rustial-engine 0.0.1

//! Smooth camera animation with easing and momentum.
//!
//! This module provides a stateful animator used by
//! [`MapState`](crate::MapState) to drive camera transitions:
//!
//! - **`fly_to`** -- the van Wijk & Nuij (2003) "optimal path" animation
//!   matching MapLibre/Mapbox `flyTo`.  Simultaneously interpolates center,
//!   zoom, bearing, and pitch along a zoom-out-then-zoom-in arc.
//! - **`ease_to`** -- simple interpolation of center, zoom, bearing, and
//!   pitch with configurable duration and easing.
//! - **Momentum** -- inertial pan that decays over time.
//! - **Simple targets** -- independent exponential-smoothed zoom / rotation
//!   targets (legacy, still useful for scroll-zoom and keyboard rotation).
//!
//! # Design
//!
//! - Time integration is explicit via [`tick`](CameraAnimator::tick)
//!   and caller-provided `dt` (seconds).
//! - The van Wijk flight uses an easing function applied to wall-clock
//!   progress `t ? [0, 1]` to produce a path parameter `k`, then derives
//!   zoom scale `w(s)` and center factor `u(s)` from the paper's
//!   closed-form expressions.
//! - Bearing and pitch are linearly interpolated in `k` (same as MapLibre).
//! - Yaw interpolation follows the shortest angular path.
//!
//! # Reference
//!
//! Van Wijk, Jarke J.; Nuij, Wim A. A.  "Smooth and efficient zooming
//! and panning."  INFOVIS '03. pp. 15-22.
//! <https://www.win.tue.nl/~vanwijk/zoompan.pdf>

use crate::camera::Camera;
use crate::camera_projection::CameraProjection;
use crate::geo_wrap::{shortest_lon_target, wrap_lon_180};
use rustial_math::{GeoCoord, WorldCoord};

// ---------------------------------------------------------------------------
// Constants
// ---------------------------------------------------------------------------

/// Approximate meters per degree of latitude (WGS-84 at equator).
const METERS_PER_DEGREE: f64 = 111_319.49;

/// Maximum latitude for Web Mercator validity.
const MERCATOR_MAX_LAT: f64 = 85.06;

/// Activity threshold for momentum (m/s).
const MOMENTUM_EPS: f64 = 0.01;

/// Distance convergence threshold (meters).
const DISTANCE_EPS: f64 = 0.1;

/// Angle convergence threshold (radians).
const ANGLE_EPS: f64 = 1e-4;

/// Earth's equatorial circumference in meters (2*pi * WGS-84 semi-major axis).
const WGS84_CIRCUMFERENCE: f64 = 2.0 * std::f64::consts::PI * 6_378_137.0;

/// Standard tile size in pixels.
const TILE_PX: f64 = 256.0;

// ---------------------------------------------------------------------------
// Easing
// ---------------------------------------------------------------------------

/// Evaluate a cubic bezier easing curve at parameter `t`.
///
/// Control points are `(0, 0)`, `(x1, y1)`, `(x2, y2)`, `(1, 1)`.
/// Returns the `y` value for the given `t` (time) input.
fn cubic_bezier(x1: f64, y1: f64, x2: f64, y2: f64, t: f64) -> f64 {
    // Newton-Raphson to find the bezier parameter `s` such that `x(s) = t`.
    let mut s = t;
    for _ in 0..8 {
        let x = bezier_component(x1, x2, s) - t;
        let dx = bezier_derivative(x1, x2, s);
        if dx.abs() < 1e-12 {
            break;
        }
        s -= x / dx;
        s = s.clamp(0.0, 1.0);
    }
    bezier_component(y1, y2, s)
}

fn bezier_component(p1: f64, p2: f64, t: f64) -> f64 {
    let t2 = t * t;
    let t3 = t2 * t;
    3.0 * p1 * t * (1.0 - t) * (1.0 - t) + 3.0 * p2 * t2 * (1.0 - t) + t3
}

fn bezier_derivative(p1: f64, p2: f64, t: f64) -> f64 {
    let t2 = t * t;
    3.0 * p1 * (1.0 - t) * (1.0 - t) + 6.0 * (p2 - p1) * t * (1.0 - t) + 3.0 * (1.0 - p2) * t2
}

/// MapLibre default easing: `cubic-bezier(0.25, 0.1, 0.25, 1.0)`.
fn default_easing(t: f64) -> f64 {
    cubic_bezier(0.25, 0.1, 0.25, 1.0, t)
}

// ---------------------------------------------------------------------------
// Zoom <-> distance helpers
// ---------------------------------------------------------------------------

/// Convert a fractional zoom level to camera distance (meters).
///
/// This inverts the `zoom = log2(circumference / (mpp * tile_px))` formula
/// where `mpp = visible_height / viewport_height` and
/// `visible_height = 2 * distance * tan(fov_y/2)` for perspective,
/// or `visible_height = 2 * distance` for orthographic.
fn zoom_to_distance(zoom: f64, fov_y: f64, viewport_height: u32, is_perspective: bool) -> f64 {
    let mpp = WGS84_CIRCUMFERENCE / (2.0_f64.powf(zoom) * TILE_PX);
    let visible_height = mpp * viewport_height.max(1) as f64;
    if is_perspective {
        visible_height / (2.0 * (fov_y / 2.0).tan())
    } else {
        visible_height / 2.0
    }
}

/// Convert camera distance to fractional zoom level.
fn distance_to_zoom(distance: f64, fov_y: f64, viewport_height: u32, is_perspective: bool) -> f64 {
    let visible_height = if is_perspective {
        2.0 * distance * (fov_y / 2.0).tan()
    } else {
        2.0 * distance
    };
    let mpp = visible_height / viewport_height.max(1) as f64;
    if mpp <= 0.0 || !mpp.is_finite() {
        return 22.0;
    }
    (WGS84_CIRCUMFERENCE / (mpp * TILE_PX))
        .log2()
        .clamp(0.0, 22.0)
}

// ---------------------------------------------------------------------------
// FlyToOptions
// ---------------------------------------------------------------------------

