[BE]: Code challenge solution - Hoang Tien Dat

.gitignore

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+.vscode/
+
+node_modules/
+dist/

package.json

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+{
+  "name": "code-challenge",
+  "version": "1.0.0",
+  "scripts": {
+    "problem4:test": "node --require ts-node/register --test src/problem4/main.test.ts",
+    "problem5:start": "yarn ts-node src/problem5/index.ts"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "devDependencies": {
+    "@types/better-sqlite3": "^7.6.13",
+    "@types/express": "^5.0.6",
+    "@types/node": "^25.6.0",
+    "ts-node": "^10.9.2",
+    "typescript": "^5.0.0"
+  },
+  "dependencies": {
+    "better-sqlite3": "^12.9.0",
+    "express": "^5.2.1",
+    "reflect-metadata": "^0.2.2",
+    "typeorm": "^0.3.28",
+    "zod": "^4.3.6"
+  }
+}

src/problem4/main.test.ts

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+import assert from 'node:assert/strict';
+import { describe, test } from 'node:test';
+import { sum_to_n_a, sum_to_n_b, sum_to_n_c } from './main';
+
+const implementations: [string, (n: number) => number][] = [
+  ['sum_to_n_a (iterative)', sum_to_n_a],
+  ['sum_to_n_b (recursive)', sum_to_n_b],
+  ['sum_to_n_c (formula)', sum_to_n_c],
+];
+
+for (const [name, fn] of implementations) {
+  describe(name, () => {
+    test('n = 1 returns 1', () => assert.equal(fn(1), 1));
+    test('n = 5 returns 15', () => assert.equal(fn(5), 15));
+    test('n = 10 returns 55', () => assert.equal(fn(10), 55));
+    test('n = 100 returns 5050', () => assert.equal(fn(100), 5050));
+    test('n = 0 returns 1 (sum 0..1)', () => assert.equal(fn(0), 1));
+    test('n = 2 returns 3', () => assert.equal(fn(2), 3));
+    test('n = 1000 returns 500500', () => assert.equal(fn(1000), 500500));
+    test('n = -1 returns 0 (sum -1..1)', () => assert.equal(fn(-1), 0));
+    test('n = -2 returns -2 (sum -2..1)', () => assert.equal(fn(-2), -2));
+    test('n = -3 returns -5 (sum -3..1)', () => assert.equal(fn(-3), -5));
+  });
+}

src/problem4/main.ts

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+/**
+ * Three implementations of summing integers from 1 to `n` (inclusive).
+ * Behavior for any integer `n`:
+ * - If n >= 1: sum = 1 + 2 + ... + n
+ * - If n < 1: sum = n + (n+1) + ... + 1
+ * Assumes result fits within Number.MAX_SAFE_INTEGER.
+ */
+
+/** Iterative approach (loop). Time: O(|n|), Space: O(1) */
+export function sum_to_n_a(n: number): number {
+  if (n >= 1) {
+    let sum = 0;
+    for (let i = 1; i <= n; i++) sum += i;
+    return sum;
+  }
+  let sum = 0;
+  for (let i = n; i <= 1; i++) sum += i;
+  return sum;
+}
+
+/** Recursive approach. Time: O(|n|), Space: O(|n|) (call stack) */
+export function sum_to_n_b(n: number): number {
+  if (n === 1) return 1;
+  if (n > 1) return n + sum_to_n_b(n - 1);
+  return n + sum_to_n_b(n + 1);
+}
+
+/** Formula approach (constant time). Time: O(1), Space: O(1) */
+export function sum_to_n_c(n: number): number {
+  if (n >= 1) return (n * (n + 1)) / 2;
+  const count = 2 - n; // number of terms from n..1
+  return ((n + 1) * count) / 2;
+}

