backend-common: add UrlReaderHostMux

packages/backend-common/src/reading/UrlReaderHostMux.ts

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2020 Spotify AB
+ *
+ * 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.
+ */
+
+import { UrlReader } from './types';
+
+/**
+ * A UrlReader implementation that selects from a set of UrlReaders
+ * based on the host in the URL.
+ */
+export class UrlReaderHostMux implements UrlReader {
+  private readonly readers = new Map<string, UrlReader>();
+
+  register(host: string, reader: UrlReader): void {
+    this.readers.set(host, reader);
+  }
+
+  read(url: string): Promise<Buffer> {
+    const parsed = new URL(url);
+
+    const reader = this.readers.get(parsed.host);
+    if (!reader) {
+      throw new Error(`No reader registered for host ${parsed.host}`);
+    }
+
+    return reader.read(url);
+  }
+}