fix: empty source exhaustion (#822)

* fix: update queue metadata after append
* refactor: improve Empty and Zero sources

Author: roderickvd

CHANGELOG.md

@@ -11,8 +11,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- `Chirp` now implements `Iterator::size_hint` and `ExactSizeIterator`.
+- `Chirp` and `Empty` now implement `Iterator::size_hint` and `ExactSizeIterator`.
 - `SamplesBuffer` now implements `ExactSizeIterator`.
+- `Zero` now implements `try_seek`, `total_duration` and `Copy`.
 - Added `Source::is_exhausted()` helper method to check if a source has no more samples.
 - Added `Red` noise generator that is more practical than `Brownian` noise.
 - Added `std_dev()` to `WhiteUniform` and `WhiteTriangular`.
@@ -38,6 +39,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Improved precision in `SkipDuration` to avoid off-by-a-few-samples errors.
 - Fixed channel misalignment in queue with non-power-of-2 channel counts (e.g., 6 channels) by ensuring frame-aligned span lengths.
 - Fixed channel misalignment when sources end before their promised span length by padding with silence to complete frames.
+- Fixed `Empty` source to properly report exhaustion.
+- Fixed `Zero::current_span_len` returning remaining samples instead of span length.
 
 ### Changed
 - `output_to_wav` renamed to `wav_to_file` and now takes ownership of the `Source`.

src/queue.rs

@@ -4,7 +4,6 @@ use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
-use crate::math::nz;
 use crate::source::{Empty, SeekError, Source, Zero};
 use crate::Sample;
 
@@ -121,9 +120,6 @@ pub struct SourcesQueueOutput {
     padding_samples_remaining: usize,
 }
 
-const SILENCE_SAMPLE_RATE: SampleRate = nz!(44100);
-const SILENCE_CHANNELS: ChannelCount = nz!(1);
-
 /// Returns a threshold span length that ensures frame alignment.
 ///
 /// Spans must end on frame boundaries (multiples of channel count) to prevent
@@ -173,27 +169,38 @@ impl Source for SourcesQueueOutput {
 
     #[inline]
     fn channels(&self) -> ChannelCount {
-        // When current source is exhausted, peek at the next source's metadata
         if !self.current.is_exhausted() {
+            // Current source is active (producing samples)
+            // - Initially: never (Empty is exhausted immediately)
+            // - 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() {
+            // 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.
             next.channels()
         } else {
-            // Queue is empty - return silence metadata
-            SILENCE_CHANNELS
+            // Queue is empty, no sources queued
+            // - Initially: Empty
+            // - With keep_alive: exhausted Zero between silence chunks (matches Empty)
+            // - Without keep_alive: Empty (will end on next())
+            self.current.channels()
         }
     }
 
     #[inline]
     fn sample_rate(&self) -> SampleRate {
-        // When current source is exhausted, peek at the next source's metadata
         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() {
+            // Current source exhausted, peek at next queued source
+            // This prevents wrong resampling setup in UniformSourceIterator
             next.sample_rate()
         } else {
-            // Queue is empty - return silence metadata
-            SILENCE_SAMPLE_RATE
+            // Queue is empty, no sources queued
+            self.current.sample_rate()
         }
     }
 
@@ -276,10 +283,11 @@ impl SourcesQueueOutput {
             let mut next = self.input.next_sounds.lock().unwrap();
 
             if next.is_empty() {
+                let channels = self.current.channels();
                 let silence = Box::new(Zero::new_samples(
-                    SILENCE_CHANNELS,
-                    SILENCE_SAMPLE_RATE,
-                    threshold(SILENCE_CHANNELS),
+                    channels,
+                    self.current.sample_rate(),
+                    threshold(channels),
                 )) as Box<_>;
                 if self.input.keep_alive_if_empty.load(Ordering::Acquire) {
                     // Play a short silence in order to avoid spinlocking.
@@ -381,6 +389,40 @@ mod tests {
         assert_eq!(rx.next(), Some(-10.0));
     }
 
+    #[test]
+    fn append_updates_metadata() {
+        for keep_alive in [false, true] {
+            let (tx, rx) = queue::queue(keep_alive);
+            assert_eq!(
+                rx.channels(),
+                nz!(1),
+                "Initial channels should be 1 (keep_alive={keep_alive})"
+            );
+            assert_eq!(
+                rx.sample_rate(),
+                nz!(48000),
+                "Initial sample rate should be 48000 (keep_alive={keep_alive})"
+            );
+
+            tx.append(SamplesBuffer::new(
+                nz!(2),
+                nz!(44100),
+                vec![0.1, 0.2, 0.3, 0.4],
+            ));
+
+            assert_eq!(
+                rx.channels(),
+                nz!(2),
+                "Channels should update to 2 (keep_alive={keep_alive})"
+            );
+            assert_eq!(
+                rx.sample_rate(),
+                nz!(44100),
+                "Sample rate should update to 44100 (keep_alive={keep_alive})"
+            );
+        }
+    }
+
     #[test]
     fn span_ending_mid_frame() {
         let mut test_source1 = TestSource::new(&[0.1, 0.2, 0.1, 0.2, 0.1])

src/source/empty.rs

@@ -6,22 +6,15 @@ use crate::math::nz;
 use crate::{Sample, Source};
 
