core: Add simple stereo mapping

psst-core/src/audio/output.rs

@@ -37,6 +37,7 @@ impl AudioOutput {
             move |this| Stream::open(device, config, callback_recv, this).unwrap()
         });
         let sink = AudioSink {
+            channel_count: supported.channels(),
             sample_rate: supported.sample_rate(),
             stream_send: handle.sender(),
             callback_send,
@@ -75,12 +76,17 @@ impl AudioOutput {
 
 #[derive(Clone)]
 pub struct AudioSink {
+    channel_count: cpal::ChannelCount,
     sample_rate: cpal::SampleRate,
     callback_send: Sender<CallbackMsg>,
     stream_send: Sender<StreamMsg>,
 }
 
 impl AudioSink {
+    pub fn channel_count(&self) -> usize {
+        self.channel_count as usize
+    }
+
     pub fn sample_rate(&self) -> u32 {
         self.sample_rate.0
     }

psst-core/src/audio/source.rs

@@ -25,3 +25,48 @@ impl AudioSource for Empty {
         0
     }
 }
+
+pub struct StereoMapper<S> {
+    source: S,
+    input_channels: usize,
+    output_channels: usize,
+    buffer: Vec<f32>,
+}
+
+impl<S> StereoMapper<S> {
+    pub fn new(
+        source: S,
+        input_channels: usize,
+        output_channels: usize,
+        max_input_size: usize,
+    ) -> Self {
+        Self {
+            source,
+            input_channels,
+            output_channels,
+            buffer: vec![0.0; (max_input_size / input_channels) * output_channels],
+        }
+    }
+
+    fn input_size(&mut self, output_size: usize) -> usize {
+        (output_size / self.output_channels) * self.input_channels
+    }
+}
+
+impl<S> AudioSource for StereoMapper<S>
+where
+    S: AudioSource,
+{
+    fn write(&mut self, output: &mut [f32]) -> usize {
+        let input_max = self.input_size(output.len()).min(self.buffer.len());
+        let written = self.source.write(&mut self.buffer[..input_max]);
+        let input = &self.buffer[..written];
+        let input_frames = input.chunks_exact(self.input_channels);
+        let output_frames = output.chunks_exact_mut(self.output_channels);
+        for (i, o) in input_frames.zip(output_frames) {
+            o[0] = i[0];
+            o[1] = i[1];
+        }
+        output.len()
+    }
+}

psst-core/src/player/worker.rs

@@ -20,7 +20,7 @@ use crate::{
         decode::AudioDecoder,
         output::AudioSink,
         resample::{AudioResampler, ResamplingQuality, ResamplingSpec},
-        source::AudioSource,
+        source::{AudioSource, StereoMapper},
     },
     error::Error,
 };
@@ -47,9 +47,14 @@ impl PlaybackManager {
 
     pub fn play(&mut self, loaded: LoadedPlaybackItem) {
         let path = loaded.file.path();
-        let source = DecoderSource::new(loaded, self.event_send.clone(), self.sink.sample_rate());
-        self.current = Some((path, source.actor.sender()));
-        self.sink.play(source);
+        let decoder = DecoderSource::new(loaded, self.event_send.clone(), self.sink.sample_rate());
+        self.current = Some((path, decoder.actor.sender()));
+        self.sink.play(StereoMapper::new(
+            decoder,
+            2,
+            self.sink.channel_count(),
+            8 * 1024,
+        ));
         self.sink.resume();
     }