/// Options for the [`fly_to`](CameraAnimator::start_fly_to) animation.
///
/// Mirrors the MapLibre/Mapbox `FlyToOptions` API.
///
/// All fields are optional.  When omitted, the animation retains the
/// camera's current value for that property.
#[derive(Debug, Clone)]
pub struct FlyToOptions {
    /// Target geographic center.  If `None`, the center does not change.
    pub center: Option<GeoCoord>,
    /// Target zoom level.  If `None`, the zoom does not change.
    pub zoom: Option<f64>,
    /// Target bearing (yaw) in **radians**.  If `None`, bearing does not change.
    ///
    /// The animation always takes the shortest angular path.
    pub bearing: Option<f64>,
    /// Target pitch in **radians**.  If `None`, pitch does not change.
    pub pitch: Option<f64>,
    /// The zooming "curve" (? in the van Wijk paper).
    ///
    /// Higher values produce more exaggerated zoom-out.
    /// Default: `1.42` (the user-study average from van Wijk 2003).
    pub curve: f64,
    /// Average speed in ?-screenfulls per second.  Default: `1.2`.
    ///
    /// Ignored when `duration` is set explicitly.
    pub speed: f64,
    /// Average speed in screenfulls per second (overrides `speed`).
    ///
    /// Ignored when `duration` is set explicitly.
    pub screen_speed: Option<f64>,
    /// Explicit animation duration in **seconds**.
    ///
    /// When set, `speed` / `screen_speed` are ignored and the animation
    /// runs for exactly this duration.
    pub duration: Option<f64>,
    /// The zoom level at the peak of the flight path.
    ///
    /// When set, `curve` is overridden to produce a zoom-out that reaches
    /// this zoom level.
    pub min_zoom: Option<f64>,
    /// Maximum allowed duration in **seconds**.
    ///
    /// If the auto-computed duration exceeds this, the animation
    /// degrades to an instant `jump_to`.
    pub max_duration: Option<f64>,
    /// Easing function `f(t) -> k` where `t` and `k` are both in `[0, 1]`.
    ///
    /// Default: `cubic-bezier(0.25, 0.1, 0.25, 1.0)` (MapLibre default).
    pub easing: Option<fn(f64) -> f64>,
}

impl Default for FlyToOptions {
    fn default() -> Self {
        Self {
            center: None,
            zoom: None,
            bearing: None,
            pitch: None,
            curve: 1.42,
            speed: 1.2,
            screen_speed: None,
            duration: None,
            min_zoom: None,
            max_duration: None,
            easing: None,
        }
    }
}

// ---------------------------------------------------------------------------
// EaseToOptions
// ---------------------------------------------------------------------------

/// Options for the [`ease_to`](CameraAnimator::start_ease_to) animation.
///
/// Simple linear interpolation of all camera properties over a fixed
/// duration, matching MapLibre's `easeTo`.
#[derive(Debug, Clone)]
pub struct EaseToOptions {
    /// Target geographic center.
    pub center: Option<GeoCoord>,
    /// Target zoom level.
    pub zoom: Option<f64>,
    /// Target bearing (yaw) in radians.
    pub bearing: Option<f64>,
    /// Target pitch in radians.
    pub pitch: Option<f64>,
    /// Duration in seconds.  Default: `0.5`.
    pub duration: f64,
    /// Easing function.  Default: `cubic-bezier(0.25, 0.1, 0.25, 1.0)`.
    pub easing: Option<fn(f64) -> f64>,
}

impl Default for EaseToOptions {
    fn default() -> Self {
        Self {
            center: None,
            zoom: None,
            bearing: None,
            pitch: None,
            duration: 0.5,
            easing: None,
        }
    }
}

// ---------------------------------------------------------------------------
// Internal flight state
// ---------------------------------------------------------------------------

/// Active van Wijk fly-to animation state.
struct FlyToState {
    // -- End state --
    end_center: GeoCoord,
    end_zoom: f64,
    end_bearing: f64,
    end_pitch: f64,

    // -- Start state (needed for per-frame interpolation) --
    start_zoom: f64,
    start_bearing: f64,
    start_pitch: f64,

    // -- Van Wijk parameters --
    /// Projected start position (world coords at initial scale).
    from_x: f64,
    from_y: f64,
    /// Projected delta (end - start in world coords at initial scale).
    delta_x: f64,
    delta_y: f64,
    /// w0: initial visible span in pixels.
    w0: f64,
    /// u1: ground-plane path length in pixels at initial scale.
    u1: f64,
    /// ?: zoom curve parameter.
    rho: f64,
    /// r?: zoom-out factor during ascent.
    r0: f64,
    /// S: total path length in ?-screenfulls.
    path_length: f64,
    /// Whether u? ? 0 (same-location zoom-only path).
    zoom_only: bool,
    /// k = w1 < w0 ? -1 : 1  (used in zoom-only fallback).
    zoom_only_sign: f64,

    // -- Timing --
    duration: f64,
    elapsed: f64,
    easing: fn(f64) -> f64,

    // -- Projection used for world-space interpolation --
    projection: CameraProjection,
    is_perspective: bool,
    fov_y: f64,
    viewport_height: u32,
}

impl FlyToState {
    fn new(camera: &Camera, opts: &FlyToOptions) -> Option<Self> {
        let projection = camera.projection();
        let is_perspective = camera.mode() == crate::camera::CameraMode::Perspective;
        let fov_y = camera.fov_y();
        let viewport_height = camera.viewport_height();
        let viewport_width = camera.viewport_width();

        let start_center = *camera.target();
        let start_zoom =
            distance_to_zoom(camera.distance(), fov_y, viewport_height, is_perspective);
        let start_bearing = camera.yaw();
        let start_pitch = camera.pitch();

        let end_center = opts.center.unwrap_or(start_center);
        let end_zoom = opts.zoom.unwrap_or(start_zoom);
        let end_bearing = match opts.bearing {
            Some(b) => shortest_bearing_target(start_bearing, b),
            None => start_bearing,
        };
        let end_pitch = opts.pitch.unwrap_or(start_pitch);

        // Project start and end centers to world coordinates.
        let from_world = projection.project(&start_center);
        let to_world = projection.project(&end_center);

