wz-phone/crates/wzp-native/src/lib.rs

//! wzp-native — standalone Android cdylib for all the C++ audio code.
//!
//! Built with `cargo ndk`, NOT `cargo tauri android build`. Loaded at
//! runtime by the Tauri desktop cdylib (`wzp-desktop`) via libloading.
//! See `docs/incident-tauri-android-init-tcb.md` for why the split exists.
//!
//! Phase 2: real Oboe audio backend.
//!
//! Architecture: Oboe runs capture + playout streams on its own high-
//! priority AAudio callback threads inside the C++ bridge. Two SPSC ring
//! buffers (capture and playout) are shared between the C++ callbacks
//! and the Rust side via atomic indices — no locks on the hot path.
//! `wzp-desktop` drains the capture ring into its Opus encoder and fills
//! the playout ring with decoded PCM.

use std::sync::atomic::{AtomicI32, Ordering};

// ─── Phase 1 smoke-test exports (kept for sanity checks) ─────────────────

/// Returns 42. Used by wzp-desktop's setup() to verify dlopen + dlsym
/// work before any audio code runs.
#[unsafe(no_mangle)]
pub extern "C" fn wzp_native_version() -> i32 {
    42
}

/// Writes a NUL-terminated string into `out` (capped at `cap`) and
/// returns bytes written excluding the NUL.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn wzp_native_hello(out: *mut u8, cap: usize) -> usize {
    const MSG: &[u8] = b"hello from wzp-native\0";
    if out.is_null() || cap == 0 {
        return 0;
    }
    let n = MSG.len().min(cap);
    unsafe {
        core::ptr::copy_nonoverlapping(MSG.as_ptr(), out, n);
        *out.add(n - 1) = 0;
    }
    n - 1
}

// ─── C++ Oboe bridge FFI ─────────────────────────────────────────────────

#[repr(C)]
struct WzpOboeConfig {
    sample_rate: i32,
    frames_per_burst: i32,
    channel_count: i32,
}

#[repr(C)]
struct WzpOboeRings {
    capture_buf: *mut i16,
    capture_capacity: i32,
    capture_write_idx: *mut AtomicI32,
    capture_read_idx: *mut AtomicI32,
    playout_buf: *mut i16,
    playout_capacity: i32,
    playout_write_idx: *mut AtomicI32,
    playout_read_idx: *mut AtomicI32,
}

// SAFETY: atomics synchronise producer/consumer; raw pointers are owned
// by the AudioBackend singleton below whose lifetime covers all calls.
unsafe impl Send for WzpOboeRings {}
unsafe impl Sync for WzpOboeRings {}

unsafe extern "C" {
    fn wzp_oboe_start(config: *const WzpOboeConfig, rings: *const WzpOboeRings) -> i32;
    fn wzp_oboe_stop();
    fn wzp_oboe_capture_latency_ms() -> f32;
    fn wzp_oboe_playout_latency_ms() -> f32;
    fn wzp_oboe_is_running() -> i32;
}

// ─── SPSC ring buffer (shared with C++ via AtomicI32) ────────────────────

/// 20 ms @ 48 kHz mono = 960 samples.
const FRAME_SAMPLES: usize = 960;
/// ~160 ms headroom at 48 kHz.
const RING_CAPACITY: usize = 7680;

struct RingBuffer {
    buf: Vec<i16>,
    capacity: usize,
    write_idx: AtomicI32,
    read_idx: AtomicI32,
}

// SAFETY: SPSC with atomic read/write cursors; producer and consumer
// are always on different threads.
unsafe impl Send for RingBuffer {}
unsafe impl Sync for RingBuffer {}

impl RingBuffer {
    fn new(capacity: usize) -> Self {
        Self {
            buf: vec![0i16; capacity],
            capacity,
            write_idx: AtomicI32::new(0),
            read_idx: AtomicI32::new(0),
        }
    }

    fn available_read(&self) -> usize {
        let w = self.write_idx.load(Ordering::Acquire);
        let r = self.read_idx.load(Ordering::Relaxed);
        let avail = w - r;
        if avail < 0 { (avail + self.capacity as i32) as usize } else { avail as usize }
    }

    fn available_write(&self) -> usize {
        self.capacity - 1 - self.available_read()
    }

    fn write(&self, data: &[i16]) -> usize {
        let count = data.len().min(self.available_write());
        if count == 0 {
            return 0;
        }
        let mut w = self.write_idx.load(Ordering::Relaxed) as usize;
        let cap = self.capacity;
        let buf_ptr = self.buf.as_ptr() as *mut i16;
        for sample in &data[..count] {
            unsafe { *buf_ptr.add(w) = *sample; }
            w += 1;
            if w >= cap { w = 0; }
        }
        self.write_idx.store(w as i32, Ordering::Release);
        count
    }

    fn read(&self, out: &mut [i16]) -> usize {
        let count = out.len().min(self.available_read());
        if count == 0 {
            return 0;
        }
        let mut r = self.read_idx.load(Ordering::Relaxed) as usize;
        let cap = self.capacity;
        let buf_ptr = self.buf.as_ptr();
        for slot in &mut out[..count] {
            unsafe { *slot = *buf_ptr.add(r); }
            r += 1;
            if r >= cap { r = 0; }
        }
        self.read_idx.store(r as i32, Ordering::Release);
        count
    }

    fn buf_ptr(&self) -> *mut i16 {
        self.buf.as_ptr() as *mut i16
    }
    fn write_idx_ptr(&self) -> *mut AtomicI32 {
        &self.write_idx as *const AtomicI32 as *mut AtomicI32
    }
    fn read_idx_ptr(&self) -> *mut AtomicI32 {
        &self.read_idx as *const AtomicI32 as *mut AtomicI32
    }
}

