bevy_lights 0.2.2

//! The pool's hand-off: which candidate holds a light, and how a light that changes hands fades.
//!
//! The pool hands its real lights to the nearest candidates, and when the ordering changes a slot
//! teleports from one prop to another. Two plain functions decide that, so the rule can be tested
//! without an app: [`hand_off`] keeps a slot on the candidate it holds until a challenger is
//! meaningfully nearer, and [`fade_factor`] dips the light's brightness through the change so the
//! teleport reads as a change of intensity rather than a jump.
//!
//! The craft behind both is in `docs/research/pool-hand-off.md`: overlapping keys, and the eye's
//! finite integration time.

/// The default hysteresis: a challenger must be at least this much nearer than the candidate a slot
/// holds before it takes the light.
///
/// `1.0` is "nearest wins outright", the behaviour before this module existed. `1.15` means a
/// challenger has to be about 13 % closer, which stops two props at nearly the same distance from
/// trading a light every frame while still letting a clearly nearer one win at once.
pub const HYSTERESIS: f32 = 1.15;

/// How long a light takes to come back to full brightness after it changes owner, in seconds.
///
/// A third of a second is 20 frames at 60 Hz: well below the eye's flicker-fusion threshold, so the
/// change is followed rather than seen as a step, and long enough to be read as a dimming.
pub const FADE_SECONDS: f32 = 0.35;

/// The candidate a pool slot should hold, given the one it holds now and its distance against the
/// nearest challenger's.
///
/// `held_distance` is the squared distance from the focus to the held candidate, and
/// `challenger_distance` the same for the nearest candidate that is not held. Distances are squared
/// because that is what the pool sorts on; the hysteresis is applied to the square roots, which is
/// the same comparison on the distances themselves.
///
/// Returns `true` when the slot should take the challenger:
///
/// - with no held candidate (a slot newly filled), `true` — the nearest wins;
/// - when the held candidate is still in reach and the challenger is not nearer by `hysteresis`
///   times, `false` — "overlapping keys": the held light stays;
/// - when the challenger is nearer by more than that, `true`;
/// - `hysteresis` at or below 1.0 makes every nearer challenger win, which is the old rule.
///
/// A held candidate that has left reach or gone is the caller's to drop: this function only compares
/// distances, so a caller that has no held candidate passes [`f32::INFINITY`] and takes the challenger
/// immediately.
///
/// ```
/// # use bevy_lights::model::pool::*;
/// // Held at 4 m, challenged at 3.9 m with 1.15 hysteresis: the held light stays.
/// assert!(!hand_off(16.0, 15.21, HYSTERESIS));
/// // Challenged at 2 m: the challenger wins.
/// assert!(hand_off(16.0, 4.0, HYSTERESIS));
/// // Nothing held: the challenger always wins.
/// assert!(hand_off(f32::INFINITY, 100.0, HYSTERESIS));
/// ```
pub fn hand_off(held_distance: f32, challenger_distance: f32, hysteresis: f32) -> bool {
    // Both `NaN`s are answered **before** the clamp: `f32::max` returns the other operand when one is
    // `NaN`, so a broken distance would silently become a zero — "nothing held", or "at the focus".
    if challenger_distance.is_nan() {
        return false;
    }
    if held_distance.is_nan() {
        return true;
    }
    // Compare on the distances, not their squares: the hysteresis is a ratio of distances.
    preferred(held_distance.max(0.0).sqrt(), challenger_distance.max(0.0).sqrt(), hysteresis)
}

/// The overlap rule itself, on any "smaller is better" quantity: `true` when `challenger` beats `held`
/// by the `hysteresis` margin. [`hand_off`] is this on distances.
///
/// A pool that ranks its candidates by something other than distance — the light each puts on the
/// subject, say — compares that quantity here, so ranking and holding are one rule rather than two. The
/// quantity is not square-rooted: the caller states it in the units the margin applies to, which for
/// [`hand_off`] is the distance and not its square.
///
/// ```
/// # use bevy_lights::model::pool::*;
/// // The light arriving at the subject, as its reciprocal: a value 15 % smaller wins.
/// assert!(preferred(1.0, 1.0 / 1.16, HYSTERESIS));
/// assert!(!preferred(1.0, 1.0 / 1.10, HYSTERESIS));
/// ```
pub fn preferred(held: f32, challenger: f32, hysteresis: f32) -> bool {
    // A challenge that is not a number cannot be compared; keep what is held rather than chatter.
    if challenger.is_nan() {
        return false;
    }
    if !held.is_finite() {
        return true;
    }
    if held.is_nan() {
        return true;
    }
    let margin = if hysteresis.is_nan() { 1.0 } else { hysteresis.max(1.0) };
    // `is_nan` on the challenger already returned; the held value is clamped, which is right for a
    // distance and for an illuminance reciprocal alike.
    challenger.max(0.0) * margin < held.max(0.0)
}

/// The brightness multiplier of a light that is **leaving** a hand-off `since` seconds after it: 1 at
/// the instant of the hand-off, falling to exactly 0 over `fade_seconds`, and 0 from then on.
///
/// The complement of [`fade_factor`], so a cross-fade's departing and arriving lights sum to exactly
/// one at every point: the subject neither brightens nor darkens as the light changes hands, which is
/// what "overlapping key lights" buys.
///
/// ```
/// # use bevy_lights::model::pool::*;
/// for since in [0.0, 0.05, 0.1, 0.2, 0.35] {
///     assert!((fade_out_factor(since, FADE_SECONDS) + fade_factor(since, FADE_SECONDS) - 1.0).abs() < 1e-6);
/// }
/// ```
pub fn fade_out_factor(since: f32, fade_seconds: f32) -> f32 {
    1.0 - fade_factor(since, fade_seconds)
}

