add initial support for playing custom audio waveform generators

with typical sine wave generator example added to the audio_playback
example project

examples/audio_playback/src/main.rs

@@ -1,3 +1,4 @@
+use std::f64::consts::PI;
 use std::path::Path;
 
 use anyhow::Result;
@@ -27,6 +28,31 @@ fn load_and_convert_wav(path: &Path, target_spec: &AudioSpec) -> Result<AudioBuf
     Ok(sound)
 }
 
+pub struct SineWaveGenerator {
+    t: usize,
+}
+
+impl SineWaveGenerator {
+    pub fn new() -> Self {
+        SineWaveGenerator {
+            t: 0
+        }
+    }
+}
+
+impl AudioGenerator for SineWaveGenerator {
+    fn gen_sample(&mut self, position: usize) -> Option<u8> {
+        const MAX_TIME: usize = AUDIO_FREQUENCY_22KHZ as usize * 3;  // 3 seconds
+        if self.t < MAX_TIME {
+            let sample = (self.t as f64 * 0.25).sin() * 80.0;
+            self.t += 1;
+            Some((sample + 128.0) as u8)
+        } else {
+            None
+        }
+    }
+}
+
 fn main() -> Result<()> {
     let mut system = SystemBuilder::new().window_title("Audio Playback").vsync(true).build()?;
 
@@ -73,6 +99,10 @@ fn main() -> Result<()> {
             audio_device.play_buffer(&sound5, false)?;
         }
 
+        if system.keyboard.is_key_pressed(Scancode::Num6) {
+            audio_device.play_generator(Box::new(SineWaveGenerator::new()), false);
+        }
+
         for index in 0..NUM_CHANNELS {
             let channel = &audio_device[index];
             let mut status = &mut statuses[index];

libretrogd/src/audio/mod.rs

@@ -55,11 +55,17 @@ impl AudioSpec {
 
 //////////////////////////////////////////////////////////////////////////////////////////////////
 
-#[derive(Debug, Clone)]
+pub trait AudioGenerator: Send {
+    fn gen_sample(&mut self, position: usize) -> Option<u8>;
+}
+
+//////////////////////////////////////////////////////////////////////////////////////////////////
+
 pub struct AudioChannel {
     pub playing: bool,
     pub loops: bool,
     pub data: Vec<u8>,
+    pub generator: Option<Box<dyn AudioGenerator>>,
     pub volume: f32,
     pub position: usize,
 }
@@ -71,10 +77,19 @@ impl AudioChannel {
             loops: false,
             volume: 1.0,
             position: 0,
+            generator: None,
             data: Vec::new(),
         }
     }
 
+    fn data_at(&mut self, position: usize) -> Option<u8> {
+        if let Some(generator) = &mut self.generator {
+            generator.gen_sample(position)
+        } else {
+            self.data.get(self.position).copied()
+        }
+    }
+
     /// Returns the next sample from this channel's buffer. If this channel's buffer is done
     /// playing or there is no buffer data at all, `None` is returned. If the next sample was
     /// successfully loaded from the buffer, the channel's current position is advanced by 1.
@@ -84,9 +99,9 @@ impl AudioChannel {
     /// instead of 0 to 255).
     #[inline]
     fn next_sample(&mut self) -> Option<i16> {
-        if let Some(sample) = self.data.get(self.position) {
+        if let Some(sample) = self.data_at(self.position) {
             self.position += 1;
-            Some((*sample as i16) - 128)
+            Some(sample as i16 - 128)
         } else {
             None
         }
@@ -104,25 +119,25 @@ impl AudioChannel {
     pub fn sample(&mut self) -> Option<i16> {
         if !self.playing {
             return None;
-        } else if self.position >= self.data.len() {
-            if self.loops {
-                self.position = 0;
-            } else {
-                self.stop();
-                return None;
-            }
         }
 
-        if let Some(raw_sample) = self.next_sample() {
-            Some((raw_sample as f32 * self.volume) as i16)
+        if let Some(sample) = self.next_sample() {
+            Some((sample as f32 * self.volume) as i16)
         } else {
-            None
+            if self.loops {
+                self.position = 0;
+                None
+            } else {
+                self.stop();
+                None
+            }
         }
     }
 
     #[inline]
     pub fn reset(&mut self) {
         self.data.clear();
+        self.generator = None;
         self.position = 0;
         self.playing = false;
     }
@@ -131,14 +146,27 @@ impl AudioChannel {
     pub fn play_buffer(&mut self, buffer: &AudioBuffer, loops: bool) {
         self.data.clear();
         self.data.extend(&buffer.data);
+        self.generator = None;
         self.position = 0;
         self.playing = true;
         self.loops = loops;
     }
 
+    pub fn play_generator(&mut self, generator: Box<dyn AudioGenerator>, loops: bool) {
+        self.data.clear();
+        self.generator = Some(generator);
+        self.position = 0;
+        self.playing = true;
+        self.loops = loops;
+    }
+
+    pub fn is_playable(&self) -> bool {
+        !self.data.is_empty() || self.generator.is_some()
+    }
+
     #[inline]
     pub fn play(&mut self, loops: bool) {
-        if !self.data.is_empty() {
+        if self.is_playable() {
             self.position = 0;
             self.playing = true;
             self.loops = loops;
@@ -222,6 +250,15 @@ impl AudioDevice {
         }
     }
 
+    pub fn play_generator(&mut self, generator: Box<dyn AudioGenerator>, loops: bool) -> Result<Option<&mut AudioChannel>, AudioDeviceError> {
+        if let Some(channel) = self.stopped_channels_iter_mut().next() {
+            channel.play_generator(generator, loops);
+            Ok(Some(channel))
+        } else {
+            Ok(None)
+        }
+    }
+
     #[inline]
     pub fn playing_channels_iter(&mut self) -> impl Iterator<Item = &AudioChannel> {
         self.channels.iter().filter(|channel| channel.playing)