perf: improve mixer and sample rate conversion

CHANGELOG.md

@@ -48,8 +48,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `Blue` noise generator uses uniform instead of Gaussian noise for better performance.
 - `Gaussian` noise generator has standard deviation of 0.6 for perceptual equivalence.
 - `Velvet` noise generator takes density in Hz as `usize` instead of `f32`.
-- Upgrade `cpal` to v0.17.
+- Upgraded `cpal` to v0.17.
 - Clarified `Source::current_span_len()` contract documentation.
+- Improved queue, mixer and sample rate conversion performance.
 
 ## Version [0.21.1] (2025-07-14)
 

src/conversions/sample_rate.rs

@@ -1,6 +1,7 @@
 use crate::common::{ChannelCount, SampleRate};
 use crate::{math, Sample};
 use num_rational::Ratio;
+use std::collections::VecDeque;
 use std::mem;
 
 /// Iterator that converts from a certain sample rate to another.
@@ -27,7 +28,7 @@ where
     /// This counter is incremented (modulo `to`) every time the iterator is called.
     next_output_span_pos_in_chunk: u32,
     /// The buffer containing the samples waiting to be output.
-    output_buffer: Vec<I::Item>,
+    output_buffer: VecDeque<I::Item>,
 }
 
 impl<I> SampleRateConverter<I>
@@ -81,7 +82,10 @@ where
             next_output_span_pos_in_chunk: 0,
             current_span: first_samples,
             next_frame: next_samples,
-            output_buffer: Vec::with_capacity(num_channels.get() as usize - 1),
+            // Capacity: worst case is upsampling where we buffer multiple frames worth of samples.
+            output_buffer: VecDeque::with_capacity(
+                (to as f32 / from as f32).ceil() as usize * num_channels.get() as usize,
+            ),
         }
     }
 
@@ -126,8 +130,8 @@ where
         }
 
         // Short circuit if there are some samples waiting.
-        if !self.output_buffer.is_empty() {
-            return Some(self.output_buffer.remove(0));
+        if let Some(sample) = self.output_buffer.pop_front() {
+            return Some(sample);
         }
 
         // The span we are going to return from this function will be a linear interpolation
@@ -172,7 +176,7 @@ where
             if off == 0 {
                 result = Some(sample);
             } else {
-                self.output_buffer.push(sample);
+                self.output_buffer.push_back(sample);
             }
         }
 
@@ -183,14 +187,10 @@ where
             result
         } else {
             // draining `self.current_span`
-            if !self.current_span.is_empty() {
-                let r = Some(self.current_span.remove(0));
-                mem::swap(&mut self.output_buffer, &mut self.current_span);
-                self.current_span.clear();
-                r
-            } else {
-                None
-            }
+            let mut current_span = self.current_span.drain(..);
+            let r = current_span.next()?;
+            self.output_buffer.extend(current_span);
+            Some(r)
         }
     }
 

src/mixer.rs

@@ -3,10 +3,14 @@
 use crate::common::{ChannelCount, SampleRate};
 use crate::source::{SeekError, Source, UniformSourceIterator};
 use crate::Sample;
-use std::sync::atomic::{AtomicBool, Ordering};
-use std::sync::{Arc, Mutex};
+use std::sync::Arc;
 use std::time::Duration;
 
+#[cfg(feature = "crossbeam-channel")]
+use crossbeam_channel::{unbounded as channel, Receiver, Sender};
+#[cfg(not(feature = "crossbeam-channel"))]
+use std::sync::mpsc::{channel, Receiver, Sender};
+
 /// Builds a new mixer.
 ///
 /// You can choose the characteristics of the output thanks to this constructor. All the sounds
@@ -19,9 +23,10 @@ use std::time::Duration;
 /// As a result, input sources added to the mixer later might not be forwarded to the sink.
 /// Add `Zero` source to prevent detaching the mixer from sink.
 pub fn mixer(channels: ChannelCount, sample_rate: SampleRate) -> (Mixer, MixerSource) {
+    let (tx, rx) = channel();
+
     let input = Mixer(Arc::new(Inner {
-        has_pending: AtomicBool::new(false),
-        pending_sources: Mutex::new(Vec::new()),
+        pending_tx: tx,
         channels,
         sample_rate,
     }));
@@ -31,7 +36,7 @@ pub fn mixer(channels: ChannelCount, sample_rate: SampleRate) -> (Mixer, MixerSo
         input: input.clone(),
         sample_count: 0,
         still_pending: vec![],
-        still_current: vec![],
+        pending_rx: rx,
     };
 
     (input, output)
@@ -42,8 +47,7 @@ pub fn mixer(channels: ChannelCount, sample_rate: SampleRate) -> (Mixer, MixerSo
 pub struct Mixer(Arc<Inner>);
 
 struct Inner {
-    has_pending: AtomicBool,
-    pending_sources: Mutex<Vec<Box<dyn Source + Send>>>,
+    pending_tx: Sender<Box<dyn Source + Send>>,
     channels: ChannelCount,
     sample_rate: SampleRate,
 }
@@ -57,12 +61,8 @@ impl Mixer {
     {
         let uniform_source =
             UniformSourceIterator::new(source, self.0.channels, self.0.sample_rate);
-        self.0
-            .pending_sources
-            .lock()
-            .unwrap()
-            .push(Box::new(uniform_source) as Box<_>);
-        self.0.has_pending.store(true, Ordering::SeqCst); // TODO: can we relax this ordering?
+        // Ignore send errors (channel dropped means MixerSource was dropped)
+        let _ = self.0.pending_tx.send(Box::new(uniform_source));
     }
 }
 
