fix(wasapi): shift i24 to/from top bits (#1309)

Co-authored-by: Roderick van Domburg <roderick@vandomburg.net>

Author: LastExceed

CHANGELOG.md

@@ -85,6 +85,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - **WASAPI**: Capture streams no longer report a spurious xrun on the first buffer after starting.
 - **WASAPI**: Output streams no longer reject formats that the built-in resampler can convert.
 - **WASAPI**: Device enumeration no longer panics if the COM enumerator fails to initialize.
+- **WASAPI**: Fix 24-bit samples not getting shifted from/to MSB.
 - **WebAudio**: Fix stale audio output when a data callback wrote a partial buffer.
 - **WebAudio**: Fix unsound `Send + Sync` on `Stream` when compiled with `+atomics`.
 - **WebAudio**: Fix `Host::is_available()` always returning `true`, even in non-window contexts.

src/host/wasapi/stream.rs

@@ -1,7 +1,7 @@
 use std::{
     mem,
     ops::ControlFlow,
-    ptr,
+    ptr, slice,
     sync::{
         Arc,
         atomic::{AtomicBool, AtomicU64, Ordering},
@@ -665,6 +665,15 @@ fn run_input(
         emit_error(error_callback, err);
     }
 
+    let stream = &run_ctxt.stream;
+    let scratch_len = if stream.sample_format == SampleFormat::I24 {
+        stream.max_frames_in_buffer as usize * stream.bytes_per_frame as usize / size_of::<i32>()
+    } else {
+        // The scratch buffer won't be used in this case.
+        0 // Vec::with_capacity(0) does not allocate.
+    };
+    let mut scratch_buffer = vec![0; scratch_len].into_boxed_slice();
+
     loop {
         match process_commands_and_await_signal(&mut run_ctxt, error_callback) {
             ControlFlow::Break(()) => break,
@@ -675,7 +684,12 @@ fn run_input(
             AudioClientFlow::Capture { ref capture_client } => capture_client.clone(),
             _ => unreachable!(),
         };
-        if let Err(err) = process_input(&run_ctxt.stream, capture_client, data_callback) {
+        if let Err(err) = process_input(
+            &run_ctxt.stream,
+            capture_client,
+            data_callback,
+            &mut scratch_buffer,
+        ) {
             emit_error(error_callback, err);
             break;
         }
@@ -815,6 +829,7 @@ fn process_input(
     stream: &StreamInner,
     capture_client: Audio::IAudioCaptureClient,
     data_callback: &mut dyn FnMut(&Data, &CallbackInfo),
+    scratch_buffer: &mut [i32],
 ) -> Result<(), Error> {
     unsafe {
         // Get the available data in the shared buffer.
@@ -850,10 +865,26 @@ fn process_input(
                 && flags & Audio::AUDCLNT_BUFFERFLAGS_DATA_DISCONTINUITY.0 as u32 != 0;
 
             debug_assert!(!buffer.is_null());
+            let byte_count = frames_available as usize * stream.bytes_per_frame as usize;
+            let data = if stream.sample_format == SampleFormat::I24 {
+                // WASAPI stores i24 in the upper bits
+                let source_data =
+                    slice::from_raw_parts(buffer.cast(), byte_count / size_of::<i32>());
+                // use a scratch buffer since the capture buffer isn't meant to be written
+                let dst = &mut scratch_buffer[..source_data.len()];
+                dst.copy_from_slice(source_data);
+                for sample in dst.iter_mut() {
+                    // On signed integers, >> is an arithmetic shift,
+                    // which ensures the correct upper bits get shifted in
+                    *sample >>= 8;
+                }
 
-            let data = buffer as *mut ();
-            let len = frames_available as usize * stream.bytes_per_frame as usize
-                / stream.sample_format.sample_size();
+                dst.as_mut_ptr().cast()
+            } else {
+                buffer.cast()
+            };
+
+            let len = byte_count / stream.sample_format.sample_size();
             let data = Data::from_parts(data, len, stream.sample_format);
 
             if !stream.draining.load(Ordering::Relaxed) {
@@ -921,6 +952,18 @@ fn process_output(
                     xrun: false,
                 },
             );
+
+            if stream.sample_format == SampleFormat::I24 {
+                // WASAPI stores i24 in the upper bits
+                #[expect(
+                    clippy::cast_ptr_alignment,
+                    reason = "WASAPI guarantees the buffer to be aligned to a frame boundary"
+                )]
+                let buffer_slice_i32 = slice::from_raw_parts_mut(buffer.cast::<i32>(), len);
+                for sample in buffer_slice_i32 {
+                    *sample <<= 8;
+                }
+            }
         }
 
         render_client.ReleaseBuffer(frames_available, 0)?;