fix(snd): replace Audio Sync solution with a faster one

src/bus.rs

@@ -66,12 +66,16 @@ impl Bus {
         self.cartridge.as_ref()?.title()
     }
 
-    pub(crate) fn pass_audio_src(&mut self, sender: AudioSender) {
+    pub(crate) fn pass_audio_src(&mut self, sender: AudioSender<f32>) {
         self.snd.set_audio_src(sender)
     }
 
-    pub(crate) fn is_mpsc_still_full(&mut self) -> bool {
-        self.snd.is_mpsc_still_full()
+    pub(crate) fn is_full(&self) -> bool {
+        self.snd.is_full()
+    }
+
+    pub(crate) fn flush_samples(&mut self) {
+        self.snd.flush_samples()
     }
 
     pub(crate) fn clock(&mut self) {

src/cpu.rs

@@ -45,7 +45,7 @@ impl Cpu {
         })
     }
 
-    pub fn set_audio_src(&mut self, sender: AudioSender) {
+    pub fn set_audio_src(&mut self, sender: AudioSender<f32>) {
         self.bus.pass_audio_src(sender)
     }
 
@@ -96,13 +96,6 @@ impl Cpu {
     }
 
     pub fn step(&mut self) -> Cycle {
-        // if !self.bus.boot_enabled() {
-        //     let out = std::io::stdout();
-        //     let handle = out.lock();
-
-        //     self.log_state(handle).unwrap();
-        // }
-
         let cycles = match self.halted() {
             Some(state) => {
                 use HaltState::*;
@@ -117,6 +110,10 @@ impl Cpu {
                 self.inc_pc();
 
                 let instr = self.decode(opcode);
+
+                // let out = std::io::stdout();
+                // let _ = self._debug_log(out.lock(), &instr);
+
                 let cycles = self.execute(instr);
 
                 self.check_ime();
@@ -125,15 +122,17 @@ impl Cpu {
             }
         };
 
-        // FIXME: Regression, this is not a proper fix,
-        // the FIFO behaves weirdly using this code
+        // TODO: With how we currently handle audio
+        // this --while being correct-- incurs a performance penalty
+        // as our emu is audio-bound.
+        let mut elapsed = 0x00;
         let pending: u32 = cycles.into();
-        let mut offset = 0;
-        for _ in 0..(pending + offset) {
-            if !self.bus.is_mpsc_still_full() {
+        while elapsed < pending {
+            if !self.bus.is_full() {
                 self.bus.clock();
+                elapsed += 1;
             } else {
-                offset += 1;
+                self.bus.flush_samples();
             }
         }
 

src/ppu.rs

@@ -369,14 +369,15 @@ impl Ppu {
                 }
                 SendToFifoTwo => {
                     let palette = &self.monochrome.bg_palette;
-                    self.fetch.send_to_fifo(&mut self.fifo, palette);
-                    self.fetch.x_pos += 1;
 
+                    if let Ok(()) = self.fetch.send_to_fifo(&mut self.fifo, palette) {
+                        self.fetch.x_pos += 1;
                         self.fetch.back.next(TileNumber);
                         self.fetch.back.tile = Default::default();
                     }
                 }
             }
+        }
 
         if self.fifo.is_enabled() {
             use RenderPriority::*;
@@ -720,7 +721,11 @@ impl PixelFetcher {
         tile_data_addr + (offset * 2)
     }
 
-    fn send_to_fifo(&self, fifo: &mut FifoRenderer, palette: &BackgroundPalette) {
+    fn send_to_fifo(&self, fifo: &mut FifoRenderer, palette: &BackgroundPalette) -> Result<(), ()> {
+        if !fifo.back.is_empty() {
+            return Err(());
+        }
+
         let (high, low) = self
             .back
             .tile
@@ -729,14 +734,14 @@ impl PixelFetcher {
 
         let tbpp = Pixels::from_bytes(high, low);
 
-        if fifo.back.is_empty() {
         for x in 0..Pixels::PIXEL_COUNT {
             let shade = palette.shade(tbpp.shade_id(x));
 
             let fifo_info = BackgroundFifoInfo { shade };
             fifo.back.push_back(fifo_info);
         }
-        }
+
+        Ok(())
     }
 
     fn get_obj_addr(attr: &ObjectAttribute, pos: &ScreenPosition, size: ObjectSize) -> u16 {

src/sound.rs

@@ -1,3 +1,5 @@
+use std::collections::VecDeque;
+
 use bitfield::bitfield;
 use crossbeam_channel::{Receiver, Sender, TrySendError};
 use rodio::Source;
@@ -7,8 +9,8 @@ use crate::emu::SM83_CLOCK_SPEED;
 const WAVE_PATTERN_RAM_LEN: usize = 0x10;
 
 const SAMPLE_RATE: u32 = 48000; // Hz
-const AUDIO_BUFFER_LEN: usize = 1024;
-const CHANNEL_COUNT: u16 = 2;
+const AUDIO_BUFFER_LEN: usize = 512;
+const CHANNEL_COUNT: usize = 2;
 const SAMPLE_INCREMENT: u64 = SAMPLE_RATE as u64;
 
 #[derive(Default, Debug, Clone)]
@@ -27,10 +29,10 @@ pub(crate) struct Sound {
     frame_seq_state: FrameSequencerState,
     div_prev: Option<u8>,
 
-    sender: Option<AudioSender>,
+    sender: Option<AudioSender<f32>>,
     sample_counter: u64,
 
