audio_samples 1.0.5

//! Structured iteration over audio sample data.
//!
//! This module defines the primary iteration abstractions for traversing
//! [`AudioSamples`] in semantically meaningful ways. Rather than exposing raw
//! indexing or layout-dependent access, the iterators in this module present
//! audio data through three conceptual lenses:
//!
//! - **Frames** — a snapshot across all channels at one time index
//!   ([`FrameIterator`])
//! - **Channels** — the full temporal sequence for one channel
//!   ([`ChannelIterator`])
//! - **Windows** — fixed-size, optionally overlapping temporal blocks
//!   ([`WindowIterator`], requires the `editing` feature)
//!
//! Audio algorithms frequently need to traverse data in ways that reflect its
//! *structure* rather than its *storage layout*. Centralising iteration logic here
//! prevents duplicated indexing and boundary-handling code throughout the crate,
//! while keeping each iterator's ownership and lifetime contract explicit and
//! documented at the iterator type level.
//!
//! For in-place or overlapping mutation, specialised methods such as
//! [`AudioSamples::apply_to_frames`], [`AudioSamples::apply_to_channel_data`], and
//! [`AudioSamples::apply_to_windows`] are provided as counterparts to the
//! read-oriented iterators defined here.
//!
//! Obtain an iterator by calling the corresponding method on any
//! [`AudioSamples`] value. The method is also available through the
//! [`AudioSampleIterators`] extension trait. Collect, chain, or consume the
//! iterator using standard [`Iterator`] combinators.
//!
//! ```
//! use audio_samples::{AudioSamples, sample_rate, iterators::AudioSampleIterators};
//! use ndarray::array;
//!
//! let audio = AudioSamples::new_multi_channel(
//!     array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
//!     sample_rate!(44100),
//! ).unwrap();
//!
//! // Iterate over time-aligned frames (one sample per channel per time step).
//! for frame in audio.frames() {
//!     assert_eq!(frame.num_channels().get(), 2);
//! }
//!
//! // Iterate over complete channels.
//! let channels: Vec<_> = audio.channels().collect();
//! assert_eq!(channels.len(), 2);
//! ```

#[cfg(feature = "editing")]
use non_empty_slice::{NonEmptyVec, non_empty_vec};

use crate::{
    AudioData, AudioSampleError, AudioSampleResult, AudioSamples, LayoutError,
    traits::StandardSample,
};

#[cfg(feature = "editing")]
use crate::AudioEditing;

use std::marker::PhantomData;

#[cfg(feature = "editing")]
use std::num::NonZeroUsize;

/// Extension trait providing iterator methods for AudioSamples.
pub trait AudioSampleIterators<'a, T>
where
    T: StandardSample,
{
    /// Returns an iterator over frames, where each frame is a snapshot of one
    /// sample from each channel at the same point in time.
    ///
    /// For mono audio, each frame contains exactly one sample. For multi-channel
    /// audio, each frame contains one sample per channel, preserving channel
    /// alignment across time.
    ///
    /// # Returns
    ///
    /// A [`FrameIterator`] that yields one [`AudioSamples`] view per time index.
    /// The total number of frames equals `self.samples_per_channel()`.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate, iterators::AudioSampleIterators};
    /// use ndarray::array;
    ///
    /// let audio = AudioSamples::new_multi_channel(
    ///     array![[1.0f32, 2.0], [3.0, 4.0]],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// // Each frame has one sample per channel; two time steps → two frames.
    /// let mut count = 0;
    /// for frame in audio.frames() {
    ///     assert_eq!(frame.num_channels().get(), 2);
    ///     count += 1;
    /// }
    /// assert_eq!(count, 2);
    /// ```
    fn frames<'iter>(&'iter self) -> FrameIterator<'iter, 'a, T>
    where
        'a: 'iter;

    /// Returns an iterator over complete channels.
    ///
    /// Each iteration yields the full temporal sequence of samples belonging to
    /// one channel. Channels are yielded in increasing channel-index order.
    ///
    /// # Returns
    ///
    /// A [`ChannelIterator`] that yields one owned [`AudioSamples`] per channel.
    /// The total number of items equals `self.num_channels()`.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate, iterators::AudioSampleIterators};
    /// use ndarray::array;
    ///
    /// let audio = AudioSamples::new_multi_channel(
    ///     array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// let channels: Vec<_> = audio.channels().collect();
    /// assert_eq!(channels.len(), 2);
    /// assert_eq!(channels[0].samples_per_channel().get(), 3);
    /// ```
    fn channels<'iter>(&'iter self) -> ChannelIterator<'iter, 'a, T>;

    #[cfg(feature = "editing")]
    /// Returns an iterator over fixed-size, optionally overlapping windows.
    ///
    /// Each window covers `window_size` samples per channel. Successive windows
    /// start `hop_size` samples apart, so windows overlap when `hop_size < window_size`.
    ///
    /// The default boundary strategy is [`PaddingMode::Zero`]. Call
    /// [`WindowIterator::with_padding_mode`] on the returned iterator to change it.
    ///
    /// # Arguments
    ///
    /// – `window_size` — number of samples per channel in each window. If zero,
    ///   no windows are yielded.
    /// – `hop_size` — number of samples to advance between window starts. If zero,
    ///   no windows are yielded.
    ///
    /// # Returns
    ///
    /// A [`WindowIterator`] that yields one owned [`AudioSamples`] per window.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// See [`AudioSamples::windows`] for a runnable usage example.
    ///
    /// ```ignore
    /// // Conceptual usage via the trait interface (usize arguments):
    /// let windows: Vec<_> = audio.windows(3_usize, 3_usize).collect();
    /// ```
    fn windows<'iter>(
        &'iter self,
        window_size: usize,
        hop_size: usize,
    ) -> WindowIterator<'iter, 'a, T>
    where
        'a: 'iter;
}

impl<'a, T> AudioSamples<'a, T>
where
    T: StandardSample,
{
    /// Returns an iterator over frames, where each frame is a snapshot of one
    /// sample from each channel at the same point in time.
    ///
    /// For mono audio, each frame contains exactly one sample. For multi-channel
    /// audio, each frame contains one sample per channel in channel-index order.
    ///
    /// # Returns
    ///
    /// A [`FrameIterator`] that yields one [`AudioSamples`] view per time index.
    /// The iterator yields exactly `self.samples_per_channel()` frames.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate};
    /// use ndarray::array;
    ///
    /// let audio = AudioSamples::new_multi_channel(
    ///     array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// // Three time steps → three frames.
    /// assert_eq!(audio.frames().count(), 3);
    ///
    /// // Each frame spans all channels.
    /// for frame in audio.frames() {
    ///     assert_eq!(frame.num_channels().get(), 2);
    /// }
    /// ```
    #[inline]
    #[must_use]
    pub fn frames<'iter>(&'iter self) -> FrameIterator<'iter, 'a, T>
    where
        'a: 'iter,
    {
        FrameIterator::new(self)
    }

