system design - Twitter

twitter.cs

/*
System Design - Twitter

1. User Registration  
- Users can register with a unique username.  
- Duplicate usernames are not allowed.  

2. Follow & Unfollow  
- Users can follow/unfollow others to control their feed.  

3. Posting Tweets  
- Tweets must be at least 200 characters.  
- Each tweet has a unique ID, timestamp, and content.  

4. Fetching Feeds  
- Shows tweets from followed users, sorted by time (latest first).  
- Only past tweets are displayed (no future timestamps).  
- Limited to the 20 most recent tweets.  

5. Testing  
- Verify registration, follow/unfollow, and tweet validation.  
- Ensure correct feed behavior and tweet filtering.

*/

//====================== Test Cases ==========================================
using System;
using System.Collections.Generic;
using System.Linq;

// Test 1: User Registration
Console.WriteLine("Test 1: User Registration");
User user1 = new("Alice");
User user2 = new("Bob");
DB.users.Add(user1);
DB.users.Add(user2);
Console.WriteLine(DB.users.Count == 2 ? "PASS" : "FAIL");

// Test 2: Duplicate Registration
Console.WriteLine("Test 2: Duplicate Registration");
Console.WriteLine(user1.Register("Alice") == false ? "PASS" : "FAIL");

// Test 3: Follow & Unfollow
Console.WriteLine("Test 3: Follow & Unfollow");
user1.Follow("Bob");
Console.WriteLine(user1.Followings.Contains("Bob") ? "PASS" : "FAIL");
user1.UnFollow("Bob");
Console.WriteLine(user1.Followings.Contains("Bob") == false ? "PASS" : "FAIL");

// Test 4: Post Tweet
Console.WriteLine("Test 4: Post Tweet");
bool success = user1.Post("This is a short tweet."); // Should fail (length < 200)
Console.WriteLine(success == false ? "PASS" : "FAIL");

success = user1.Post(new string('A', 201)); // Should succeed (length > 200)
Console.WriteLine(success == true ? "PASS" : "FAIL");

// Test 5: Get Feeds (No Following)
Console.WriteLine("Test 5: Get Feeds (No Following)");
var feeds = user1.GetFeeds();
Console.WriteLine(!feeds.Any() ? "PASS" : "FAIL");

// Test 6: Get Feeds (With Following)
Console.WriteLine("Test 6: Get Feeds (With Following)");
user1.Follow("Bob");
user2.Post(new string('B', 201)); // Bob posts a tweet
var feeds2 = user1.GetFeeds();
Console.WriteLine(feeds2.Any() ? "PASS" : "FAIL");

// Test 7: Unfollow and check feeds
Console.WriteLine("Test 7: Unfollow and check feeds");
user1.UnFollow("Bob");
var feeds3 = user1.GetFeeds();
Console.WriteLine(!feeds3.Any() ? "PASS" : "FAIL");

// Test 8: Multiple users posting and fetching feeds
Console.WriteLine("Test 8: Multiple users posting and fetching feeds");
User user3 = new("Charlie");
DB.users.Add(user3);
user1.Follow("Bob");
user1.Follow("Charlie");
user3.Post(new string('C', 201));
var feeds4 = user1.GetFeeds();
Console.WriteLine(feeds4.Count() == 2 ? "PASS" : "FAIL");

// Test 9: Posting multiple tweets and retrieving latest 20
Console.WriteLine("Test 9: Posting multiple tweets and retrieving latest 20");
for (int i = 0; i < 25; i++)
{
    user2.Post(new string('D', 201));
}
var feeds5 = user1.GetFeeds();
Console.WriteLine(feeds5.Count() == 20 ? "PASS" : "FAIL");

// Test 10: Ensure only past tweets are shown
Console.WriteLine("Test 10: Ensure only past tweets are shown");
user3.tweets.Add(Guid.NewGuid().ToString(), (DateTime.Now.AddMinutes(10), "Future tweet"));
var feeds6 = user1.GetFeeds();
Console.WriteLine(!feeds6.Contains("Future tweet") ? "PASS" : "FAIL");

Console.WriteLine("All tests completed.");


//====================== Code ==========================================

static class DB
{
    public static List<User> users = [];
}

public class User
{
    public string UserName { get; set; }
    public User(string userName)
    {
        UserName = userName;
    }
    public HashSet<string> Followings = [];
    public Dictionary<string, (DateTime, string)> tweets = [];
    public bool Register(string userName)
    {
        if (DB.users.Any(x => x.UserName == userName)) return false;
        DB.users.Add(new User(userName));
        return true;
    }
    public void Follow(string userName)
    {
        Followings.Add(userName);
    }
    public void UnFollow(string userName)
    {
        Followings.Remove(userName);
    }
    public bool Post(string tweet)
    {
        if (string.IsNullOrWhiteSpace(tweet) || tweet.Length < 200) return false;
        string tweetId = Guid.NewGuid().ToString();
        DateTime tweetDate = new();
        tweets.Add(tweetId, (tweetDate, tweet));
        return true;
    }

    public IEnumerable<string> GetFeeds()
    {
        if (Followings.Any() == false) return [];
        var followings = DB.users.Where(x => Followings.Any(y => y == x.UserName));
        var latestTweets = followings.SelectMany(x => x.tweets)
            .Where(x => x.Value.Item1 <= DateTime.Now)
            .OrderByDescending(x => x.Value.Item1)
            .Take(20)
            .Select(x => x.Value.Item2);
        return latestTweets;
    }
}