main.java

import java.net.*;
import java.io.*;
import java.util.*;
import java.util.HashMap;   

import java.util.TimerTask;
import java.util.Timer;
import java.util.List;  

public class main {
        public static HashMap<String, String> dataStore = new HashMap<>(); 
        public static List<String> listStore = new ArrayList<>();
         public static HashMap<String, List<String>> listMap = new HashMap<>();
         public static HashMap<String, HashSet<String>> setStore = new HashMap<>();   
        
        public static void setKey(String key, String value) {
            dataStore.put(key, value);
        }

        public static String getKey(String key){
            return dataStore.get(key);
        }

        public static void deleteKey(String key){
            if(dataStore.containsKey(key)){
                dataStore.remove(key);  
            }
        }

        public static boolean keyExists(String key){
            return dataStore.containsKey(key);
        }   

        public static void increment(String key){
            if(dataStore.containsKey(key)){
                int val = Integer.parseInt(dataStore.get(key));
                dataStore.put(key, String.valueOf(val +1)); 
            }

        }

        public static void expireKey(String key, int seconds){
            if(dataStore.containsKey(key)){
                Timer timer = new Timer();

                timer.schedule(new TimerTask(){
                    @Override
                    public void run(){
                        dataStore.remove(key);
                    }
                }, seconds * 1000);
            }
        }

        public static void rpush(String key, String value){
            if(!listMap.containsKey(key)){
                listMap.put(key, new ArrayList<>());    
            }
            listMap.get(key).add(value);
        }

        public static void lpush(String key, String value){
            if(!listMap.containsKey(key)){
                listMap.put(key, new ArrayList<>());    
            }
            listMap.get(key).add(0, value);
        }

        public static List<String> lrange(String key, int start, int end){
            if(!listMap.containsKey(key)){
                return new ArrayList<>();   
            }
            if(end == -1){
                end = listMap.get(key).size() -1;  
            }
            List<String> list = listMap.get(key);
            if(start<0 || end >= list.size() || start > end){
                return new ArrayList<>();   
            }
            return list.subList(start, end +1);
        }

        public static void sadd(String key, String value){
            if(!setStore.containsKey(key)){
                setStore.put(key, new HashSet<>());    
            }
            setStore.get(key).add(value);
        }

        public static List<String> smembers(String key){
            if(!setStore.containsKey(key)){
                return new ArrayList<>();    
            }
            List<String> members = new ArrayList<>(setStore.get(key));

            return members;
        }
           
    public static void main(String[] args) {
      
        try {
            ServerSocket ss = new ServerSocket(6379);
            System.out.println("Redis server started on port 6379...");
            
             Socket s = ss.accept(); 
             if(s != null){
                System.out.println("Client connected...");}
                DataInputStream in = new DataInputStream(s.getInputStream());
                 DataOutputStream out = new DataOutputStream(s.getOutputStream());
                String msg = "";
          
            while(!msg.equals("exit")){
               try
                {
                    msg = in.readUTF();
                    String[] parts = msg.split(" ");
                    String command = parts[0].toUpperCase();
                    System.out.println("Received command: " + msg); 
                    if(command.equals("SET") && parts.length == 3){
                        String key = parts[1];
                        String value = parts[2];
                        setKey(key, value);
                        out.writeUTF("OK");
                        System.out.println("Set key: " + key + " with value: " + value);
                    } else if(command.equals("GET") && parts.length == 2){
                        String key = parts[1];
                        String value = getKey(key);
                            if(value != null){
                                out.writeUTF(value);
                            } else {
                                out.writeUTF("NULL");
                            }
                    } else if(command.equals("DEL") && parts.length == 2){
                        String key = parts[1];
                        deleteKey(key);
                        out.writeUTF("OK");
                    }
                    else if(command.equals("INCR") && parts.length == 2){
                        String key = parts[1];
                        increment(key);
                        out.writeUTF("OK");
                    }
                    else if(command.equals("EXPIRE") && parts.length == 3){
                        String key = parts[1];
                        int sec = Integer.parseInt(parts[2]);  
                        expireKey(key, sec);
                        out.writeUTF("OK");
                    }
                    else if(command.equals("EXISTS") && parts.length == 2){
                        String key = parts[1];
                        boolean exists = keyExists(key);
                        out.writeUTF(exists ? "1" : "0");
                    }
                     else if(command.equals("EXIT")){
                        out.writeUTF("Goodbye!");

                    }
                    else if(command.equals("RPUSH") && parts.length == 3){
                        String key = parts[1];  
                        String value = parts[2];
                        rpush(key , value);
                        out.writeUTF("OK"); 
                     }
                        else if(command.equals("LPUSH") && parts.length == 3){
                            String key =parts[1];
                            String value = parts[2];
                            lpush(key, value);
                            out.writeUTF("OK"); 
                        }
                        else if(command.equals("LRANGE") && parts.length == 4){
                            String key = parts[1];
                            int start = Integer.parseInt(parts[2]);
                            int end = Integer.parseInt(parts[3]);
                            List<String> range = lrange(key , start, end);
                            out.writeUTF(String.join(",", range)); 
                        }  
                            else if(command.equals("SADD") && parts.length == 3){
                                String key = parts[1];
                                String value = parts[2];
                                sadd(key, value);
                                out.writeUTF("OK"); 
                            }
                            else if(command.equals("SMEMBERS") && parts.length == 2){
                                String key = parts[1];
                                List<String> members = smembers(key);
                                out.writeUTF(String.join(",", members)); 
                            } 
                     else {
                        out.writeUTF("ERROR: Invalid command");
                    }
                }
                catch(IOException i)
                {
                    System.out.println(i);
                    break;
                } 
            }
              s.close();     
        }catch (Exception e) {
            e.printStackTrace();
    }
}
}