Fix VAD interface to support audio chunks of any length (#218)

The webrtcvad library expects frames of fixed size, but Genie will
pass the buffers as they come from nodejs streaming code without
any slicing. Do the slicing and concatenation of frames here so
we only call the library with frames of fixed size.

src/service/platform/index.ts

@@ -245,10 +245,12 @@ export class SoundEffectsApi implements Tp.Capabilities.SoundEffectsApi {
 
 class VAD implements Tp.Capabilities.VadApi {
     private _instance : webrtcvad_|null;
+    private _previousChunk : Buffer|null;
     frameSize : number;
 
     constructor() {
         this._instance = null;
+        this._previousChunk = null;
         this.frameSize = 0;
     }
 
@@ -270,11 +272,22 @@ class VAD implements Tp.Capabilities.VadApi {
     process(chunk : Buffer) {
         if (!this._instance)
             return false;
-        const n = chunk.length % this.frameSize;
-        let r = false;
-        for (let i = 0; i < n; i++)
-            r ||= this._instance.process(chunk.slice(i * this.frameSize, this.frameSize));
-        return r;
+
+        if (this._previousChunk)
+            chunk = Buffer.concat([this._previousChunk, chunk], this._previousChunk.length + chunk.length);
+
+        let anySound = false;
+        let offset : number;
+        for (offset = 0; offset < chunk.length && offset + this.frameSize <= chunk.length; offset += this.frameSize) {
+            const sliced = chunk.slice(offset, offset + this.frameSize);
+            const sound = this._instance.process(sliced);
+            anySound = anySound || sound;
+        }
+        if (offset < chunk.length)
+            this._previousChunk = chunk.slice(offset);
+        else
+            this._previousChunk = null;
+        return anySound;
     }
 }