-    is_mpsc_full: bool,
+    buffer: AudioBuffer<(f32, f32)>,
 }
 
 impl Sound {
@@ -70,7 +72,7 @@ impl Sound {
 
         self.div_prev = Some(bit_5);
 
-        if self.sample_counter >= SM83_CLOCK_SPEED {
+        if self.sender.is_some() && self.sample_counter >= SM83_CLOCK_SPEED {
             self.sample_counter %= SM83_CLOCK_SPEED;
             // Sample the APU
 
@@ -93,27 +95,33 @@ impl Sound {
             let left = (ch1_left + ch2_left + ch3_left + ch4_left) / 4.0;
             let right = (ch1_right + ch2_right + ch3_right + ch4_right) / 4.0;
 
-            if let Some(s) = self.sender.as_ref() {
-                let _ = s.send_samples(left, right).map_err(|e| match e {
-                    TrySendError::Full(_) => self.is_mpsc_full = true,
-                    TrySendError::Disconnected(_) => {
-                        panic!("Audio Sample MPSC Channel Disconnected")
-                    }
-                });
-            }
+            self.buffer.push_back((left, right));
         }
     }
 
-    pub(crate) fn set_audio_src(&mut self, sender: AudioSender) {
+    pub(crate) fn set_audio_src(&mut self, sender: AudioSender<f32>) {
         self.sender = Some(sender);
     }
 
-    pub(crate) fn is_mpsc_still_full(&mut self) -> bool {
-        if let Some(sender) = self.sender.as_ref() {
-            self.is_mpsc_full = sender.is_full();
+    pub(crate) fn is_full(&self) -> bool {
+        self.buffer.len() >= AUDIO_BUFFER_LEN * CHANNEL_COUNT
     }
 
-        self.is_mpsc_full
+    pub(crate) fn flush_samples(&mut self) {
+        if let Some(sender) = self.sender.as_ref() {
+            while self.buffer.len() >= CHANNEL_COUNT {
+                match self.buffer.pop_front() {
+                    Some((left, right)) => {
+                        sender
+                            .send_samples(left, right)
+                            .expect("Successfully sent samples across threads");
+                    }
+                    None => unreachable!(
+                        "While loop ensures that there are at least two items in AudioBuffer"
+                    ),
+                }
+            }
+        }
     }
 
     fn clock_length(freq_hi: &FrequencyHigh, length_timer: &mut u16, enabled: &mut bool) {
@@ -1175,35 +1183,34 @@ impl From<ChannelControl> for u8 {
 pub struct AudioMPSC;
 
 impl AudioMPSC {
-    pub fn new() -> (AudioSender, AudioReceiver) {
-        let (send, recv) = crossbeam_channel::bounded(AUDIO_BUFFER_LEN * 2);
+    pub fn new() -> (AudioSender<f32>, AudioReceiver<f32>) {
+        // TODO: Can we provide an upper limit for this?
+        // The larger this channel is, the more lag there is between the Audio and
+        // Emulator
+        let (send, recv) = crossbeam_channel::unbounded();
 
         (AudioSender { inner: send }, AudioReceiver { inner: recv })
     }
 }
 
 #[derive(Debug, Clone)]
-pub struct AudioSender {
-    inner: Sender<f32>,
+pub struct AudioSender<T> {
+    inner: Sender<T>,
 }
 
-impl AudioSender {
-    fn send_samples(&self, left: f32, right: f32) -> Result<(), TrySendError<f32>> {
+impl<T> AudioSender<T> {
+    fn send_samples(&self, left: T, right: T) -> Result<(), TrySendError<T>> {
         self.inner.try_send(left).and(self.inner.try_send(right))?;
         Ok(())
     }
-
-    fn is_full(&self) -> bool {
-        self.inner.is_full()
-    }
 }
 
-pub struct AudioReceiver {
-    inner: Receiver<f32>,
+pub struct AudioReceiver<T> {
+    inner: Receiver<T>,
 }
 
-impl Iterator for AudioReceiver {
-    type Item = f32;
+impl<T> Iterator for AudioReceiver<T> {
+    type Item = T;
 
     fn next(&mut self) -> Option<Self::Item> {
         // TODO: Should this never return none?
@@ -1211,7 +1218,7 @@ impl Iterator for AudioReceiver {
     }
 }
 
-impl Source for AudioReceiver {
+impl<T: rodio::Sample> Source for AudioReceiver<T> {
     fn current_frame_len(&self) -> Option<usize> {
         // A frame changes when the samples rate or
         // number of channels change. This will never happen, so
@@ -1221,7 +1228,7 @@ impl Source for AudioReceiver {
 
     fn channels(&self) -> u16 {
         // The Gameboy supports two channels
-        CHANNEL_COUNT
+        CHANNEL_COUNT as u16
     }
 
     fn sample_rate(&self) -> u32 {
@@ -1233,3 +1240,30 @@ impl Source for AudioReceiver {
         None
     }
 }
+
+#[derive(Debug, Clone)]
+struct AudioBuffer<T> {
+    inner: VecDeque<T>,
+}
+
+impl<T> Default for AudioBuffer<T> {
+    fn default() -> Self {
+        Self {
+            inner: VecDeque::with_capacity(AUDIO_BUFFER_LEN * CHANNEL_COUNT),
+        }
+    }
+}
+
+impl<T> AudioBuffer<T> {
+    pub fn push_back(&mut self, value: T) {
+        self.inner.push_back(value)
+    }
+
+    pub fn pop_front(&mut self) -> Option<T> {
+        self.inner.pop_front()
+    }
+
+    pub fn len(&self) -> usize {
+        self.inner.len()
+    }
+}