add missing src/etag-cache-leveldb.mjs from template

src/etag-cache-leveldb.mjs

@@ -0,0 +1,119 @@
+import { streamToUint8Array } from "browser-stream-util";
+
+/**
+ * Stores etags and bodies into leveldb.
+ * Reconstructs response with body if etag or url matches.
+ * Will store in the cache:
+ * url : etag
+ * etag : body
+ */
+export class ETagCacheLevelDB {
+  #db;
+  #numberOfStoredRequests = 0;
+  #numberOfStoredBytes = 0;
+  #numberOfLoadedRequests = 0;
+  #numberOfLoadedBytes = 0;
+
+  constructor(db) {
+    this.#db = db;
+  }
+
+  /**
+   * Adds the "If-None-Match" header if etag is found for the url.
+   * @param {string|URL} url
+   * @param {Object} headers
+   * @returns {Promise<boolean>} true if etag was found in cache and header has been added
+   */
+  async addHeaders(url, headers) {
+    try {
+      const entry = await this.#db.get(url);
+      headers["If-None-Match"] = entry.toString();
+      return true;
+    } catch {}
+
+    return false;
+  }
+
+  /**
+   * Deliver statisics data.
+   * @return {Object}
+   */
+  get statistics() {
+    return {
+      numberOfStoredRequests: this.#numberOfStoredRequests,
+      numberOfStoredBytes: this.#numberOfStoredBytes,
+      numberOfLoadedRequests: this.#numberOfLoadedRequests,
+      numberOfLoadedBytes: this.#numberOfLoadedBytes
+    };
+  }
+
+  /**
+   * Stores response in the cache.
+   * Two entries are stored:
+   * - url : etag
+   * - etag : body
+   * @param {Response} response as produced by fetch
+   * @returns {Promise<any>}
+   */
+  async storeResponse(response) {
+    if (response.ok) {
+      try {
+        response = response.clone();
+
+        const etag = rawTagData(response.headers.get("etag"));
+
+        if (etag) {
+          const promiseA = this.#db.put(response.url, etag);
+          const body = await streamToUint8Array(response.body);
+          this.#numberOfStoredRequests++;
+          this.#numberOfStoredBytes += body.length;
+          return Promise.all([promiseA, this.#db.put(etag, Buffer.from(body))]);
+        }
+      } catch (e) {
+        console.error(e);
+      }
+    }
+  }
+
+  /**
+   * Constructs a new Response feed from the cache if a matching etag is found in the cache.
+   * @param {Response} response as provided by fetch
+   * @returns {Promise<Response>}
+   */
+  async loadResponse(response) {
+    try {
+      let etag = rawTagData(response.headers.get("etag"));
+
+      if (!etag) {
+        etag = await this.#db.get(response.url);
+      }
+
+      const entry = await this.#db.get(etag);
+
+      if (entry) {
+        this.#numberOfLoadedRequests++;
+        this.#numberOfLoadedBytes += entry.length;
+        return new Response( entry, {
+          status: 200,
+          statusText: "OK from cache"
+        });
+      }
+    } catch (e) {
+      console.error(e);
+      return new Response(undefined, {
+        status: 404,
+        statusText: e.message
+      });
+    }
+    return response;
+  }
+}
+
+/**
+ * Strips away etag flags (weak and the like)
+ * @param {string|null} etag
+ * @returns {string|undefined} raw etag
+ */
+export function rawTagData(etag) {
+  return etag?.replace(/W\//, "");
+}