    /// Returns an iterator over complete channels.
    ///
    /// Each iteration yields the full temporal sequence of samples belonging to
    /// one channel. Channels are yielded in increasing channel-index order.
    ///
    /// Each yielded value is an owned [`AudioSamples`] instance containing exactly
    /// one mono channel. This involves allocation and data copying.
    ///
    /// # Returns
    ///
    /// A [`ChannelIterator`] yielding one owned [`AudioSamples`] per channel.
    /// The iterator yields exactly `self.num_channels()` items.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate};
    /// use ndarray::array;
    ///
    /// let audio = AudioSamples::new_multi_channel(
    ///     array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// let channels: Vec<_> = audio.channels().collect();
    /// assert_eq!(channels.len(), 2);
    /// assert_eq!(channels[0].samples_per_channel().get(), 3);
    /// ```
    #[inline]
    #[must_use]
    pub fn channels<'iter>(&'iter self) -> ChannelIterator<'iter, 'a, T> {
        ChannelIterator::new(self)
    }

    #[cfg(feature = "editing")]
    /// Returns an iterator over fixed-size, optionally overlapping windows.
    ///
    /// Each window covers `window_size` samples per channel. Successive windows
    /// start `hop_size` samples apart, so windows overlap when `hop_size < window_size`.
    ///
    /// The default boundary strategy is [`PaddingMode::Zero`], which zero-pads the
    /// last window when the signal does not divide evenly. Call
    /// [`WindowIterator::with_padding_mode`] on the returned iterator to change
    /// this behaviour.
    ///
    /// # Arguments
    ///
    /// – `window_size` — number of samples per channel in each window.
    /// – `hop_size` — number of samples to advance between window starts.
    ///
    /// # Returns
    ///
    /// A [`WindowIterator`] yielding one owned [`AudioSamples`] per window.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// # #[cfg(feature = "editing")] {
    /// use audio_samples::{AudioSamples, sample_rate};
    /// use ndarray::array;
    /// use std::num::NonZeroUsize;
    ///
    /// let audio = AudioSamples::new_mono(
    ///     array![1.0f32, 2.0, 3.0, 4.0, 5.0, 6.0],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// // Non-overlapping windows of size 3.
    /// let windows: Vec<_> = audio
    ///     .windows(NonZeroUsize::new(3).unwrap(), NonZeroUsize::new(3).unwrap())
    ///     .collect();
    /// assert_eq!(windows.len(), 2);
    /// assert_eq!(windows[0].samples_per_channel().get(), 3);
    /// # }
    /// ```
    #[inline]
    #[must_use]
    pub fn windows<'iter>(
        &'iter self,
        window_size: NonZeroUsize,
        hop_size: NonZeroUsize,
    ) -> WindowIterator<'iter, 'a, T>
    where
        'a: 'iter,
    {
        WindowIterator::new(self, window_size, hop_size)
    }

    /// Applies a mutable function to every frame without requiring a borrowing-safe iterator.
    ///
    /// The callback receives the frame index and a mutable slice containing the samples for
    /// that frame across all channels. For mono audio the slice has length 1. For
    /// multi-channel audio the slice is a temporary buffer ordered by channel index;
    /// changes are written back into the underlying storage after the callback returns.
    ///
    /// Use this method when in-place, frame-wise mutation is needed and the immutable
    /// [`AudioSamples::frames`] iterator is insufficient.
    ///
    /// # Arguments
    ///
    /// – `f` — a closure of the form `FnMut(frame_index: usize, frame_samples: &mut [T])`.
    ///   – `frame_index` — zero-based index of the current frame.
    ///   – `frame_samples` — mutable slice of length `num_channels()` for the current frame.
    ///
    /// # Returns
    ///
    /// `()` — the audio is modified in place.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate};
    /// use ndarray::array;
    ///
    /// let mut audio = AudioSamples::new_multi_channel(
    ///     array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// // Double every sample frame-by-frame.
    /// audio.apply_to_frames(|_frame_idx, frame| {
    ///     for s in frame { *s *= 2.0; }
    /// });
    ///
    /// assert_eq!(
    ///     audio.as_multi_channel().unwrap(),
    ///     &array![[2.0f32, 4.0, 6.0], [8.0, 10.0, 12.0]],
    /// );
    /// ```
    #[inline]
    pub fn apply_to_frames<F>(&mut self, mut f: F)
    where
        F: FnMut(usize, &mut [T]), // (frame_index, frame_samples)
    {
        match &mut self.data {
            AudioData::Mono(arr) => {
                for (frame_idx, sample) in arr.iter_mut().enumerate() {
                    f(frame_idx, std::slice::from_mut(sample));
                }
            }
            AudioData::Multi(arr) => {
                let (channels, samples_per_channel) = arr.dim();

                for frame_idx in 0..samples_per_channel.get() {
                    let mut frame = Vec::with_capacity(channels.get());
                    for ch in 0..channels.get() {
                        frame.push(arr[[ch, frame_idx]]);
                    }

                    f(frame_idx, &mut frame);

                    for ch in 0..channels.get() {
                        arr[[ch, frame_idx]] = frame[ch];
                    }
                }
            }
        }
    }

