//! Reverse operation: reverse frame order while preserving channel layout.

/// Reverse sample data in-place, preserving channel order within each frame.
///
/// For stereo audio, this reverses the order of L/R frame pairs without
/// swapping L and R within a frame.
pub fn apply_reverse(samples: &mut [f32], channels: u16) {
    let ch = channels as usize;
    if ch == 0 || samples.is_empty() {
        return;
    }

    let num_frames = samples.len() / ch;
    // Swap frames from outside in
    for i in 0..num_frames / 2 {
        let j = num_frames - 1 - i;
        for c in 0..ch {
            samples.swap(i * ch + c, j * ch + c);
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn reverse_mono() {
        let mut samples = vec![1.0, 2.0, 3.0, 4.0, 5.0];
        apply_reverse(&mut samples, 1);
        assert_eq!(samples, vec![5.0, 4.0, 3.0, 2.0, 1.0]);
    }

    #[test]
    fn reverse_stereo() {
        // 3 frames: [L0,R0, L1,R1, L2,R2]
        let mut samples = vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0];
        apply_reverse(&mut samples, 2);
        // Reversed frames: [L2,R2, L1,R1, L0,R0]
        assert_eq!(samples, vec![5.0, 6.0, 3.0, 4.0, 1.0, 2.0]);
    }

    #[test]
    fn reverse_single_frame() {
        let mut samples = vec![1.0, 2.0];
        apply_reverse(&mut samples, 2);
        assert_eq!(samples, vec![1.0, 2.0]);
    }

    #[test]
    fn reverse_empty() {
        let mut samples: Vec<f32> = vec![];
        apply_reverse(&mut samples, 1);
        assert!(samples.is_empty());
    }

    #[test]
    fn reverse_double_is_identity() {
        let original = vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0];
        let mut samples = original.clone();
        apply_reverse(&mut samples, 2);
        apply_reverse(&mut samples, 2);
        assert_eq!(samples, original);
    }
}