denoize 0.72.0

//! Band-limited, channel-synchronous sample-rate conversion.

use rubato::{FftFixedIn, Resampler};

use crate::audio::sanitize_sample;
use crate::config::MAX_SAMPLE_RATE;

const CHUNK_FRAMES: usize = 1024;
const SUB_CHUNKS: usize = 2;
const MAX_RESAMPLE_WORKING_BYTES: u128 = 512 * 1024 * 1024;
// Covers rubato's FFT plans, spectra, filters, scratch buffers, and temporary
// constructor storage in addition to the explicitly-sized buffers below.
const FFT_PLAN_SCALAR_SAFETY_FACTOR: u128 = 64;

/// A channel-synchronous converter for a continuous stream of arbitrary-sized
/// blocks. Unlike [`resample_channels`], this keeps both the FFT overlap and
/// the fractional sample clock between calls.
///
/// Output may be empty while enough input is accumulated to cover the fixed
/// FFT block and filter delay. [`finish`](Self::finish) emits the remaining
/// delayed samples and fixes the total length to the rounded rate ratio.
pub(crate) struct StreamingResampler {
    channels: usize,
    from_rate: u32,
    to_rate: u32,
    converter: Option<FftFixedIn<f64>>,
    pending: Vec<Vec<f64>>,
    delay_remaining: usize,
    total_input_frames: usize,
    emitted_output_frames: usize,
    finished: bool,
}

impl StreamingResampler {
    pub(crate) fn new(channels: usize, from_rate: u32, to_rate: u32) -> Result<Self, String> {
        validate_sample_rates(from_rate, to_rate)?;
        validate_resampler_plan(channels, from_rate, to_rate)?;

        let mut pending = empty_channels(channels, CHUNK_FRAMES)?;
        for channel in &mut pending {
            debug_assert!(channel.capacity() >= CHUNK_FRAMES);
        }
        let converter = if from_rate == to_rate {
            None
        } else {
            Some(
                FftFixedIn::<f64>::new(
                    from_rate as usize,
                    to_rate as usize,
                    CHUNK_FRAMES,
                    SUB_CHUNKS,
                    channels,
                )
                .map_err(|error| format!("failed to create sample-rate converter: {error}"))?,
            )
        };
        let delay_remaining = converter.as_ref().map_or(0, Resampler::output_delay);

        Ok(Self {
            channels,
            from_rate,
            to_rate,
            converter,
            pending,
            delay_remaining,
            total_input_frames: 0,
            emitted_output_frames: 0,
            finished: false,
        })
    }

    /// Convert another equal-length block. Every returned channel has exactly
    /// the same length, but that length can be zero because of filter latency.
    pub(crate) fn process(&mut self, input: &[Vec<f64>]) -> Result<Vec<Vec<f64>>, String> {
        let frames = validate_stream_channels(input, self.channels)?;
        if self.finished {
            return Err(
                "sample-rate conversion stream is finished; reset it before processing more input"
                    .into(),
            );
        }
        let total_input_frames = self
            .total_input_frames
            .checked_add(frames)
            .ok_or_else(|| "sample-rate conversion input length overflow".to_string())?;
        let output_target =
            planned_output_frames(total_input_frames, self.from_rate, self.to_rate)?;
        let reserve = output_target
            .checked_sub(self.emitted_output_frames)
            .ok_or_else(|| "sample-rate conversion stream exceeded its output clock".to_string())?;
        let mut output = empty_channels(self.channels, reserve)?;

        if frames == 0 {
            return Ok(output);
        }

        if self.converter.is_none() {
            for (destination, source) in output.iter_mut().zip(input) {
                destination.extend(source.iter().copied().map(sanitize_sample));
            }
            self.total_input_frames = total_input_frames;
            self.emitted_output_frames = output_target;
            return Ok(output);
        }