        // World-size at the *initial* zoom level, used to convert between
        // "world pixels" (MapLibre's coordinate space) and meters.
        // At zoom z, world_size = tile_px * 2^z  (in pixels).
        // We need a pixel-space path length: divide meters by mpp.
        let pixels_per_meter = 2.0_f64.powf(start_zoom) * TILE_PX / WGS84_CIRCUMFERENCE;

        let from_px_x = from_world.position.x * pixels_per_meter;
        let from_px_y = from_world.position.y * pixels_per_meter;
        let to_px_x = to_world.position.x * pixels_per_meter;
        let to_px_y = to_world.position.y * pixels_per_meter;

        let delta_x = to_px_x - from_px_x;
        let delta_y = to_px_y - from_px_y;

        // u1: ground-plane path length in pixels at initial scale.
        let u1 = (delta_x * delta_x + delta_y * delta_y).sqrt();

        // w0: initial visible span (max of width, height) in pixels.
        let w0 = viewport_width.max(viewport_height).max(1) as f64;

        // scale = 2^(endZoom - startZoom)  (MapLibre: zoomScale(zoom - startZoom))
        let scale_of_zoom = 2.0_f64.powf(end_zoom - start_zoom);

        // w1: final visible span measured in pixels at the *initial* scale.
        // MapLibre: w1 = w0 / scale
        let w1 = w0 / scale_of_zoom;

        // rho: curve parameter.
        let mut rho = opts.curve;

        // If minZoom is specified, override rho.
        // MapLibre: const minZoom = clamp(Math.min(options.minZoom, startZoom, zoom), ...)
        if let Some(min_z) = opts.min_zoom {
            let min_z = min_z.min(start_zoom).min(end_zoom);
            let scale_of_min_zoom = 2.0_f64.powf(min_z - start_zoom);
            let w_max = w0 / scale_of_min_zoom;
            if u1 > 0.0 {
                rho = (w_max / u1 * 2.0).sqrt();
            }
        }

        let rho2 = rho * rho;

        // r(i): zoom-out factor at one end of the animation.
        // MapLibre/Mapbox:
        //   function r(i) {
        //     const b = (w1*w1 - w0*w0 + (i ? -1 : 1) * rho2*rho2 * u1*u1)
        //               / (2 * (i ? w1 : w0) * rho2 * u1);
        //     return Math.log(Math.sqrt(b*b + 1) - b);
        //   }
        // descent=true corresponds to i=1 (descent), descent=false to i=0 (ascent).
        let zoom_out_factor = |descent: bool| -> f64 {
            let (w, sign) = if descent { (w1, -1.0) } else { (w0, 1.0) };
            let b = (w1 * w1 - w0 * w0 + sign * rho2 * rho2 * u1 * u1) / (2.0 * w * rho2 * u1);
            ((b * b + 1.0).sqrt() - b).ln()
        };

        let r0;
        let path_length;
        let zoom_only;
        let zoom_only_sign;

        // Check if the ground-plane path is effectively zero or if the
        // full van Wijk formula produces a non-finite result.
        // MapLibre/Mapbox: if (Math.abs(u1) < 0.000001 || !isFinite(S))
        let is_degenerate = if u1.abs() < 0.000001 {
            true
        } else {
            let trial_r0 = zoom_out_factor(false);
            let trial_r1 = zoom_out_factor(true);
            let trial_s = (trial_r1 - trial_r0) / rho;
            !trial_s.is_finite()
        };

        if is_degenerate {
            // Same-location or degenerate path.
            // MapLibre/Mapbox: if (Math.abs(w0 - w1) < 0.000001) return this.easeTo(...)
            if (w0 - w1).abs() < 0.000001 {
                // No zoom change and no center change -- nothing to animate
                // unless bearing or pitch changed.
                if (end_bearing - start_bearing).abs() < ANGLE_EPS
                    && (end_pitch - start_pitch).abs() < ANGLE_EPS
                {
                    return None;
                }
            }

            // MapLibre/Mapbox: const k = w1 < w0 ? -1 : 1;
            //                  S = Math.abs(Math.log(w1 / w0)) / rho;
            //                  w = function(s) { return Math.exp(k * rho * s); };
            zoom_only = true;
            zoom_only_sign = if w1 < w0 { -1.0 } else { 1.0 };
            r0 = 0.0;
            path_length = if w1 > 0.0 && w0 > 0.0 {
                (w1 / w0).ln().abs() / rho
            } else {
                0.0
            };
        } else {
            zoom_only = false;
            zoom_only_sign = 0.0;
            // MapLibre/Mapbox: r0 = r(0);  S = (r(1) - r0) / rho;
            r0 = zoom_out_factor(false);
            let r1 = zoom_out_factor(true);
            path_length = (r1 - r0) / rho;
        }

        // Compute duration.
        // MapLibre/Mapbox: duration = 1000 * S / V  (milliseconds)
        // We store duration in seconds.
        let duration = if let Some(d) = opts.duration {
            d
        } else {
            let v = if let Some(ss) = opts.screen_speed {
                ss / rho
            } else {
                opts.speed
            };
            if v > 0.0 {
                path_length / v
            } else {
                1.0
            }
        };

        // Check max_duration.
        // MapLibre/Mapbox: if (options.maxDuration && options.duration > options.maxDuration)
        //                      options.duration = 0;
        if let Some(max_dur) = opts.max_duration {
            if duration > max_dur {
                return None; // Degrade to instant jump.
            }
        }

        let easing = opts.easing.unwrap_or(default_easing);

        Some(FlyToState {
            start_zoom,
            start_bearing,
            start_pitch,
            end_center,
            end_zoom,
            end_bearing,
            end_pitch,
            from_x: from_px_x,
            from_y: from_px_y,
            delta_x,
            delta_y,
            w0,
            u1,
            rho,
            r0,
            path_length,
            zoom_only,
            zoom_only_sign,
            duration,
            elapsed: 0.0,
            easing,
            projection,
            is_perspective,
            fov_y,
            viewport_height,
        })
    }

    /// Evaluate the van Wijk w(s) function -- visible span at path distance s.
    fn w(&self, s: f64) -> f64 {
        if self.zoom_only {
            (self.zoom_only_sign * self.rho * s).exp()
        } else {
            self.r0.cosh() / (self.r0 + self.rho * s).cosh()
        }
    }