    /// Applies a mutable function to each channel's contiguous sample slice.
    ///
    /// This is the fallible counterpart to [`AudioSamples::apply_to_channel_data`].
    /// It requires that the underlying ndarray storage is contiguous in memory.
    /// Non-contiguous layouts (such as after certain in-place reversals or
    /// non-standard strides) will cause this method to return an error.
    ///
    /// The callback receives the channel index and a mutable slice of all samples
    /// for that channel.
    ///
    /// # Arguments
    ///
    /// – `f` — a closure of the form `FnMut(channel_index: usize, channel_samples: &mut [T])`.
    ///   – `channel_index` — zero-based index of the channel being processed.
    ///   – `channel_samples` — mutable slice of all samples belonging to that channel.
    ///
    /// # Returns
    ///
    /// `Ok(())` if all channels were processed successfully.
    ///
    /// # Errors
    ///
    /// Returns [crate::AudioSampleError::Layout] with variant `NonContiguous` if the
    /// underlying multi-channel storage is not contiguous in memory.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate};
    /// use ndarray::array;
    ///
    /// let mut audio = AudioSamples::new_multi_channel(
    ///     array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// // Halve channel 0, double channel 1.
    /// audio.try_apply_to_channel_data(|ch, samples| {
    ///     let gain = if ch == 0 { 0.5 } else { 2.0 };
    ///     for s in samples { *s *= gain; }
    /// }).unwrap();
    ///
    /// assert_eq!(
    ///     audio.as_multi_channel().unwrap(),
    ///     &array![[0.5f32, 1.0, 1.5], [8.0, 10.0, 12.0]],
    /// );
    /// ```
    #[inline]
    pub fn try_apply_to_channel_data<F>(&mut self, mut f: F) -> AudioSampleResult<()>
    where
        F: FnMut(usize, &mut [T]), // (channel_index, channel_samples)
    {
        match &mut self.data {
            AudioData::Mono(arr) => {
                let slice = arr.as_slice_mut();
                f(0, slice);
            }
            AudioData::Multi(arr) => {
                let (channels, samples_per_channel) = arr.dim();
                let samples_per_channel = samples_per_channel.get();
                let slice = arr.as_slice_mut().ok_or_else(|| {
                    AudioSampleError::Layout(LayoutError::NonContiguous {
                        operation: "multi-channel iterator access".to_string(),
                        layout_type: "non-contiguous multi-channel data".to_string(),
                    })
                })?;

                for ch in 0..channels.get() {
                    let start_idx = ch * samples_per_channel;
                    let channel_slice = &mut slice[start_idx..start_idx + samples_per_channel];
                    f(ch, channel_slice);
                }
            }
        }
        Ok(())
    }

    /// Applies a mutable function to each channel's contiguous sample slice.
    ///
    /// This is the infallible counterpart to [`AudioSamples::try_apply_to_channel_data`].
    /// It panics if the underlying storage is not contiguous; prefer the fallible
    /// variant when working with audio that may have non-standard memory layouts.
    ///
    /// The callback receives the channel index and a mutable slice of all samples
    /// for that channel.
    ///
    /// # Arguments
    ///
    /// – `f` — a closure of the form `FnMut(channel_index: usize, channel_samples: &mut [T])`.
    ///   – `channel_index` — zero-based index of the channel being processed.
    ///   – `channel_samples` — mutable slice of all samples belonging to that channel.
    ///
    /// # Returns
    ///
    /// `()` — the audio is modified in place.
    ///
    /// # Panics
    ///
    /// Panics if the underlying storage is not contiguous in memory. Use
    /// [`AudioSamples::try_apply_to_channel_data`] to handle non-contiguous inputs
    /// without panicking.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate};
    /// use ndarray::array;
    ///
    /// let mut audio = AudioSamples::new_mono(
    ///     array![1.0f32, 2.0, 3.0, 4.0],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// // Add 10.0 to every sample.
    /// audio.apply_to_channel_data(|_ch, samples| {
    ///     for s in samples { *s += 10.0; }
    /// });
    ///
    /// assert_eq!(audio.as_mono().unwrap(), &array![11.0f32, 12.0, 13.0, 14.0]);
    /// ```
    #[inline]
    pub fn apply_to_channel_data<F>(&mut self, mut f: F)
    where
        F: FnMut(usize, &mut [T]), // (channel_index, channel_samples)
    {
        self.try_apply_to_channel_data(|ch, data| f(ch, data))
            .expect("apply_to_channel_data requires contiguous storage; use try_apply_to_channel_data to handle non-contiguous inputs");
    }

    /// Applies a mutable function to each temporal window of audio data.
    ///
    /// For mono audio, the callback receives a mutable slice directly into the
    /// underlying buffer for each window. For multi-channel audio, the callback
    /// receives a temporary interleaved buffer of length `window_size * num_channels`
    /// laid out as `[ch0_s0, ch1_s0, …, ch0_s1, ch1_s1, …]`; changes are
    /// written back into the underlying storage after the callback returns.
    ///
    /// Only fully-contained windows are visited; trailing samples that do not
    /// form a complete window are not passed to the callback.
    ///
    /// Use this method for in-place windowed processing, such as applying window
    /// functions or block-wise gain changes, when the read-only
    /// [`AudioSamples::windows`] iterator is not sufficient.
    ///
    /// # Arguments
    ///
    /// – `window_size` — number of samples per channel in each window. If zero,
    ///   the method returns immediately.
    /// – `hop_size` — number of samples to advance between window starts. If zero,
    ///   the method returns immediately.
    /// – `f` — a closure of the form `FnMut(window_index: usize, window_samples: &mut [T])`.
    ///   – `window_index` — zero-based index of the current window.
    ///   – `window_samples` — mutable slice for the current window. For mono audio,
    ///     length equals `window_size`. For multi-channel audio, length equals
    ///     `window_size * num_channels`, laid out in interleaved channel order.
    ///
    /// # Returns
    ///
    /// `()` — the audio is modified in place.
    ///
    /// # Panics
    ///
    /// Does not panic.
    ///
    /// ## Examples
    ///
    /// ```
    /// use audio_samples::{AudioSamples, sample_rate};
    /// use ndarray::array;
    ///
    /// let mut audio = AudioSamples::new_mono(
    ///     array![1.0f32, 2.0, 3.0, 4.0, 5.0, 6.0],
    ///     sample_rate!(44100),
    /// ).unwrap();
    ///
    /// // Halve every sample using non-overlapping windows of size 3.
    /// audio.apply_to_windows(3, 3, |_window_idx, window| {
    ///     for s in window { *s *= 0.5; }
    /// });
    ///
    /// assert_eq!(
    ///     audio.as_mono().unwrap(),
    ///     &array![0.5f32, 1.0, 1.5, 2.0, 2.5, 3.0],
    /// );
    /// ```
    #[inline]
    pub fn apply_to_windows<F>(&mut self, window_size: usize, hop_size: usize, mut f: F)
    where
        F: FnMut(usize, &mut [T]), // (window_index, window_samples)
    {
        let total_samples = self.samples_per_channel().get();
        if total_samples == 0 || window_size == 0 {
            return;
        }

        match &mut self.data {
            AudioData::Mono(arr) => {
                let mut window_idx = 0;
                let mut pos = 0;

                while pos + window_size <= total_samples {
                    let slice = arr.as_slice_mut();
                    let window_slice = &mut slice[pos..pos + window_size];
                    f(window_idx, window_slice);
                    pos += hop_size;
                    window_idx += 1;
                }
            }
            AudioData::Multi(arr) => {
                let (rows, cols) = arr.dim();
                let rows = rows.get();
                let samples_per_channel = cols;

                let mut pos = 0;
                let mut window_idx = 0;

                while pos + window_size <= samples_per_channel.get() {
                    // Create a temporary buffer for the interleaved window
                    let mut window_data = vec![T::zero(); window_size * rows];