        // Keep direct, finite blocks allocation-free. Invalid input is copied
        // through the same checked sanitization path as the offline converter.
        let sanitized;
        let input = if input
            .iter()
            .flatten()
            .any(|sample| !sample.is_finite() || *sample < -1.0 || *sample > 1.0)
        {
            sanitized = clone_channels(input, true)?;
            &sanitized
        } else {
            input
        };

        let mut position = 0usize;
        let pending_frames = self.pending.first().map_or(0, Vec::len);
        if pending_frames > 0 {
            let copied = (CHUNK_FRAMES - pending_frames).min(frames);
            for (pending, source) in self.pending.iter_mut().zip(input) {
                pending.extend_from_slice(&source[..copied]);
            }
            position = copied;
            if pending_frames + copied == CHUNK_FRAMES {
                let converted = self
                    .converter
                    .as_mut()
                    .expect("non-identity converter")
                    .process(&self.pending, None)
                    .map_err(|error| format!("sample-rate conversion failed: {error}"))?;
                append_streaming(
                    &mut output,
                    &converted,
                    &mut self.delay_remaining,
                    &mut self.emitted_output_frames,
                    output_target,
                    false,
                )?;
                for pending in &mut self.pending {
                    pending.clear();
                }
            }
        }

        let mut chunk = Vec::new();
        chunk
            .try_reserve_exact(self.channels)
            .map_err(|_| "unable to reserve sample-rate conversion input channels".to_string())?;
        while frames - position >= CHUNK_FRAMES {
            chunk.clear();
            for channel in input {
                chunk.push(&channel[position..position + CHUNK_FRAMES]);
            }
            let converted = self
                .converter
                .as_mut()
                .expect("non-identity converter")
                .process(&chunk, None)
                .map_err(|error| format!("sample-rate conversion failed: {error}"))?;
            append_streaming(
                &mut output,
                &converted,
                &mut self.delay_remaining,
                &mut self.emitted_output_frames,
                output_target,
                false,
            )?;
            position += CHUNK_FRAMES;
        }

        if position < frames {
            for (pending, source) in self.pending.iter_mut().zip(input) {
                pending.extend_from_slice(&source[position..]);
            }
        }
        self.total_input_frames = total_input_frames;
        Ok(output)
    }

    /// Flush the final partial FFT block and overlap. This is idempotent; after
    /// it succeeds, [`process`](Self::process) rejects input until reset.
    #[allow(dead_code)]
    pub(crate) fn finish(&mut self) -> Result<Vec<Vec<f64>>, String> {
        let output_target =
            planned_output_frames(self.total_input_frames, self.from_rate, self.to_rate)?;
        let reserve = output_target
            .checked_sub(self.emitted_output_frames)
            .ok_or_else(|| "sample-rate conversion stream exceeded its output clock".to_string())?;
        let mut output = empty_channels(self.channels, reserve)?;
        if self.finished {
            return Ok(output);
        }
        if self.converter.is_none() || output_target == self.emitted_output_frames {
            self.finished = true;
            return Ok(output);
        }

        if self
            .pending
            .first()
            .is_some_and(|channel| !channel.is_empty())
        {
            let converted = self
                .converter
                .as_mut()
                .expect("non-identity converter")
                .process_partial(Some(&self.pending), None)
                .map_err(|error| format!("sample-rate conversion flush failed: {error}"))?;
            append_streaming(
                &mut output,
                &converted,
                &mut self.delay_remaining,
                &mut self.emitted_output_frames,
                output_target,
                true,
            )?;
            for pending in &mut self.pending {
                pending.clear();
            }
        }

        while self.emitted_output_frames < output_target {
            let before = (self.delay_remaining, self.emitted_output_frames);
            let converted = self
                .converter
                .as_mut()
                .expect("non-identity converter")
                .process_partial::<&[f64]>(None, None)
                .map_err(|error| format!("sample-rate conversion flush failed: {error}"))?;
            append_streaming(
                &mut output,
                &converted,
                &mut self.delay_remaining,
                &mut self.emitted_output_frames,
                output_target,
                true,
            )?;
            if before == (self.delay_remaining, self.emitted_output_frames) {
                return Err("sample-rate conversion flush made no progress".into());
            }
        }
        self.finished = true;
        Ok(output)
    }

