Typos, grammatical errors fix and improve player check in join

Lottery.sol

@@ -21,6 +21,8 @@ contract Lottery {
 
     bool public open;
 
+    mapping(uint => mapping(address => bool)) private entered;
+
     constructor () {
         owner = msg.sender;
     } 
@@ -61,11 +63,10 @@ contract Lottery {
         require(msg.value == entryAmount, "Insufficient Funds");
 
         //Check if user is already a player
-        for(uint i=0; i < players.length; i++){
-            if (msg.sender == players[i]){
-                revert reEntry();
-            }
+        if(entered[lotteryId][msg.sender]){
+            revert reEntry();
         }
+        entered[lotteryId][msg.sender] = true;
         players.push(msg.sender);
     }
 

README.md

@@ -1,44 +1,46 @@
-# Introduction
+# How to generate and work with randomness in Solidity using Witnet
+
+## Introduction
 - Randomness is a core functionality for many applications
 - Games, trait-based NFTs, and luck-based financial applications rely heavily on randomness
-- Welcome! In this tutorial, we are going to learn about random number generation and Working with it
+- Welcome! In this tutorial, we are going to learn about random number generation and working with it
 ---
-# Pre requisites
-- To understand and utilize this tutorial you need to have the understanding of:
-    - Basic knowledge of developing smart contracts in Solidity
-    - Basic experience with Remix IDE 
-    - Working with Celo Alfajores testnet
+## Pre requisites
+- To get the most out of this tutorial you need to have a basic understanding of:
+    - Developing smart contracts in Solidity
+    - Working with the Remix IDE 
+    - Working with the Celo Alfajores testnet
 ---
-# Requirements
+## Requirements
 - Metamask
 - Celo Alfajores testnet
 ---
-# Let's Get started!
-## What we are going to build
-- A lottery contract where users can join the lottery by paying an entry amount (just like buying a lottery in real life)
-- A random winner will be chosen from the players and will be rewarded the entire lottery amount 
-- The Lottery will be conducted by an owner who would have access to start and end the lottery
-- The owner will not have access to the funds
+## Let's Get started!
+
+### What we are going to build
+- A lottery smart contract where users can join the lottery by paying an entry fee
+- A random winner will be chosen amongst the participants and will be rewarded the entire lottery amount 
+- The Lottery will be conducted by the lottery's owner who would have access to start and end the lottery
+- The lottery's owner will not have access to the funds
 - The winner will be picked randomly based on the random number generated by an oracle
-- Before starting to code, we must understand some basics first
+- Before starting to code, we must first understand some basics 
 - We will be covering
-    - Randomness and its requirement in blockchain
-    - Problem with generating on-chain randomness
-    - About oracles and Random Number Generation
-    - About Witnet Oracle
-    - Random number generation contract from Witnet
-## Requirement of Randomness
+    - Use cases of randomness in blockchain applications
+    - Problems with generating randomness on-chain
+    - Achieving randomness through oracles
+    - An Introduction to the WitnetRandomness contract provided by Witnet
+### Requirement of Randomness
 - Most applications require randomness in some sort
 - Let's take for example a gambling contract(like our own), these contracts award people based on luck, and luck is random
 - For these luck-based applications to work, the randomness must be tamper-proof so that no one can exploit
-- While in the field of blockchain and smart contracts, there are some problems 
-## Problem with On-Chain Randomness
+- However, in the field of blockchain and smart contracts, there are some problems 
+### Problem with On-Chain Randomness
 - When people get started they use randomness by using the block number or the block timestamp
 - But everything in the blockchain is deterministic
 - This leads to big security concerns where a party can gain an unfair advantage
 - The only bypass to this situation is to generate a number through a trusted source that is outside of the blockchain
 - Enter Oracles!!
-## Oracles and Random Number generation
+### Oracles and Random Number generation
 - Oracles act as a bridge between smart contracts and the outer world
 - Oracles are a reliable source of information
 - They allow the blockchain to interact with external data
@@ -49,14 +51,14 @@
 - Through oracles, smart contracts can obtain tamper-proof random numbers
 - Every oracle has a different way of generating the randomness
 - We are going to learn how to generate a random number using Witnet
-## Witnet Oracle
+### Witnet Oracle
 - Witnet is a multichain oracle that gives smart contract access to real-world information
 - It is one of the oracles that are available in the cell network
 - Witnet provides us with a contract using which we can call and obtain randomness
 - How this work is, nodes from the witnet are randomly selected to generate a random byte that is cryptographically committed
 - It is called crowd-witnessing
-- To learn more about randomness in witnet - https://docs.witnet.io/smart-contracts/witnet-randomness-oracle
-## Using Randomness from witnet
+- To learn more about randomness in Witnet - https://docs.witnet.io/smart-contracts/witnet-randomness-oracle
+### Using Randomness from Witnet
 - Before starting to code our lottery contract, let's understand the randomness functions provided by Witnet
 - You can find their code and the explanation here - https://docs.witnet.io/smart-contracts/witnet-randomness-oracle/code-examples
 
