[BE] Code challenge solution - GIANG TRAN

.gitignore

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+# Node Dependencies
+node_modules/
+npm-debug.log
+
+# Compiled TypeScript Output
+dist/
+build/
+
+# Local SQLite Database (from Problem 5)
+app.db
+*.db
+
+# Environment Variables
+.env
+
+# OS/Editor Generated Files
+.DS_Store
+.vscode/

src/problem4/README.md

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+# Problem 4: Three ways to sum to n
+
+# Overview
+This repository contains three unique implementations in TypeScript to calculate the summation of integers from 1 to `n` (e.g., `sum_to_n(5) === 1 + 2 + 3 + 4 + 5 === 15`). 
+
+Each implementation explores a different algorithmic approach, varying in time and space complexity.
+
+# Implementation A: Iterative Approach
+This approach uses a standard `for` loop to incrementally add each number from 1 to `n` to a running total.
+
+# Implementation B: Mathematical Formula (Optimal)
+This approach utilizes the arithmetic progression sum formula: `n * (n + 1) / 2`.
+
+# Implementation C: Recursive Approach
+This approach solves the problem by breaking it down into smaller sub-problems, calling itself with `n - 1` until it reaches the base case (`n <= 1`).
+
+## How to Run
+
+Ensure you have [Node.js](https://nodejs.org/) and TypeScript installed.
+
+1. Install dependencies (if testing libraries are configured):
+   npm install
+
+2. Compile the TypeScript file:
+  npx tsc src/sum.ts
+
+3. Run tests (assuming Jest is set up from your earlier configurations):
+  npm test

src/problem4/jest.config.js

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+const { createDefaultPreset } = require("ts-jest");
+
+const tsJestTransformCfg = createDefaultPreset().transform;
+
+/** @type {import("jest").Config} **/
+module.exports = {
+  testEnvironment: "node",
+  transform: {
+    ...tsJestTransformCfg,
+  },
+};

src/problem4/package.json

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+{
+  "name": "code-challenge",
+  "version": "1.0.0",
+  "scripts": {
+    "build": "tsc",
+    "test": "jest"
+  },
+  "devDependencies": {
+    "@types/jest": "^29.5.14",
+    "jest": "^29.7.0",
+    "ts-jest": "^29.4.9",
+    "typescript": "^5.0.0"
+  }
+}

src/problem4/src/index.ts

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+import { sum_to_n_a, sum_to_n_b, sum_to_n_c } from "./sum";
+
+const n = 5;
+
+console.log("Iterative:", sum_to_n_a(n));
+console.log("Formula:", sum_to_n_b(n));
+console.log("Recursive:", sum_to_n_c(n));

src/problem4/src/sum.ts

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+// Implementation A: Iterative
+export function sum_to_n_a(n: number): number {
+  if (n < 0) throw new Error("n must be non-negative");
+
+  let sum = 0;
+  for (let i = 1; i <= n; i++) {
+    sum += i;
+  }
+  return sum;
+}
+
+// Implementation B: Formula (Optimal)
+export function sum_to_n_b(n: number): number {
+  if (n < 0) throw new Error("n must be non-negative");
+
+  return (n * (n + 1)) / 2;
+}
+
+// Implementation C: Recursion
+export function sum_to_n_c(n: number): number {
+  if (n < 0) throw new Error("n must be non-negative");
+  if (n <= 1) return n;
+
+  return n + sum_to_n_c(n - 1);
+}

src/problem4/tests/sum.test.ts

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+import { sum_to_n_a, sum_to_n_b, sum_to_n_c } from "../src/sum";
+
+describe("Sum to n", () => {
+  const testCases = [
+    { input: 0, expected: 0 },
+    { input: 1, expected: 1 },
+    { input: 5, expected: 15 },
+    { input: 10, expected: 55 },
+  ];
+
+  testCases.forEach(({ input, expected }) => {
+    test(`n = ${input}`, () => {
+      expect(sum_to_n_a(input)).toBe(expected);
+      expect(sum_to_n_b(input)).toBe(expected);
+      expect(sum_to_n_c(input)).toBe(expected);
+    });
+  });
+});

