feat: implement Hash/Eq for host Device types (#1068)

Add Eq/Hash implementations for Device across ALSA, WASAPI,
WebAudio, CoreAudio (macOS/iOS), AudioWorklet, and Null. Hashing is
based on backend device identifiers and, for ALSA, PCM ID + direction.
WASAPI hashes the COM device ID string obtained via GetId.

Author: roderickvd

CHANGELOG.md

@@ -5,13 +5,14 @@
 - Add `Display` and `FromStr` implementations for `HostId`.
 - Add support for custom `Host`s, `Device`s, and `Stream`s.
 - Add `Sample::bits_per_sample` method.
-- Add `Copy` impl to `InputCallbackInfo` and `OutputCallbackInfo`.
+- Add `Copy` implementation to `InputCallbackInfo` and `OutputCallbackInfo`.
 - Add `StreamError::StreamInvalidated` variant for when stream must be rebuilt.
 - Add `StreamError::BufferUnderrun` variant for buffer underrun/overrun notifications.
+- Add `Hash` implementation to `Device` for all backends.
 - Change `SampleRate` from struct to `u32` type alias.
 - Update `audio_thread_priority` to 0.34.
 - AAudio: Configure buffer to ensure consistent callback buffer sizes.
-- AAudio: Make `Stream` implement `Send` and `Sync`.
+- AAudio: Add `Send` and `Sync` implementations to `Stream`.
 - AAudio: Fix the buffer size range detection by querying the AudioService property correctly.
 - AAudio: Add support for 12 and 24 kHz sample rates.
 - ALSA: Improve `BufferSize::Fixed` precision and audio callback performance.
@@ -23,6 +24,7 @@
 - ALSA: Update `alsa` to 0.10.
 - ALSA: Pass `silent=true` to `PCM.try_recover`, so it doesn't write to stderr.
 - ALSA: Report buffer underruns/overruns via `StreamError::BufferUnderrun`.
+- ALSA: Add `Eq` and `PartialEq` implementations to `Device`.
 - ASIO: Fix linker flags for MinGW cross-compilation.
 - ASIO: Add packed(4) to representation of ASIO time structs in bindings.
 - ASIO: Add handling for `kAsioResetRequest` message to prevent driver UI becoming unresponsive.
@@ -38,8 +40,8 @@
 - CoreAudio: Update `mach2` to 0.6.
 - CoreAudio: Configure device buffer to ensure predictable callback buffer sizes.
 - CoreAudio: Fix timestamp accuracy.
-- CoreAudio: Make `Stream` implement `Send`.
-- CoreAudio: Remove `Clone` impl from `Stream`.
+- CoreAudio: Add `Send` implementation to `Stream`.
+- CoreAudio: Remove `Clone` implementation from `Stream`.
 - CoreAudio: Fix segfaults when enumerating devices.
 - CoreAudio: Fix undefined behavior related to null pointers and aligned reads.
 - Emscripten: Add `BufferSize::Fixed` validation against supported range.
@@ -52,9 +54,9 @@
 - WASAPI: Expose `IMMDevice` from WASAPI host Device.
 - WASAPI: Add `I24` and `U24` sample format support (24-bit samples stored in 4 bytes).
 - WASAPI: Update `windows` to >= 0.59, <= 0.62.
-- WASAPI: Make `Stream` implement `Send` and `Sync`.
-- Wasm: Removed optional `wee-alloc` feature for security reasons.
-- Wasm: Make `Stream` implement `Send` and `Sync`.
+- WASAPI: Add `Send` and `Sync` implementations to `Stream`.
+- WebAudio: Removed optional `wee-alloc` feature for security reasons.
+- WebAudio: Add `Send` and `Sync` implementations to `Stream`.
 - WebAudio: Add `BufferSize::Fixed` validation against supported range.
 
 # Version 0.16.0 (2025-06-07)

src/host/alsa/mod.rs

@@ -314,6 +314,29 @@ pub struct Device {
     handles: Arc<Mutex<DeviceHandles>>,
 }
 
+impl PartialEq for Device {
+    fn eq(&self, other: &Self) -> bool {
+        // Devices are equal if they have the same PCM ID and direction.
+        // The handles field is not part of device identity.
+        self.pcm_id == other.pcm_id && self.direction == other.direction
+    }
+}
+
+impl Eq for Device {}
+
+impl std::hash::Hash for Device {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        // Hash based on PCM ID and direction for consistency with PartialEq
+        self.pcm_id.hash(state);
+        // Manually hash direction since alsa::Direction doesn't implement Hash
+        match self.direction {
+            Some(alsa::Direction::Capture) => 0u8.hash(state),
+            Some(alsa::Direction::Playback) => 1u8.hash(state),
+            None => 2u8.hash(state),
+        }
+    }
+}
+
 impl Device {
     fn build_stream_inner(
         &self,

src/host/audioworklet/mod.rs

@@ -23,7 +23,7 @@ use std::time::Duration;
 /// Content is false if the iterator is empty.
 pub struct Devices(bool);
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Device;
 
 pub struct Host;

src/host/coreaudio/ios/mod.rs

@@ -32,7 +32,7 @@ pub mod enumerate;
 // These days the default of iOS is now F32 and no longer I16
 const SUPPORTED_SAMPLE_FORMAT: SampleFormat = SampleFormat::F32;
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Device;
 
 pub struct Host;

src/host/coreaudio/macos/device.rs

@@ -337,7 +337,7 @@ impl DeviceTrait for Device {
     }
 }
 
-#[derive(Clone, Eq, PartialEq)]
+#[derive(Clone, Eq, Hash, PartialEq)]
 pub struct Device {
     pub(crate) audio_device_id: AudioDeviceID,
 }

src/host/null/mod.rs

@@ -15,7 +15,7 @@ use crate::{
 #[derive(Default)]
 pub struct Devices;
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Device;
 
 pub struct Host;

src/host/wasapi/device.rs

@@ -940,6 +940,38 @@ impl PartialEq for Device {
 
 impl Eq for Device {}
 
+impl std::hash::Hash for Device {
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        // Hash the device ID for consistency with PartialEq
+        // SAFETY: GetId only fails with E_OUTOFMEMORY, which is unrecoverable.
+        // We need consistent hash/eq behavior.
+        unsafe {
+            use windows::Win32::System::Com;
+
+            struct IdRAII(windows::core::PWSTR);
+            impl Drop for IdRAII {
+                fn drop(&mut self) {
+                    unsafe { Com::CoTaskMemFree(Some(self.0 .0 as *mut _)) }
+                }
+            }
+
+            let id = self.device.GetId().expect("cpal: GetId failure");
+            let id = IdRAII(id);
+
+            // Hash the 16-bit null-terminated string
+            let mut offset = 0;
+            loop {
+                let w: u16 = *(id.0).0.offset(offset);
+                if w == 0 {
+                    break;
+                }
+                w.hash(state);
+                offset += 1;
+            }
+        }
+    }
+}
+
 impl fmt::Debug for Device {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.debug_struct("Device")

src/host/webaudio/mod.rs

@@ -27,7 +27,7 @@ type ClosureHandle = Arc<RwLock<Option<Closure<dyn FnMut()>>>>;
 /// Content is false if the iterator is empty.
 pub struct Devices(bool);
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Device;
 
 pub struct Host;