[BE]: Code challenge solution - Nguyen Trung Dung

.gitignore

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+node_modules/
+dist/
+.env
+coverage/
+package-lock.json

src/problem4/README.md

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+# Problem 4: Three Ways to Sum to n
+
+This folder provides three unique TypeScript implementations of `sum_to_n`.
+
+## Implementations
+
+- `sum_to_n_a`: iterative loop, simple and explicit, `O(|n|)` time and `O(1)` space.
+- `sum_to_n_b`: arithmetic-series formula, fastest option at `O(1)` time and `O(1)` space.
+- `sum_to_n_c`: recursive implementation, valid but less efficient because it uses `O(|n|)` stack space.
+
+For negative numbers, the functions sum from `0` down to `n` so the behavior stays deterministic for any integer input.
+
+## Test
+
+```bash
+cd src/problem4
+npm install
+npm test
+```

src/problem4/package.json

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+{
+  "name": "problem4",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "test": "node --import tsx --test ./test/sum-to-n.test.ts",
+    "build": "tsc -p tsconfig.json"
+  },
+  "devDependencies": {
+    "@types/node": "^24.6.0",
+    "tsx": "^4.20.6",
+    "typescript": "^5.9.3"
+  }
+}

src/problem4/src/index.ts

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+export function sum_to_n_a(n: number): number {
+  // Iterative approach: O(|n|) time, O(1) space.
+  let total = 0;
+
+  if (n >= 0) {
+    for (let value = 1; value <= n; value += 1) {
+      total += value;
+    }
+
+    return total;
+  }
+
+  for (let value = 0; value >= n; value -= 1) {
+    total += value;
+  }
+
+  return total;
+}
+
+export function sum_to_n_b(n: number): number {
+  // Closed-form arithmetic series: O(1) time, O(1) space.
+  if (n >= 0) {
+    return (n * (n + 1)) / 2;
+  }
+
+  return (Math.abs(n) * (n - 1)) / 2;
+}
+
+export function sum_to_n_c(n: number): number {
+  // Recursive walk: O(|n|) time, O(|n|) call stack space.
+  const step = n >= 0 ? 1 : -1;
+  const stop = n >= 0 ? n + 1 : n - 1;
+
+  const walk = (current: number, total: number): number => {
+    if (current === stop) {
+      return total;
+    }
+
+    return walk(current + step, total + current);
+  };
+
+  return walk(step > 0 ? 1 : 0, 0);
+}

src/problem4/test/sum-to-n.test.ts

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+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { sum_to_n_a, sum_to_n_b, sum_to_n_c } from "../src/index.js";
+
+const cases = [
+  { input: 0, expected: 0 },
+  { input: 1, expected: 1 },
+  { input: 5, expected: 15 },
+  { input: -3, expected: -6 },
+  { input: 10, expected: 55 }
+] as const;
+
+for (const fn of [sum_to_n_a, sum_to_n_b, sum_to_n_c]) {
+  test(`${fn.name} returns the expected summation`, () => {
+    for (const { input, expected } of cases) {
+      assert.equal(fn(input), expected);
+    }
+  });
+}

src/problem4/tsconfig.json

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+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "strict": true,
+    "declaration": true,
+    "outDir": "dist",
+    "rootDir": ".",
+    "esModuleInterop": true,
+    "skipLibCheck": true
+  },
+  "include": [
+    "src/**/*.ts",
+    "test/**/*.ts"
+  ]
+}

src/problem5/.env.example

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+PORT=3000
+MONGODB_URI=mongodb://127.0.0.1:27017/code_challenge_problem5
+AUTH_TOKEN=replace-with-a-strong-token