    /// Evaluate the van Wijk u(s) function -- center interpolation factor.
    fn u(&self, s: f64) -> f64 {
        if self.zoom_only {
            0.0
        } else {
            let rho2 = self.rho * self.rho;
            self.w0 * ((self.r0.cosh() * (self.r0 + self.rho * s).tanh() - self.r0.sinh()) / rho2)
                / self.u1
        }
    }

    /// Advance the flight by `dt` seconds.  Returns `true` when complete.
    fn tick(&mut self, camera: &mut Camera, dt: f64) -> bool {
        self.elapsed += dt;
        let t = (self.elapsed / self.duration.max(1e-9)).clamp(0.0, 1.0);
        let k = (self.easing)(t);

        let done = t >= 1.0;

        if done {
            // Snap to final state.
            camera.set_target(self.end_center);
            camera.set_distance(zoom_to_distance(
                self.end_zoom,
                self.fov_y,
                self.viewport_height,
                self.is_perspective,
            ));
            camera.set_yaw(self.end_bearing);
            camera.set_pitch(self.end_pitch);
            return true;
        }

        // Path distance in ?-screenfulls.
        let s = k * self.path_length;

        // Scale factor: 1/w(s).
        let scale = 1.0 / self.w(s);

        // Center interpolation factor.
        let center_factor = self.u(s);

        // Interpolate zoom via scale.
        let zoom = self.start_zoom + scale.log2();
        camera.set_distance(zoom_to_distance(
            zoom,
            self.fov_y,
            self.viewport_height,
            self.is_perspective,
        ));

        // Interpolate center in projected pixel space, then unproject.
        let pixels_per_meter = 2.0_f64.powf(self.start_zoom) * TILE_PX / WGS84_CIRCUMFERENCE;
        let cx_px = self.from_x + self.delta_x * center_factor;
        let cy_px = self.from_y + self.delta_y * center_factor;
        // Convert from "initial-scale pixels" back to meters, then to the
        // current-scale center (the scale multiplication here accounts for
        // the zoom change and corresponds to MapLibre's
        // `from.add(delta.mult(centerFactor)).mult(scale)` followed by
        // `unprojectFromWorldCoordinates(tr.worldSize, ...)`).
        //
        // In MapLibre, `tr.worldSize` changes with zoom, which is why they
        // multiply by `scale` before unprojecting.  We achieve the same
        // result by converting from initial-scale pixels to meters (dividing
        // by the initial pixels_per_meter) and then letting the projection
        // unproject from meter-space.
        let cx_m = cx_px / pixels_per_meter;
        let cy_m = cy_px / pixels_per_meter;
        let new_center = self.projection.unproject(&WorldCoord::new(cx_m, cy_m, 0.0));
        camera.set_target(new_center);

        // Interpolate bearing (shortest path).
        let bearing = self.start_bearing + (self.end_bearing - self.start_bearing) * k;
        camera.set_yaw(bearing);

        // Interpolate pitch linearly.
        let pitch = self.start_pitch + (self.end_pitch - self.start_pitch) * k;
        camera.set_pitch(pitch);

        false
    }
}

// ---------------------------------------------------------------------------
// Internal ease-to state
// ---------------------------------------------------------------------------

/// Active ease-to animation state.
struct EaseToState {
    start_center: GeoCoord,
    start_zoom: f64,
    start_bearing: f64,
    start_pitch: f64,

    end_center: GeoCoord,
    end_zoom: f64,
    end_bearing: f64,
    end_pitch: f64,

    duration: f64,
    elapsed: f64,
    easing: fn(f64) -> f64,

    is_perspective: bool,
    fov_y: f64,
    viewport_height: u32,
}

impl EaseToState {
    fn new(camera: &Camera, opts: &EaseToOptions) -> Self {
        let is_perspective = camera.mode() == crate::camera::CameraMode::Perspective;
        let start_zoom = distance_to_zoom(
            camera.distance(),
            camera.fov_y(),
            camera.viewport_height(),
            is_perspective,
        );
        let start_bearing = camera.yaw();

        let end_center = opts.center.unwrap_or(*camera.target());
        let wrapped_end_center = GeoCoord::from_lat_lon(
            end_center.lat,
            shortest_lon_target(camera.target().lon, end_center.lon),
        );

        Self {
            start_center: *camera.target(),
            start_zoom,
            start_bearing,
            start_pitch: camera.pitch(),
            end_center: wrapped_end_center,
            end_zoom: opts.zoom.unwrap_or(start_zoom),
            end_bearing: match opts.bearing {
                Some(b) => shortest_bearing_target(start_bearing, b),
                None => start_bearing,
            },
            end_pitch: opts.pitch.unwrap_or(camera.pitch()),
            duration: opts.duration,
            elapsed: 0.0,
            easing: opts.easing.unwrap_or(default_easing),
            is_perspective,
            fov_y: camera.fov_y(),
            viewport_height: camera.viewport_height(),
        }
    }

    fn tick(&mut self, camera: &mut Camera, dt: f64) -> bool {
        self.elapsed += dt;
        let t = (self.elapsed / self.duration.max(1e-9)).clamp(0.0, 1.0);
        let k = (self.easing)(t);
        let done = t >= 1.0;

        if done {
            camera.set_target(self.end_center);
            camera.set_distance(zoom_to_distance(
                self.end_zoom,
                self.fov_y,
                self.viewport_height,
                self.is_perspective,
            ));
            camera.set_yaw(self.end_bearing);
            camera.set_pitch(self.end_pitch);
            return true;
        }

        // Center: lerp in geographic space.
        let lat = self.start_center.lat + (self.end_center.lat - self.start_center.lat) * k;
        let lon = self.start_center.lon + (self.end_center.lon - self.start_center.lon) * k;
        camera.set_target(GeoCoord::from_lat_lon(lat, wrap_lon_180(lon)));

        // Zoom.
        let zoom = self.start_zoom + (self.end_zoom - self.start_zoom) * k;
        camera.set_distance(zoom_to_distance(
            zoom,
            self.fov_y,
            self.viewport_height,
            self.is_perspective,
        ));

        // Bearing (shortest path, already normalized).
        camera.set_yaw(self.start_bearing + (self.end_bearing - self.start_bearing) * k);

