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//! True peak metering with 4x oversampling via cubic interpolation.
//!
//! `TruePeakMeter` provides a self-contained, stateless measurement of the
//! inter-sample peak level in a block of audio samples.
//!
//! ## Algorithm
//!
//! The signal is virtually upsampled 4× by inserting three interpolated
//! samples between every pair of original samples using **cubic (Catmull-Rom)**
//! interpolation. The peak of the upsampled signal is the "true peak" — the
//! maximum amplitude that would appear after digital-to-analogue conversion.
//!
//! Catmull-Rom spline between `p1` and `p2` with parameter `t ∈ (0, 1)`:
//!
//! ```text
//! q(t) = 0.5 · [ (2p1)
//! + (-p0 + p2)·t
//! + (2p0 - 5p1 + 4p2 - p3)·t²
//! + (-p0 + 3p1 - 3p2 + p3)·t³ ]
//! ```
//!
//! ## Example
//!
//! ```rust
//! use oximedia_metering::true_peak_meter::TruePeakMeter;
//!
//! let meter = TruePeakMeter::new();
//! let samples: Vec<f32> = (0..1024)
//! .map(|i| (2.0 * std::f32::consts::PI * 997.0 * i as f32 / 48000.0).sin())
//! .collect();
//! let peak = meter.measure(&samples);
//! assert!(peak >= 0.99 && peak <= 1.05, "997 Hz sine peak ≈ 1.0, got {peak}");
//! ```
/// True peak meter with 4× cubic oversampling.
#[derive(Debug, Clone, Default)]
pub struct TruePeakMeter;
impl TruePeakMeter {
/// Create a new `TruePeakMeter`.
#[must_use]
pub fn new() -> Self {
Self
}
/// Measure the true peak of `samples` using 4× cubic interpolation.
///
/// Returns the maximum absolute amplitude found in the oversampled signal.
/// Returns `0.0` if `samples` is empty.
#[must_use]
pub fn measure(&self, samples: &[f32]) -> f32 {
let n = samples.len();
if n == 0 {
return 0.0;
}
// Catmull-Rom helper: interpolate between p1 and p2 at t ∈ (0, 1).
let catmull_rom = |p0: f32, p1: f32, p2: f32, p3: f32, t: f32| -> f32 {
let t2 = t * t;
let t3 = t2 * t;
0.5 * ((2.0 * p1)
+ (-p0 + p2) * t
+ (2.0 * p0 - 5.0 * p1 + 4.0 * p2 - p3) * t2
+ (-p0 + 3.0 * p1 - 3.0 * p2 + p3) * t3)
};
// Helper to get a sample with reflect-boundary padding.
let s = |i: isize| -> f32 {
if i < 0 {
samples[0]
} else if i >= n as isize {
samples[n - 1]
} else {
samples[i as usize]
}
};
let mut peak = 0.0_f32;
for i in 0..n {
let ii = i as isize;
let p0 = s(ii - 1);
let p1 = s(ii);
let p2 = s(ii + 1);
let p3 = s(ii + 2);
// Original sample.
peak = peak.max(p1.abs());
// Three interpolated sub-samples at t = 0.25, 0.50, 0.75.
for &t in &[0.25_f32, 0.50, 0.75] {
let v = catmull_rom(p0, p1, p2, p3, t);
peak = peak.max(v.abs());
}
}
peak
}
}
// ─── Tests ────────────────────────────────────────────────────────────────────
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_empty_input_returns_zero() {
let meter = TruePeakMeter::new();
assert_eq!(meter.measure(&[]), 0.0);
}
#[test]
fn test_dc_signal() {
let meter = TruePeakMeter::new();
let dc = vec![0.5_f32; 256];
let peak = meter.measure(&dc);
assert!(
(peak - 0.5).abs() < 0.01,
"DC signal peak should ≈ 0.5, got {peak}"
);
}
#[test]
fn test_sine_997hz_peak_near_one() {
let meter = TruePeakMeter::new();
let samples: Vec<f32> = (0..4096)
.map(|i| (2.0 * std::f32::consts::PI * 997.0 * i as f32 / 48000.0).sin())
.collect();
let peak = meter.measure(&samples);
assert!(
peak >= 0.99 && peak <= 1.05,
"997 Hz sine at 0 dBFS should have true peak ≈ 1.0, got {peak}"
);
}
#[test]
fn test_peak_at_least_as_large_as_sample_peak() {
let meter = TruePeakMeter::new();
let samples: Vec<f32> = (0..512)
.map(|i| (2.0 * std::f32::consts::PI * 440.0 * i as f32 / 48000.0).sin() * 0.8)
.collect();
let sample_peak = samples.iter().map(|v| v.abs()).fold(0.0_f32, f32::max);
let true_peak = meter.measure(&samples);
assert!(
true_peak >= sample_peak,
"True peak ({true_peak}) must be ≥ sample peak ({sample_peak})"
);
}
#[test]
fn test_single_impulse() {
let meter = TruePeakMeter::new();
let mut samples = vec![0.0_f32; 64];
samples[32] = 0.9;
let peak = meter.measure(&samples);
assert!(peak >= 0.9, "Peak must capture the impulse, got {peak}");
}
#[test]
fn test_negative_impulse() {
let meter = TruePeakMeter::new();
let mut samples = vec![0.0_f32; 64];
samples[16] = -0.7;
let peak = meter.measure(&samples);
assert!(
peak >= 0.7,
"Absolute peak of negative impulse should be ≥ 0.7, got {peak}"
);
}
}