 /// An empty source.
-#[derive(Debug, Copy, Clone)]
-pub struct Empty();
-
-impl Default for Empty {
-    #[inline]
-    fn default() -> Self {
-        Self::new()
-    }
-}
+#[derive(Debug, Default, Copy, Clone)]
+pub struct Empty;
 
 impl Empty {
     /// An empty source that immediately ends without ever returning a sample to
     /// play
     #[inline]
-    pub fn new() -> Empty {
-        Empty()
+    pub fn new() -> Self {
+        Self
     }
 }
 
@@ -32,12 +25,19 @@ impl Iterator for Empty {
     fn next(&mut self) -> Option<Self::Item> {
         None
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
 }
 
+impl ExactSizeIterator for Empty {}
+
 impl Source for Empty {
     #[inline]
     fn current_span_len(&self) -> Option<usize> {
-        None
+        Some(0)
     }
 
     #[inline]
@@ -52,7 +52,7 @@ impl Source for Empty {
 
     #[inline]
     fn total_duration(&self) -> Option<Duration> {
-        Some(Duration::new(0, 0))
+        Some(Duration::ZERO)
     }
 
     #[inline]

src/source/zero.rs

@@ -1,40 +1,46 @@
 use std::time::Duration;
 
+use dasp_sample::Sample as DaspSample;
+
 use super::SeekError;
 use crate::common::{ChannelCount, SampleRate};
 use crate::{Sample, Source};
 
 /// An source that produces samples with value zero (silence). Depending on if
 /// it where created with [`Zero::new`] or [`Zero::new_samples`] it can be never
 /// ending or finite.
-#[derive(Clone, Debug)]
+#[derive(Copy, Clone, Debug)]
 pub struct Zero {
     channels: ChannelCount,
     sample_rate: SampleRate,
-    num_samples: Option<usize>,
+    total_samples: Option<usize>,
+    position: usize,
 }
 
 impl Zero {
     /// Create a new source that never ends and produces total silence.
     #[inline]
-    pub fn new(channels: ChannelCount, sample_rate: SampleRate) -> Zero {
-        Zero {
+    pub fn new(channels: ChannelCount, sample_rate: SampleRate) -> Self {
+        Self {
             channels,
             sample_rate,
-            num_samples: None,
+            total_samples: None,
+            position: 0,
         }
     }
+
     /// Create a new source that never ends and produces total silence.
     #[inline]
     pub fn new_samples(
         channels: ChannelCount,
         sample_rate: SampleRate,
         num_samples: usize,
-    ) -> Zero {
-        Zero {
+    ) -> Self {
+        Self {
             channels,
             sample_rate,
-            num_samples: Some(num_samples),
+            total_samples: Some(num_samples),
+            position: 0,
         }
     }
 }
@@ -44,23 +50,33 @@ impl Iterator for Zero {
 
     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
-        if let Some(num_samples) = self.num_samples {
-            if num_samples > 0 {
-                self.num_samples = Some(num_samples - 1);
-                Some(0.0)
+        if let Some(total_samples) = self.total_samples {
+            if self.position < total_samples {
+                self.position += 1;
             } else {
-                None
+                return None;
+            }
+        }
+
+        Some(Sample::EQUILIBRIUM)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        match self.total_samples {
+            Some(total_samples) => {
+                let remaining = total_samples - self.position;
+                (remaining, Some(remaining))
             }
-        } else {
-            Some(0.0)
+            None => (usize::MAX, None),
         }
     }
 }
 
 impl Source for Zero {
     #[inline]
     fn current_span_len(&self) -> Option<usize> {
-        self.num_samples
+        self.total_samples
     }
 
     #[inline]
@@ -73,13 +89,33 @@ impl Source for Zero {
         self.sample_rate
     }
 
-    #[inline]
     fn total_duration(&self) -> Option<Duration> {
-        None
+        self.total_samples.map(|total| {
+            let sample_rate = self.sample_rate.get() as u64;
+            let frames = total / self.channels.get() as usize;
+            let secs = frames as u64 / sample_rate;
+            let nanos = ((frames as u64 % sample_rate) * 1_000_000_000) / sample_rate;
+            Duration::new(secs, nanos as u32)
+        })
     }
 
-    #[inline]
-    fn try_seek(&mut self, _: Duration) -> Result<(), SeekError> {
+    fn try_seek(&mut self, pos: Duration) -> Result<(), SeekError> {
+        if let (Some(total_samples), Some(total_duration)) =
+            (self.total_samples, self.total_duration())
+        {
+            let mut target = pos;
+            if target > total_duration {
+                target = total_duration;
+            }
+
+            let target_samples = (target.as_secs_f32()
+                * self.sample_rate.get() as f32
+                * self.channels.get() as f32) as usize;
+            let target_samples = target_samples.min(total_samples);
+
+            self.position = target_samples;
+        }
+
         Ok(())
     }
 }