Create README.md

Author: SysSn13

README.md

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+<!-- markdownlint-disable MD033 -->
+
+<h1 align="center">
+  <a href="https://github.com/SysSn13/leetcode-rating-predictor">
+    <img src="./data/screenshots/icon.png" alt="Logo" width="125" height="125" style="background:white">
+  </a>
+</h1>
+
+<div align="center">
+  <h3>LC Predictor</h3>
+  Want to see your leetcode rating change right after the contest? Well, you are in the right place!
+  <br />
+  <br />
+</div>
+
+<br />
+<div align="center">
+  <a href="https://chrome.google.com/webstore/detail/lc-predictor/jfhgaegpgiepniiebglgjhhfnjcibphh"><img
+      src="https://i.imgur.com/iswHnpJ.png" alt="Chrome Web Store" height="50">
+    </img>
+  </a>
+</div>
+
+<div align="center">
+  <a href="https://github.com/SysSn13/leetcode-rating-predictor/blob/main/LICENSE"><img
+      src="https://img.shields.io/badge/license-MIT-blue.svg" alt="license" /></a>
+  <a href="https://github.com/SysSn13/leetcode-rating-predictor/graphs/contributors" alt="Contributors">
+    <img src="https://img.shields.io/github/contributors/SysSn13/leetcode-rating-predictor" />
+  </a>
+  <a href="https://chrome.google.com/webstore/detail/leethub/aciombdipochlnkbpcbgdpjffcfdbggi"><img
+      src="https://img.shields.io/chrome-web-store/v/jfhgaegpgiepniiebglgjhhfnjcibphh.svg" alt="chrome-webstore" /></a>
+
+</div>
+<br />
+
+# About
+
+It takes about 4-5 days for leetcode to update the contest ratings of participants. So you have to wait for a long time to know your rating changes. This application predicts accurate leetcode rating changes for all the contestants within a few minutes of completion of the contest.
+
+# Getting started
+
+This project consists of two types of user interfaces. You can either use browser extension or the website to get your rating changes.
+
+## Chrome extension
+
+You can install the extension from [Chrome Web Store](https://chrome.google.com/webstore/detail/lc-predictor/jfhgaegpgiepniiebglgjhhfnjcibphh). It adds the rating changes on leetcode ranking pages itself.
+
+<div align="center">
+  <img src="./data/screenshots/extension_demo.gif" alt="extension preview">
+</div>
+
+## Website
+
+You can also visit [lcpredictor.herokuapp.com](https://lcpredictor.herokuapp.com/) to get your rating changes.
+
+<div align="center">
+  <img src="./data/screenshots/web_demo.gif" alt="website preview">
+</div>
+
+# How It Works
+
+This project is written in Node + MongoDB + Redis tech stack. We can divide it into three microservices.
+
+|  #  |    Name    | Languages |
+| :-: | :--------: | :-------: |
+| 1.  | Background |  Js, Cpp  |
+| 2.  |  Website   |  Js, Ejs  |
+| 3.  |    API     |    Js     |
+
+## Backgound
+
+It is the most important part of the project. It's job is to fetch the data from leetcode and predict the contest ratings periodically.
+
+### Rating prediction
+
+Rating prediction is a cpu intensive task. Therefore a [**nodejs C++ addon**](https://nodejs.org/api/addons.html) is implemented for this task so that we can utilize threading with better performance using **C++**. For performance measurement we got these results :
+
+|           | No. of Threads |      Contest       | Time taken to make predictions(s) |
+| :-------: | :------------: | :----------------: | :-------------------------------: |
+|    Js     |       1        | Weekly contest 242 |              186.589              |
+| C++ addon |       1        | Weekly contest 242 |              39.607               |
+| C++ addon |       2        | Weekly contest 242 |              19.963               |
+| C++ addon |       4        | Weekly contest 242 |              11.401               |
+| C++ addon |       8        | Weekly contest 242 |              20.304               |
+
+#### Machine configuration :
+
+| Property  | Value                                   |
+| :-------- | :-------------------------------------- |
+| Processor | Intel® Core™ i5-8250U CPU @ 1.60GHz × 8 |
+| Memory    | 7.7 GB                                  |
+| OS        | Ubuntu 21.04                            |
