Add support for database

src/database/deserializer.ts

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+import fs from "fs";
+import { NotificationDatabase } from "../types/notification";
+
+/**
+ * Loads and deserializes database from JSON formatted file.
+ */
+export function loadDatabase(file: string): NotificationDatabase {
+  const text = fs.readFileSync(file).toString();
+  return deserializeDatabase(text);
+}
+
+/**
+ * Deserializes database from JSON format.
+ */
+export function deserializeDatabase(json: string): NotificationDatabase {
+  return JSON.parse(json) as NotificationDatabase;
+}

src/database/index.ts

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+export { serializeDatabase } from "./serializer";
+export { deserializeDatabase } from "./deserializer";

src/database/serializer.ts

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+import fs from "fs";
+import { Task } from "../types/task";
+import { Notification, NotificationDatabase } from "../types/notification";
+
+/**
+ * Applies the mapper for each task from list, groups them by time and dumps the data into JSON formatted file.
+ */
+export function dumpDatabase(file: string, tasks: Task[], mapper: (task: Task) => Notification[]) {
+  const data = serializeDatabase(tasks, mapper);
+  fs.writeFileSync(file, data);
+}
+
+/**
+ * Applies the mapper for each task from list, groups them by time and serializes into JSON format.
+ */
+export function serializeDatabase(tasks: Task[], mapper: (task: Task) => Notification[]): string {
+  const output = tasks.flatMap(wrapWithTimeFiller(mapper)).reduce((acc, n) => {
+    if (n.time) {
+      (acc[n.time] = (acc[n.time] || [])).push(n);
+    };
+    
+    return acc;
+  }, {} as NotificationDatabase);
+
+  return JSON.stringify(output);
+}
+
+function wrapWithTimeFiller(mapper: (task: Task) => Notification[]): (task: Task) => Notification[] {
+  return (task: Task) => mapper(task)
+    .map(notification => ({
+      ...notification,
+      time: task.reminder ?? notification.time,
+    }));
+}

src/types/notification.ts

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+import dayjs, { Dayjs } from "dayjs"
+
+export type Notification = {
+  time?: string;
+  title: string;
+  text: string;
+};
+
+export type NotificationDatabase = Record<string, Notification[]>;