max / audiofiles

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1	//! Channel conversion: mono-to-stereo and stereo-to-mono destructive edits.
2
3	use crate::error::CoreError;
4
5	/// Convert mono interleaved samples to stereo by duplicating each sample.
6	///
7	/// Returns the new channel count (2). Caller must update the stored channel count.
8	/// Rejects input with more than 1 channel to prevent garbled output.
9	pub fn apply_mono_to_stereo(samples: &mut Vec<f32>, channels: u16) -> Result<u16, CoreError> {
10	if channels != 1 {
11	return Err(CoreError::Internal(format!(
12	"mono_to_stereo: expected 1 channel, got {channels}"
13	)));
14	}
15	let n = samples.len();
16	let mut stereo = Vec::with_capacity(n * 2);
17	for &s in samples.iter() {
18	stereo.push(s);
19	stereo.push(s);
20	}
21	*samples = stereo;
22	Ok(2)
23	}
24
25	/// Convert stereo interleaved samples to mono by averaging L+R per frame.
26	///
27	/// Returns the new channel count (1). Caller must update the stored channel count.
28	pub fn apply_stereo_to_mono(samples: &mut Vec<f32>, channels: u16) -> Result<u16, CoreError> {
29	if channels < 2 {
30	return Ok(channels); // already mono, no-op
31	}
32	let ch = channels as usize;
33	let num_frames = samples.len() / ch;
34	let mut mono = Vec::with_capacity(num_frames);
35	for frame in 0..num_frames {
36	let mut sum = 0.0f32;
37	for c in 0..ch {
38	sum += samples[frame * ch + c];
39	}
40	mono.push(sum / ch as f32);
41	}
42	*samples = mono;
43	Ok(1)
44	}
45
46	#[cfg(test)]
47	mod tests {
48	use super::*;
49
50	#[test]
51	fn mono_to_stereo() {
52	let mut samples = vec![0.1, 0.2, 0.3];
53	let ch = apply_mono_to_stereo(&mut samples, 1).unwrap();
54	assert_eq!(ch, 2);
55	assert_eq!(samples, vec![0.1, 0.1, 0.2, 0.2, 0.3, 0.3]);
56	}
57
58	#[test]
59	fn stereo_to_mono() {
60	let mut samples = vec![0.4, 0.6, 0.2, 0.8];
61	let ch = apply_stereo_to_mono(&mut samples, 2).unwrap();
62	assert_eq!(ch, 1);
63	assert_eq!(samples.len(), 2);
64	assert!((samples[0] - 0.5).abs() < 1e-6);
65	assert!((samples[1] - 0.5).abs() < 1e-6);
66	}
67
68	#[test]
69	fn mono_to_mono_noop() {
70	let mut samples = vec![0.1, 0.2];
71	let ch = apply_stereo_to_mono(&mut samples, 1).unwrap();
72	assert_eq!(ch, 1);
73	assert_eq!(samples, vec![0.1, 0.2]);
74	}
75
76	#[test]
77	fn roundtrip_preserves_energy() {
78	let mut samples = vec![0.5, -0.5, 0.3, -0.3];
79	let _ = apply_stereo_to_mono(&mut samples, 2).unwrap();
80	// Mono: [0.0, 0.0] — opposing channels cancel
81	assert!((samples[0]).abs() < 1e-6);
82	}
83
84	#[test]
85	fn empty_input() {
86	let mut samples: Vec<f32> = vec![];
87	let ch = apply_mono_to_stereo(&mut samples, 1).unwrap();
88	assert_eq!(ch, 2);
89	assert!(samples.is_empty());
90	}
91	}
92