src/problem5/.gitignore

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+database.sqlite

src/problem5/README.md

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+# Problem 5 — CRUD REST API
+
+A backend REST API built with **ExpressJS**, **TypeORM**, and **SQLite** (via `better-sqlite3`) in TypeScript.
+
+## Requirements
+
+- Node.js >= 16
+- Yarn (or npm)
+
+## Setup & Run
+
+```bash
+# From the project root
+yarn install
+
+# Start the server
+yarn problem5:start
+```
+
+The server starts on `http://localhost:3000` by default.
+Override the port with the `PORT` environment variable:
+
+```bash
+PORT=8080 yarn problem5:start
+```
+
+The SQLite database file is created automatically at `src/problem5/database.sqlite` on first run.
+
+---
+
+## API Reference
+
+### Resource schema
+
+| Field       | Type   | Description                          |
+|-------------|--------|--------------------------------------|
+| id          | number | Auto-generated primary key           |
+| name        | string | Required. Name of the resource       |
+| description | string | Optional description                 |
+| status      | string | `active` (default) or `inactive`     |
+| createdAt   | date   | Auto-set on creation                 |
+| updatedAt   | date   | Auto-updated on change               |
+
+---
+
+### Endpoints
+
+#### Create a resource
+```bash
+curl -X POST http://localhost:3000/resources \
+  -H "Content-Type: application/json" \
+  -d '{"name": "My resource", "description": "Some info", "status": "active"}'
+```
+**Response:** `201 Created`
+```json
+{ "id": 1, "name": "My resource", "description": "Some info", "status": "active", "createdAt": "...", "updatedAt": "..." }
+```
+
+---
+
+#### List resources
+```bash
+curl "http://localhost:3000/resources?name=my&status=active&page=1&size=10"
+```
+Query params (all optional):
+- `name` — partial-match filter on name
+- `status` — exact-match filter on status (`active` or `inactive`)
+- `page` — page number (default: 1)
+- `size` — items per page, max 100 (default: 10)
+
+**Response:** `200 OK`
+```json
+{ "data": [...], "total": 42, "page": 1, "limit": 10 }
+```
+
+---
+
+#### Get a resource
+```bash
+curl http://localhost:3000/resources/1
+```
+**Response:** `200 OK` with the resource object, or `404` if not found.
+
+---
+
+#### Update a resource
+```bash
+curl -X PUT http://localhost:3000/resources/1 \
+  -H "Content-Type: application/json" \
+  -d '{"name": "Updated name", "status": "inactive"}'
+```
+All fields are optional — only provided fields are updated.
+**Response:** `200 OK` with the updated resource, or `404` if not found.
+
+---
+
+#### Delete a resource
+```bash
+curl -X DELETE http://localhost:3000/resources/1
+```
+**Response:** `204 No Content`, or `404` if not found.
+
+---
+
+#### Health check
+```bash
+curl http://localhost:3000/health
+```
+**Response:** `200 OK`
+```json
+{ "status": "ok" }
+```

src/problem5/constants/http.ts

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+export enum HttpStatus {
+  Ok = 200,
+  Created = 201,
+  NoContent = 204,
+  BadRequest = 400,
+  NotFound = 404,
+  InternalServerError = 500,
+}