        // Pitch.
        camera.set_pitch(self.start_pitch + (self.end_pitch - self.start_pitch) * k);

        false
    }
}

// ---------------------------------------------------------------------------
// CameraAnimator
// ---------------------------------------------------------------------------

/// Drives smooth camera transitions.
///
/// Supports three animation modes that may be active simultaneously:
///
/// 1. **Fly-to / ease-to** -- a coordinated multi-property transition
///    (only one may be active at a time; starting one cancels the other).
/// 2. **Simple targets** -- independent exponential-smoothed zoom / yaw /
///    pitch targets (useful for scroll-zoom, keyboard rotation).
/// 3. **Momentum** -- inertial pan that decays over time.
///
/// Call [`tick`](Self::tick) every frame with the elapsed `dt`.
pub struct CameraAnimator {
    // -- Coordinated flight / ease state --
    fly_to: Option<FlyToState>,
    ease_to: Option<EaseToState>,

    // -- Simple independent targets (legacy) --
    target_distance: Option<f64>,
    target_yaw: Option<f64>,
    target_pitch: Option<f64>,

    // -- Momentum --
    momentum: (f64, f64),

    /// Momentum decay factor per second (0..1, 0 = instant stop).
    pub momentum_decay: f64,
    /// Smoothing factor for simple zoom/rotate targets
    /// (0 = instant, higher = smoother).
    pub smoothing: f64,
}

impl Default for CameraAnimator {
    fn default() -> Self {
        Self {
            fly_to: None,
            ease_to: None,
            target_distance: None,
            target_yaw: None,
            target_pitch: None,
            momentum: (0.0, 0.0),
            momentum_decay: 0.05,
            smoothing: 8.0,
        }
    }
}

impl CameraAnimator {
    /// Create a new camera animator with default settings.
    pub fn new() -> Self {
        Self::default()
    }

    // -- Coordinated animations -------------------------------------------

    /// Start a van Wijk fly-to animation.
    ///
    /// This implements the "optimal path" algorithm from van Wijk & Nuij
    /// (2003), matching MapLibre/Mapbox `flyTo` behavior.  The camera
    /// simultaneously interpolates center, zoom, bearing, and pitch along
    /// a zoom-out-then-zoom-in arc.
    ///
    /// Cancels any active fly-to, ease-to, simple targets, and momentum.
    ///
    /// If the requested transition is degenerate (no change or exceeds
    /// `max_duration`), a `jump_to` is performed instead.
    pub fn start_fly_to(&mut self, camera: &mut Camera, options: &FlyToOptions) {
        self.cancel();

        match FlyToState::new(camera, options) {
            Some(state) => {
                self.fly_to = Some(state);
            }
            None => {
                // Degenerate: apply final state immediately.
                Self::apply_jump(
                    camera,
                    options.center,
                    options.zoom,
                    options.bearing,
                    options.pitch,
                );
            }
        }
    }

    /// Start a simple ease-to animation.
    ///
    /// Linearly interpolates center, zoom, bearing, and pitch over the
    /// specified duration with configurable easing.
    ///
    /// Cancels any active fly-to, ease-to, simple targets, and momentum.
    pub fn start_ease_to(&mut self, camera: &mut Camera, options: &EaseToOptions) {
        self.cancel();

        if options.duration <= 0.0 {
            Self::apply_jump(
                camera,
                options.center,
                options.zoom,
                options.bearing,
                options.pitch,
            );
            return;
        }

        self.ease_to = Some(EaseToState::new(camera, options));
    }

    /// Apply final state immediately (jump_to helper).
    fn apply_jump(
        camera: &mut Camera,
        center: Option<GeoCoord>,
        zoom: Option<f64>,
        bearing: Option<f64>,
        pitch: Option<f64>,
    ) {
        if let Some(c) = center {
            camera.set_target(c);
        }
        if let Some(z) = zoom {
            let is_perspective = camera.mode() == crate::camera::CameraMode::Perspective;
            camera.set_distance(zoom_to_distance(
                z,
                camera.fov_y(),
                camera.viewport_height(),
                is_perspective,
            ));
        }
        if let Some(b) = bearing {
            camera.set_yaw(b);
        }
        if let Some(p) = pitch {
            camera.set_pitch(p);
        }
    }

    // -- Simple independent targets (legacy) ------------------------------

    /// Set a zoom animation target (distance in meters).
    ///
    /// Non-finite or non-positive distances are ignored.
    pub fn animate_zoom(&mut self, target_distance: f64) {
        if target_distance.is_finite() && target_distance > 0.0 {
            self.target_distance = Some(target_distance);
        }
    }

    /// Set rotation animation targets.
    ///
    /// Non-finite angles are ignored independently.
    pub fn animate_rotate(&mut self, target_yaw: f64, target_pitch: f64) {
        if target_yaw.is_finite() {
            self.target_yaw = Some(target_yaw);
        }
        if target_pitch.is_finite() {
            self.target_pitch = Some(target_pitch);
        }
    }

    /// Apply pan momentum in world meters/second.
    ///
    /// Non-finite inputs are ignored.
    pub fn apply_momentum(&mut self, vx: f64, vy: f64) {
        if vx.is_finite() && vy.is_finite() {
            self.momentum = (vx, vy);
        }
    }

    /// Cancel all animations and momentum.
    pub fn cancel(&mut self) {
        self.fly_to = None;
        self.ease_to = None;
        self.target_distance = None;
        self.target_yaw = None;
        self.target_pitch = None;
        self.momentum = (0.0, 0.0);
    }

    /// Whether any animation or momentum is active.
    pub fn is_active(&self) -> bool {
        self.fly_to.is_some()
            || self.ease_to.is_some()
            || self.target_distance.is_some()
            || self.target_yaw.is_some()
            || self.target_pitch.is_some()
            || self.momentum.0.abs() > MOMENTUM_EPS
            || self.momentum.1.abs() > MOMENTUM_EPS
    }

    /// Whether a coordinated fly-to or ease-to animation is active.
    pub fn is_flying(&self) -> bool {
        self.fly_to.is_some()
    }

    /// Whether a coordinated ease-to animation is active.
    pub fn is_easing(&self) -> bool {
        self.ease_to.is_some()
    }

