feat(audio): capture transactional fixed-rate stereo samples

crates/gba-core/src/audio.rs

@@ -1,5 +1,7 @@
 //! Nominal Direct Sound and PSG channels, including single-bank wave playback. No host output.
 
+pub(crate) mod capture;
+pub use capture::{AUDIO_QUEUE_CAPACITY, AUDIO_SAMPLE_RATE};
 mod modulation;
 mod noise;
 mod pulse;

crates/gba-core/src/audio/capture.rs

@@ -0,0 +1,103 @@
+//! Optional fixed-rate digital capture, separate from hardware channel state.
+use super::StereoLevel;
+use std::collections::VecDeque;
+
+/// Nominal stereo capture rate. This is not the GBA PWM/resolution model.
+pub const AUDIO_SAMPLE_RATE: u32 = 32768;
+/// Maximum committed stereo frames retained before the caller drains them.
+pub const AUDIO_QUEUE_CAPACITY: usize = 4096;
+const PERIOD: u32 = 512;
+pub(crate) const STEP_SAMPLES: usize = 8;
+
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct SampleClock(Option<u16>);
+
+impl SampleClock {
+    /// Advance devices at every retained sample boundary. Once capacity is
+    /// exhausted, advance the remainder in bulk and count discarded samples.
+    pub(crate) fn advance(
+        &mut self,
+        mut cycles: u32,
+        mut capacity: usize,
+        mut advance: impl FnMut(u32, bool),
+    ) -> u64 {
+        let Some(mut phase) = self.0 else {
+            advance(cycles, false);
+            return 0;
+        };
+        while cycles != 0 && capacity != 0 {
+            let count = cycles.min(PERIOD - u32::from(phase));
+            phase += count as u16;
+            cycles -= count;
+            let sample = u32::from(phase) == PERIOD;
+            if sample {
+                phase = 0;
+                capacity -= 1;
+            }
+            advance(count, sample);
+        }
+        let discarded = (u64::from(phase) + u64::from(cycles)) / u64::from(PERIOD);
+        if cycles != 0 {
+            advance(cycles, false);
+        }
+        self.0 = Some(((u64::from(phase) + u64::from(cycles)) % u64::from(PERIOD)) as u16);
+        discarded
+    }
+}
+
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+pub(crate) struct SampleBatch {
+    pub(crate) samples: [StereoLevel; STEP_SAMPLES],
+    pub(crate) len: usize,
+    pub(crate) dropped: u64,
+}
+
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
+pub(crate) struct AudioCapture {
+    pub(crate) clock: SampleClock,
+    queue: VecDeque<StereoLevel>,
+    pub(crate) dropped: u64,
+}
+
+impl AudioCapture {
+    pub(crate) fn set_enabled(&mut self, enabled: bool) {
+        if enabled == self.clock.0.is_some() {
+            return;
+        }
+        self.clock = SampleClock(enabled.then_some(0));
+        self.queue = if enabled {
+            VecDeque::with_capacity(AUDIO_QUEUE_CAPACITY)
+        } else {
+            VecDeque::new()
+        };
+        self.dropped = 0;
+    }
+
+    pub(crate) fn capacity(&self) -> usize {
+        AUDIO_QUEUE_CAPACITY - self.queue.len()
+    }
+
+    pub(crate) fn push(&mut self, sample: StereoLevel) {
+        if self.queue.len() < AUDIO_QUEUE_CAPACITY {
+            self.queue.push_back(sample);
+        } else {
+            self.dropped = self.dropped.saturating_add(1);
+        }
+    }
+
+    pub(crate) fn commit(&mut self, clock: SampleClock, batch: SampleBatch) {
+        self.clock = clock;
+        self.dropped = self.dropped.saturating_add(batch.dropped);
+        for sample in &batch.samples[..batch.len] {
+            self.push(*sample);
+        }
+    }
+
+    pub(crate) fn drain(&mut self, output: &mut [StereoLevel]) -> usize {
+        let count = output.len().min(self.queue.len());
+        for slot in &mut output[..count] {
+            *slot = self.queue.pop_front().unwrap();
+        }
+        count
+    }
+}