src/problem5/README.md

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+# Problem 5: A Crude Server
+
+A RESTful CRUD API server built with Express.js and TypeScript, using MongoDB for persistence and Bearer token authentication for protected endpoints.
+
+## Features
+
+- CRUD operations for resource creation, listing, detail lookup, update, and deletion.
+- Basic filtering with `name` and `status` query parameters.
+- MongoDB persistence through Mongoose.
+- Bearer token authentication controlled by environment configuration.
+- Interactive Swagger/OpenAPI documentation for browser-based testing.
+- TypeScript-based codebase with integration tests for the API flow.
+
+## Tech Stack
+
+- **Runtime**: Node.js
+- **Framework**: Express.js
+- **Language**: TypeScript
+- **Database**: MongoDB with Mongoose
+- **Documentation**: Swagger/OpenAPI 3.0 with Swagger UI
+- **Testing**: Node test runner, Supertest, mongodb-memory-server
+
+## Resource Model
+
+```json
+{
+  "id": "ObjectId",
+  "name": "Test Resource",
+  "value": "xxxxx",
+  "description": "A test resource",
+  "status": 1,
+  "createdAt": "2026-01-25T10:00:00.000Z",
+  "updatedAt": "2026-01-25T10:00:00.000Z",
+  "detail": {}
+}
+```
+
+## Endpoints
+
+- `POST /resources` creates a resource.
+- `GET /resources?name=abc&status=1` lists resources with basic filters.
+- `GET /resources/:id` returns a resource detail.
+- `PUT /resources/:id` updates a resource.
+- `DELETE /resources/:id` deletes a resource.
+
+All endpoints require `Authorization: Bearer <AUTH_TOKEN>`.
+
+## Installation
+
+1. Move into the problem folder:
+
+```bash
+cd src/problem5
+```
+
+2. Install dependencies:
+
+```bash
+npm install
+```
+
+3. Copy the environment file:
+
+```bash
+cp .env.example .env
+```
+
+4. Update the values in `.env`:
+
+```env
+PORT=3000
+MONGODB_URI=mongodb://127.0.0.1:27017/code_challenge_problem5
+AUTH_TOKEN=replace-with-a-strong-token
+```
+
+## Running the Application
+
+### Development Mode
+
+```bash
+npm run dev
+```
+
+### Production Build
+
+```bash
+npm run build
+```
+
+If you want to start the compiled output manually:
+
+```bash
+node dist/src/server.js
+```
+
+The server starts on `http://localhost:3000` by default.
+
+## API Documentation
+
+### Swagger UI
+
+Once the server is running, open:
+
+- [http://localhost:3000/api-docs](http://localhost:3000/api-docs)
+
+Swagger is public for convenience, but the actual CRUD endpoints still require Bearer authentication.
+
+How to test in Swagger UI:
+
+1. Open the Swagger page.
+2. Expand any `/resources` endpoint.
+3. Click `Authorize`.
+4. Enter the token as plain text from `AUTH_TOKEN`.
+5. Run requests directly from the browser.
+
+Useful supporting endpoint:
+
+- [http://localhost:3000/openapi.json](http://localhost:3000/openapi.json) exposes the raw OpenAPI document.
+
+## API Endpoints
+
+- `POST /resources` creates a resource.
+- `GET /resources?name=abc&status=1` lists resources with basic filters.
+- `GET /resources/:id` returns a resource detail.
+- `PUT /resources/:id` updates a resource.
+- `DELETE /resources/:id` deletes a resource.
+
+## Testing with cURL
+
+All examples below assume:
+
+```bash
+set TOKEN=replace-with-a-strong-token
+```
+
+If you are using PowerShell, use:
+
+```powershell
+$TOKEN = "replace-with-a-strong-token"
+```
+
+### 1. Create a resource
+
+```bash
+curl -X POST http://localhost:3000/resources \
+  -H "Authorization: Bearer %TOKEN%" \
+  -H "Content-Type: application/json" \
+  -d "{\"name\":\"Test Resource\",\"value\":\"xxxxx\",\"description\":\"A test resource\",\"status\":1,\"detail\":{\"source\":\"curl\"}}"
+```
+
+PowerShell example:
+
+```powershell
+curl.exe -X POST http://localhost:3000/resources `
+  -H "Authorization: Bearer $TOKEN" `
+  -H "Content-Type: application/json" `
+  -d "{\"name\":\"Test Resource\",\"value\":\"xxxxx\",\"description\":\"A test resource\",\"status\":1,\"detail\":{\"source\":\"powershell\"}}"
+```
+
+### 2. List resources
+
+```bash
+curl "http://localhost:3000/resources?name=Test&status=1" \
+  -H "Authorization: Bearer %TOKEN%"
+```
+
+### 3. Get resource detail
+
+```bash
+curl http://localhost:3000/resources/<resource_id> \
+  -H "Authorization: Bearer %TOKEN%"
+```
+
+### 4. Update a resource
+
+```bash
+curl -X PUT http://localhost:3000/resources/<resource_id> \
+  -H "Authorization: Bearer %TOKEN%" \
+  -H "Content-Type: application/json" \
+  -d "{\"description\":\"Updated description\",\"status\":0}"
+```
+
+### 5. Delete a resource
+
+```bash
+curl -X DELETE http://localhost:3000/resources/<resource_id> \
+  -H "Authorization: Bearer %TOKEN%"
+```
+
+## Test
+
+```bash
+npm test
+```
+
+## Project Structure
+
+```text
+src/problem5/
+├── src/
+│   ├── config/
+│   │   ├── database.ts
+│   │   ├── env.ts
+│   │   └── swagger.ts
+│   ├── controllers/
+│   │   └── resource-controller.ts
+│   ├── middleware/
+│   │   └── auth.ts
+│   ├── models/
+│   │   └── resource.ts
+│   ├── routes/
+│   │   └── resource-routes.ts
+│   ├── types/
+│   │   └── express.d.ts
+│   ├── app.ts
+│   └── server.ts
+├── test/
+│   └── resource-api.test.ts
+├── .env.example
+├── package.json
+├── tsconfig.json
+└── README.md
+```

src/problem5/package.json

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+{
+  "name": "problem5",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "tsx watch src/server.ts",
+    "test": "node --import tsx --test ./test/resource-api.test.ts",
+    "build": "tsc -p tsconfig.json"
+  },
+  "dependencies": {
+    "dotenv": "^17.2.3",
+    "express": "^5.1.0",
+    "mongoose": "^8.19.1",
+    "swagger-jsdoc": "^6.2.8",
+    "swagger-ui-express": "^5.0.1"
+  },
+  "devDependencies": {
+    "@types/express": "^5.0.3",
+    "@types/node": "^24.6.0",
+    "@types/supertest": "^6.0.3",
+    "@types/swagger-jsdoc": "^6.0.4",
+    "@types/swagger-ui-express": "^4.1.8",
+    "mongodb-memory-server": "^10.2.3",
+    "supertest": "^7.1.4",
+    "tsx": "^4.20.6",
+    "typescript": "^5.9.3"
+  }
+}