    /// Advance the animation by `dt` seconds, mutating the camera.
    ///
    /// Non-finite or non-positive `dt` values are ignored.
    pub fn tick(&mut self, camera: &mut Camera, dt: f64) {
        if !dt.is_finite() || dt <= 0.0 {
            return;
        }

        // Coordinated fly-to takes priority.
        if let Some(ref mut state) = self.fly_to {
            if state.tick(camera, dt) {
                self.fly_to = None;
            }
            return;
        }

        // Coordinated ease-to.
        if let Some(ref mut state) = self.ease_to {
            if state.tick(camera, dt) {
                self.ease_to = None;
            }
            return;
        }

        // Simple independent targets.
        let smoothing = self.smoothing.max(0.0);
        let t = 1.0 - (-smoothing * dt).exp();

        if let Some(target) = self.target_distance {
            let d = camera.distance() + (target - camera.distance()) * t;
            camera.set_distance(d);
            if (camera.distance() - target).abs() < DISTANCE_EPS {
                camera.set_distance(target);
                self.target_distance = None;
            }
        }

        if let Some(target) = self.target_yaw {
            let delta = shortest_angle_delta(camera.yaw(), target);
            camera.set_yaw(camera.yaw() + delta * t);
            if delta.abs() < ANGLE_EPS {
                camera.set_yaw(target);
                self.target_yaw = None;
            }
        }

        if let Some(target) = self.target_pitch {
            let p = camera.pitch() + (target - camera.pitch()) * t;
            camera.set_pitch(p);
            if (camera.pitch() - target).abs() < ANGLE_EPS {
                camera.set_pitch(target);
                self.target_pitch = None;
            }
        }

        // Pan momentum.
        if self.momentum.0.abs() > MOMENTUM_EPS || self.momentum.1.abs() > MOMENTUM_EPS {
            let dx_deg = self.momentum.0 * dt
                / (METERS_PER_DEGREE * camera.target().lat.to_radians().cos().max(0.001));
            let dy_deg = self.momentum.1 * dt / METERS_PER_DEGREE;

            let mut target = *camera.target();
            target.lon += dx_deg;
            target.lat += dy_deg;

            target.lat = target.lat.clamp(-MERCATOR_MAX_LAT, MERCATOR_MAX_LAT);
            if target.lon > 180.0 {
                target.lon -= 360.0;
            }
            if target.lon < -180.0 {
                target.lon += 360.0;
            }
            camera.set_target(target);

            let decay = self.momentum_decay.clamp(0.0, 1.0).powf(dt);
            self.momentum.0 *= decay;
            self.momentum.1 *= decay;
        }
    }
}

// ---------------------------------------------------------------------------
// Helpers
// ---------------------------------------------------------------------------

/// Smallest signed angular delta from `from` to `to` in radians.
///
/// Result is in `[-PI, PI]`.
fn shortest_angle_delta(from: f64, to: f64) -> f64 {
    let two_pi = std::f64::consts::TAU;
    let mut d = (to - from) % two_pi;
    if d > std::f64::consts::PI {
        d -= two_pi;
    }
    if d < -std::f64::consts::PI {
        d += two_pi;
    }
    d
}

/// Normalize target bearing so that interpolation takes the shortest path.
///
/// Returns a target value numerically close to `from` that yields the
/// same final bearing modulo 2?.
fn shortest_bearing_target(from: f64, to: f64) -> f64 {
    from + shortest_angle_delta(from, to)
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

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

    // -- Legacy simple animation tests ------------------------------------

    #[test]
    fn zoom_converges() {
        let mut cam = Camera::default();
        cam.set_distance(10_000_000.0);
        let mut anim = CameraAnimator::new();
        anim.animate_zoom(1_000_000.0);

        for _ in 0..60 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }
        assert!((cam.distance() - 1_000_000.0).abs() < 100_000.0);

        for _ in 0..240 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }
        assert!((cam.distance() - 1_000_000.0).abs() < 1.0);
        assert!(!anim.is_active());
    }

    #[test]
    fn momentum_decays() {
        let mut cam = Camera::default();
        let mut anim = CameraAnimator::new();
        anim.apply_momentum(100_000.0, 0.0);

        assert!(anim.is_active());

        for _ in 0..600 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }

        assert!(!anim.is_active());
    }

    #[test]
    fn cancel_stops_all() {
        let mut anim = CameraAnimator::new();
        anim.animate_zoom(100.0);
        anim.apply_momentum(10.0, 10.0);
        assert!(anim.is_active());
        anim.cancel();
        assert!(!anim.is_active());
    }

    #[test]
    fn shortest_angle_delta_wraps() {
        let from = std::f64::consts::PI - 0.0174533;
        let to = -std::f64::consts::PI + 0.0174533;
        let d = shortest_angle_delta(from, to);
        assert!(d.abs() < 0.1);
    }

    #[test]
    fn rotate_uses_shortest_path() {
        let mut cam = Camera::default();
        cam.set_yaw(std::f64::consts::PI - 0.01);
        let mut anim = CameraAnimator::new();
        anim.animate_rotate(-std::f64::consts::PI + 0.01, cam.pitch());

        let before = cam.yaw();
        anim.tick(&mut cam, 1.0 / 60.0);
        let moved = (cam.yaw() - before).abs();
        assert!(
            moved < 0.2,
            "yaw moved too far; likely long-path interpolation"
        );
    }

    #[test]
    fn tick_ignores_invalid_dt() {
        let mut cam = Camera::default();
        let mut anim = CameraAnimator::new();
        anim.animate_zoom(1000.0);
        let before = cam.distance();

        anim.tick(&mut cam, f64::NAN);
        assert_eq!(cam.distance(), before);

        anim.tick(&mut cam, 0.0);
        assert_eq!(cam.distance(), before);

        anim.tick(&mut cam, -1.0);
        assert_eq!(cam.distance(), before);
    }

    #[test]
    fn animate_zoom_rejects_invalid_values() {
        let mut anim = CameraAnimator::new();
        anim.animate_zoom(f64::NAN);
        assert!(!anim.is_active());
        anim.animate_zoom(-10.0);
        assert!(!anim.is_active());
        anim.animate_zoom(0.0);
        assert!(!anim.is_active());
    }