src/problem4/tsconfig.json

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+{
+  "compilerOptions": {
+    "target": "ES6",
+    "module": "commonjs",
+    "strict": true,
+    "outDir": "dist",
+    "esModuleInterop": true,
+    "types": ["jest"]
+  },
+  "include": ["src", "tests"]
+}

src/problem5/README.md

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+# Problem 5: A Crude Server
+
+This is a backend RESTful API built with ExpressJS and TypeScript. It implements a set of CRUD interfaces for a generic "Resource" entity and uses a local SQLite database for simple, reliable data persistence.
+
+## Prerequisites
+
+- [Node.js](https://nodejs.org/) (v14 or higher recommended)
+- npm or yarn
+
+## Setup and Configuration
+
+1. **Install dependencies:**
+   Navigate to the project root and run:
+   ```bash
+  npm init -y
+  npm install express better-sqlite3
+  npm install --save-dev typescript @types/express @types/node @types/better-sqlite3 ts-node nodemon
+  ```
+
+2. **Database:**
+   This application uses `better-sqlite3`. Upon starting the server for the first time, an `app.db` SQLite file will be automatically generated in the root directory. The database tables will be created automatically if they do not exist.
+
+## How to Run the Application
+
+**Development Mode (with auto-reload):**
+```bash
+npm run dev
+```
+
+**Production Mode:**
+```bash
+npm run build
+npm start
+```
+
+The server will start on **http://localhost:3000**.
+
+## Testing with Postman
+
+You can use API testing tools like [Postman](https://www.postman.com/) or the VS Code REST Client to interact with the server. Ensure the server is running locally on port 3000 before sending requests.
+
+### 1. Create a Resource
+- **Method:** `POST`
+- **URL:** `http://localhost:3000/resources`
+- **Headers:** `Content-Type: application/json`
+- **Body (raw JSON):**
+  ```json
+  {
+    "name": "Sample Resource",
+    "description": "This is a test",
+    "status": "active"
+  }
+  ```
+
+### 2. List Resources
+- **Method:** `GET`
+- **URL:** `http://localhost:3000/resources`
+- **Query Filters (Optional):** You can filter by status using `?status=active` or `?status=inactive` (e.g., `http://localhost:3000/resources?status=active`).
+
+### 3. Get Details of a Resource
+- **Method:** `GET`
+- **URL:** `http://localhost:3000/resources/1` *(Replace '1' with the ID of an existing resource)*
+
+### 4. Update Resource Details
+- **Method:** `PUT`
+- **URL:** `http://localhost:3000/resources/1`
+- **Headers:** `Content-Type: application/json`
+- **Body (raw JSON):**
+  ```json
+  {
+    "name": "Updated Name",
+    "status": "inactive"
+  }
+  ```
+
+### 5. Delete a Resource
+- **Method:** `DELETE`
+- **URL:** `http://localhost:3000/resources/1`

src/problem5/package.json

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+{
+  "name": "problem-5",
+  "version": "1.0.0",
+  "main": "index.js",
+  "type": "module",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1",
+    "start": "node --loader ts-node/esm src/index.ts",
+    "dev": "nodemon --exec node --loader ts-node/esm src/index.ts",
+    "build": "tsc"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "description": "This is a backend RESTful API built with ExpressJS and TypeScript. It implements a set of CRUD interfaces for a generic \"Resource\" entity and uses a local SQLite database for simple, reliable data persistence.",
+  "dependencies": {
+    "better-sqlite3": "^11.10.0",
+    "express": "^5.2.1"
+  },
+  "devDependencies": {
+    "@types/better-sqlite3": "^7.6.13",
+    "@types/express": "^5.0.6",
+    "@types/node": "^25.6.0",
+    "nodemon": "^3.1.14",
+    "ts-node": "^10.9.2",
+    "typescript": "^6.0.3"
+  }
+}