crates/gba-core/src/io.rs

@@ -161,6 +161,7 @@ enum PowerState {
 pub(crate) struct Io {
     pub(crate) serial: Serial,
     pub(crate) audio: Audio,
+    pub(crate) audio_capture: crate::audio::capture::AudioCapture,
     timers: [Timer; 4],
     // Low ten system-clock bits cover every supported divider. STOP freezes this phase.
     timer_phase: u16,
@@ -430,6 +431,7 @@ impl Io {
 
     pub(crate) fn commit_timer_step(&mut self, step: TimerStep) {
         self.audio = step.audio;
+        self.audio_capture.commit(step.sample_clock, step.samples);
         self.serial = step.serial;
         self.dma.trigger_sound(self.audio.take_requests());
         self.timers = step.timers;
@@ -493,12 +495,20 @@ impl Io {
         self.pending |= self.display.advance(cycles);
         if advance_timers {
             self.pending |= self.serial.advance(cycles);
-            self.pending |= Self::advance_timer_bank(
-                &mut self.timers,
-                &mut self.timer_phase,
-                &mut self.audio,
-                cycles,
-            );
+            let mut clock = self.audio_capture.clock;
+            let dropped = clock.advance(cycles, self.audio_capture.capacity(), |count, sample| {
+                self.pending |= Self::advance_timer_bank(
+                    &mut self.timers,
+                    &mut self.timer_phase,
+                    &mut self.audio,
+                    count,
+                );
+                if sample {
+                    self.audio_capture.push(self.audio.level());
+                }
+            });
+            self.audio_capture.clock = clock;
+            self.audio_capture.dropped = self.audio_capture.dropped.saturating_add(dropped);
             self.dma.trigger_sound(self.audio.take_requests());
         }
         self.wake_if_requested();

crates/gba-core/src/io/timer_step.rs

@@ -7,6 +7,8 @@ pub(super) const STAGED_IRQ_MASK: u16 = 0xf8;
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub(crate) struct TimerStep {
     pub(crate) audio: crate::audio::Audio,
+    pub(crate) sample_clock: crate::audio::capture::SampleClock,
+    pub(crate) samples: crate::audio::capture::SampleBatch,
     pub(crate) serial: super::Serial,
     pub(super) timers: [Timer; 4],
     pub(super) phase: u16,
@@ -18,6 +20,8 @@ impl TimerStep {
     pub(super) fn new(io: &Io) -> Self {
         Self {
             audio: io.audio,
+            sample_clock: io.audio_capture.clock,
+            samples: Default::default(),
             serial: io.serial,
             timers: io.timers,
             phase: io.timer_phase,
@@ -29,12 +33,23 @@ impl TimerStep {
     pub(crate) fn advance_to(&mut self, elapsed: u32) {
         assert!(elapsed >= self.elapsed, "timer step time must be monotonic");
         self.pending |= self.serial.advance(elapsed - self.elapsed);
-        self.pending |= Io::advance_timer_bank(
-            &mut self.timers,
-            &mut self.phase,
-            &mut self.audio,
+        let dropped = self.sample_clock.advance(
             elapsed - self.elapsed,
+            crate::audio::capture::STEP_SAMPLES - self.samples.len,
+            |count, sample| {
+                self.pending |= Io::advance_timer_bank(
+                    &mut self.timers,
+                    &mut self.phase,
+                    &mut self.audio,
+                    count,
+                );
+                if sample {
+                    self.samples.samples[self.samples.len] = self.audio.level();
+                    self.samples.len += 1;
+                }
+            },
         );
+        self.samples.dropped = self.samples.dropped.saturating_add(dropped);
         self.elapsed = elapsed;
     }
 

crates/gba-core/src/machine.rs

@@ -77,7 +77,7 @@ impl Error for FrameRunError {
 }
 
 /// Owns the CPU and memory bus. Use this instead of Cpu::step to advance device
-/// clocks, execute DMA, honor HALT/STOP, and deliver interrupts. No audio yet.
+/// clocks, execute DMA, honor HALT/STOP, and deliver interrupts. Host audio is external.
 pub struct Machine {
     cpu: Cpu,
     memory: Memory,

crates/gba-core/src/memory.rs

@@ -1,5 +1,7 @@
 use std::{cell::Cell, error::Error, fmt};
 
+#[cfg(test)]
+mod audio_capture_tests;
 #[cfg(test)]
 mod audio_step_tests;
 #[cfg(test)]
@@ -279,6 +281,28 @@ impl Memory {
         self.io.audio.level()
     }
 
