Initial untested VecDeque concept.

Cargo.toml

@@ -63,6 +63,7 @@ alsa-backend = ["librespot-playback/alsa-backend"]
 portaudio-backend = ["librespot-playback/portaudio-backend"]
 pulseaudio-backend = ["librespot-playback/pulseaudio-backend"]
 jackaudio-backend = ["librespot-playback/jackaudio-backend"]
+cpal-backend = ["librespot-playback/cpal-backend"]
 
 with-tremor = ["librespot-audio/with-tremor"]
 with-vorbis = ["librespot-audio/with-vorbis"]

playback/Cargo.toml

@@ -20,9 +20,11 @@ portaudio-rs    = { version = "0.3.0", optional = true }
 libpulse-sys    = { version = "0.0.0", optional = true }
 jack            = { version = "0.5.3", optional = true }
 libc            = { version = "0.2", optional = true }
+cpal            = { version = "0.8.2", optional = true }
 
 [features]
 alsa-backend = ["alsa"]
 portaudio-backend = ["portaudio-rs"]
 pulseaudio-backend = ["libpulse-sys", "libc"]
 jackaudio-backend = ["jack"]
+cpal-backend = ["cpal"]

playback/src/audio_backend/cpal.rs

@@ -0,0 +1,92 @@
+use super::{Open, Sink};
+extern crate cpal;
+use std::io;
+use std::thread;
+use std::collections::VecDeque;
+
+pub struct CpalSink {
+    event_loop: cpal::EventLoop,
+    buffer: mut VecDeque<i16>,
+    stream_id: Option<cpal::StreamId>,
+}
+
+impl Open for CpalSink {
+    fn open(device: Option<String>) -> CpalSink {
+        info!("Using cpal sink");
+
+        if device.is_some() {
+            // N.B. This is perfectly possible to support.
+            // TODO: First need to enable listing of devices.
+                // Remember to filter to those which support Stereo 16bit 44100Hz
+            // TODO: Choose cpal sink by name.
+            panic!("cpal sink does not support specifying a device name");
+        }
+
+        let event_loop = cpal::EventLoop::new();
+
+        CpalSink {
+            // Allow an (arbitrary) 2 second buffer before resizing.
+            buffer: VecDeque::with_capacity(44100 * 2 * 2),
+            event_loop: event_loop,
+        }
+    }
+}
+
+impl Sink for CpalSink {
+    fn start(&mut self) -> io::Result<()> {
+
+        if self.stream_id.is_none() {
+
+            let device = cpal::default_output_device().expect("no output device available");
+            // TODO: Support more formats.
+            let format = cpal::Format(2, 44100, cpal::SampleFormat::I16);
+
+            self.stream_id = self.event_loop.build_output_stream(&device, &format)?;
+
+            self.event_loop.play_stream(self.stream_id.clone());
+        }
+
+        if self.thread.is_none() {
+            let event_loop = self.event_loop;
+            let source  = self.buffer;
+            thread::spawn(move |event_loop, source| {
+                event_loop.run(move |_stream_id, mut stream_data| {
+                    match data {
+                        cpal::StreamData::Output { buffer: cpal::UnknownTypeOutputBuffer::I16(mut buffer) } => {
+                            let sl = source.len();
+                            if (sl > buffer.len()) {
+                                sl = buffer.len();
+                            }
+                            // let u: Vec<_> = source.drain(..sl).collect();
+                            // buffer[..s1].copy_from_slice(u[..s1]);
+
+                            for (sample, data) in buffer.iter_mut().zip(source.drain(..sl)) {
+                                *sample = data;
+                            }
+                        },
+                        _ => (),
+                    }
+                });
+            })
+        }
+
+        Ok(())
+    }
+
+    fn stop(&mut self) -> io::Result<()> {
+        if !self.stream_id.is_none() {
+            self.event_loop.destroy_stream(self.stream_id);
+            self.stream_id = None;
+            self.buffer.clear();
+        }
+        Ok(())
+    }
+
+    fn write(&mut self, data: &[i16]) -> io::Result<()> {
+        // self.0.as_mut().unwrap().write_interleaved(&data).unwrap();
+        // self.buffer.reserve(data.len()); // Unneccessary?
+        // self.buffer.extend_from_slice(data);
+        self.buffer.extend(data);
+        Ok(())
+    }
+}

playback/src/audio_backend/mod.rs

@@ -34,6 +34,11 @@ mod jackaudio;
 #[cfg(feature = "jackaudio-backend")]
 use self::jackaudio::JackSink;
 
+#[cfg(feature = "cpal-backend")]
+mod cpal;
+#[cfg(feature = "cpal-backend")]
+use self::cpal::CpalSink;
+
 mod pipe;
 use self::pipe::StdoutSink;
 
@@ -46,6 +51,8 @@ pub const BACKENDS: &'static [(&'static str, fn(Option<String>) -> Box<Sink>)] =
     ("pulseaudio", mk_sink::<PulseAudioSink>),
     #[cfg(feature = "jackaudio-backend")]
     ("jackaudio", mk_sink::<JackSink>),
+    #[cfg(feature = "cpal-backend")]
+    ("cpal", mk_sink::<CpalSink>),
     ("pipe", mk_sink::<StdoutSink>),
 ];