refactor: repartition server-side and client-side code

app/.server/llm/switchable-stream.ts

@@ -0,0 +1,66 @@
+export default class SwitchableStream extends TransformStream {
+  private _controller: TransformStreamDefaultController | null = null;
+  private _currentReader: ReadableStreamDefaultReader | null = null;
+  private _switches = 0;
+
+  constructor() {
+    let controllerRef: TransformStreamDefaultController | undefined;
+
+    super({
+      start(controller) {
+        controllerRef = controller;
+      },
+    });
+
+    if (controllerRef === undefined) {
+      throw new Error('Controller not properly initialized');
+    }
+
+    this._controller = controllerRef;
+  }
+
+  async switchSource(newStream: ReadableStream) {
+    if (this._currentReader) {
+      await this._currentReader.cancel();
+    }
+
+    this._currentReader = newStream.getReader();
+
+    this._pumpStream();
+
+    this._switches++;
+  }
+
+  private async _pumpStream() {
+    if (!this._currentReader || !this._controller) {
+      throw new Error('Stream is not properly initialized');
+    }
+
+    try {
+      while (true) {
+        const { done, value } = await this._currentReader.read();
+
+        if (done) {
+          break;
+        }
+
+        this._controller.enqueue(value);
+      }
+    } catch (error) {
+      console.log(error);
+      this._controller.error(error);
+    }
+  }
+
+  close() {
+    if (this._currentReader) {
+      this._currentReader.cancel();
+    }
+
+    this._controller?.terminate();
+  }
+
+  get switches() {
+    return this._switches;
+  }
+}