// ─── AudioBackend singleton ──────────────────────────────────────────────
//
// There is one global AudioBackend instance because Oboe's C++ side
// holds its own singleton of the streams. The `Box::leak`'d statics own
// the ring buffers for the lifetime of the process — dropping them while
// Oboe is still running would cause use-after-free in the audio callback.

use std::sync::OnceLock;

struct AudioBackend {
    capture: RingBuffer,
    playout: RingBuffer,
    started: std::sync::Mutex<bool>,
}

static BACKEND: OnceLock<&'static AudioBackend> = OnceLock::new();

fn backend() -> &'static AudioBackend {
    BACKEND.get_or_init(|| {
        Box::leak(Box::new(AudioBackend {
            capture: RingBuffer::new(RING_CAPACITY),
            playout: RingBuffer::new(RING_CAPACITY),
            started: std::sync::Mutex::new(false),
        }))
    })
}

// ─── C FFI for wzp-desktop ───────────────────────────────────────────────

/// Start the Oboe audio streams. Returns 0 on success, non-zero on error.
/// Idempotent — calling while already running is a no-op that returns 0.
#[unsafe(no_mangle)]
pub extern "C" fn wzp_native_audio_start() -> i32 {
    let b = backend();
    let mut started = match b.started.lock() {
        Ok(g) => g,
        Err(_) => return -1,
    };
    if *started {
        return 0;
    }

    let config = WzpOboeConfig {
        sample_rate: 48_000,
        frames_per_burst: FRAME_SAMPLES as i32,
        channel_count: 1,
    };
    let rings = WzpOboeRings {
        capture_buf: b.capture.buf_ptr(),
        capture_capacity: b.capture.capacity as i32,
        capture_write_idx: b.capture.write_idx_ptr(),
        capture_read_idx: b.capture.read_idx_ptr(),
        playout_buf: b.playout.buf_ptr(),
        playout_capacity: b.playout.capacity as i32,
        playout_write_idx: b.playout.write_idx_ptr(),
        playout_read_idx: b.playout.read_idx_ptr(),
    };
    let ret = unsafe { wzp_oboe_start(&config, &rings) };
    if ret != 0 {
        return ret;
    }
    *started = true;
    0
}

/// Stop Oboe. Idempotent. Safe to call from any thread.
#[unsafe(no_mangle)]
pub extern "C" fn wzp_native_audio_stop() {
    let b = backend();
    if let Ok(mut started) = b.started.lock() {
        if *started {
            unsafe { wzp_oboe_stop() };
            *started = false;
        }
    }
}

/// Read captured PCM samples from the capture ring. Returns the number
/// of `i16` samples actually copied into `out` (may be less than
/// `out_len` if the ring is empty).
#[unsafe(no_mangle)]
pub unsafe extern "C" fn wzp_native_audio_read_capture(out: *mut i16, out_len: usize) -> usize {
    if out.is_null() || out_len == 0 {
        return 0;
    }
    let slice = unsafe { std::slice::from_raw_parts_mut(out, out_len) };
    backend().capture.read(slice)
}

/// Write PCM samples into the playout ring. Returns the number of
/// samples actually enqueued (may be less than `in_len` if the ring
/// is nearly full — in practice the caller should pace to 20 ms
/// frames and spin briefly if the ring is full).
#[unsafe(no_mangle)]
pub unsafe extern "C" fn wzp_native_audio_write_playout(input: *const i16, in_len: usize) -> usize {
    if input.is_null() || in_len == 0 {
        return 0;
    }
    let slice = unsafe { std::slice::from_raw_parts(input, in_len) };
    backend().playout.write(slice)
}

/// Current capture latency reported by Oboe, in milliseconds. Returns
/// NaN / 0.0 if the stream isn't running.
#[unsafe(no_mangle)]
pub extern "C" fn wzp_native_audio_capture_latency_ms() -> f32 {
    unsafe { wzp_oboe_capture_latency_ms() }
}

/// Current playout latency reported by Oboe, in milliseconds.
#[unsafe(no_mangle)]
pub extern "C" fn wzp_native_audio_playout_latency_ms() -> f32 {
    unsafe { wzp_oboe_playout_latency_ms() }
}

/// Non-zero if both Oboe streams are currently running.
#[unsafe(no_mangle)]
pub extern "C" fn wzp_native_audio_is_running() -> i32 {
    unsafe { wzp_oboe_is_running() }
}