    #[allow(dead_code)]
    pub(crate) fn reset(&mut self) {
        if let Some(converter) = &mut self.converter {
            converter.reset();
            self.delay_remaining = converter.output_delay();
        } else {
            self.delay_remaining = 0;
        }
        for pending in &mut self.pending {
            pending.clear();
        }
        self.total_input_frames = 0;
        self.emitted_output_frames = 0;
        self.finished = false;
    }
}

pub fn resample(input: &[f64], from_rate: u32, to_rate: u32) -> Result<Vec<f64>, String> {
    validate_sample_rates(from_rate, to_rate)?;
    validate_resampler_plan(1, from_rate, to_rate)?;
    let mut channel = Vec::new();
    channel
        .try_reserve_exact(input.len())
        .map_err(|_| "unable to reserve sample-rate conversion input".to_string())?;
    channel.extend_from_slice(input);
    let channels = resample_channels(&[channel], from_rate, to_rate)?;
    Ok(channels.into_iter().next().unwrap_or_default())
}

/// Resample every channel through one shared clock so stereo phase and timing
/// cannot drift. The FFT resampler includes a band-limiting filter, unlike the
/// linear interpolation previously used here.
pub fn resample_channels(
    input: &[Vec<f64>],
    from_rate: u32,
    to_rate: u32,
) -> Result<Vec<Vec<f64>>, String> {
    validate_sample_rates(from_rate, to_rate)?;
    if input.is_empty() {
        return Ok(Vec::new());
    }
    let frames = input[0].len();
    if input.iter().any(|channel| channel.len() != frames) {
        return Err("all channels must contain the same number of frames".into());
    }
    if frames == 0 {
        return clone_channels(input, false);
    }
    validate_resampler_plan(input.len(), from_rate, to_rate)?;

    // The converter performs floating-point FFT arithmetic, so one invalid
    // input sample would otherwise contaminate an entire block.  Keep the
    // common finite path allocation-free while still making direct callers
    // safe for NaN, infinity, and out-of-range amplitudes.
    let needs_sanitization = input
        .iter()
        .flatten()
        .any(|sample| !sample.is_finite() || *sample < -1.0 || *sample > 1.0);
    let sanitized;
    let input = if needs_sanitization {
        sanitized = clone_channels(input, true)?;
        &sanitized
    } else {
        input
    };
    if from_rate == to_rate {
        return clone_channels(input, false);
    }

    let expected = planned_output_frames(frames, from_rate, to_rate)?;
    let mut converter = FftFixedIn::<f64>::new(
        from_rate as usize,
        to_rate as usize,
        CHUNK_FRAMES,
        SUB_CHUNKS,
        input.len(),
    )
    .map_err(|error| format!("failed to create sample-rate converter: {error}"))?;
    let delay = converter.output_delay();
    let output_target = expected
        .checked_add(delay)
        .ok_or_else(|| "sample-rate conversion output capacity overflow".to_string())?;
    // A call may return substantially more than CHUNK_FRAMES when upsampling
    // (for example, 48 kHz -> 768 kHz returns up to 16,384 frames). Keep a
    // checked bound for the final flush call, but retain only the samples that
    // belong to the delayed output target in the accumulator.
    let append_limit = output_target
        .checked_add(converter.output_frames_max())
        .ok_or_else(|| "sample-rate conversion output capacity overflow".to_string())?;
    let mut output = Vec::new();
    output
        .try_reserve_exact(input.len())
        .map_err(|_| "unable to reserve sample-rate conversion channels".to_string())?;
    for _ in 0..input.len() {
        let mut channel = Vec::new();
        channel
            .try_reserve_exact(output_target)
            .map_err(|_| "unable to reserve sample-rate conversion output".to_string())?;
        output.push(channel);
    }
    let mut position = 0;

    while frames - position >= converter.input_frames_next() {
        let count = converter.input_frames_next();
        let chunk: Vec<&[f64]> = input
            .iter()
            .map(|channel| &channel[position..position + count])
            .collect();
        let converted = converter
            .process(&chunk, None)
            .map_err(|error| format!("sample-rate conversion failed: {error}"))?;
        append(&mut output, &converted, output_target, append_limit)?;
        position += count;
    }
    if position < frames {
        let tail: Vec<&[f64]> = input.iter().map(|channel| &channel[position..]).collect();
        let converted = converter
            .process_partial(Some(&tail), None)
            .map_err(|error| format!("sample-rate conversion failed: {error}"))?;
        append(&mut output, &converted, output_target, append_limit)?;
    }
    while output.first().map_or(0, Vec::len) < output_target {
        let converted = converter
            .process_partial::<&[f64]>(None, None)
            .map_err(|error| format!("sample-rate conversion flush failed: {error}"))?;
        append(&mut output, &converted, output_target, append_limit)?;
    }