    #[test]
    fn animate_rotate_rejects_non_finite_independently() {
        let mut anim = CameraAnimator::new();
        anim.animate_rotate(f64::NAN, 0.1);
        assert!(anim.is_active());
        anim.cancel();

        anim.animate_rotate(0.2, f64::NAN);
        assert!(anim.is_active());
    }

    #[test]
    fn momentum_clamps_and_wraps_geo() {
        let mut cam = Camera::default();
        cam.set_target(GeoCoord::from_lat_lon(85.0, 179.9));
        let mut anim = CameraAnimator::new();
        anim.apply_momentum(100_000.0, 100_000.0);

        anim.tick(&mut cam, 1.0);

        assert!(cam.target().lat <= MERCATOR_MAX_LAT);
        assert!(cam.target().lon >= -180.0 && cam.target().lon <= 180.0);
    }

    #[test]
    fn decay_is_clamped_to_valid_range() {
        let mut cam = Camera::default();
        let mut anim = CameraAnimator::new();
        anim.apply_momentum(100.0, 0.0);
        anim.momentum_decay = 2.0;

        let before = cam.target().lon;
        anim.tick(&mut cam, 1.0);
        let after = cam.target().lon;
        assert!(after != before);
        let after2_before = cam.target().lon;
        anim.tick(&mut cam, 1.0);
        assert!((cam.target().lon - after2_before).abs() < 1.0);
    }

    // -- Fly-to tests -----------------------------------------------------

    #[test]
    fn fly_to_changes_center_and_zoom() {
        let mut cam = Camera::default();
        cam.set_target(GeoCoord::from_lat_lon(0.0, 0.0));
        cam.set_distance(10_000_000.0);
        cam.set_viewport(800, 600);

        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                center: Some(GeoCoord::from_lat_lon(48.8566, 2.3522)), // Paris
                zoom: Some(12.0),
                ..Default::default()
            },
        );

        assert!(anim.is_active());
        assert!(anim.is_flying());

        // Run for a long time to ensure completion.
        for _ in 0..6000 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }

        assert!(!anim.is_active());
        assert!(
            (cam.target().lat - 48.8566).abs() < 0.01,
            "lat={}",
            cam.target().lat
        );
        assert!(
            (cam.target().lon - 2.3522).abs() < 0.01,
            "lon={}",
            cam.target().lon
        );
    }

    #[test]
    fn fly_to_animates_bearing_shortest_path() {
        let mut cam = Camera::default();
        cam.set_distance(1_000_000.0);
        cam.set_viewport(800, 600);
        cam.set_yaw(std::f64::consts::PI - 0.1);

        let target_yaw = -std::f64::consts::PI + 0.1;
        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                bearing: Some(target_yaw),
                zoom: Some(5.0),
                ..Default::default()
            },
        );

        // The first frame should take the short path (< 0.3 radians of
        // total delta, so any individual step should be small).
        let before = cam.yaw();
        anim.tick(&mut cam, 1.0 / 60.0);
        let step = (cam.yaw() - before).abs();
        assert!(step < 0.5, "yaw step too large: {step}");
    }

    #[test]
    fn fly_to_same_location_different_zoom_works() {
        let mut cam = Camera::default();
        cam.set_distance(10_000_000.0);
        cam.set_viewport(800, 600);
        let start = *cam.target();

        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                zoom: Some(14.0),
                ..Default::default()
            },
        );

        assert!(anim.is_active());

        for _ in 0..6000 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }

        assert!(!anim.is_active());
        // Center should not have moved significantly.
        assert!((cam.target().lat - start.lat).abs() < 0.01);
        assert!((cam.target().lon - start.lon).abs() < 0.01);
    }

    #[test]
    fn fly_to_cancel_stops_animation() {
        let mut cam = Camera::default();
        cam.set_distance(10_000_000.0);
        cam.set_viewport(800, 600);

        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                center: Some(GeoCoord::from_lat_lon(51.5, -0.12)),
                zoom: Some(10.0),
                ..Default::default()
            },
        );

        anim.tick(&mut cam, 0.1);
        assert!(anim.is_active());
        anim.cancel();
        assert!(!anim.is_active());
    }

    #[test]
    fn fly_to_explicit_duration() {
        let mut cam = Camera::default();
        cam.set_distance(10_000_000.0);
        cam.set_viewport(800, 600);

        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                center: Some(GeoCoord::from_lat_lon(35.6762, 139.6503)),
                zoom: Some(10.0),
                duration: Some(2.0),
                ..Default::default()
            },
        );

        // Run for slightly more than 2 seconds to account for floating point.
        for _ in 0..130 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }
        assert!(!anim.is_active());
        assert!((cam.target().lat - 35.6762).abs() < 0.01);
    }

    #[test]
    fn fly_to_max_duration_degrades_to_jump() {
        let mut cam = Camera::default();
        cam.set_distance(10_000_000.0);
        cam.set_viewport(800, 600);

        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                center: Some(GeoCoord::from_lat_lon(35.6762, 139.6503)),
                zoom: Some(10.0),
                max_duration: Some(0.001), // Very short max => instant jump.
                ..Default::default()
            },
        );

        // Should have jumped immediately.
        assert!(!anim.is_active());
        assert!((cam.target().lat - 35.6762).abs() < 0.01);
    }

    // -- Ease-to tests ----------------------------------------------------

    #[test]
    fn ease_to_basic() {
        let mut cam = Camera::default();
        cam.set_distance(10_000_000.0);
        cam.set_viewport(800, 600);

        let mut anim = CameraAnimator::new();
        anim.start_ease_to(
            &mut cam,
            &EaseToOptions {
                center: Some(GeoCoord::from_lat_lon(40.7128, -74.0060)),
                zoom: Some(12.0),
                duration: 0.5,
                ..Default::default()
            },
        );

        assert!(anim.is_active());
        assert!(anim.is_easing());

        for _ in 0..60 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }

        assert!(!anim.is_active());
        assert!((cam.target().lat - 40.7128).abs() < 0.01);
    }

    #[test]
    fn ease_to_zero_duration_is_instant() {
        let mut cam = Camera::default();
        cam.set_distance(10_000_000.0);
        cam.set_viewport(800, 600);

        let mut anim = CameraAnimator::new();
        anim.start_ease_to(
            &mut cam,
            &EaseToOptions {
                center: Some(GeoCoord::from_lat_lon(51.5, -0.12)),
                duration: 0.0,
                ..Default::default()
            },
        );