                    // Copy window data from each channel into interleaved buffer.
                    for ch in 0..rows {
                        for sample_idx in 0..window_size {
                            let dst_idx = sample_idx * rows + ch; // Interleaved layout
                            window_data[dst_idx] = arr[[ch, pos + sample_idx]];
                        }
                    }

                    // Call the user function
                    f(window_idx, &mut window_data);

                    // Copy modified data back to original channels.
                    for ch in 0..rows {
                        for sample_idx in 0..window_size {
                            let src_idx = sample_idx * rows + ch; // Interleaved layout
                            arr[[ch, pos + sample_idx]] = window_data[src_idx];
                        }
                    }

                    pos += hop_size;
                    window_idx += 1;
                }
            }
        }
    }
}

/// Iterates over time-aligned frames of an [`AudioSamples`] instance.
///
/// A *frame* represents the set of samples across all channels at a single
/// time index. For mono audio, each frame contains exactly one sample. For
/// multi-channel audio, each frame contains one sample per channel, preserving
/// channel alignment.
///
/// ## Purpose
///
/// `FrameIterator` provides a structured, time-centric view of audio data.
/// It is intended for algorithms that operate on synchronous samples across
/// channels, such as frame-wise feature extraction, analysis, or inspection.
///
/// The iterator yields immutable views into the underlying audio data. No
/// reordering, resampling, or interpolation is performed.
///
/// ## Invariants
///
/// - Frames are yielded in strictly increasing temporal order.
/// - Each yielded frame corresponds to exactly one time index.
/// - The number of frames is equal to the number of samples per channel.
/// - All channels are assumed to have equal length.
///
/// ## Assumptions and Limitations
///
/// This iterator assumes that the underlying [`AudioSamples`] instance is
/// channel-aligned and immutable for the lifetime of the iterator. It is not
/// suitable for in-place mutation or algorithms that require overlapping or
/// non-sequential access.
///
/// Use higher-level windowed or transformation APIs when temporal context
/// beyond a single frame is required.
pub struct FrameIterator<'iter, 'a, T>
where
    T: StandardSample,
    'a: 'iter,
{
    /// The source audio data over which frames are iterated.
    audio: &'iter AudioSamples<'a, T>,
    current_frame: usize,
    total_frames: usize,
    _phantom: PhantomData<T>,
}

impl<'iter, 'a, T> FrameIterator<'iter, 'a, T>
where
    T: StandardSample,
    'a: 'iter,
{
    /// Constructs a new frame iterator over the given audio.
    ///
    /// ## Purpose
    ///
    /// This constructor establishes a frame-wise traversal over the provided
    /// audio data, yielding one frame per time index.
    ///
    /// # Arguments
    ///
    /// - `audio`: The source audio to iterate over. All channels must be
    ///   time-aligned.
    ///
    /// ## Behavioural Guarantees
    ///
    /// - The iterator will yield exactly `audio.samples_per_channel()` frames.
    /// - Frames are yielded in deterministic order.
    ///
    /// # Panics
    ///
    /// This function does not panic.
    #[inline]
    #[must_use]
    pub fn new(audio: &'iter AudioSamples<'a, T>) -> Self {
        let total_frames = audio.samples_per_channel().get();
        Self {
            audio,
            current_frame: 0,
            total_frames,
            _phantom: PhantomData,
        }
    }
}

impl<'iter, 'a, T> Iterator for FrameIterator<'iter, 'a, T>
where
    T: StandardSample,
    'a: 'iter,
{
    type Item = AudioSamples<'iter, T>;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        if self.current_frame >= self.total_frames {
            return None;
        }

        let frame_range = self.current_frame..self.current_frame + 1;
        self.current_frame += 1;
        // Copy the &'iter reference so that slice_samples returns AudioSamples<'iter, T>
        // rather than a shorter-lived borrow through &mut self.
        let audio: &'iter AudioSamples<'a, T> = self.audio;
        audio.slice_samples(frame_range).ok()
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.total_frames - self.current_frame;
        (remaining, Some(remaining))
    }
}

impl<T> ExactSizeIterator for FrameIterator<'_, '_, T> where T: StandardSample {}

/// Iterates over complete channels of an [`AudioSamples`] instance.
///
/// Each iteration yields the full sequence of samples belonging to a single
/// channel, independent of other channels. Channels are yielded sequentially
/// in channel index order.
///
/// ## Purpose
///
/// `ChannelIterator` provides a channel-centric view of audio data. It is
/// intended for workflows that process or analyse channels independently,
/// such as per-channel filtering, statistics, or visualisation.
///
/// Unlike frame-based iteration, this iterator exposes the *entire temporal
/// extent* of one channel at a time.
///
/// ## Behaviour and Ownership
///
/// Each yielded item is an owned [`AudioSamples`] instance containing exactly
/// one channel. This reflects the fact that channel-wise slicing produces
/// independent audio objects rather than borrowed views.
///
/// As a result, channel iteration involves allocation and data copying.
/// Callers should take this into account when iterating over large audio
/// buffers or when allocation-free access is required.
///
/// ## Invariants
///
/// - Channels are yielded in increasing channel index order.
/// - Each channel is yielded exactly once.
/// - The number of yielded items is equal to the number of channels.
/// - All samples within a yielded item belong to the same channel.
pub struct ChannelIterator<'iter, 'data, T>
where
    T: StandardSample,
{
    /// The source audio from which channels are extracted.
    audio: &'iter AudioSamples<'data, T>,
    current_channel: usize,
    total_channels: usize,
}

impl<'iter, 'data, T> ChannelIterator<'iter, 'data, T>
where
    T: StandardSample,
{
    /// Constructs a new iterator over the channels of the given audio.
    ///
    /// ## Purpose
    ///
    /// This constructor establishes a channel-wise traversal over the provided
    /// audio data, yielding one complete channel per iteration.
    ///
    /// # Arguments
    ///
    /// - `audio`: The source audio whose channels will be iterated.
    ///
    /// ## Behavioural Guarantees
    ///
    /// - The iterator will yield exactly `audio.num_channels()` items.
    /// - Channels are yielded in deterministic order.
    ///
    /// # Panics
    ///
    /// This function does not panic.
    #[inline]
    #[must_use]
    pub fn new(audio: &'iter AudioSamples<'data, T>) -> Self {
        let total_channels = audio.num_channels().get();