    for channel in &mut output {
        channel.drain(..delay.min(channel.len()));
        channel.truncate(expected);
        channel.resize(expected, 0.0);
        for sample in channel {
            *sample = sanitize_sample(*sample);
        }
    }
    Ok(output)
}

fn validate_sample_rates(from_rate: u32, to_rate: u32) -> Result<(), String> {
    if from_rate == 0 || to_rate == 0 || from_rate > MAX_SAMPLE_RATE || to_rate > MAX_SAMPLE_RATE {
        return Err(format!(
            "sample rates must be between 1 and {MAX_SAMPLE_RATE} Hz"
        ));
    }
    Ok(())
}

fn planned_output_frames(frames: usize, from_rate: u32, to_rate: u32) -> Result<usize, String> {
    let numerator = (frames as u128)
        .checked_mul(to_rate as u128)
        .and_then(|value| value.checked_add(from_rate as u128 / 2))
        .ok_or_else(|| "sample-rate conversion output length overflow".to_string())?;
    usize::try_from(numerator / from_rate as u128)
        .map_err(|_| "sample-rate conversion output length is too large".to_string())
}

pub(crate) fn validate_resampler_plan(
    channels: usize,
    from_rate: u32,
    to_rate: u32,
) -> Result<(), String> {
    resampler_plan_bytes(channels, from_rate, to_rate).map(|_| ())
}

pub(crate) fn resampler_plan_bytes(
    channels: usize,
    from_rate: u32,
    to_rate: u32,
) -> Result<u64, String> {
    validate_sample_rates(from_rate, to_rate)?;
    if channels == 0 {
        return Err("sample-rate conversion requires at least one channel".into());
    }
    if from_rate == to_rate {
        return Ok(0);
    }
    let gcd = greatest_common_divisor(from_rate as u128, to_rate as u128);
    let minimum_input_chunk = from_rate as u128 / gcd;
    let wanted_subchunk = (CHUNK_FRAMES / SUB_CHUNKS) as u128;
    let fft_chunks = wanted_subchunk
        .checked_add(minimum_input_chunk - 1)
        .map(|value| value / minimum_input_chunk)
        .ok_or_else(|| "sample-rate conversion FFT plan overflow".to_string())?;
    let fft_size_in = fft_chunks
        .checked_mul(from_rate as u128 / gcd)
        .ok_or_else(|| "sample-rate conversion FFT plan overflow".to_string())?;
    let fft_size_out = fft_chunks
        .checked_mul(to_rate as u128 / gcd)
        .ok_or_else(|| "sample-rate conversion FFT plan overflow".to_string())?;
    let maximum_available = fft_size_in
        .checked_sub(1)
        .and_then(|value| value.checked_add(CHUNK_FRAMES as u128))
        .ok_or_else(|| "sample-rate conversion FFT plan overflow".to_string())?;
    let maximum_output = (maximum_available / fft_size_in)
        .checked_mul(fft_size_out)
        .ok_or_else(|| "sample-rate conversion FFT plan overflow".to_string())?;

    let per_channel = (CHUNK_FRAMES as u128)
        .checked_add(fft_size_in)
        .and_then(|value| value.checked_add(fft_size_out))
        .and_then(|value| value.checked_add(maximum_output))
        .ok_or_else(|| "sample-rate conversion buffer plan overflow".to_string())?;
    let channel_samples = per_channel
        .checked_mul(channels as u128)
        .ok_or_else(|| "sample-rate conversion buffer plan overflow".to_string())?;
    let shared_samples = fft_size_in
        .checked_add(fft_size_out)
        .and_then(|value| value.checked_mul(FFT_PLAN_SCALAR_SAFETY_FACTOR))
        .ok_or_else(|| "sample-rate conversion FFT plan overflow".to_string())?;
    let bytes = channel_samples
        .checked_add(shared_samples)
        .and_then(|value| value.checked_mul(std::mem::size_of::<f64>() as u128))
        .ok_or_else(|| "sample-rate conversion working-set plan overflow".to_string())?;
    if bytes > MAX_RESAMPLE_WORKING_BYTES {
        return Err(format!(
            "sample-rate conversion working set requires {bytes} bytes, limit is {MAX_RESAMPLE_WORKING_BYTES} bytes"
        ));
    }
    u64::try_from(bytes).map_err(|_| "sample-rate conversion working-set plan overflow".to_string())
}

