SJF.java

/**
*  <h2>Shortest job first.</h2>
* <p>Shortest job first (SJF) or shortest job next, is a scheduling policy 
* that selects the waiting process with the smallest execution time to execute next
* Shortest Job first has the advantage of having minimum average waiting time among all scheduling algorithms.
* It is a Greedy Algorithm.
* It may cause starvation if shorter processes keep coming.
* This problem has been solved using the concept of aging.</p>
* @author shivg7706
* @since 2018/10/27
*/

import java.util.Scanner;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.*;

class Process {

    public int pid;
    public int arrivalTime;
    public int burstTime;
    public int priority;
    public int turnAroundTime;
    public int waitTime;
    public int remainingTime;
}

class Schedule {

    private int noOfProcess;
    private int timer = 0;
    private ArrayList<Process> processes;
    private ArrayList<Process> remainingProcess;
    private ArrayList<Integer> gantChart;
    private float burstAll;
    private Map<Integer, ArrayList<Process>> arrivals;

    Schedule() {
        Scanner in = new Scanner(System.in);

        processes = new ArrayList<Process>();
        remainingProcess = new ArrayList<Process>();

        gantChart = new ArrayList<>();
        arrivals = new HashMap<>();

        System.out.print("Enter the no. of processes: ");
        noOfProcess = in.nextInt();
        System.out.println("Enter the arrival, burst and priority of processes");
        for (int i = 0; i < noOfProcess; i++) {
            Process p = new Process();
            p.pid = i;
            p.arrivalTime = in.nextInt();
            p.burstTime = in.nextInt();
            p.priority = in.nextInt();
            p.turnAroundTime = 0;
            p.waitTime = 0;
            p.remainingTime = p.burstTime;

            if (arrivals.get(p.arrivalTime) == null) {
                arrivals.put(p.arrivalTime, new ArrayList<Process>());
            }
            arrivals.get(p.arrivalTime).add(p);
            processes.add(p);
            burstAll += p.burstTime;
        }

    }


    void startScheduling() {

        
        processes.sort(new Comparator<Process>() {
            @Override
            public int compare (Process a, Process b) {
                return a.arrivalTime - b.arrivalTime;
            }
        });

        while(!(arrivals.size() == 0 && remainingProcess.size() == 0)) {
            removeFinishedProcess();
            if(arrivals.get(timer) != null) {    
                remainingProcess.addAll(arrivals.get(timer));
                arrivals.remove(timer);
            }

            remainingProcess.sort(new Comparator<Process>() {
                private int alpha = 6;
                private int beta = 1;

                @Override
                public int compare (Process a, Process b) {
                    int aRem = a.remainingTime;
                    int bRem = b.remainingTime;
                    int aprior = a.priority;
                    int bprior = b.priority;
                    return (alpha*aRem + beta*aprior) - (alpha*bRem + beta*bprior);
                }
            });

            int k = timeElapsed(timer);
            ageing(k);
            timer++;
        }

        System.out.println("Total time required: " + (timer-1));
    }

    void removeFinishedProcess() {
        ArrayList<Integer> completed = new ArrayList<Integer>();
        for (int i = 0; i < remainingProcess.size(); i++) {
            if(remainingProcess.get(i).remainingTime == 0) {
                completed.add(i);
            }
        }

        for (int i = 0; i < completed.size(); i++) {
            int pid = remainingProcess.get(completed.get(i)).pid;
            processes.get(pid).waitTime = remainingProcess.get(completed.get(i)).waitTime;
            remainingProcess.remove(remainingProcess.get(completed.get(i)));
        }


    }

    public int timeElapsed(int i) {
        if(!remainingProcess.isEmpty()) {
            gantChart.add(i, remainingProcess.get(0).pid);
            remainingProcess.get(0).remainingTime--;
            return 1;
        }
        return 0;
    }

    public void ageing(int k) {
        for (int i = k; i < remainingProcess.size(); i++) {
            remainingProcess.get(i).waitTime++;
            if (remainingProcess.get(i).waitTime % 7 == 0) {
                remainingProcess.get(i).priority--;
            }
        }   
    }


    public void solve() {
        System.out.println("Gant chart ");
        for (int i = 0; i < gantChart.size(); i++) {
            System.out.print(gantChart.get(i) + " ");
        }
        System.out.println();

        float waitTimeTot = 0;
        float tatTime = 0;

        for (int i = 0; i < noOfProcess; i++) {
            processes.get(i).turnAroundTime = processes.get(i).waitTime + processes.get(i).burstTime;

            waitTimeTot += processes.get(i).waitTime;
            tatTime += processes.get(i).turnAroundTime;

            System.out.println("Process no.: " + i + " Wait time: " + processes.get(i).waitTime + " Turn Around Time: " + processes.get(i).turnAroundTime);
        }

        System.out.println("Average Waiting Time: " + waitTimeTot/noOfProcess);
        System.out.println("Average TAT Time: " + tatTime/noOfProcess);
        System.out.println("Throughput: " + (float)noOfProcess/(timer - 1));
    }
    
}

public class SJF {
    public static void main(String[] args) {
        Schedule s = new Schedule();
        s.startScheduling();
        s.solve();
    }
}