/// The brightness multiplier of a light `since` seconds after it changed owner: 0 at the instant of
/// the hand-off, rising to exactly 1 over [`FADE_SECONDS`], and exactly 1 from then on.
///
/// Smoothstep, so the multiplier's derivative is zero at both ends and the fade has no edge to catch
/// the eye at either end. A negative or `NaN` time is 0 (the fade has just begun); a time past the fade
/// is 1.
///
/// ```
/// # use bevy_lights::model::pool::*;
/// assert_eq!(fade_factor(0.0, FADE_SECONDS), 0.0);
/// assert_eq!(fade_factor(FADE_SECONDS, FADE_SECONDS), 1.0);
/// assert_eq!(fade_factor(9.0, FADE_SECONDS), 1.0);
/// assert!(fade_factor(0.1, FADE_SECONDS) < fade_factor(0.2, FADE_SECONDS));
/// ```
pub fn fade_factor(since: f32, fade_seconds: f32) -> f32 {
    if since.is_nan() || fade_seconds <= 0.0 {
        return if since.is_nan() { 0.0 } else { 1.0 };
    }
    let s = (since / fade_seconds).clamp(0.0, 1.0);
    s * s * (3.0 - 2.0 * s)
}

#[cfg(test)]
#[allow(clippy::expect_used, clippy::unwrap_used, clippy::panic)]
mod tests {
    use super::*;

    #[test]
    fn a_held_light_stays_until_a_challenger_is_meaningfully_nearer() {
        // Held at 4 m. The challenger's distance at which it wins is 4 / 1.15 = 3.478 m.
        let held = 16.0;
        assert!(!hand_off(held, 15.5, HYSTERESIS), "3.937 m does not win");
        assert!(!hand_off(held, 12.25, HYSTERESIS), "3.5 m does not win");
        assert!(hand_off(held, 12.0, HYSTERESIS), "3.464 m wins");
        assert!(hand_off(held, 4.0, HYSTERESIS), "2 m wins");
    }

    #[test]
    fn no_held_candidate_takes_the_nearest_at_once() {
        assert!(hand_off(f32::INFINITY, 100.0, HYSTERESIS));
        assert!(hand_off(f32::NAN, 100.0, HYSTERESIS));
        assert!(!hand_off(1.0, f32::NAN, HYSTERESIS), "a broken distance does not displace a held light");
    }

    #[test]
    fn a_hysteresis_of_one_is_the_old_nearest_wins() {
        assert!(hand_off(16.0, 15.99, 1.0), "any nearer challenger wins at 1.0");
        assert!(!hand_off(16.0, 16.01, 1.0), "a further one never does");
        assert!(hand_off(16.0, 15.99, 0.5), "below 1.0 clamps to the nearest-wins rule");
    }

    #[test]
    fn a_cross_fade_sums_to_one_at_every_point() {
        let mut since = 0.0;
        while since <= FADE_SECONDS {
            let out = fade_out_factor(since, FADE_SECONDS);
            let incoming = fade_factor(since, FADE_SECONDS);
            assert!((out + incoming - 1.0).abs() < 1e-6, "at {since}: {out} + {incoming}");
            assert!((0.0..=1.0).contains(&out), "{out} at {since}");
            since += 0.005;
        }
        assert_eq!(fade_out_factor(0.0, FADE_SECONDS), 1.0, "a departing light starts whole");
        assert_eq!(fade_out_factor(FADE_SECONDS, FADE_SECONDS), 0.0, "and ends gone");
    }

    #[test]
    fn the_fade_runs_from_nothing_to_full_and_stays_there() {
        assert_eq!(fade_factor(0.0, FADE_SECONDS), 0.0);
        assert!((fade_factor(FADE_SECONDS / 2.0, FADE_SECONDS) - 0.5).abs() < 1e-5, "half at half time");
        assert_eq!(fade_factor(FADE_SECONDS, FADE_SECONDS), 1.0);
        assert_eq!(fade_factor(FADE_SECONDS * 3.0, FADE_SECONDS), 1.0);
        assert_eq!(fade_factor(-1.0, FADE_SECONDS), 0.0, "a negative age is the start of the fade");
        assert!(fade_factor(f32::NAN, FADE_SECONDS) == 0.0);
        // A zero-length fade is no fade at all.
        assert_eq!(fade_factor(0.0, 0.0), 1.0);

        // Monotone, and smooth at both ends.
        let mut last = 0.0;
        let mut t = 0.0;
        while t <= FADE_SECONDS {
            let f = fade_factor(t, FADE_SECONDS);
            assert!(f >= last, "the fade fell at {t}");
            assert!((0.0..=1.0).contains(&f), "{f} at {t}");
            last = f;
            t += 0.005;
        }
        assert!(fade_factor(0.001, FADE_SECONDS) < 1e-4, "the start has no step");
        assert!(fade_factor(FADE_SECONDS - 0.001, FADE_SECONDS) > 1.0 - 1e-4, "nor the end");
    }
}