@@ -80,8 +80,8 @@ pub struct MixerSource {
     // A temporary vec used in start_pending_sources.
     still_pending: Vec<Box<dyn Source + Send>>,
 
-    // A temporary vec used in sum_current_sources.
-    still_current: Vec<Box<dyn Source + Send>>,
+    // Receiver for pending sources from the channel.
+    pending_rx: Receiver<Box<dyn Source + Send>>,
 }
 
 impl Source for MixerSource {
@@ -118,9 +118,7 @@ impl Iterator for MixerSource {
 
     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
-        if self.input.0.has_pending.load(Ordering::SeqCst) {
-            self.start_pending_sources();
-        }
+        self.start_pending_sources();
 
         self.sample_count += 1;
 
@@ -145,9 +143,7 @@ impl MixerSource {
     // in-step with the modulo of the samples produced so far. Otherwise, the
     // sound will play on the wrong channels, e.g. left / right will be reversed.
     fn start_pending_sources(&mut self) {
-        let mut pending = self.input.0.pending_sources.lock().unwrap(); // TODO: relax ordering?
-
-        for source in pending.drain(..) {
+        while let Ok(source) = self.pending_rx.try_recv() {
             let in_step = self
                 .sample_count
                 .is_multiple_of(source.channels().get() as usize);
@@ -158,24 +154,19 @@ impl MixerSource {
                 self.still_pending.push(source);
             }
         }
-        std::mem::swap(&mut self.still_pending, &mut pending);
-
-        let has_pending = !pending.is_empty();
-        self.input
-            .0
-            .has_pending
-            .store(has_pending, Ordering::SeqCst); // TODO: relax ordering?
     }
 
     fn sum_current_sources(&mut self) -> Sample {
         let mut sum = 0.0;
-        for mut source in self.current_sources.drain(..) {
-            if let Some(value) = source.next() {
-                sum += value;
-                self.still_current.push(source);
+        self.current_sources.retain_mut(|source| {
+            match source.next() {
+                Some(value) => {
+                    sum += value;
+                    true // Keep this source
+                }
+                None => false, // Remove exhausted source
             }
-        }
-        std::mem::swap(&mut self.still_current, &mut self.current_sources);
+        });
 
         sum
     }

src/queue.rs

@@ -1,5 +1,6 @@
 //! Queue that plays sounds one after the other.
 
+use std::collections::VecDeque;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
@@ -26,7 +27,7 @@ use std::sync::mpsc::{channel, Receiver, Sender};
 ///
 pub fn queue(keep_alive_if_empty: bool) -> (Arc<SourcesQueueInput>, SourcesQueueOutput) {
     let input = Arc::new(SourcesQueueInput {
-        next_sounds: Mutex::new(Vec::new()),
+        next_sounds: Mutex::new(VecDeque::new()),
         keep_alive_if_empty: AtomicBool::new(keep_alive_if_empty),
     });
 
@@ -48,7 +49,7 @@ type SignalDone = Option<Sender<()>>;
 
 /// The input of the queue.
 pub struct SourcesQueueInput {
-    next_sounds: Mutex<Vec<(Sound, SignalDone)>>,
+    next_sounds: Mutex<VecDeque<(Sound, SignalDone)>>,
 
     // See constructor.
     keep_alive_if_empty: AtomicBool,
@@ -64,7 +65,7 @@ impl SourcesQueueInput {
         self.next_sounds
             .lock()
             .unwrap()
-            .push((Box::new(source) as Box<_>, None));
+            .push_back((Box::new(source) as Box<_>, None));
     }
 
     /// Adds a new source to the end of the queue.
@@ -81,7 +82,7 @@ impl SourcesQueueInput {
         self.next_sounds
             .lock()
             .unwrap()
-            .push((Box::new(source) as Box<_>, Some(tx)));
+            .push_back((Box::new(source) as Box<_>, Some(tx)));
         rx
     }
 
@@ -175,7 +176,7 @@ impl Source for SourcesQueueOutput {
             // - After append: the appended source while playing
             // - With keep_alive: Zero (silence) while playing
             self.current.channels()
-        } else if let Some((next, _)) = self.input.next_sounds.lock().unwrap().first() {
+        } else if let Some((next, _)) = self.input.next_sounds.lock().unwrap().front() {
             // Current source exhausted, peek at next queued source
             // This is critical: UniformSourceIterator queries metadata during append,
             // before any samples are pulled. We must report the next source's metadata.
@@ -194,7 +195,7 @@ impl Source for SourcesQueueOutput {
         if !self.current.is_exhausted() {
             // Current source is active (producing samples)
             self.current.sample_rate()
-        } else if let Some((next, _)) = self.input.next_sounds.lock().unwrap().first() {
+        } else if let Some((next, _)) = self.input.next_sounds.lock().unwrap().front() {
             // Current source exhausted, peek at next queued source
             // This prevents wrong resampling setup in UniformSourceIterator
             next.sample_rate()
@@ -282,7 +283,9 @@ impl SourcesQueueOutput {
         let (next, signal_after_end) = {
             let mut next = self.input.next_sounds.lock().unwrap();
 
-            if next.is_empty() {
+            if let Some(next) = next.pop_front() {
+                next
+            } else {
                 let channels = self.current.channels();
                 let silence = Box::new(Zero::new_samples(
                     channels,
@@ -295,8 +298,6 @@ impl SourcesQueueOutput {
                 } else {
                     return Err(());
                 }
-            } else {
-                next.remove(0)
             }
         };