example: port AudioWorklet / wasm example from GoogleChromeLabs, cf. h…

…ttps://github.com/GoogleChromeLabs/web-audio-samples/tree/main/src/audio-worklet/design-pattern/wasm/

examples/audio-worklet-webassembly.mjs

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+// https://github.com/GoogleChromeLabs/web-audio-samples/tree/main/src/audio-worklet/design-pattern/wasm
+//
+// Copyright (c) 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import { AudioContext, OscillatorNode, AudioWorkletNode } from '../index.mjs';
+
+const audioContext = new AudioContext();
+
+await audioContext.audioWorklet.addModule('./worklets/wasm-worklet-processor.mjs');
+const oscillator = new OscillatorNode(audioContext);
+const bypasser = new AudioWorkletNode(audioContext, 'wasm-worklet-processor');
+oscillator.connect(bypasser).connect(audioContext.destination);
+oscillator.start();
+

examples/worklets/Makefile

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+DEPS = SimpleKernel.cc
+
+build: $(DEPS)
+	@emcc --bind -O1 \
+	  -s WASM=1 \
+		-s BINARYEN_ASYNC_COMPILATION=0 \
+		-s SINGLE_FILE=1 \
+		-s ENVIRONMENT=node \
+		-s EXPORT_ES6=1 \
+        -s EXPORTED_FUNCTIONS="['_malloc']" \
+		SimpleKernel.cc \
+		-o simple-kernel.wasmmodule.mjs
+
+clean:
+	@rm -f simple-kernel.wasmmodule.js

examples/worklets/SimpleKernel.cc

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+// adapted from https://github.com/GoogleChromeLabs/web-audio-samples/tree/main/src/audio-worklet/design-pattern/wasm
+
+/**
+ * Copyright 2018 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+#include "emscripten/bind.h"
+
+using namespace emscripten;
+
+const unsigned kRenderQuantumFrames = 128;
+const unsigned kBytesPerChannel = kRenderQuantumFrames * sizeof(float);
+
+// The "kernel" is an object that processes a audio stream, which contains
+// one or more channels. It is supposed to obtain the frame data from an
+// |input|, process and fill an |output| of the AudioWorkletProcessor.
+//
+//       AudioWorkletProcessor Input(multi-channel, 128-frames)
+//                                 |
+//                                 V
+//                               Kernel
+//                                 |
+//                                 V
+//       AudioWorkletProcessor Output(multi-channel, 128-frames)
+//
+// In this implementation, the kernel operates based on 128-frames, which is
+// the render quantum size of Web Audio API.
+class SimpleKernel {
+ public:
+  SimpleKernel() {}
+
+  void Process(uintptr_t input_ptr, uintptr_t output_ptr,
+               unsigned channel_count) {
+    float* input_buffer = reinterpret_cast<float*>(input_ptr);
+    float* output_buffer = reinterpret_cast<float*>(output_ptr);
+
+    // Bypasses the data. By design, the channel count will always be the same
+    // for |input_buffer| and |output_buffer|.
+    for (unsigned channel = 0; channel < channel_count; ++channel) {
+      float* destination = output_buffer + channel * kRenderQuantumFrames;
+      float* source = input_buffer + channel * kRenderQuantumFrames;
+      memcpy(destination, source, kBytesPerChannel);
+    }
+  }
+};
+
+EMSCRIPTEN_BINDINGS(CLASS_SimpleKernel) {
+  class_<SimpleKernel>("SimpleKernel")
+      .constructor()
+      .function("process",
+                &SimpleKernel::Process,
+                allow_raw_pointers());
+}