feat: add non-preemptive SJF scheduling

non_preemptive_sjf_scheduling.cpp

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+/**
+ * @file
+ * @brief Implementation of SJF CPU scheduling algorithm
+ * @details
+ * shortest job first (SJF), also known as shortest job next (SJN), is a scheduling policy 
+ * that selects for execution the waiting process with the smallest execution time.
+ * SJN is a non-preemptive algorithm. Shortest remaining time is a preemptive variant of SJN.
+ * @link https://www.guru99.com/shortest-job-first-sjf-scheduling.html
+ * @author [Lakshmi Srikumar](https://github.com/LakshmiSrikumar)
+ */
+
+#include <algorithm>         /// for sorting
+#include <cassert>          /// for assert
+#include <cstdlib>          /// random number generation
+#include <ctime>            /// for time   
+#include <iomanip>          /// for formatting the output
+#include <iostream>         /// for IO operations
+#include <queue>            /// for std::priority_queue
+#include <unordered_set>    /// for std::unordered_set
+#include <vector>           /// for std::vector     
+
+using std::cin;
+using std::cout;
+using std::endl;
+using std::get;
+using std::left;
+using std::make_tuple;
+using std::priority_queue;
+using std::rand;
+using std::srand;
+using std::tuple;
+using std::unordered_set;
+using std::vector;
+
+/**
+ * @brief Comparator function for sorting a vector
+ * @tparam S Data type of Process ID
+ * @tparam T Data type of Arrival time
+ * @tparam E Data type of Burst time
+ * @param t1 First tuple
+ * @param t2 Second tuple
+ * @returns true if t1 and t2 are in the CORRECT order
+ * @returns false if t1 and t2 are in the INCORRECT order
+ */
+template <typename S, typename T, typename E>
+bool sortcol(tuple<S, T, E>& t1, tuple<S, T, E>& t2) {
+    if (get<1>(t1) < get<1>(t2)) {
+        return true;
+    } else if (get<1>(t1) == get<1>(t2) && get<0>(t1) < get<0>(t2)) {
+        return true;
+    }
+    return false;
+}
+
+/**
+ * @class Compare
+ * @brief Comparator class for priority queue
+ * @tparam S Data type of Process ID
+ * @tparam T Data type of Arrival time
+ * @tparam E Data type of Burst time
+ */
+
+template <typename S, typename T, typename E>
+class Compare {
+ public:
+
+ /**
+     * @param t1 First tuple
+     * @param t2 Second tuple
+     * @brief A comparator function that checks whether to swap the two tuples
+     * or not.
+     * @link Refer to
+     * https://www.geeksforgeeks.org/comparator-class-in-c-with-examples/ for
+     * detailed description of comparator
+     * @returns true if the tuples SHOULD be swapped
+     * @returns false if the tuples SHOULDN'T be swapped
+     */
+
+    bool operator()(tuple<S, T, E, double, double, double>& t1,
+                    tuple<S, T, E, double, double, double>& t2) {
+        // Compare burst times for SJF
+        if (get<2>(t2) < get<2>(t1)) {
+            return true;
+        }
+        // If burst times are the same, compare arrival times
+        else if (get<2>(t2) == get<2>(t1)) {
+            return get<1>(t2) < get<1>(t1);
+        }
+        return false;
+    }
+};
+
+/**
+ * @class SJF
+ * @brief Class which implements the SJF scheduling algorithm
+ * @tparam S Data type of Process ID
+ * @tparam T Data type of Arrival time
+ * @tparam E Data type of Burst time
+ */
+template <typename S, typename T, typename E>
+class SJF {
+/**
+     * Priority queue of schedules(stored as tuples) of processes.
+     * In each tuple
+     * 1st element: Process ID
+     * 2nd element: Arrival Time
+     * 3rd element: Burst time
+     * 4th element: Completion time
+     * 5th element: Turnaround time
+     * 6th element: Waiting time
+     */
+    priority_queue<tuple<S, T, E, double, double, double>,
+                   vector<tuple<S, T, E, double, double, double>>,
+                   Compare<S, T, E>> schedule;
+
+    // Stores final status of all the processes after completing the execution.
+    vector<tuple<S, T, E, double, double, double>> result;
+
+    // Stores process IDs. Used for confirming absence of a process while  it.
+    unordered_set<S> idList;
+
+ public:
+ /**
+     * @brief Adds the process to the ready queue if it isn't already there
+     * @param id Process ID
+     * @param arrival Arrival time of the process
+     * @param burst Burst time of the process
+     * @returns void
+     *
+     */
+    void addProcess(S id, T arrival, E burst) {
+    // Add if a process with process ID as id is not found in idList.
+        if (idList.find(id) == idList.end()) {
+            tuple<S, T, E, double, double, double> t =
+                make_tuple(id, arrival, burst, 0, 0, 0);
+            schedule.push(t);
+            idList.insert(id);
+        }
+    }
+
+      /**
+     * @brief Algorithm for scheduling CPU processes according to the Shortest Job
+     First (SJF) scheduling algorithm.
+     *
+     * @details Non pre-emptive SJF is an algorithm that schedules processes based on the length 
+     * of their burst times. The process with the smallest burst time is executed first.