fn greatest_common_divisor(mut lhs: u128, mut rhs: u128) -> u128 {
    while rhs != 0 {
        let remainder = lhs % rhs;
        lhs = rhs;
        rhs = remainder;
    }
    lhs
}

fn clone_channels(input: &[Vec<f64>], sanitize: bool) -> Result<Vec<Vec<f64>>, String> {
    let mut cloned = Vec::new();
    cloned
        .try_reserve_exact(input.len())
        .map_err(|_| "unable to reserve sample-rate conversion channels".to_string())?;
    for source in input {
        let mut channel = Vec::new();
        channel
            .try_reserve_exact(source.len())
            .map_err(|_| "unable to reserve sample-rate conversion samples".to_string())?;
        if sanitize {
            channel.extend(source.iter().copied().map(sanitize_sample));
        } else {
            channel.extend_from_slice(source);
        }
        cloned.push(channel);
    }
    Ok(cloned)
}

fn empty_channels(channels: usize, capacity: usize) -> Result<Vec<Vec<f64>>, String> {
    let mut output = Vec::new();
    output
        .try_reserve_exact(channels)
        .map_err(|_| "unable to reserve sample-rate conversion channels".to_string())?;
    for _ in 0..channels {
        let mut channel = Vec::new();
        channel
            .try_reserve_exact(capacity)
            .map_err(|_| "unable to reserve sample-rate conversion samples".to_string())?;
        output.push(channel);
    }
    Ok(output)
}

fn validate_stream_channels(input: &[Vec<f64>], channels: usize) -> Result<usize, String> {
    if input.len() != channels {
        return Err(format!(
            "sample-rate conversion expected {channels} channels, received {}",
            input.len()
        ));
    }
    let frames = input.first().map_or(0, Vec::len);
    if input.iter().any(|channel| channel.len() != frames) {
        return Err("all channels must contain the same number of frames".into());
    }
    Ok(frames)
}

fn append_streaming(
    output: &mut [Vec<f64>],
    chunk: &[Vec<f64>],
    delay_remaining: &mut usize,
    emitted_output_frames: &mut usize,
    output_target: usize,
    allow_final_truncation: bool,
) -> Result<(), String> {
    if chunk.len() != output.len() {
        return Err("sample-rate conversion returned an invalid channel count".into());
    }
    let frames = chunk.first().map_or(0, Vec::len);
    if chunk.iter().any(|channel| channel.len() != frames) {
        return Err("sample-rate conversion returned unaligned channels".into());
    }
    let skipped = (*delay_remaining).min(frames);
    *delay_remaining -= skipped;
    let available = frames - skipped;
    let remaining = output_target
        .checked_sub(*emitted_output_frames)
        .ok_or_else(|| "sample-rate conversion stream exceeded its output clock".to_string())?;
    if available > remaining && !allow_final_truncation {
        return Err("sample-rate conversion produced samples ahead of its input clock".into());
    }
    let retained = available.min(remaining);
    for (destination, source) in output.iter_mut().zip(chunk) {
        destination
            .try_reserve_exact(retained)
            .map_err(|_| "unable to grow sample-rate conversion output".to_string())?;
        destination.extend(
            source[skipped..skipped + retained]
                .iter()
                .copied()
                .map(sanitize_sample),
        );
    }
    *emitted_output_frames = emitted_output_frames
        .checked_add(retained)
        .ok_or_else(|| "sample-rate conversion output length overflow".to_string())?;
    Ok(())
}