+
+It implements leetcode's latest rating prediction algorithm. Rating predictions are very close to the original rating but the accuracy may not be 100% due to changes in contest rankings after the completion of contest (leetcode rejudges some submissons).
+
+These are the results for the predictions of weekly-contest-242:
+
+|  Measure  |       Value        |
+| :-------: | :----------------: |
+|    MSE    | 167.7947072739485  |
+| R-squared | 0.9988091420057561 |
+
+### Job scheduling
+
+Job scheduling is required for processing jobs on desired time. Leetcode contests are weekly and biweekly. We can schedule them by scheduling a repeated job. But for making it more generic, job schedulers are implemented who schedules prediction and data update jobs. These job schedulers are scheduled as a repeated job. It is accomplished by using [bull](https://github.com/OptimalBits/bull), a redis based queue for Node. A bull dashboard is also integrated using [bull-board](https://github.com/felixmosh/bull-board).
+
+<div align="center">
+  <img src="./data/screenshots/bull-dashboard1.png" alt="bull dashboard">
+  <img src="./data/screenshots/bull-dashboard2.png" alt="bull dashboard">
+</div>
+
+## Website
+
+It is built using [express](https://expressjs.com/) framework. Ejs is used for writing templates. It contains a table for contests and ranking pages with predicted rating changes for all the contests. Pagination is added to ranking pages for better user experience and performace.
+
+## API
+
+It is also implemented using [express](https://expressjs.com/) framework. It contains an endpoint for fetching users'
+predicted rating changes which is used in the browser extension.
+
+IP based rate limit is enforced for both the API and the website using [express-rate-limit](https://github.com/nfriedly/express-rate-limit).
+
+# Development
+
+## Setup
+
+-   Clone the repository
+    ```bash
+    git clone https://github.com/SysSn13/leetcode-rating-predictor
+    ```
+-   Install the dependencies
+    ```bash
+    npm install
+    ```
+-   Setup environment variables
+
+    ```bash
+    cp .env.example .env
+    ```
+
+    Fill in the required values in the `.env` file.
+
+-   Build the predict-addon (if you are using different node version)
+    ```bash
+    npm run buildAddon
+    ```
+-   Start the project
+    ```bash
+    npm start
+    ```
+-   Or start the development server by:
+    ```bash
+    npm run dev
+    ```
+
+## Environment variables
+
+```
+DATABASE_URL: Connection string for mongodb.
+
+# for web
+WEB: Whether to run the website or not. (0 or 1)
+
+RATE_LIMIT_WINDOW: Window size for rate limit in milliseconds (default: 10000).
+
+RATE_LIMIT: Number of requests allowed in the window (default: 50).
+
+
+# for api
+API_DISABLED: Whether to disable the API or not. (0 or 1)
+
+API_RATE_LIMIT_WINDOW: Window size for API rate limit in milliseconds (default: 10000).
+
+API_RATE_LIMIT: Number of API requests allowed in the window (default: 20).
+
+
+# for background
+BACKGROUND: Whether to run the background or not. (0 or 1)
+
+REDIS_URL: Connection string for redis.
+
+THREAD_CNT: Number of threads for prediction.(default: 4)
+```
+
+## Browser extension
+
+Current only chrome browser is suported. It uses manifest V3. See [this](https://developer.chrome.com/docs/extensions/mv3/getstarted/) for getting started with extension development. Source code for the extension is in `./chrome-extension`.
+
+## Contributing
+
+You can contribute by creating issues, feature/ pull requests. Any meaningful contributions you make are greatly appreciated.
+
+For contributing in the source code, please follow these steps:
+
+-   Fork the Project
+-   Create your new Branch
+    ```bash
+    git checkout -b feature/AmazingFeature
+    ```
+-   Commit your Changes
+    ```
+    git commit -m 'Add some AmazingFeature'
+    ```
+-   Push to the Branch
+    ```
+    git push origin feature/AmazingFeature
+    ```
+-   Open a Pull Request
+
+# License
+
+Distributed under the MIT License. See [LICENSE](/LICENSE) for more information.