Add MLFQ Scheduler (#5575)

src/main/java/com/thealgorithms/scheduling/MLFQScheduler.java

@@ -0,0 +1,148 @@
+package com.thealgorithms.scheduling;
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Queue;
+
+/**
+ * The Multi-Level Feedback Queue (MLFQ) Scheduler class.
+ * This class simulates scheduling using multiple queues, where processes move
+ * between queues depending on their CPU burst behavior.
+ */
+public class MLFQScheduler {
+    private List<Queue<Process>> queues; // Multi-level feedback queues
+    private int[] timeQuantum; // Time quantum for each queue level
+    private int currentTime; // Current time in the system
+
+    /**
+     * Constructor to initialize the MLFQ scheduler with the specified number of
+     * levels and their corresponding time quantums.
+     *
+     * @param levels       Number of queues (priority levels)
+     * @param timeQuantums Time quantum for each queue level
+     */
+    public MLFQScheduler(int levels, int[] timeQuantums) {
+        queues = new ArrayList<>(levels);
+        for (int i = 0; i < levels; i++) {
+            queues.add(new LinkedList<>());
+        }
+        timeQuantum = timeQuantums;
+        currentTime = 0;
+    }
+
+    /**
+     * Adds a new process to the highest priority queue (queue 0).
+     *
+     * @param p The process to be added to the scheduler
+     */
+    public void addProcess(Process p) {
+        queues.get(0).add(p);
+    }
+
+    /**
+     * Executes the scheduling process by running the processes in all queues,
+     * promoting or demoting them based on their completion status and behavior.
+     * The process continues until all queues are empty.
+     */
+    public void run() {
+        while (!allQueuesEmpty()) {
+            for (int i = 0; i < queues.size(); i++) {
+                Queue<Process> queue = queues.get(i);
+                if (!queue.isEmpty()) {
+                    Process p = queue.poll();
+                    int quantum = timeQuantum[i];
+
+                    // Execute the process for the minimum of the time quantum or the remaining time
+                    int timeSlice = Math.min(quantum, p.remainingTime);
+                    p.execute(timeSlice);
+                    currentTime += timeSlice; // Update the system's current time
+
+                    if (p.isFinished()) {
+                        System.out.println("Process " + p.pid + " finished at time " + currentTime);
+                    } else {
+                        if (i < queues.size() - 1) {
+                            p.priority++; // Demote the process to the next lower priority queue
+                            queues.get(i + 1).add(p); // Add to the next queue level
+                        } else {
+                            queue.add(p); // Stay in the same queue if it's the last level
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    /**
+     * Helper function to check if all the queues are empty (i.e., no process is
+     * left to execute).
+     *
+     * @return true if all queues are empty, otherwise false
+     */
+    private boolean allQueuesEmpty() {
+        for (Queue<Process> queue : queues) {
+            if (!queue.isEmpty()) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    /**
+     * Retrieves the current time of the scheduler, which reflects the total time
+     * elapsed during the execution of all processes.
+     *
+     * @return The current time in the system
+     */
+    public int getCurrentTime() {
+        return currentTime;
+    }
+}
+
+/**
+ * Represents a process in the Multi-Level Feedback Queue (MLFQ) scheduling
+ * algorithm.
+ */
+class Process {
+    int pid;
+    int burstTime;
+    int remainingTime;
+    int arrivalTime;
+    int priority;
+
+    /**
+     * Constructor to initialize a new process.
+     *
+     * @param pid         Process ID
+     * @param burstTime   CPU Burst Time (time required for the process)
+     * @param arrivalTime Arrival time of the process
+     */
+    Process(int pid, int burstTime, int arrivalTime) {
+        this.pid = pid;
+        this.burstTime = burstTime;
+        this.remainingTime = burstTime;
+        this.arrivalTime = arrivalTime;
+        this.priority = 0;
+    }
+
+    /**
+     * Executes the process for a given time slice.
+     *
+     * @param timeSlice The amount of time the process is executed
+     */
+    public void execute(int timeSlice) {
+        remainingTime -= timeSlice;
+        if (remainingTime < 0) {
+            remainingTime = 0;
+        }
+    }
+
+    /**
+     * Checks if the process has finished execution.
+     *
+     * @return true if the process is finished, otherwise false
+     */
+    public boolean isFinished() {
+        return remainingTime == 0;
+    }
+}