t=0 of the new transition

        let b = f.nodes.iter().find(|n| n.index == 1).expect("node 1 still fading");
        assert_eq!(b.pos, [10.0, 10.0], "exiting node should stay at its last simulated position while fading, not teleport to a stale scene-authored one");
    }

    #[test]
    fn mid_flight_retarget_starts_from_the_displayed_position_not_the_old_target() {
        // Retargeting while a transition is in flight must start the new
        // transition from wherever the node is currently displayed (the
        // in-flight interpolated position), not from the previous target
        // (`to`) it was heading toward but had not yet reached.
        let mut a = Animator::new();
        let mut f = Frame::default();
        a.set_target(scene_with(&[(0, node([0.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true); // snaps (displayed was empty)
        a.set_target(scene_with(&[(0, node([100.0, 0.0], 10.0, 1.0))]), 0.0, 200.0, Easing::Linear, true);
        a.tick_into(100.0, &mut f); // halfway: displayed pos[0] == 50.0, still mid-flight

        a.set_target(scene_with(&[(0, node([300.0, 0.0], 10.0, 1.0))]), 100.0, 200.0, Easing::Linear, true);
        a.tick_into(100.0, &mut f); // t=0 of the new transition
        assert!((f.nodes[0].pos[0] - 50.0).abs() < 1e-4, "new transition should start from the displayed position (50), not the old target (100)");
    }

    #[test]
    fn frame_buffers_do_not_reallocate_when_warm() {
        let mut a = Animator::new();
        let mut s = Scene::default();
        for i in 0..100u32 { s.set(i, node([i as f32, 0.0], 5.0, 1.0)); }
        for i in 0..99u32 {
            s.edges.push(SceneEdge { a: i, b: i + 1, color: [1.0; 4], width: 1.0, opacity: 1.0, label: EMPTY_LABEL });
        }
        // First target snaps (displayed is empty); warm up one frame.
        a.set_target(s.clone(), 0.0, 100.0, Easing::Linear, true);
        let mut f = Frame::default();
        a.tick_into(0.0, &mut f);
        // Second target actually animates; duration long enough that every
        // tick below stays mid-transition, so `advance`'s animating body
        // (interpolate_into + scratch + node clone) runs each frame.
        let mut s2 = s.clone();
        for n in s2.nodes.iter_mut().flatten() { n.pos[0] += 50.0; }
        a.set_target(s2, 0.0, 1e9, Easing::Linear, true);
        a.tick_into(1.0, &mut f); // capture capacities only after an animating tick
        let caps = (f.nodes.capacity(), f.edges.capacity(), f.halos.capacity());
        for t in 2..150 { a.tick_into(t as f64, &mut f); }
        // Mid-loop retarget exercises the warm `from_buf.clone_from` path.
        a.set_target(s, 150.0, 1e9, Easing::Linear, true);
        for t in 151..300 { a.tick_into(t as f64, &mut f); }
        assert_eq!(caps, (f.nodes.capacity(), f.edges.capacity(), f.halos.capacity()));
    }

    #[test]
    fn dense_gaps_are_skipped_not_emitted() {
        let mut s = Scene::default();
        s.set(5, node([1.0, 2.0], 5.0, 1.0)); // indices 0..5 are None
        let mut a = Animator::new();
        a.set_target(s, 0.0, 0.0, Easing::Linear, false);
        let mut f = Frame::default();
        a.tick_into(0.0, &mut f);
        assert_eq!(f.nodes.len(), 1);
        assert_eq!(f.nodes[0].index, 5);
    }
}