Empty Source type should report 0 channels

move channel from fix to constructor, add test

Empty Source should not have a sample rate. Sample Rate Conversion to handle Empty source properly

add more doc around Empty

examples/channel_volume.rs

@@ -0,0 +1,12 @@
+use rodio::source::ChannelVolume;
+
+fn main() {
+    let (_stream, handle) = rodio::OutputStream::try_default().unwrap();
+    let sink = rodio::Sink::try_new(&handle).unwrap();
+
+    let input = rodio::source::SineWave::new(440.0);
+    let chan_vol = ChannelVolume::new(input, vec![0.01, 0.01, 0.0, 0.0, 0.0, 0.0]);
+    sink.append(chan_vol);
+
+    sink.sleep_until_end();
+}

src/conversions/channels.rs

@@ -27,8 +27,11 @@ where
         from: cpal::ChannelCount,
         to: cpal::ChannelCount,
     ) -> ChannelCountConverter<I> {
-        assert!(from >= 1);
         assert!(to >= 1);
+        let from = match from {
+            0 => to,
+            n => n,
+        };
 
         ChannelCountConverter {
             input,
@@ -139,4 +142,11 @@ mod test {
         let output = ChannelCountConverter::new(input.into_iter(), 2, 1);
         assert_eq!(output.len(), 2);
     }
+
+    #[test]
+    fn zero_input() {
+        let input = vec![1u16, 2, 3, 4, 5, 6];
+        let output = ChannelCountConverter::new(input.into_iter(), 0, 3);
+        assert_eq!(output.len(), 6);
+    }
 }

src/conversions/sample_rate.rs

@@ -47,7 +47,10 @@ where
         to: cpal::SampleRate,
         num_channels: cpal::ChannelCount,
     ) -> SampleRateConverter<I> {
-        let from = from.0;
+        let from = match from.0 {
+            0 => to.0,
+            n => n,
+        };
         let to = to.0;
 
         assert!(from >= 1);

src/source/empty.rs

@@ -4,6 +4,9 @@ use std::time::Duration;
 use crate::{Sample, Source};
 
 /// An empty source.
+///
+/// The empty source is special in that it will never return any data.
+/// It also reports 0 channels, a sample rate of 0, and a Duration of 0.
 #[derive(Debug, Copy, Clone)]
 pub struct Empty<S>(PhantomData<S>);
 
@@ -36,17 +39,17 @@ where
 {
     #[inline]
     fn current_frame_len(&self) -> Option<usize> {
-        None
+        Some(0)
     }
 
     #[inline]
     fn channels(&self) -> u16 {
-        1
+        0
     }
 
     #[inline]
     fn sample_rate(&self) -> u32 {
-        48000
+        0
     }
 
     #[inline]

src/source/sine.rs

@@ -17,7 +17,7 @@ impl SineWave {
     #[inline]
     pub fn new(freq: f32) -> SineWave {
         SineWave {
-            freq: freq,
+            freq,
             num_sample: 0,
         }
     }

src/source/uniform.rs

@@ -56,7 +56,10 @@ where
         // Limit the frame length to something reasonable
         let frame_len = input.current_frame_len().map(|x| x.min(32768));
 
-        let from_channels = input.channels();
+        let from_channels = match input.channels() {
+            n if n == 0 => target_channels,
+            n => n,
+        };
         let from_sample_rate = input.sample_rate();
 
         let input = Take {