src/problem5/controllers/resource.controller.ts

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+import { Request, Response } from 'express';
+import { HttpStatus } from '../constants/http';
+import { datasource } from '../datasource';
+import { Resource } from '../entities';
+import { CreateDTO, ModifyDTO, ResourceIdDTO, ListDTO } from '../dto/resource.dto';
+
+export const create = async (req: Request, res: Response) => {
+  const parsed = CreateDTO.safeParse(req.body);
+  if (!parsed.success) {
+    res.status(HttpStatus.BadRequest).json({ error: parsed.error.issues[0]?.message });
+    return;
+  }
+
+  const repo = datasource.getRepository(Resource);
+  const resource = repo.create(parsed.data);
+  const saved = await repo.save(resource);
+  res.status(HttpStatus.Created).json(saved);
+};
+
+export const list = async (req: Request, res: Response) => {
+  const parsed = ListDTO.safeParse(req.query);
+  if (!parsed.success) {
+    res.status(HttpStatus.BadRequest).json({ error: parsed.error.issues[0]?.message });
+    return;
+  }
+
+  const { name, status, page, size } = parsed.data;
+  const qb = datasource.getRepository(Resource).createQueryBuilder('resource');
+
+  if (name) {
+    qb.andWhere('resource.name LIKE :name', { name: `%${name}%` });
+  }
+  if (status) {
+    qb.andWhere('resource.status = :status', { status });
+  }
+
+  qb.offset((page - 1) * size).limit(size);
+
+  const [items, total] = await qb.getManyAndCount();
+  res.json({ data: items, total, page, limit: size });
+};
+
+export const get = async (req: Request, res: Response) => {
+  const parsed = ResourceIdDTO.safeParse(req.params['id']);
+  if (!parsed.success) {
+    res.status(HttpStatus.BadRequest).json({ error: parsed.error.issues[0]?.message });
+    return;
+  }
+
+  const resource = await datasource.getRepository(Resource).findOneBy({ id: parsed.data });
+  if (!resource) {
+    res.status(HttpStatus.NotFound).json({ error: 'Resource not found' });
+    return;
+  }
+
+  res.json(resource);
+};
+
+export const modify = async (req: Request, res: Response) => {
+  const idParsed = ResourceIdDTO.safeParse(req.params['id']);
+  if (!idParsed.success) {
+    res.status(HttpStatus.BadRequest).json({ error: idParsed.error.issues[0]?.message });
+    return;
+  }
+
+  const parsed = ModifyDTO.safeParse(req.body);
+  if (!parsed.success) {
+    res.status(HttpStatus.BadRequest).json({ error: parsed.error.issues[0]?.message });
+    return;
+  }
+
+  const repo = datasource.getRepository(Resource);
+  const resource = await repo.findOneBy({ id: idParsed.data });
+
+  if (!resource) {
+    res.status(HttpStatus.NotFound).json({ error: 'Resource not found' });
+    return;
+  }
+
+  const { name, description, status } = parsed.data;
+  if (name !== undefined) {
+    resource.name = name;
+  }
+  if (description !== undefined) {
+    resource.description = description;
+  }
+  if (status !== undefined) {
+    resource.status = status;
+  }
+
+  const updated = await repo.save(resource);
+  res.json(updated);
+};
+
+export const remove = async (req: Request, res: Response) => {
+  const parsed = ResourceIdDTO.safeParse(req.params['id']);
+  if (!parsed.success) {
+    res.status(HttpStatus.BadRequest).json({ error: parsed.error.issues[0]?.message });
+    return;
+  }
+
+  const repo = datasource.getRepository(Resource);
+  const resource = await repo.findOneBy({ id: parsed.data });
+
+  if (!resource) {
+    res.status(HttpStatus.NotFound).json({ error: 'Resource not found' });
+    return;
+  }
+
+  await repo.remove(resource);
+  res.status(HttpStatus.NoContent).send();
+};

src/problem5/datasource.ts

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+import 'reflect-metadata';
+import { DataSource } from 'typeorm';
+import { Resource } from './entities';
+
+export const datasource = new DataSource({
+  type: 'better-sqlite3',
+  database: 'src/problem5/database.sqlite',
+  synchronize: true,
+  logging: false,
+  entities: [Resource],
+});

src/problem5/dto/resource.dto.ts

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+import { z } from 'zod';
+
+export const StatusEnum = z.enum(['active', 'inactive']);
+
+export const CreateDTO = z.object({
+  name: z.string().trim().min(1, 'name is required'),
+  description: z.string().optional(),
+  status: StatusEnum.optional(),
+});
+
+export const ModifyDTO = z.object({
+  name: z.string().trim().min(1, 'name must be a non-empty string').optional(),
+  description: z.string().optional(),
+  status: StatusEnum.optional(),
+});
+
+export const ResourceIdDTO = z.coerce.number().int().positive('id must be a positive integer');
+
+export const ListDTO = z.object({
+  name: z.string().optional(),
+  status: StatusEnum.optional(),
+  page: z.coerce.number().int().min(1).default(1),
+  size: z.coerce.number().int().min(1).max(100).default(10),
+});

src/problem5/entities/index.ts

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+export * from './resource.entity';

src/problem5/entities/resource.entity.ts

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+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  CreateDateColumn,
+  UpdateDateColumn,
+} from 'typeorm';
+
+@Entity()
+export class Resource {
+  @PrimaryGeneratedColumn()
+  id!: number;
+
+  @Column()
+  name!: string;
+
+  @Column({ nullable: true })
+  description!: string;
+
+  @Column({ default: 'active' })
+  status!: string;
+
+  @CreateDateColumn()
+  createdAt!: Date;
+
+  @UpdateDateColumn()
+  updatedAt!: Date;
+}