Improve the emscripten backend (#172)

* Use the js! macro from stdweb

* Rework the Buffer::finish method

* Use references from stdweb

* Fix emscripten warnings

* Rework the run() method to use stdweb

* Adjust timings

* Add entry in CHANGELOG

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+- Changed the emscripten backend to consume less CPU.
+
 # Version 0.5.1 (2017-10-21)
 
 - Added `Sample::to_i16()`, `Sample::to_u16()` and `Sample::from`.

Cargo.toml

@@ -22,3 +22,6 @@ libc = "0.2"
 
 [target.'cfg(any(target_os = "macos", target_os = "ios"))'.dependencies]
 coreaudio-rs = "0.7.0"
+
+[target.'cfg(target_os = "emscripten")'.dependencies]
+stdweb = { version = "0.1.3", default-features = false }

src/emscripten/mod.rs

@@ -1,7 +1,12 @@
-use std::marker::PhantomData;
+use std::mem;
-use std::os::raw::c_char;
-use std::os::raw::c_int;
 use std::os::raw::c_void;
+use std::slice::from_raw_parts;
+use std::sync::Mutex;
+use stdweb;
+use stdweb::Reference;
+use stdweb::unstable::TryInto;
+use stdweb::web::set_timeout;
+use stdweb::web::TypedArray;
 
 use CreationError;
 use Format;
@@ -10,12 +15,6 @@ use Sample;
 use SupportedFormat;
 use UnknownTypeBuffer;
 
-extern {
-    fn emscripten_set_main_loop_arg(_: extern fn(*mut c_void), _: *mut c_void, _: c_int, _: c_int);
-    fn emscripten_run_script(script: *const c_char);
-    fn emscripten_run_script_int(script: *const c_char) -> c_int;
-}
-
 // The emscripten backend works by having a global variable named `_cpal_audio_contexts`, which
 // is an array of `AudioContext` objects. A voice ID corresponds to an entry in this array.
 //
@@ -25,121 +24,115 @@ extern {
 // that is in each buffer ; this is obviously bad, and also the schedule is too tight and there may
 // be underflows
 
-pub struct EventLoop;
+pub struct EventLoop {
+    voices: Mutex<Vec<Option<Reference>>>,
+}
+
 impl EventLoop {
     #[inline]
     pub fn new() -> EventLoop {
-        EventLoop
+        stdweb::initialize();
+
+        EventLoop {
+            voices: Mutex::new(Vec::new()),
+        }
     }
 
     #[inline]
-    pub fn run<F>(&self, mut callback: F) -> !
+    pub fn run<F>(&self, callback: F) -> !
+        where F: FnMut(VoiceId, UnknownTypeBuffer)
+    {
+        // The `run` function uses `set_timeout` to invoke a Rust callback repeatidely. The job
+        // of this callback is to fill the content of the audio buffers.
+
+        // The first argument of the callback function (a `void*`) is a casted pointer to `self`
+        // and to the `callback` parameter that was passed to `run`.
+
+        fn callback_fn<F>(user_data_ptr: *mut c_void)
             where F: FnMut(VoiceId, UnknownTypeBuffer)
         {
             unsafe {
-            // The `run` function uses `emscripten_set_main_loop_arg` to invoke a Rust callback
+                let user_data_ptr2 = user_data_ptr as *mut (&EventLoop, F);
-            // repeatidely. The job of this callback is to fill the content of the audio buffers.
+                let user_data = &mut *user_data_ptr2;
+                let user_cb = &mut user_data.1;
 
-            // The first argument of the callback function (a `void*`) is a casted pointer to the
+                let voices = user_data.0.voices.lock().unwrap().clone();
-            // `callback` parameter that was passed to `run`.
+                for (voice_id, voice) in voices.iter().enumerate() {
-
+                    let voice = match voice.as_ref() {
-            extern "C" fn callback_fn<F>(callback_ptr: *mut c_void)
+                        Some(v) => v,
-                where F: FnMut(VoiceId, UnknownTypeBuffer)
+                        None => continue,
-            {
+                    };
-                unsafe {
-                    let num_contexts = emscripten_run_script_int("(function() {
-                        if (window._cpal_audio_contexts)
-                            return window._cpal_audio_contexts.length;
-                        else
-                            return 0;
-                        })()\0".as_ptr() as *const _);
-
-                    // TODO: this processes all the voices, even those from maybe other event loops
-                    // this is not a problem yet, but may become one in the future?
-                    for voice_id in 0 .. num_contexts {
-                        let callback_ptr = &mut *(callback_ptr as *mut F);
 
                     let buffer = Buffer {
                         temporary_buffer: vec![0.0; 44100 * 2 / 3],
-                            voice_id: voice_id,
+                        voice: &voice,
-                            marker: PhantomData,
                     };
 