fn append(
    output: &mut [Vec<f64>],
    chunk: &[Vec<f64>],
    output_target: usize,
    append_limit: usize,
) -> Result<(), String> {
    for (output, chunk) in output.iter_mut().zip(chunk) {
        let produced = output
            .len()
            .checked_add(chunk.len())
            .ok_or_else(|| "sample-rate conversion output length overflow".to_string())?;
        if produced > append_limit {
            return Err("sample-rate conversion output exceeded its planned bound".into());
        }
        let remaining = output_target
            .checked_sub(output.len())
            .ok_or_else(|| "sample-rate conversion output exceeded its target".to_string())?;
        let retained = remaining.min(chunk.len());
        output
            .try_reserve_exact(retained)
            .map_err(|_| "unable to grow sample-rate conversion output".to_string())?;
        output.extend_from_slice(&chunk[..retained]);
    }
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::f64::consts::TAU;

    #[test]
    fn same_rate_is_an_exact_identity() {
        let input = vec![0.25, -0.5, 1.0];
        assert_eq!(resample(&input, 48_000, 48_000).unwrap(), input);
    }

    #[test]
    fn preserves_requested_duration() {
        let input = vec![0.0; 44_100];
        assert_eq!(resample(&input, 44_100, 16_000).unwrap().len(), 16_000);
        assert_eq!(resample(&input, 44_100, 48_000).unwrap().len(), 48_000);
    }

    #[test]
    fn high_ratio_flush_retains_only_the_requested_output() {
        let input = vec![0.0; CHUNK_FRAMES * 2];
        let output = resample(&input, 48_000, 768_000).unwrap();
        assert_eq!(output.len(), input.len() * 16);
    }

    #[test]
    fn append_discards_flush_overshoot_at_the_checked_target() {
        let mut output = vec![Vec::new()];
        output[0].try_reserve_exact(4).unwrap();
        let capacity = output[0].capacity();
        append(&mut output, &[vec![1.0; 16]], 4, 20).unwrap();
        assert_eq!(output[0], vec![1.0; 4]);
        assert_eq!(output[0].capacity(), capacity);
    }

    #[test]
    fn downsampling_rejects_content_above_nyquist() {
        let tone = |frequency: f64| {
            (0..48_000)
                .map(|i| (TAU * frequency * i as f64 / 48_000.0).sin())
                .collect::<Vec<_>>()
        };
        let passband = resample(&tone(1_000.0), 48_000, 16_000).unwrap();
        let stopband = resample(&tone(12_000.0), 48_000, 16_000).unwrap();
        let rms = |samples: &[f64]| {
            (samples.iter().map(|x| x * x).sum::<f64>() / samples.len() as f64).sqrt()
        };
        assert!(rms(&stopband) < rms(&passband) * 0.01);
    }

    #[test]
    fn linked_channels_remain_sample_identical() {
        let channel: Vec<f64> = (0..4_410)
            .map(|i| (TAU * 997.0 * i as f64 / 44_100.0).sin())
            .collect();
        let output = resample_channels(&[channel.clone(), channel], 44_100, 48_000).unwrap();
        assert_eq!(output[0], output[1]);
    }

    #[test]
    fn streaming_chunks_match_offline_conversion_exactly() {
        let left: Vec<f64> = (0..10_003)
            .map(|i| (TAU * 997.0 * i as f64 / 44_100.0).sin() * 0.5)
            .collect();
        let right: Vec<f64> = left.iter().map(|sample| -*sample * 0.75).collect();
        let input = vec![left, right];
        let expected = resample_channels(&input, 44_100, 48_000).unwrap();
        let mut converter = StreamingResampler::new(2, 44_100, 48_000).unwrap();
        let mut actual = vec![Vec::new(), Vec::new()];
        let chunks = [1usize, 17, 479, 2_049, 3, 1_024, 511];
        let mut position = 0usize;
        let mut chunk_index = 0usize;
        while position < input[0].len() {
            let end = (position + chunks[chunk_index % chunks.len()]).min(input[0].len());
            let block = input
                .iter()
                .map(|channel| channel[position..end].to_vec())
                .collect::<Vec<_>>();
            let converted = converter.process(&block).unwrap();
            for (destination, source) in actual.iter_mut().zip(converted) {
                destination.extend(source);
            }
            position = end;
            chunk_index += 1;
        }
        let tail = converter.finish().unwrap();
        for (destination, source) in actual.iter_mut().zip(tail) {
            destination.extend(source);
        }
        assert_eq!(actual, expected);
    }