        // Should have jumped immediately.
        assert!(!anim.is_active());
        assert!((cam.target().lat - 51.5).abs() < 0.01);
    }

    // -- Easing function test ---------------------------------------------

    #[test]
    fn default_easing_endpoints() {
        assert!((default_easing(0.0)).abs() < 1e-6);
        assert!((default_easing(1.0) - 1.0).abs() < 1e-6);
    }

    #[test]
    fn default_easing_monotonic() {
        let mut prev = 0.0;
        for i in 1..=100 {
            let t = i as f64 / 100.0;
            let v = default_easing(t);
            assert!(v >= prev - 1e-9, "non-monotonic at t={t}: {v} < {prev}");
            prev = v;
        }
    }

    // -- Van Wijk algorithm correctness -----------------------------------

    #[test]
    fn fly_to_zooms_out_then_in() {
        // The van Wijk algorithm should zoom out during the middle of the
        // flight (lower zoom = higher distance) and then zoom back in.
        let mut cam = Camera::default();
        cam.set_target(GeoCoord::from_lat_lon(0.0, 0.0));
        cam.set_distance(zoom_to_distance(
            5.0,
            cam.fov_y(),
            cam.viewport_height(),
            true,
        ));
        cam.set_viewport(800, 600);

        let start_dist = cam.distance();
        let end_zoom = 5.0; // Same zoom, different center => pure zoom-out arc.

        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                center: Some(GeoCoord::from_lat_lon(40.0, 30.0)),
                zoom: Some(end_zoom),
                duration: Some(4.0),
                easing: Some(|t| t), // Linear easing for predictable sampling.
                ..Default::default()
            },
        );

        // Sample distance at mid-flight.  With same start/end zoom and a
        // large center displacement, the camera should be farther away
        // (zoomed out) at the midpoint.
        let mut max_dist: f64 = 0.0;
        for _ in 0..240 {
            anim.tick(&mut cam, 1.0 / 60.0);
            max_dist = max_dist.max(cam.distance());
        }
        // Complete the rest.
        for _ in 0..300 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }

        assert!(
            max_dist > start_dist * 1.5,
            "mid-flight distance ({max_dist:.0}) should be well above start ({start_dist:.0})"
        );
        // Final distance should be back near the start (same zoom).
        let final_dist = cam.distance();
        assert!(
            (final_dist - start_dist).abs() < start_dist * 0.05,
            "final distance ({final_dist:.0}) should be near start ({start_dist:.0})"
        );
    }

    #[test]
    fn fly_to_van_wijk_r_function_matches_maplibre() {
        // Direct unit test of the zoom_out_factor (r) function against
        // known MapLibre values.
        //
        // For w0=800, w1=400 (zoom in by 1 level), u1=1000, rho=1.42:
        //   rho2 = 2.0164
        //   r(0) = ln(sqrt(b0^2+1) - b0)  where b0 = (w1^2-w0^2+rho2^2*u1^2) / (2*w0*rho2*u1)
        //   r(1) = ln(sqrt(b1^2+1) - b1)  where b1 = (w1^2-w0^2-rho2^2*u1^2) / (2*w1*rho2*u1)
        let w0: f64 = 800.0;
        let w1: f64 = 400.0;
        let u1: f64 = 1000.0;
        let rho: f64 = 1.42;
        let rho2 = rho * rho;

        let r = |descent: bool| -> f64 {
            let (w, sign) = if descent { (w1, -1.0) } else { (w0, 1.0) };
            let b = (w1 * w1 - w0 * w0 + sign * rho2 * rho2 * u1 * u1) / (2.0 * w * rho2 * u1);
            ((b * b + 1.0).sqrt() - b).ln()
        };

        let r0 = r(false);
        let r1 = r(true);
        let s = (r1 - r0) / rho;

        assert!(r0.is_finite(), "r0 should be finite");
        assert!(r1.is_finite(), "r1 should be finite");
        assert!(s > 0.0, "path length S should be positive, got {s}");
        assert!(s.is_finite(), "path length S should be finite");

        // Verify that w(0) = cosh(r0)/cosh(r0) = 1.0 (unit visible span at start).
        let w_at_0 = r0.cosh() / (r0 + rho * 0.0).cosh();
        assert!(
            (w_at_0 - 1.0).abs() < 1e-10,
            "w(0) should be 1.0, got {w_at_0}"
        );

        // Verify that w(S) = w1/w0 (MapLibre: scale = 1/w(S), so zoom = startZoom + log2(scale)).
        let w_at_s = r0.cosh() / (r0 + rho * s).cosh();
        let expected_w_at_s = w1 / w0; // 0.5
        assert!(
            (w_at_s - expected_w_at_s).abs() < 0.01,
            "w(S) should be {expected_w_at_s}, got {w_at_s}"
        );
    }

    #[test]
    fn fly_to_degenerate_same_center_uses_w1_not_zoom() {
        // Regression: the degenerate path should compare w0 vs w1 (pixel
        // spans), not w0 vs end_zoom (a zoom level number).
        let mut cam = Camera::default();
        cam.set_target(GeoCoord::from_lat_lon(10.0, 20.0));
        cam.set_distance(zoom_to_distance(
            3.0,
            cam.fov_y(),
            cam.viewport_height(),
            true,
        ));
        cam.set_viewport(800, 600);

        let start_dist = cam.distance();

        let mut anim = CameraAnimator::new();
        anim.start_fly_to(
            &mut cam,
            &FlyToOptions {
                // Same center, different zoom => zoom-only degenerate path.
                zoom: Some(10.0),
                duration: Some(1.0),
                ..Default::default()
            },
        );

        assert!(
            anim.is_active(),
            "animation should be active for zoom-only flight"
        );

        // Run to completion.
        for _ in 0..70 {
            anim.tick(&mut cam, 1.0 / 60.0);
        }

        assert!(!anim.is_active());
        // Should have zoomed in significantly.
        assert!(
            cam.distance() < start_dist * 0.01,
            "should have zoomed in: start={start_dist:.0}, end={:.0}",
            cam.distance()
        );
    }
}