-                        callback_ptr(VoiceId(voice_id), ::UnknownTypeBuffer::F32(::Buffer { target: Some(buffer) }));
+                    user_cb(VoiceId(voice_id), ::UnknownTypeBuffer::F32(::Buffer { target: Some(buffer) }));
                 }
+
+                set_timeout(|| callback_fn::<F>(user_data_ptr), 330);
             }
         }
 
-            let callback_ptr = &mut callback as *mut F as *mut c_void;
+        let mut user_data = (self, callback);
-            emscripten_set_main_loop_arg(callback_fn::<F>, callback_ptr, 3, 1);
+        let user_data_ptr = &mut user_data as *mut (_, _);
 
-            unreachable!()
+        set_timeout(|| callback_fn::<F>(user_data_ptr as *mut _), 10);
-        }
+
+        stdweb::event_loop();
     }
 
     #[inline]
-    pub fn build_voice(&self, _: &Endpoint, format: &Format)
+    pub fn build_voice(&self, _: &Endpoint, _format: &Format)
                        -> Result<VoiceId, CreationError>
     {
-        // TODO: find an empty element in the array first, instead of pushing at the end, in case
+        let voice = js!(return new AudioContext()).into_reference().unwrap();
-        // the user creates and destroys lots of voices?
 
-        let num = unsafe {
+        let mut voices = self.voices.lock().unwrap();
-            emscripten_run_script_int(concat!(r#"(function() {
+        let voice_id = if let Some(pos) = voices.iter().position(|v| v.is_none()) {
-                if (!window._cpal_audio_contexts)
+            voices[pos] = Some(voice);
-                    window._cpal_audio_contexts = new Array();
+            pos
-                window._cpal_audio_contexts.push(new AudioContext());
+        } else {
-                return window._cpal_audio_contexts.length - 1;
+            let l = voices.len();
-                })()"#, "\0").as_ptr() as *const _)
+            voices.push(Some(voice));
+            l
         };
 
-        Ok(VoiceId(num))
+        Ok(VoiceId(voice_id))
     }
 
     #[inline]
     pub fn destroy_voice(&self, voice_id: VoiceId) {
-        unsafe {
+        self.voices.lock().unwrap()[voice_id.0] = None;
-            let script = format!("
-                if (window._cpal_audio_contexts)
-                    window._cpal_audio_contexts[{}] = null;\0", voice_id.0);
-            emscripten_run_script(script.as_ptr() as *const _)
-        }
     }
 
     #[inline]
     pub fn play(&self, voice_id: VoiceId) {
-        unsafe {
+        let voices = self.voices.lock().unwrap();
-            let script = format!("
+        let voice = voices.get(voice_id.0).and_then(|v| v.as_ref()).expect("invalid voice ID");
-                if (window._cpal_audio_contexts)
+        js!(@{voice}.resume());
-                    if (window._cpal_audio_contexts[{v}])
-                        window._cpal_audio_contexts[{v}].resume();\0", v = voice_id.0);
-            emscripten_run_script(script.as_ptr() as *const _)
-        }
     }
 
     #[inline]
     pub fn pause(&self, voice_id: VoiceId) {
-        unsafe {
+        let voices = self.voices.lock().unwrap();
-            let script = format!("
+        let voice = voices.get(voice_id.0).and_then(|v| v.as_ref()).expect("invalid voice ID");
-                if (window._cpal_audio_contexts)
+        js!(@{voice}.suspend());
-                    if (window._cpal_audio_contexts[{v}])
-                        window._cpal_audio_contexts[{v}].suspend();\0", v = voice_id.0);
-            emscripten_run_script(script.as_ptr() as *const _)
-        }
     }
 }
 
-// Index within the `_cpal_audio_contexts` global variable in Javascript.
+// Index within the `voices` array of the events loop.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct VoiceId(c_int);
+pub struct VoiceId(usize);
 
