Submission problem4 5 6 fork

.gitignore

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+node_modules/
+dist/
+.env
+data/*.tmp

README.md

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+# 99Tech Backend Assessment
+
+This repository contains my backend assessment solutions for 99Tech.
+
+## Problem 4
+
+File: `src/problem4.ts`
+
+The task is to provide three unique TypeScript implementations of:
+
+```ts
+function sum_to_n(n: number): number
+```
+
+For example:
+
+```ts
+sum_to_n(5) === 1 + 2 + 3 + 4 + 5 === 15
+```
+
+### Assumption
+
+The prompt says `n` can be any integer, but only defines the positive case.
+For `n <= 0`, I interpret "sum to n" as walking from `1` down to `n`.
+
+Example:
+
+```ts
+sum_to_n(-3) === 1 + 0 + (-1) + (-2) + (-3) === -5
+```
+
+The prompt also states that the result will always be less than
+`Number.MAX_SAFE_INTEGER`.
+
+### Implementations
+
+- `sum_to_n_a`: arithmetic formula, `O(1)` time and `O(1)` space.
+- `sum_to_n_b`: iterative loop, `O(|n|)` time and `O(1)` space.
+- `sum_to_n_c`: recursion, `O(|n|)` time and `O(|n|)` space.
+
+### Run
+
+```bash
+npm install
+npm run build
+npm run problem4
+```
+
+## Problem 5
+
+File: `src/problem5/server.ts`
+
+The task is to develop an ExpressJS backend server with TypeScript, CRUD APIs,
+basic filters, and simple data persistence.
+
+This solution uses a `Book` resource and stores data in `data/books.json`.
+The JSON file acts as a simple file-based database, so data remains after the
+server restarts.
+
+### Book Resource
+
+```ts
+type Book = {
+  id: string;
+  title: string;
+  author: string;
+  status: "available" | "borrowed" | "archived";
+  description?: string;
+  createdAt: string;
+  updatedAt: string;
+};
+```
+
+### Run Server
+
+```bash
+npm install
+npm run dev
+```
+
+The server starts on:
+
+```txt
+http://localhost:3000
+```
+
+You can change the port with:
+
+```bash
+PORT=4000 npm run dev
+```
+
+### API Endpoints
+
+Health check:
+
+```bash
+curl http://localhost:3000/health
+```
+
+Create a book:
+
+```bash
+curl -X POST http://localhost:3000/books \
+  -H "Content-Type: application/json" \
+  -d '{"title":"Clean Code","author":"Robert C. Martin","status":"available","description":"A book about writing readable code."}'
+```
+
+List books:
+
+```bash
+curl "http://localhost:3000/books"
+```
+
+List books with filters:
+
+```bash
+curl "http://localhost:3000/books?status=available&author=robert&q=clean&limit=10&offset=0"
+```
+
+Get book details:
+
+```bash
+curl http://localhost:3000/books/<book-id>
+```
+
+Update a book:
+
+```bash
+curl -X PATCH http://localhost:3000/books/<book-id> \
+  -H "Content-Type: application/json" \
+  -d '{"status":"borrowed"}'
+```
+
+Delete a book:
+
+```bash
+curl -X DELETE http://localhost:3000/books/<book-id>
+```
+
+### Build And Run Compiled Code
+
+```bash
+npm run build
+npm start
+```
+
+## Problem 6
+
+File: `problem6/README.md`
+
+The task is to write an architecture specification for a live scoreboard module.
+The documentation includes API contracts, data model, security requirements,
+execution flow, realtime update strategy, and improvement notes.

data/books.json

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+{
+  "books": [
+    {
+      "id": "846c1e8d-dc2e-45ad-b444-274f03df7aa2",
+      "title": "Clean Code",
+      "author": "Robert C. Martin",
+      "status": "available",
+      "description": "A book about writing readable code.",
+      "createdAt": "2026-04-17T07:55:08.185Z",
+      "updatedAt": "2026-04-17T07:55:08.185Z"
+    }
+  ]
+}