@@ -96,14 +98,14 @@ contract Witnet {
 - It takes around 5 to 10 minutes to generate the random number
 - After 5 - 10 minutes, we can call the fetch function to obtain the random number
 - This number is generated earlier and it is fetched into the contract whenever you call the fetch function
-## Setup
-- To make this tutorial as simple as possible, we are going to use only remix to write and test the contract
+### Setup
+- To make this tutorial as simple as possible, we are going to use only Remix to write and test the contract
 - You can also use local development by using your favorite code editor
 - You can download the package for the interface of the randomness contract through this command:
     npm I witnet-solidity-bridge
-- For the rest of us, we are going to use remix IDE
+- For the rest of us, we are going to use Remix IDE
 - Open up remix IDE and create a new file -> Lottery.sol
-## Building the contract
+### Building the contract
 - If you're curious, the entire code for the contract can be viewed by [CLICKING HERE!](https://github.com/KishoreVB70/Lottery-Tutorial/blob/main/Lottery.sol)
 - Before starting to code a project, we must have an outline of all the functionality that will be in the contract
 - This contract will behave like a traditional lottery where there is an owner who will start and end the lottery
@@ -130,7 +132,7 @@ contract Lottery {
 ```
 - As usual, we write the license and the pragma solidity version
 - We import the interface for the randomness contract
-- We set the address of the randomness contract in Celo alfajores and create the instance of the interface.
+- We set the address of the randomness contract hosted on the Celo Alfajores testnet and create the instance of the interface.
 ---
 ```
     uint256 entryAmount;
@@ -143,6 +145,8 @@ contract Lottery {
     address owner;
 
     bool open;
+
+    mapping(uint => mapping(address => bool)) private entered;
 ```
 - Let's look at all the variables we will be going through
 - Firstly we have the `entryAmount` so that the players can know the amount
@@ -210,20 +214,19 @@ contract Lottery {
         require(msg.value == entryAmount, "Insufficient Funds");
 
         //Check if user is already a player
-        for(uint i=0; i < players.length; i++){
-            if (msg.sender == players[i]){
-                revert reEntry();
-            }
+        if(entered[lotteryId][msg.sender]){
+            revert reEntry();
         }
+        entered[lotteryId][msg.sender] = true;
         players.push(msg.sender);
     }
 ```
 - Then we have the join function which is payable and can only be accessed if there is an active lottery(`onlyIfOpen`)
 - First, we have a require statement to check if the user has sent the right amount
-- Then we perform a check to see if the user is already entered in the lottery
-- We have employed a simple for loop which iterated over the `players` array and checks if the caller is already a part of it
-- If the caller is already in the array, then the function call is reverted by the custom error
-- If not, the player is added to the array
+- Then we perform a check to see if the user has already entered in the lottery
+- We use a mapping to store a boolean value about whether a user is already participating in the current lottery
+- If the caller is already participating in the lottery, then the function call is reverted with a custom error
+- If not, the player is added to the array and the entered value of the caller is set to true for the current lottery 
 ---
 ```
     function requestRandomness() external onlyOwner onlyIfOpen{
@@ -278,22 +281,22 @@ contract Lottery {
 - Finally, we emit the `Ended` event to log the information of the lottery Id, winner amount, and the winner's address
 - As our contract is handling funds, we implement a receive function
 ---
-## Testing the contract
+### Testing the contract
 - Finally, we are done coding the contract
 - You can find the entire code of the contract by [CLICKING HERE!](https://github.com/KishoreVB70/Lottery-Tutorial/blob/main/Lottery.sol)
 - Now we are going to use remix IDE and metamask to deploy the contract to the Celo alfajores network
 - I have made a simple video to show you guys the functionality
 [![Testing Video](https://i.postimg.cc/ZYFzPrvF/tutorial-yt.jpg)](https://www.youtube.com/watch?v=DPsizPEkZZk "Testing Video")
 ---
-# Conclusion
+## Conclusion
 - Congratulations!, you have learned another new implementation in the Web3 world
 - In this tutorial, we have learned a reliable way  to generate random numbers and built a practical contract
 ---
-# What's next
+## What's next
 -  You can use this random number generator to make complex contracts for games
 - Explore other oracles and use randomness generator from them
 - You can learn more about oracles and access other forms of data they provide
 ---
-# References
+## References
 - Witnet - https://witnet.io/
 - Witnet Randomness - https://docs.witnet.io/intro/tutorials/randomness