 // Detects whether the `AudioContext` global variable is available.
 fn is_webaudio_available() -> bool {
-    unsafe {
+    stdweb::initialize();
-        emscripten_run_script_int(concat!(r#"(function() {
+
-                if (!AudioContext) { return 0; } else { return 1; }
+    js!(
-            })()"#, "\0").as_ptr() as *const _) != 0
+        if (!AudioContext) { return false; } else { return true; }
-    }
+    ).try_into().unwrap()
 }
 
 // Content is false if the iterator is empty.
@@ -201,8 +194,7 @@ pub type SupportedFormatsIterator = ::std::vec::IntoIter<SupportedFormat>;
 
 pub struct Buffer<'a, T: 'a> where T: Sample {
     temporary_buffer: Vec<T>,
-    voice_id: c_int,
+    voice: &'a Reference,
-    marker: PhantomData<&'a mut T>,
 }
 
 impl<'a, T> Buffer<'a, T> where T: Sample {
@@ -218,37 +210,36 @@ impl<'a, T> Buffer<'a, T> where T: Sample {
 
     #[inline]
     pub fn finish(self) {
-        unsafe {
+        // TODO: directly use a TypedArray<f32> once this is supported by stdweb
-            // TODO: **very** slow
-            let src_data = self.temporary_buffer.iter().map(|&b| b.to_f32().to_string() + ", ").fold(String::new(), |mut a, b| { a.push_str(&b); a });
 
-            debug_assert_eq!(self.temporary_buffer.len() % 2, 0);       // TODO: num channels
+        let typed_array = {
+            let t_slice: &[T] = self.temporary_buffer.as_slice();
+            let u8_slice: &[u8] = unsafe { from_raw_parts(t_slice.as_ptr() as *const _, t_slice.len() * mem::size_of::<T>()) };
+            let typed_array: TypedArray<u8> = u8_slice.into();
+            typed_array
+        };
 
-            let script = format!("(function() {{
+        let num_channels = 2u32;       // TODO: correct value
-                    if (!window._cpal_audio_contexts)
+        debug_assert_eq!(self.temporary_buffer.len() % num_channels as usize, 0);
-                        return;
+
-                    var context = window._cpal_audio_contexts[{voice_id}];
+        js!(
-                    if (!context)
+            var src_buffer = new Float32Array(@{typed_array}.buffer);
-                        return;
+            var context = @{self.voice};
-                    var buffer = context.createBuffer({num_channels}, {buf_len} / {num_channels}, 44100);
+            var buf_len = @{self.temporary_buffer.len() as u32};
-                    var src = [{src_data}];
+            var num_channels = @{num_channels};
-                    for (var channel = 0; channel < {num_channels}; ++channel) {{
+
+            var buffer = context.createBuffer(num_channels, buf_len / num_channels, 44100);
+            for (var channel = 0; channel < num_channels; ++channel) {
                 var buffer_content = buffer.getChannelData(channel);
-                        for (var i = 0; i < {buf_len} / {num_channels}; ++i) {{
+                for (var i = 0; i < buf_len / num_channels; ++i) {
-                            buffer_content[i] = src[i * {num_channels} + channel];
+                    buffer_content[i] = src_buffer[i * num_channels + channel];
-                        }}
+                }
-                    }}
+            }
+
             var node = context.createBufferSource();
             node.buffer = buffer;
             node.connect(context.destination);
             node.start();
-                }})()\0",
+        );
-                    num_channels = 2,
-                    voice_id = self.voice_id,
-                    buf_len = self.temporary_buffer.len(),
-                    src_data = src_data);
-
-            emscripten_run_script(script.as_ptr() as *const _)
-        }
     }
 }

src/lib.rs

@@ -38,9 +38,16 @@ from time to time.
 
 */
 
+#![recursion_limit = "512"]
+
 #[macro_use]
 extern crate lazy_static;
 
+// Extern crate declarations with `#[macro_use]` must unfortunately be at crate root.
+#[cfg(target_os = "emscripten")]
+#[macro_use]
+extern crate stdweb;
+
 pub use samples_formats::{Sample, SampleFormat};
 
 #[cfg(all(not(windows), not(target_os = "linux"), not(target_os = "freebsd"),