src/problem5/src/index.ts

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+// src/index.ts
+import express, { type Request, type Response } from 'express';
+import Database from 'better-sqlite3';
+
+const app = express();
+const port = process.env.PORT || 3000;
+
+// Middleware
+app.use(express.json());
+
+// Initialize SQLite Database (creates 'app.db' file in the root)
+const db = new Database('app.db', { verbose: console.log });
+
+// Create the table if it doesn't exist
+db.exec(`
+  CREATE TABLE IF NOT EXISTS resources (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    name TEXT NOT NULL,
+    description TEXT,
+    status TEXT DEFAULT 'active'
+  )
+`);
+
+// Prepared Statements for performance and security
+const insertResource = db.prepare('INSERT INTO resources (name, description, status) VALUES (?, ?, ?)');
+const selectAllResources = db.prepare('SELECT * FROM resources');
+const selectResourcesByStatus = db.prepare('SELECT * FROM resources WHERE status = ?');
+const selectResourceById = db.prepare('SELECT * FROM resources WHERE id = ?');
+const updateResource = db.prepare('UPDATE resources SET name = ?, description = ?, status = ? WHERE id = ?');
+const deleteResource = db.prepare('DELETE FROM resources WHERE id = ?');
+
+// 1. Create a resource
+app.post('/resources', (req: Request, res: Response) => {
+  const { name, description, status } = req.body;
+  if (!name) {
+    return res.status(400).json({ error: 'Name is required' });
+  }
+
+  try {
+    const info = insertResource.run(name, description || null, status || 'active');
+    res.status(201).json({ id: info.lastInsertRowid, name, description, status });
+  } catch (error) {
+    res.status(500).json({ error: 'Failed to create resource' });
+  }
+});
+
+// 2. List resources with basic filters (e.g., ?status=active)
+app.get('/resources', (req: Request, res: Response) => {
+  const { status } = req.query;
+
+  try {
+    let resources;
+    if (status && typeof status === 'string') {
+      resources = selectResourcesByStatus.all(status);
+    } else {
+      resources = selectAllResources.all();
+    }
+    res.status(200).json(resources);
+  } catch (error) {
+    res.status(500).json({ error: 'Failed to fetch resources' });
+  }
+});
+
+// 3. Get details of a resource
+app.get('/resources/:id', (req: Request, res: Response) => {
+  const { id } = req.params;
+
+  try {
+    const resource = selectResourceById.get(id);
+    if (!resource) {
+      return res.status(404).json({ error: 'Resource not found' });
+    }
+    res.status(200).json(resource);
+  } catch (error) {
+    res.status(500).json({ error: 'Failed to fetch resource' });
+  }
+});
+
+// 4. Update resource details
+app.put('/resources/:id', (req: Request, res: Response) => {
+  const { id } = req.params;
+  const { name, description, status } = req.body;
+
+  try {
+    const existing = selectResourceById.get(id) as any;
+    if (!existing) {
+      return res.status(404).json({ error: 'Resource not found' });
+    }
+
+    const updatedName = name !== undefined ? name : existing.name;
+    const updatedDesc = description !== undefined ? description : existing.description;
+    const updatedStatus = status !== undefined ? status : existing.status;
+
+    updateResource.run(updatedName, updatedDesc, updatedStatus, id);
+    res.status(200).json({ id: Number(id), name: updatedName, description: updatedDesc, status: updatedStatus });
+  } catch (error) {
+    res.status(500).json({ error: 'Failed to update resource' });
+  }
+});
+
+// 5. Delete a resource
+app.delete('/resources/:id', (req: Request, res: Response) => {
+  const { id } = req.params;
+
+  try {
+    const info = deleteResource.run(id);
+    if (info.changes === 0) {
+      return res.status(404).json({ error: 'Resource not found' });
+    }
+    res.status(204).send(); // 204 No Content
+  } catch (error) {
+    res.status(500).json({ error: 'Failed to delete resource' });
+  }
+});
+
+// Start Server
+app.listen(port, () => {
+  console.log(`Server is running on http://localhost:${port}`);
+});