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1	//! BPM and musical key detection via stratum-dsp, with minimum-duration gating.
2
3	use tracing::instrument;
4
5	/// BPM and key detection results from stratum-dsp.
6	pub struct BpmKeyResult {
7	pub bpm: Option<f64>,
8	pub bpm_confidence: Option<f32>,
9	pub key: Option<String>,
10	pub key_confidence: Option<f32>,
11	}
12
13	/// Detect BPM and musical key using stratum-dsp's `analyze_audio`.
14	///
15	/// Skips samples shorter than `min_duration` seconds (caller typically passes 2.0)
16	/// since tempo detection needs enough rhythmic content to find a reliable beat grid.
17	#[instrument(skip_all)]
18	pub fn detect_bpm_key(samples: &[f32], sample_rate: u32, min_duration: f64) -> BpmKeyResult {
19	if sample_rate == 0 {
20	return BpmKeyResult {
21	bpm: None,
22	bpm_confidence: None,
23	key: None,
24	key_confidence: None,
25	};
26	}
27	let duration = samples.len() as f64 / sample_rate as f64;
28	if duration < min_duration {
29	return BpmKeyResult {
30	bpm: None,
31	bpm_confidence: None,
32	key: None,
33	key_confidence: None,
34	};
35	}
36
37	// stratum-dsp handles the heavy lifting (autocorrelation-based BPM, chroma-based key).
38	// On error (corrupt audio, unsupported format), return None for both rather than
39	// propagating — callers treat missing BPM/key as "not detected".
40	let config = stratum_dsp::AnalysisConfig::default();
41	let result = match stratum_dsp::analyze_audio(samples, sample_rate, config) {
42	Ok(r) => r,
43	Err(_) => {
44	return BpmKeyResult {
45	bpm: None,
46	bpm_confidence: None,
47	key: None,
48	key_confidence: None,
49	}
50	}
51	};
52
53	// Filter out implausible BPM values. Musically meaningful tempos range from
54	// ~30 BPM (very slow ambient) to ~300 BPM (extreme speedcore). Values outside
55	// 0-300 are detection artifacts (silence, noise, etc.).
56	let bpm = if result.bpm > 0.0 && result.bpm < 300.0 {
57	Some(result.bpm as f64)
58	} else {
59	None
60	};
61
62	let key = {
63	let name = result.key.name();
64	if name.is_empty() \|\| name == "Unknown" {
65	None
66	} else {
67	Some(name)
68	}
69	};
70
71	BpmKeyResult {
72	bpm,
73	bpm_confidence: bpm.map(\|_\| result.bpm_confidence),
74	key,
75	key_confidence: Some(result.key_confidence),
76	}
77	}
78
79	#[cfg(test)]
80	mod tests {
81	use super::*;
82
83	#[test]
84	fn short_sample_returns_none() {
85	let short = vec![0.0f32; 44100]; // 1 second
86	let result = detect_bpm_key(&short, 44100, 2.0);
87	assert!(result.bpm.is_none());
88	assert!(result.key.is_none());
89	}
90
91	#[test]
92	fn silence_bpm() {
93	let silence = vec![0.0f32; 44100 * 4];
94	let result = detect_bpm_key(&silence, 44100, 2.0);
95	// Silence shouldn't have a meaningful BPM
96	// (stratum may return 0 or garbage — we filter those)
97	if let Some(bpm) = result.bpm {
98	assert!(bpm > 0.0 && bpm < 300.0);
99	}
100	}
101	}
102