        Self {
            audio,
            current_channel: 0,
            total_channels: total_channels as usize,
        }
    }
}

impl<T> Iterator for ChannelIterator<'_, '_, T>
where
    T: StandardSample,
{
    type Item = AudioSamples<'static, T>;
    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        if self.current_channel >= self.total_channels {
            return None;
        }

        let channel = match self
            .audio
            .clone()
            .into_owned()
            .slice_channels(self.current_channel..=self.current_channel)
        {
            Ok(ch) => ch,
            Err(e) => {
                eprintln!("Error slicing channel {}: {}", self.current_channel, e);
                return None;
            }
        };

        self.current_channel += 1;

        Some(channel)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.total_channels - self.current_channel;
        (remaining, Some(remaining))
    }
}

impl<T> ExactSizeIterator for ChannelIterator<'_, '_, T> where T: StandardSample {}

/// Defines how window iteration behaves when a window extends beyond the
/// available audio data.
///
/// `PaddingMode` controls the treatment of trailing windows whose span exceeds
/// the number of samples per channel. The selected mode determines whether such
/// windows are padded, truncated, or omitted entirely.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
#[non_exhaustive]
pub enum PaddingMode {
    /// Pads incomplete windows with zeros so that all yielded windows have
    /// identical length.
    ///
    /// This mode guarantees a fixed window size and a deterministic number of
    /// windows, which is required by many spectral and frame-based algorithms.
    #[default]
    Zero,

    /// Yields trailing windows without padding.
    ///
    /// Windows near the end of the signal may be shorter than the configured
    /// window size. Callers must be prepared to handle variable-length windows.
    None,

    /// Omits any window that would extend beyond the available data.
    ///
    /// Only fully contained windows are yielded. This mode produces no padding
    /// and no partial windows.
    Skip,
}
/// Iterates over fixed-size temporal windows of audio data.
///
/// Each iteration yields a contiguous block of samples spanning all channels
/// over a fixed temporal extent. Successive windows are offset by a configurable
/// hop size and may overlap depending on the chosen parameters.
///
/// ## Purpose
///
/// `WindowIterator` provides a structured abstraction for windowed audio
/// processing. It is intended for algorithms that operate on local temporal
/// context, such as spectral analysis, feature extraction, and block-based
/// transformations.
///
/// Window iteration is defined in terms of *time*, not storage layout. All
/// windows preserve channel alignment and temporal ordering.
///
/// ## Window Boundaries and Padding
///
/// When a window would extend beyond the available data, its treatment is
/// determined by the configured [`PaddingMode`]. Depending on this mode,
/// trailing windows may be padded, truncated, or skipped entirely. This choice
/// directly affects both the number and shape of yielded windows.
///
/// ## Ownership and Allocation
///
/// Each yielded window is returned as an owned [`AudioSamples`] instance. This
/// allows windows to be processed independently but implies allocation and
/// copying **may** be performed. Whether or not this occurs is down to whether
/// the data is already owned or not. If not then yes, it will allocate,
/// otherwise the a borrow is used. For in-place or allocation-free processing,
/// prefer specialised higher-level APIs where available.
///
/// ## Invariants
///
/// - Windows are yielded in strictly increasing temporal order.
/// - All channels within a window remain time-aligned.
/// - The hop size between successive windows is constant.
/// - The iterator yields a finite, deterministic number of windows.
///
/// ## Assumptions and Limitations
///
/// This iterator assumes a fixed sampling rate for the
/// lifetime of iteration. It is not suitable for overlapping mutable access or
/// algorithms that require shared ownership of window data.
#[cfg(feature = "editing")]
pub struct WindowIterator<'iter, 'a, T>
where
    T: StandardSample,
    'a: 'iter,
{
    /// The source audio from which windows are extracted.
    audio: &'iter AudioSamples<'a, T>,
    window_size: NonZeroUsize,
    hop_size: NonZeroUsize,
    current_position: usize,
    total_samples: NonZeroUsize,
    total_windows: NonZeroUsize,
    current_window: usize,
    padding_mode: PaddingMode,
    _phantom: PhantomData<T>,
}

#[cfg(feature = "editing")]
impl<'iter, 'a, T> WindowIterator<'iter, 'a, T>
where
    T: StandardSample,
    'a: 'iter,
{
    /// Constructs a new window iterator over the given audio.
    ///
    /// ## Purpose
    ///
    /// This constructor establishes a windowed traversal over the provided
    /// audio data using the specified window and hop sizes.
    ///
    /// # Arguments
    ///
    /// - `audio`: The source audio to iterate over.
    /// - `window_size`: The number of samples per channel in each window.
    /// - `hop_size`: The number of samples between the starts of successive windows.
    ///
    /// ## Behavioural Guarantees
    ///
    /// - Windows are generated deterministically from the start of the signal.
    /// - The default padding mode is [`PaddingMode::Zero`].
    ///
    /// ## Degenerate Parameters
    ///
    /// If either `window_size` or `hop_size` is zero, the iterator yields no
    /// windows.
    ///
    /// # Panics
    ///
    /// This function does not panic.
    fn new(
        audio: &'iter AudioSamples<'a, T>,
        window_size: NonZeroUsize,
        hop_size: NonZeroUsize,
    ) -> Self {
        let total_samples = audio.samples_per_channel();

        let total_windows =
            Self::calculate_total_windows(total_samples, window_size, hop_size, PaddingMode::Zero);

