Rewrite buffer around the actual period size

This prevents over or underestimating of the period.
While it is unlikely, with comparitively small period
sizes overestimating can cause buffer underruns and
underestimating causes more writes than necessary.
It also properly accounts for the number of channels,
which I had overlooked.

playback/src/audio_backend/alsa.rs

@@ -7,14 +7,13 @@ use std::ffi::CString;
 use std::io;
 use std::process::exit;
 
-const PERIOD_SIZE: usize = 5512; // Period of roughly 125ms
-const BUFFERED_PERIODS: usize = 4; // ~ 0.5s latency
+const PREFERED_PERIOD_SIZE: Frames = 5512; // Period of roughly 125ms
+const BUFFERED_PERIODS: Frames = 4;
 
 pub struct AlsaSink {
     pcm: Option<PCM>,
     device: String,
-    buffer: [i16; PERIOD_SIZE as usize],
-    buffered_data: usize,
+    buffer: Vec<i16>,
 }
 
 fn list_outputs() {
@@ -34,8 +33,9 @@ fn list_outputs() {
     }
 }
 
-fn open_device(dev_name: &str) -> Result<(PCM), Box<Error>> {
+fn open_device(dev_name: &str) -> Result<(PCM, Frames), Box<Error>> {
     let pcm = PCM::new(dev_name, Direction::Playback, false)?;
+    let period_size = PREFERED_PERIOD_SIZE;
     // http://www.linuxjournal.com/article/6735?page=0,1#N0x19ab2890.0x19ba78d8
     // latency = period_size * periods / (rate * bytes_per_frame)
     // For 16 Bit stereo data, one frame has a length of four bytes.
@@ -50,8 +50,8 @@ fn open_device(dev_name: &str) -> Result<(PCM), Box<Error>> {
         hwp.set_format(Format::s16())?;
         hwp.set_rate(44100, ValueOr::Nearest)?;
         hwp.set_channels(2)?;
-        hwp.set_period_size_near(PERIOD_SIZE as Frames, ValueOr::Nearest)?;
-        hwp.set_buffer_size_near((PERIOD_SIZE * BUFFERED_PERIODS) as Frames)?;
+        let period_size = hwp.set_period_size_near(period_size, ValueOr::Greater)?;
+        hwp.set_buffer_size_near(period_size * BUFFERED_PERIODS)?;
         pcm.hw_params(&hwp)?;
 
         let swp = pcm.sw_params_current()?;
@@ -59,7 +59,7 @@ fn open_device(dev_name: &str) -> Result<(PCM), Box<Error>> {
         pcm.sw_params(&swp)?;
     }
 
-    Ok(pcm)
+    Ok((pcm, period_size))
 }
 
 impl Open for AlsaSink {
@@ -80,8 +80,7 @@ impl Open for AlsaSink {
         AlsaSink {
             pcm: None,
             device: name,
-            buffer: [0; PERIOD_SIZE],
-            buffered_data: 0,
+            buffer: vec![],
         }
     }
 }
@@ -91,7 +90,11 @@ impl Sink for AlsaSink {
         if self.pcm.is_none() {
             let pcm = open_device(&self.device);
             match pcm {
-                Ok(p) => self.pcm = Some(p),
+                Ok((p, period_size)) => {
+                    self.pcm = Some(p);
+                    // Create a buffer for all samples for a full period
+                    self.buffer = Vec::with_capacity((period_size * 2) as usize);
+                }
                 Err(e) => {
                     error!("Alsa error PCM open {}", e);
                     return Err(io::Error::new(
@@ -111,14 +114,13 @@ impl Sink for AlsaSink {
             // Write any leftover data in the period buffer
             // before draining the actual buffer
             let io = pcm.io_i16().unwrap();
-            match io.writei(&self.buffer[..self.buffered_data]) {
+            match io.writei(&self.buffer[..]) {
                 Ok(_) => (),
                 Err(err) => pcm.try_recover(err, false).unwrap(),
             }
             pcm.drain().unwrap();
         }
         self.pcm = None;
-        self.buffered_data = 0;
         Ok(())
     }
 
@@ -126,22 +128,20 @@ impl Sink for AlsaSink {
         let mut processed_data = 0;
         while processed_data < data.len() {
             let data_to_buffer = min(
-                PERIOD_SIZE - self.buffered_data,
+                self.buffer.capacity() - self.buffer.len(),
                 data.len() - processed_data,
             );
-            let buffer_slice =
-                &mut self.buffer[self.buffered_data..self.buffered_data + data_to_buffer];
-            buffer_slice.copy_from_slice(&data[processed_data..processed_data + data_to_buffer]);
-            self.buffered_data += data_to_buffer;
+            self.buffer
+                .extend_from_slice(&data[processed_data..processed_data + data_to_buffer]);
             processed_data += data_to_buffer;
-            if self.buffered_data == PERIOD_SIZE {
-                self.buffered_data = 0;
+            if self.buffer.len() == self.buffer.capacity() {
                 let pcm = self.pcm.as_mut().unwrap();
                 let io = pcm.io_i16().unwrap();
                 match io.writei(&self.buffer) {
                     Ok(_) => (),
                     Err(err) => pcm.try_recover(err, false).unwrap(),
                 }
+                self.buffer.clear();
             }
         }