Add autocomplete history store

assets/js/autocomplete/history/store.ts

@@ -0,0 +1,95 @@
+import store from '../../utils/store';
+/**
+ * The root JSON object that contains the history records and is persisted to disk.
+ */
+interface History {
+  /**
+   * Used to track the version of the schema layout just in case we do any
+   * breaking changes to this schema so that we can properly migrate old
+   * search history data. It's also used to prevent older versions of
+   * the frontend code from trying to use the newer incompatible schema they
+   * know nothing about (extremely improbable, but just in case).
+   */
+  schemaVersion: 1;
+
+  /**
+   * The list of history records sorted from the last recently used to the oldest unused.
+   */
+  records: string[];
+}
+
+/**
+ * History store backend is responsible for parsing and serializing the data
+ * to/from `localStorage`. It handles versioning of the schema, and transparently
+ * disables writing to the storage if the schema version is unknown to prevent
+ * data loss (extremely improbable, but just in case).
+ */
+export class HistoryStore {
+  private writable: boolean = true;
+  private readonly key: string;
+
+  constructor(key: string) {
+    this.key = key;
+  }
+
+  read(): string[] {
+    return this.extractRecords(store.get<History>(this.key));
+  }
+
+  write(records: string[]): void {
+    if (!this.writable) {
+      return;
+    }
+
+    const history: History = {
+      schemaVersion: 1,
+      records,
+    };
+
+    const start = performance.now();
+    store.set(this.key, history);
+
+    const end = performance.now();
+    console.debug(
+      `Writing ${records.length} history records to the localStorage took ${end - start}ms. ` +
+        `Records: ${records.length}`,
+    );
+  }
+
+  /**
+   * Extracts the records from the history. To do this, we first need to migrate
+   * the history object to the latest schema version if necessary.
+   */
+  private extractRecords(history: History | null): string[] {
+    // `null` here means we are starting from the initial state (empty list of records).
+    if (history === null) {
+      return [];
+    }
+
+    // We have only one version at the time of this writing, so we don't need
+    // to do any migration yet. Hopefully we never need to do a breaking change
+    // and this stays at version `1` forever.
+    const latestSchemaVersion = 1;
+
+    switch (history.schemaVersion) {
+      case latestSchemaVersion:
+        return history.records;
+      default:
+        // It's very unlikely that we ever hit this branch.
+        console.warn(
+          `Unknown search history schema version: '${history.schemaVersion}'. ` +
+            `This frontend code was built with the maximum supported schema version ` +
+            `'${latestSchemaVersion}'. The search history will be disabled for this ` +
+            `session to prevent potential history data loss. The cause of the version ` +
+            `mismatch may be that a newer version of the frontend code is running in a ` +
+            `separate tab, or you were mistakenly served with an older version of the ` +
+            `frontend code.`,
+        );
+
+        // Disallow writing to the storage to prevent data loss.
+        this.writable = false;
+
+        return [];
+    }
+  }
+}