        Self {
            audio,
            window_size,
            hop_size,
            current_position: 0,
            total_samples,
            total_windows,
            current_window: 0,
            padding_mode: PaddingMode::Zero,
            _phantom: PhantomData,
        }
    }

    const fn calculate_total_windows(
        total_samples: NonZeroUsize,
        window_size: NonZeroUsize,
        hop_size: NonZeroUsize,
        padding_mode: PaddingMode,
    ) -> NonZeroUsize {
        // Calculate the maximum number of windows we could have
        // This is the ceiling of total_samples / hop_size
        let max_windows = total_samples.get().div_ceil(hop_size.get());
        let max_windows = unsafe {
            // safety: div_ceil on NonZeroUsize is not stable yet. Convert to usize, do div_ceil, and then go back. Never not valid
            NonZeroUsize::new_unchecked(max_windows)
        };

        match padding_mode {
            PaddingMode::Zero => {
                // With zero padding, we can always create max_windows
                max_windows
            }
            PaddingMode::None => {
                // With no padding, count windows that have at least some real data
                let mut count = 0;
                let mut pos = 0;
                while pos < total_samples.get() {
                    count += 1;
                    pos += hop_size.get();
                }
                // safety: count is at least 1 because total_samples is non-zero
                unsafe { NonZeroUsize::new_unchecked(count) }
            }
            PaddingMode::Skip => {
                // With skip, only count complete windows
                // safety: we have already checked that window_size and hop_size are non-zero
                unsafe {
                    NonZeroUsize::new_unchecked(
                        1 + (total_samples.get() - window_size.get()) / hop_size.get(),
                    )
                }
            }
        }
    }

    /// Sets the padding strategy used for trailing windows.
    ///
    /// ## Purpose
    ///
    /// This method allows callers to control how incomplete windows at the end
    /// of the signal are handled.
    ///
    /// Changing the padding mode affects both the number of yielded windows and
    /// the shape of the final windows.
    ///
    /// ## Behavioural Guarantees
    ///
    /// - The iterator’s internal window count is updated consistently with the
    ///   selected mode.
    #[inline]
    #[must_use]
    pub const fn with_padding_mode(mut self, mode: PaddingMode) -> Self {
        self.padding_mode = mode;
        self.total_windows = Self::calculate_total_windows(
            self.total_samples,
            self.window_size,
            self.hop_size,
            mode,
        );
        self
    }
}

#[cfg(feature = "editing")]
impl<T> Iterator for WindowIterator<'_, '_, T>
where
    T: StandardSample,
{
    type Item = AudioSamples<'static, T>;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        if self.current_window >= self.total_windows.get() {
            return None;
        }

        let start_pos = self.current_position;
        let end_pos = start_pos + self.window_size.get();
        // Copy the stored reference so that slice_samples borrows from the
        // audio data rather than from the short-lived &mut self borrow.
        let audio = self.audio;

        let window = if end_pos <= self.total_samples.get() {
            // Complete window within bounds
            audio
                .slice_samples(start_pos..end_pos)
                .ok()
                .map(super::repr::AudioSamples::into_owned)
        } else {
            // Window extends beyond available data
            match self.padding_mode {
                PaddingMode::Zero => {
                    // Zero-pad to maintain consistent window size
                    let available_samples = self.total_samples.get().saturating_sub(start_pos);
                    match &audio.data {
                        AudioData::Mono(_) => {
                            // Add available samples
                            let starting_slice = if available_samples > 0 {
                                let slice = audio
                                    .slice_samples(start_pos..self.total_samples.get())
                                    .ok()?
                                    .into_owned();
                                Some(slice)
                            } else {
                                None
                            };

                            let silence_samples = self.window_size.get() - available_samples;
                            let length = NonZeroUsize::new(silence_samples)?;
                            let silence = if silence_samples > 0 {
                                let silence =
                                    AudioSamples::<T>::zeros_mono(length, audio.sample_rate);
                                Some(silence)
                            } else {
                                return starting_slice;
                            };

                            match (starting_slice, silence) {
                                (None, None) => None,
                                (None, Some(silence)) => Some(silence),
                                (Some(starting_slice), None) => Some(starting_slice),
                                (Some(s), Some(z)) => {
                                    let slices = vec![s, z];
                                    let slices = NonEmptyVec::new(slices).ok()?;
                                    Some(AudioSamples::concatenate_owned(slices).ok()?)
                                }
                            }
                        }
                        AudioData::Multi(_) => {
                            let interleaved_slice = if available_samples > 0 {
                                let slice = audio
                                    .slice_samples(start_pos..self.total_samples.get())
                                    .ok()?
                                    .into_owned();
                                Some(slice)
                            } else {
                                None
                            };

                            // Zero-pad remainder
                            let remaining_samples = self.window_size.get() - available_samples;
                            if remaining_samples == 0 {
                                return interleaved_slice;
                            }

                            let length = NonZeroUsize::new(remaining_samples)?;

                            let silence = AudioSamples::<T>::zeros_multi_channel(
                                audio.num_channels(),
                                length,
                                audio.sample_rate,
                            );

                            match interleaved_slice {
                                None => Some(silence),
                                Some(slice) => {
                                    AudioSamples::concatenate_owned(non_empty_vec![slice, silence])
                                        .ok()
                                }
                            }
                        }
                    }
                }
                PaddingMode::None => {
                    // Return available samples without padding
                    let available_samples = self.total_samples.get().saturating_sub(start_pos);
                    if available_samples == 0 {
                        return None;
                    }

                    audio
                        .slice_samples(start_pos..self.total_samples.get())
                        .ok()
                        .map(super::repr::AudioSamples::into_owned)
                }
                PaddingMode::Skip => {
                    // Skip incomplete windows
                    return None;
                }
            }
        };

        self.current_position += self.hop_size.get();
        self.current_window += 1;
        window
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let remaining = self.total_windows.get() - self.current_window;
        (remaining, Some(remaining))
    }
}

#[cfg(feature = "editing")]
impl<T> ExactSizeIterator for WindowIterator<'_, '_, T> where T: StandardSample {}

#[cfg(test)]
mod tests {
    use crate::AudioSamples;
    #[cfg(feature = "editing")]
    use crate::PaddingMode;
    use crate::sample_rate;
    use ndarray::{Array1, array};
    use non_empty_slice::non_empty_vec;

    #[test]
    fn test_frame_iterator_mono() {
        let audio = AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0, 5.0], sample_rate!(44100))
            .unwrap();
        audio
            .frames()
            .zip([1.0f32, 2.0, 3.0, 4.0, 5.0])
            .for_each(|(f, x)| {
                assert_eq!(f.to_interleaved_vec(), non_empty_vec![x]);
            });
    }

    #[test]
    fn test_frame_iterator_stereo() {
        let audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
            sample_rate!(44100),
        )
        .unwrap();