    #[test]
    fn streaming_finish_is_exact_and_reset_starts_a_new_clock() {
        let input = vec![vec![0.25; 9_997], vec![-0.25; 9_997]];
        let expected = resample_channels(&input, 48_000, 44_100).unwrap();
        let mut converter = StreamingResampler::new(2, 48_000, 44_100).unwrap();

        for iteration in 0..2 {
            let mut output = converter.process(&input).unwrap();
            let tail = converter.finish().unwrap();
            for (destination, source) in output.iter_mut().zip(tail) {
                destination.extend(source);
            }
            assert_eq!(output, expected, "iteration {iteration}");
            assert!(converter.finish().unwrap().iter().all(Vec::is_empty));
            assert!(converter.process(&input).is_err());
            converter.reset();
        }
    }

    #[test]
    fn streaming_identity_sanitizes_and_validates_shape() {
        let mut converter = StreamingResampler::new(2, 48_000, 48_000).unwrap();
        let output = converter
            .process(&[vec![f64::NAN, 2.0], vec![f64::NEG_INFINITY, -2.0]])
            .unwrap();
        assert_eq!(output, vec![vec![0.0, 1.0], vec![0.0, -1.0]]);
        assert!(converter.process(&[vec![0.0]]).is_err());
        assert!(converter.process(&[vec![0.0], vec![]]).is_err());
    }

    #[test]
    fn supports_arbitrary_sample_rate_pairs() {
        // Include both conventional rates and a deliberately non-standard
        // rate to make sure conversion does not rely on a fixed codec table.
        let rates = [8_000, 12_345, 22_050, 32_000, 44_100, 48_000, 96_000];
        let frames = 1_001usize;
        let input: Vec<f64> = (0..frames)
            .map(|frame| (TAU * 997.0 * frame as f64 / 44_100.0).sin() * 0.5)
            .collect();

        for &from_rate in &rates {
            for &to_rate in &rates {
                if from_rate == to_rate {
                    continue;
                }
                let output = resample(&input, from_rate, to_rate).unwrap_or_else(|error| {
                    panic!("{from_rate} Hz -> {to_rate} Hz conversion failed: {error}")
                });
                let expected = ((frames as u128 * to_rate as u128 + from_rate as u128 / 2)
                    / from_rate as u128) as usize;
                assert_eq!(
                    output.len(),
                    expected,
                    "{from_rate} Hz -> {to_rate} Hz output length"
                );
                assert!(
                    output.iter().all(|sample| sample.is_finite()),
                    "{from_rate} Hz -> {to_rate} Hz produced a non-finite sample"
                );
            }
        }
    }

    #[test]
    fn sanitizes_nonfinite_and_extreme_samples_before_conversion() {
        let input = vec![vec![
            f64::NAN,
            f64::INFINITY,
            f64::NEG_INFINITY,
            2.0,
            -2.0,
            0.25,
        ]];
        let output = resample_channels(&input, 48_000, 48_000).unwrap();
        assert_eq!(output[0], vec![0.0, 0.0, 0.0, 1.0, -1.0, 0.25]);
    }

    #[test]
    fn hostile_rate_and_capacity_plans_fail_without_allocating() {
        assert!(resample(&[0.0], 48_000, MAX_SAMPLE_RATE + 1).is_err());
        assert!(resample(&[], 0, 48_000).is_err());
        assert!(validate_resampler_plan(usize::MAX, 1, MAX_SAMPLE_RATE).is_err());
        let tiny_many_channel_input = vec![vec![0.0]; 100];
        let error = resample_channels(&tiny_many_channel_input, MAX_SAMPLE_RATE, 1).unwrap_err();
        assert!(error.contains("working set"), "unexpected error: {error}");
        if usize::BITS < 128 {
            assert!(planned_output_frames(usize::MAX, 1, MAX_SAMPLE_RATE).is_err());
        }
    }
}