Polyfill crypto to shim std::random_device in wasm builds

wasm/CMakeLists.txt

@@ -20,16 +20,18 @@ target_compile_features(${TargetName} PRIVATE
 target_link_libraries(${TargetName} PRIVATE
   elem::runtime)
 
+set(PRE "${CMAKE_CURRENT_SOURCE_DIR}/pre.js")
+
 # We use these flags for compiling our wasm bundle such that the exported Module
 # factory function asynchronously initializes the module. It's important for the browser
 # and node.js contexts, whereas we want synchronous initialization for the webaudio worklet
-set(EM_FLAGS_ASYNC "-lembind --closure 1 -s WASM=1 -s WASM_ASYNC_COMPILATION=1 -s MODULARIZE=1 -s SINGLE_FILE=1 -s ALLOW_MEMORY_GROWTH=1")
+set(EM_FLAGS_ASYNC "--pre-js ${PRE} -lembind --closure 1 -s WASM=1 -s WASM_ASYNC_COMPILATION=1 -s MODULARIZE=1 -s ENVIRONMENT=shell -s SINGLE_FILE=1 -s ALLOW_MEMORY_GROWTH=1")
 
 # We use these flags for compiling our wasm bundle such that the exported Module
 # factory function _synchronously_ initializes the module. We use this specifically in the
 # initialization of the web audio worklet where we don't necessarily have room to wait for
 # an asynchronous init
-set(EM_FLAGS_SYNC "-lembind --closure 1 -s WASM=1 -s WASM_ASYNC_COMPILATION=0 -s MODULARIZE=1 -s SINGLE_FILE=1 -s ALLOW_MEMORY_GROWTH=1")
+set(EM_FLAGS_SYNC "--pre-js ${PRE} -lembind --closure 1 -s WASM=1 -s WASM_ASYNC_COMPILATION=0 -s MODULARIZE=1 -s ENVIRONMENT=shell -s SINGLE_FILE=1 -s ALLOW_MEMORY_GROWTH=1")
 
 if ($ENV{ELEM_BUILD_ASYNC})
   set_target_properties(${TargetName}

wasm/pre.js

@@ -0,0 +1,16 @@
+// We use this to shim a crypto library in the WebAudioWorklet environment, which
+// is necessary for Emscripten's implementation of std::random_device, which we need
+// after pulling in signalsmith-stretch.
+//
+// I've seen some issues on esmcripten that seem to suggest an insecure polyfill like
+// this should already be provided, but I'm getting runtime assertion errors that it's
+// not available. An alternative solution would be to upstream a change to the stretch
+// library to rely on an injected random device in C++ territory, so that for our case
+// we could inject some simple PRNG.
+globalThis.crypto = {
+  getRandomValues: (array) => {
+    for (var i = 0; i < array.length; i++) {
+      array[i] = (Math.random() * 256) | 0;
+    }
+  }
+};