        // Borrowing-first behavior: work with frames directly, don't collect into Vec
        let expected_frames = vec![vec![1.0, 4.0], vec![2.0, 5.0], vec![3.0, 6.0]];

        for (i, frame) in audio.frames().enumerate() {
            assert_eq!(frame.to_interleaved_vec().to_vec(), expected_frames[i]);
        }
    }

    #[test]
    fn test_channel_iterator_mono() {
        let audio =
            AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0], sample_rate!(44100)).unwrap();

        // Borrowing-first behavior: work with channels directly
        let mut channel_count = 0;
        for channel in audio.channels() {
            channel_count += 1;
            assert_eq!(
                channel.to_interleaved_vec(),
                non_empty_vec![1.0, 2.0, 3.0, 4.0]
            );
        }
        assert_eq!(channel_count, 1);
    }

    #[test]
    fn test_channel_iterator_stereo() {
        let audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
            sample_rate!(44100),
        )
        .unwrap();

        // Borrowing-first behavior: work with channels directly
        let expected_channels = vec![vec![1.0, 2.0, 3.0], vec![4.0, 5.0, 6.0]];

        for (i, channel) in audio.channels().enumerate() {
            assert_eq!(channel.as_mono().unwrap().to_vec(), expected_channels[i]);
        }
    }

    #[cfg(feature = "editing")]
    #[test]
    fn test_window_iterator_no_overlap() {
        let audio =
            AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0, 5.0, 6.0], sample_rate!(44100))
                .unwrap();
        let windows: Vec<AudioSamples<f32>> =
            audio.windows(crate::nzu!(3), crate::nzu!(3)).collect();

        assert_eq!(windows.len(), 2);
        assert_eq!(windows[0].as_mono().unwrap().to_vec(), vec![1.0, 2.0, 3.0]);
        assert_eq!(windows[1].as_mono().unwrap().to_vec(), vec![4.0, 5.0, 6.0]);
    }

    #[cfg(feature = "editing")]
    #[test]
    fn test_window_iterator_with_overlap() {
        let audio =
            AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0, 5.0, 6.0], sample_rate!(44100))
                .unwrap();
        let windows: Vec<AudioSamples<f32>> =
            audio.windows(crate::nzu!(4), crate::nzu!(2)).collect();

        // For 6 samples, window_size=4, hop_size=2:
        // Window 1: position 0-3 (samples 0,1,2,3)
        // Window 2: position 2-5 (samples 2,3,4,5)
        // Window 3: position 4-7 (samples 4,5 + 2 zeros for padding)
        assert_eq!(windows.len(), 3);
        assert_eq!(
            windows[0].as_mono().unwrap().to_vec(),
            vec![1.0, 2.0, 3.0, 4.0]
        );
        assert_eq!(
            windows[1].as_mono().unwrap().to_vec(),
            vec![3.0, 4.0, 5.0, 6.0]
        );
        assert_eq!(
            windows[2].as_mono().unwrap().to_vec(),
            vec![5.0, 6.0, 0.0, 0.0]
        );
    }

    #[cfg(feature = "editing")]
    #[test]
    fn test_window_iterator_zero_padding() {
        let audio = AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0, 5.0], sample_rate!(44100))
            .unwrap();
        let windows: Vec<AudioSamples<f32>> = audio
            .windows(crate::nzu!(4), crate::nzu!(3))
            .with_padding_mode(PaddingMode::Zero)
            .collect();

        assert_eq!(windows.len(), 2);
        assert_eq!(
            windows[0].as_mono().unwrap().to_vec(),
            vec![1.0, 2.0, 3.0, 4.0]
        );
        assert_eq!(
            windows[1].as_mono().unwrap().to_vec(),
            vec![4.0, 5.0, 0.0, 0.0]
        ); // Zero-padded
    }

    #[cfg(feature = "editing")]
    #[test]
    fn test_window_iterator_no_padding() {
        let audio = AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0, 5.0], sample_rate!(44100))
            .unwrap();
        let windows: Vec<AudioSamples<f32>> = audio
            .windows(crate::nzu!(4), crate::nzu!(3))
            .with_padding_mode(PaddingMode::None)
            .collect();

        assert_eq!(windows.len(), 2);
        assert_eq!(
            windows[0].as_mono().unwrap().to_vec(),
            vec![1.0, 2.0, 3.0, 4.0]
        );
        assert_eq!(windows[1].as_mono().unwrap().to_vec(), vec![4.0, 5.0]); // Incomplete window
    }

    #[cfg(feature = "editing")]
    #[test]
    fn test_window_iterator_skip_padding() {
        let audio = AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0, 5.0], sample_rate!(44100))
            .unwrap();
        let windows: Vec<AudioSamples<f32>> = audio
            .windows(crate::nzu!(4), crate::nzu!(3))
            .with_padding_mode(PaddingMode::Skip)
            .collect();

        assert_eq!(windows.len(), 1);
        assert_eq!(
            windows[0].as_mono().unwrap().to_vec(),
            vec![1.0, 2.0, 3.0, 4.0]
        );
    }

    #[cfg(feature = "editing")]
    #[test]
    fn test_window_iterator_stereo_interleaved() {
        let audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0, 4.0], [5.0, 6.0, 7.0, 8.0]],
            sample_rate!(44100),
        )
        .unwrap();
        let windows: Vec<AudioSamples<f32>> =
            audio.windows(crate::nzu!(2), crate::nzu!(2)).collect();

        assert_eq!(windows.len(), 2);
        // First window: samples 0,1 interleaved across channels
        assert_eq!(
            windows[0].to_interleaved_vec(),
            non_empty_vec![1.0, 5.0, 2.0, 6.0]
        );
        // Second window: samples 2,3 interleaved across channels
        assert_eq!(
            windows[1].to_interleaved_vec(),
            non_empty_vec![3.0, 7.0, 4.0, 8.0]
        );
    }

    #[test]
    fn test_exact_size_iterators() {
        let audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
            sample_rate!(44100),
        )
        .unwrap();

        let frame_iter = audio.frames();
        assert_eq!(frame_iter.len(), 3);

        let channel_iter = audio.channels();
        assert_eq!(channel_iter.len(), 2);

        #[cfg(feature = "editing")]
        {
            let window_iter = audio.windows(crate::nzu!(2), crate::nzu!(1));
            assert_eq!(window_iter.len(), 3); // (3-2)/1 + 1 = 2, plus padding = 3
        }
    }

    #[test]
    fn test_multiple_iterators_from_same_audio() {
        // This test verifies that our raw pointer approach allows multiple iterators
        let audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
            sample_rate!(44100),
        )
        .unwrap();

