バッハの平均律「2の12乗根」とホールの残響2.0秒は、なぜRustの非同期処理（対位法）と美しく共鳴するのか？

#![forbid(unsafe_code)]

/// Computes Eyring-Norris reverberation time RT_60 (in seconds) for an acoustic hall.
/// `volume_m3`: Room volume V (m^3), `surface_m2`: Total surface area S (m^2),
/// `mean_alpha`: Mean absorption coefficient α in (0.0, 0.99),
/// `air_absorption_m`: Air attenuation coefficient m (m^-1, typically ~0.0015 at 2kHz).
#[inline]
pub fn eyring_reverberation_rt60(
    volume_m3: f64,
    surface_m2: f64,
    mean_alpha: f64,
    air_absorption_m: f64,
) -> f64 {
    let alpha = mean_alpha.clamp(0.01, 0.99);
    let denom = -surface_m2 * (1.0 - alpha).ln() + 4.0 * air_absorption_m * volume_m3;
    (0.161 * volume_m3) / denom.max(1e-6)
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum CounterpointViolation {
    VoiceCrossing,
    ParallelPerfectUnisonOrOctave,
    ParallelPerfectFifth,
    DirectHiddenFifthOrOctaveInSimilarMotion,
}

/// Verifies a transition `(lower_prev, upper_prev) -> (lower_next, upper_next)` (MIDI pitch numbers)
/// against strict First-Species Counterpoint rules (Fux "Gradus ad Parnassum").
pub fn verify_counterpoint_step(
    lower_prev: i16,
    upper_prev: i16,
    lower_next: i16,
    upper_next: i16,
) -> Result<(), CounterpointViolation> {
    if lower_next > upper_next {
        return Err(CounterpointViolation::VoiceCrossing);
    }
    let prev_int = (upper_prev - lower_prev).rem_euclid(12);
    let next_int = (upper_next - lower_next).rem_euclid(12);
    let dl = lower_next - lower_prev;
    let du = upper_next - upper_prev;

    // Both voices moving in the same non-zero direction (similar or parallel motion)
    let similar_motion = (dl > 0 && du > 0) || (dl < 0 && du < 0);

    if (dl != 0 || du != 0) && prev_int == next_int {
        if next_int == 0 {
            return Err(CounterpointViolation::ParallelPerfectUnisonOrOctave);
        }
        if next_int == 7 {
            return Err(CounterpointViolation::ParallelPerfectFifth);
        }
    }
    // Hidden (direct) fifths or octaves approached by similar motion with leap in upper voice
    if similar_motion && (next_int == 0 || next_int == 7) && du.abs() > 2 {
        return Err(CounterpointViolation::DirectHiddenFifthOrOctaveInSimilarMotion);
    }
    Ok(())
}