+    /// Enable optional 32768 Hz stereo capture. A toggle clears queued frames,
+    /// resets its synthetic phase/drop counter, and does not reset sound hardware.
+    /// Call between machine steps. Repeating the current setting preserves state.
+    pub fn set_audio_capture(&mut self, enabled: bool) {
+        assert!(
+            self.timer_step.get().is_none(),
+            "audio capture configuration during a step"
+        );
+        self.io.audio_capture.set_enabled(enabled);
+    }
+
+    /// Drain committed frames, oldest first. Failed steps never publish samples.
+    /// Each sample is a signed ten-bit digital level, not normalized host PCM.
+    pub fn drain_audio_samples(&mut self, output: &mut [StereoLevel]) -> usize {
+        self.io.audio_capture.drain(output)
+    }
+
+    /// Frames discarded because the bounded capture buffers were full.
+    pub fn audio_dropped_samples(&self) -> u64 {
+        self.io.audio_capture.dropped
+    }
+
     fn serial_state(&self) -> Serial {
         self.timer_step
             .get()

crates/gba-core/src/memory/audio_capture_tests.rs

@@ -0,0 +1,69 @@
+use super::{timing_event_tests::prepared, *};
+use crate::{
+    audio::StereoLevel,
+    io::{SOUNDBIAS, SOUNDCNT_X},
+    machine::Machine,
+};
+
+#[test]
+fn samples_before_store_completion_keep_old_level_and_publish_only_once() {
+    let (cpu, mut memory) = prepared(false, &[0xe5810000], &[(0, 0x202), (1, SOUNDBIAS)]);
+    memory.write16(SOUNDCNT_X, 0x80).unwrap();
+    memory.write16(SOUNDBIAS, 0x200).unwrap();
+    memory.set_audio_capture(true);
+    memory.advance_cycles(505);
+    let mut machine = Machine::new(cpu, memory);
+    machine.step().unwrap(); // Sample at cycle 512, then bias write at same bus phase.
+    let mut output = [StereoLevel::default(); 4];
+    assert_eq!(machine.memory_mut().drain_audio_samples(&mut output), 1);
+    assert_eq!(output[0].left, 0);
+    assert_eq!(machine.memory().audio_level().left, 2);
+    machine.memory_mut().advance_cycles(512);
+    assert_eq!(machine.memory_mut().drain_audio_samples(&mut output), 1);
+    assert_eq!(output[0].left, 2);
+}
+
+#[test]
+fn failed_cpu_and_dma_steps_discard_speculative_samples_and_phase() {
+    for dma in [false, true] {
+        let (cpu, mut memory) = prepared(false, &[0xe8910005], &[(1, SOUNDBIAS)]); // LDM crosses unmapped 8c.
+        memory.set_audio_capture(true);
+        memory.advance_cycles(508);
+        if dma {
+            memory.write32(DMA_BASE, 0x02000000).unwrap();
+            memory.write32(DMA_BASE + 4, 0x0400008c).unwrap();
+            memory.write32(DMA_BASE + 8, 0x84000001).unwrap();
+        }
+        let before = memory.io.audio_capture.clone();
+        let mut machine = Machine::new(cpu, memory);
+        for _ in 0..2 {
+            assert!(machine.step().is_err());
+            assert_eq!(machine.memory().io.audio_capture, before);
+            assert_eq!(machine.cycles(), 508);
+        }
+        machine.memory_mut().advance_cycles(4);
+        assert_eq!(
+            machine
+                .memory_mut()
+                .drain_audio_samples(&mut [StereoLevel::default(); 2]),
+            1
+        );
+    }
+}
+
+#[test]
+fn preflight_cannot_publish_samples_or_advance_the_sample_clock() {
+    let (_, mut memory) = prepared(false, &[0xe1a00000], &[]);
+    memory.set_audio_capture(true);
+    memory.advance_cycles(511);
+    let before = memory.io.audio_capture.clone();
+    assert!(memory
+        .write_words(&[(SOUNDBIAS, 0x202), (0x0400008c, 0)])
+        .is_err());
+    assert_eq!(memory.io.audio_capture, before);
+    memory.begin_timer_step();
+    memory.advance_timer_step(512);
+    assert_eq!(memory.io.audio_capture, before);
+    memory.discard_timer_step();
+    assert_eq!(memory.io.audio_capture, before);
+}