        // This should compile and work correctly
        let frames = audio.frames();
        let channels = audio.channels();
        #[cfg(feature = "editing")]
        let windows = audio.windows(crate::nzu!(2), crate::nzu!(1));

        // Verify they all work independently
        assert_eq!(frames.len(), 3);
        assert_eq!(channels.len(), 2);
        #[cfg(feature = "editing")]
        assert_eq!(windows.len(), 3);
    }

    // ==============================
    // MUTABLE ITERATOR TESTS
    // ==============================

    #[test]
    fn test_frame_iterator_mut_stereo() {
        let mut audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
            sample_rate!(44100),
        )
        .unwrap();

        let expected = array![[0.5f32, 1.0, 1.5], [6.0, 7.5, 9.0]];

        // Apply different processing to each channel
        audio.apply_to_channel_data(|ch, channel_data| {
            let gain = if ch == 0 { 0.5 } else { 1.5 };
            for sample in channel_data {
                *sample *= gain;
            }
        });

        assert_eq!(audio.as_multi_channel().unwrap(), &expected);
    }

    #[test]
    fn test_frame_iterator_mut_individual_access() {
        let mut audio =
            AudioSamples::new_multi_channel(array![[1.0f32, 2.0], [3.0, 4.0]], sample_rate!(44100))
                .unwrap();

        let expected = array![[10.0f32, 20.0], [3.0, 4.0]];

        // Modify only the left channel (channel 0)
        audio.apply_to_channel_data(|ch, channel_data| {
            if ch == 0 {
                for sample in channel_data {
                    *sample *= 10.0;
                }
            }
            // Leave right channel unchanged
        });

        assert_eq!(audio.as_multi_channel().unwrap(), &expected);
    }

    #[test]
    fn test_channel_iterator_mut_mono() {
        let mut audio =
            AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0], sample_rate!(44100)).unwrap();

        audio.apply_to_channel_data(|_ch, channel_data| {
            for sample in channel_data {
                *sample += 10.0;
            }
        });

        assert_eq!(audio.as_mono().unwrap(), &array![11.0f32, 12.0, 13.0, 14.0]);
    }

    #[test]
    fn test_channel_iterator_mut_stereo() {
        let mut audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0], [4.0, 5.0, 6.0]],
            sample_rate!(44100),
        )
        .unwrap();

        let expected = array![[0.5f32, 1.0, 1.5], [8.0, 10.0, 12.0]];

        // Apply different processing to each channel
        audio.apply_to_channel_data(|ch, channel_data| {
            let gain = if ch == 0 { 0.5 } else { 2.0 };
            for sample in channel_data {
                *sample *= gain;
            }
        });

        assert_eq!(audio.as_multi_channel().unwrap(), &expected);
    }

    #[test]
    fn test_window_iterator_mut_mono() {
        let mut audio =
            AudioSamples::new_mono(array![1.0f32, 2.0, 3.0, 4.0, 5.0, 6.0], sample_rate!(44100))
                .unwrap();

        // Apply windowed processing (non-overlapping)
        audio.apply_to_windows(3, 3, |_window_idx, window_data| {
            for sample in window_data {
                *sample *= 0.5;
            }
        });

        assert_eq!(
            audio.as_mono().unwrap(),
            &array![0.5f32, 1.0, 1.5, 2.0, 2.5, 3.0]
        );
    }

    #[test]
    fn test_window_iterator_mut_stereo() {
        let mut audio = AudioSamples::new_multi_channel(
            array![[1.0f32, 2.0, 3.0, 4.0], [5.0, 6.0, 7.0, 8.0]],
            sample_rate!(44100),
        )
        .unwrap();

        let expected = array![[0.8f32, 1.6, 2.4, 3.2], [6.0, 7.2, 8.4, 9.6]];

        // Apply windowed processing (non-overlapping)
        // For multi-channel, apply_to_windows provides interleaved data
        audio.apply_to_windows(2, 2, |_window_idx, window_data| {
            // window_data is interleaved: [L0, R0, L1, R1, ...] for 2-sample window
            let samples_per_channel = window_data.len() / 2; // 2 channels
            for sample_idx in 0..samples_per_channel {
                let left_idx = sample_idx * 2;
                let right_idx = sample_idx * 2 + 1;
                window_data[left_idx] *= 0.8; // Left channel gain
                window_data[right_idx] *= 1.2; // Right channel gain
            }
        });

        let result = audio.as_multi_channel().unwrap();
        for (i, (&actual, &expected)) in result.iter().zip(expected.iter()).enumerate() {
            assert!(
                (actual - expected).abs() < 1e-6,
                "Mismatch at index {}: {} != {} (diff: {})",
                i,
                actual,
                expected,
                (actual - expected).abs()
            );
        }
    }

    #[test]
    fn test_window_function_application() {
        let mut audio =
            AudioSamples::new_mono(array![1.0f32, 1.0, 1.0, 1.0], sample_rate!(44100)).unwrap();

        // Apply Hann window function
        audio.apply_to_windows(4, 4, |_window_idx, window_data| {
            let window_size = window_data.len();
            for (i, sample) in window_data.iter_mut().enumerate() {
                let hann_weight = 0.5
                    * (1.0
                        - (2.0 * std::f32::consts::PI * i as f32 / (window_size - 1) as f32).cos());
                *sample *= hann_weight;
            }
        });

        // Check that Hann window was applied (values should be different from 1.0)
        let result = audio.as_mono().unwrap();
        assert!(result[0] < 1.0); // Should be close to 0
        assert!(result[1] > 0.5); // Should be around 0.75
        assert!(result[2] > 0.5); // Should be around 0.75
        assert!(result[3] < 1.0); // Should be close to 0
    }

    #[test]
    fn test_performance_comparison_apply_vs_iterator() {
        // This test demonstrates when to use each approach
        let mut audio1 =
            AudioSamples::new_mono(Array1::<f32>::ones(1000), sample_rate!(44100)).unwrap();
        let mut audio2 = audio1.clone();

        // Method 1: Using optimized apply (recommended for simple operations)
        audio1.apply(|sample| sample * 0.5);

        // Method 2: Using convenience method (alternative for complex operations)
        audio2.apply_to_frames(|_frame_idx, frame_data| {
            for sample in frame_data {
                *sample *= 0.5;
            }
        });

        // Results should be identical
        assert_eq!(audio1.as_mono().unwrap(), audio2.as_mono().unwrap());
    }
}