+     * In a non-preemptive scheduling algorithm, once a process starts executing, 
+     * it runs to completion without being interrupted.
+     *
+     * I used a min priority queue because it allows you to efficiently pick the process 
+     * with the smallest burst time in constant time, by maintaining a priority order where 
+     * the shortest burst process is always at the front.
+     *
+     * @returns void
+     */
+
+    vector<tuple<S, T, E, double, double, double>> scheduleForSJF() {
+      // Variable to keep track of time elapsed so far
+        double timeElapsed = 0;
+
+        while (!schedule.empty()) {
+            tuple<S, T, E, double, double, double> cur = schedule.top();
+
+           // If the current process arrived at time t2, the last process
+            // completed its execution at time t1, and t2 > t1.
+            if (get<1>(cur) > timeElapsed) {
+                timeElapsed += get<1>(cur) - timeElapsed;
+            }
+
+            // Add Burst time to time elapsed
+            timeElapsed += get<2>(cur);
+
+            // Completion time of the current process will be same as time
+            // elapsed so far
+            get<3>(cur) = timeElapsed;
+
+            // Turnaround time = Completion time - Arrival time
+            get<4>(cur) = get<3>(cur) - get<1>(cur);
+
+            // Waiting time = Turnaround time - Burst time
+            get<5>(cur) = get<4>(cur) - get<2>(cur);
+
+            result.push_back(cur);
+            schedule.pop();
+        }
+        return result;
+    }
+    /**
+     * @brief Utility function for printing the status of each process after
+     * execution
+     * @returns void
+     */
+
+    void printResult() {
+        cout << "Status of all processes post completion is as follows:" << endl;
+
+        cout << std::setw(17) << left << "Process ID" << std::setw(17) << left
+             << "Arrival Time" << std::setw(17) << left << "Burst Time"
+             << std::setw(17) << left << "Completion Time" << std::setw(17)
+             << left << "Turnaround Time" << std::setw(17) << left
+             << "Waiting Time" << endl;
+
+        for (size_t i{}; i < result.size(); i++) {
+            cout << std::setprecision(2) << std::fixed << std::setw(17) << left
+                 << get<0>(result[i]) << std::setw(17) << left
+                 << get<1>(result[i]) << std::setw(17) << left
+                 << get<2>(result[i]) << std::setw(17) << left
+                 << get<3>(result[i]) << std::setw(17) << left
+                 << get<4>(result[i]) << std::setw(17) << left
+                 << get<5>(result[i]) << endl;
+        }
+    }
+};
+
+    /**
+   * @brief Computes the final status of processes after applying non-preemptive SJF scheduling
+   * @tparam S Data type of Process ID
+   * @tparam T Data type of Arrival time
+   * @tparam E Data type of Burst time
+   * @param input A vector of tuples containing Process ID, Arrival time, and Burst time
+   * @returns A vector of tuples containing Process ID, Arrival time, Burst time,
+   *         Completion time, Turnaround time, and Waiting time
+   */
+
+    template <typename S, typename T, typename E>
+  vector<tuple<S, T, E, double, double, double>> get_final_status(
+      vector<tuple<S, T, E>> input) {
+
+      // Sort the processes based on Arrival time and then Burst time
+      sort(input.begin(), input.end(), sortcol<S, T, E>);
+
+      // Result vector to hold the final status of each process
+      vector<tuple<S, T, E, double, double, double>> result(input.size());
+      double timeElapsed = 0;
+
+      for (size_t i = 0; i < input.size(); i++) {
+        // Extract Arrival time and Burst time
+          T arrival = get<1>(input[i]);
+          E burst = get<2>(input[i]);
+
+          // If the CPU is idle, move time to the arrival of the next process
+          if (arrival > timeElapsed) {
+              timeElapsed = arrival;
+          }
+
+          // Update timeElapsed by adding the burst time
+          timeElapsed += burst;
+
+          // Calculate Completion time, Turnaround time, and Waiting time
+          double completion = timeElapsed;
+          double turnaround = completion - arrival;
+          double waiting = turnaround - burst;
+
+          // Store the results in the result vector
+          result[i] = make_tuple(get<0>(input[i]), arrival, burst, completion, turnaround, waiting);
+      }
+
+      return result;
+  }
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test() {
+    for (int i{}; i < 1000; i++) {
+        srand(time(nullptr));
+        uint32_t n = 1 + rand() % 1000;
+        SJF<uint32_t, uint32_t, uint32_t> readyQueue;
+        vector<tuple<uint32_t, uint32_t, uint32_t>> input(n);
+
+        for (uint32_t i{}; i < n; i++) {
+            get<0>(input[i]) = i;
+            srand(time(nullptr));
+            get<1>(input[i]) = 1 + rand() % 10000;
+            srand(time(nullptr));
+            get<2>(input[i]) = 1 + rand() % 10000;
+        }
+
+        for (uint32_t i{}; i < n; i++) {
+            readyQueue.addProcess(get<0>(input[i]), get<1>(input[i]),
+                                  get<2>(input[i]));
+        }
+        readyQueue.scheduleForSJF();
+        //readyQueue.printResult();
+    }
+    cout << "All the tests have successfully passed!" << endl;
+}
+
+/**
+ * @brief Entry point of the program
+ */
+int main() {
+    test();
+    return 0;
+}