Added preemptive (priority + sjf) scheduling algorithms

Author: krishnaverma001

scheduling/preemptive_sjf(srtf).py

@@ -0,0 +1,87 @@
+"""
+Shortest Remaining Time First (SRTF) scheduling is a preemptive version of Shortest Job First (SJF).
+At every unit of time, it selects the process with the smallest remaining burst time
+among the processes that have arrived.
+https://en.wikipedia.org/wiki/Shortest_job_next#Preemptive_SJF_(SRTF)
+"""
+
+from statistics import mean
+from typing import List
+
+def calculate_srtf_waiting_time(arrival: List[int], burst: List[int]) -> List[int]:
+    """
+    Calculate waiting time for each process using Shortest Remaining Time First (SRTF).
+
+    Args:
+        arrival: List of arrival times of processes.
+        burst: List of burst times of processes.
+
+    Returns:
+        List of waiting times for each process.
+
+    >>> calculate_srtf_waiting_time([0, 1, 2, 3], [8, 4, 9, 5])
+    [9, 0, 15, 2]
+    """
+    n = len(arrival)
+    remaining = burst.copy()
+    waiting = [0] * n
+    completed = 0
+    t = 0
+    last_executed = -1
+    completion_time = [0] * n
+
+    while completed < n:
+        # Find process with smallest remaining time that has arrived
+        idx = -1
+        min_remaining = float('inf')
+        for i in range(n):
+            if arrival[i] <= t and remaining[i] > 0 and remaining[i] < min_remaining:
+                min_remaining = remaining[i]
+                idx = i
+
+        if idx == -1:
+            t += 1  # No process ready, advance time
+            continue
+
+        # Execute process for 1 unit of time
+        remaining[idx] -= 1
+        t += 1
+
+        # If process finished, record completion and calculate waiting
+        if remaining[idx] == 0:
+            completed += 1
+            completion_time[idx] = t
+            waiting[idx] = completion_time[idx] - arrival[idx] - burst[idx]
+
+    return waiting
+
+def calculate_srtf_turnaround_time(arrival: List[int], burst: List[int], waiting: List[int]) -> List[int]:
+    """
+    Calculate turnaround time for each process using waiting time.
+
+    Args:
+        arrival: List of arrival times.
+        burst: List of burst times.
+        waiting: List of waiting times.
+
+    Returns:
+        List of turnaround times.
+
+    >>> calculate_srtf_turnaround_time([0,1,2,3],[8,4,9,5],[6,0,12,0])
+    [14, 4, 21, 5]
+    """
+    return [burst[i] + waiting[i] for i in range(len(burst))]
+
+if __name__ == "__main__":
+    arrival_time = [0, 1, 2, 3]
+    burst_time = [8, 4, 9, 5]
+
+    waiting_time = calculate_srtf_waiting_time(arrival_time, burst_time)
+    turnaround_time = calculate_srtf_turnaround_time(arrival_time, burst_time, waiting_time)
+
+    print("PID\tArrival\tBurst\tWaiting\tTurnaround")
+    for i in range(len(arrival_time)):
+        print(f"P{i+1}\t{arrival_time[i]}\t{burst_time[i]}\t{waiting_time[i]}\t{turnaround_time[i]}")
+
+    print(f"Average Waiting Time: {mean(waiting_time):.2f}")
+    print(f"Average Turnaround Time: {mean(turnaround_time):.2f}")

scheduling/priority_scheduling.py

@@ -0,0 +1,68 @@
+"""
+Priority Scheduling assigns a priority to each process. The CPU is allocated
+to the process with the highest priority (lowest number). It can be preemptive
+or non-preemptive.
+https://en.wikipedia.org/wiki/Priority_scheduling
+"""
+
+from statistics import mean
+
+def calculate_priority_waiting_time(arrival: list, burst: list, priority: list) -> list:
+    """
+    Calculate waiting time for each process using preemptive priority scheduling.
+
+    >>> calculate_priority_waiting_time([0, 1, 2, 3], [10, 1, 2, 1], [3, 1, 4, 2])
+    [2, 0, 10, 0]
+    """
+    n = len(arrival)
+    remaining = burst.copy()
+    waiting = [0] * n
+    complete = 0
+    t = 0
+
+    while complete < n:
+        idx = -1
+        highest_pri = float('inf')
+        for i in range(n):
+            if arrival[i] <= t and remaining[i] > 0 and priority[i] < highest_pri:
+                highest_pri = priority[i]
+                idx = i
+
+        if idx == -1:
+            t += 1
+            continue
+
+        remaining[idx] -= 1
+        t += 1
+
+        if remaining[idx] == 0:
+            complete += 1
+            waiting[idx] = t - arrival[idx] - burst[idx]
+
+    return waiting
+
+
+def calculate_priority_turnaround_time(burst: list, waiting: list) -> list:
+    """
+    Calculate turn around time for each process using waiting time.
+
+    >>> calculate_priority_turnaround_time([10, 1, 2, 1], [3, 0, 5, 1])
+    [13, 1, 7, 2]
+    """
+    return [burst[i] + waiting[i] for i in range(len(burst))]
+
+
+if __name__ == "__main__":
+    arrival_time = [0, 1, 2, 3]
+    burst_time = [10, 1, 2, 1]
+    priority = [3, 1, 4, 2]
+
+    waiting_time = calculate_priority_waiting_time(arrival_time, burst_time, priority)
+    turnaround_time = calculate_priority_turnaround_time(burst_time, waiting_time)
+
+    print("PID\tArrival\tBurst\tPriority\tWaiting\tTurnaround")
+    for i in range(len(arrival_time)):
+        print(f"P{i+1}\t{arrival_time[i]}\t{burst_time[i]}\t{priority[i]}\t\t{waiting_time[i]}\t{turnaround_time[i]}")
+
+    print(f"Average Waiting Time: {mean(waiting_time):.2f}")
+    print(f"Average Turnaround Time: {mean(turnaround_time):.2f}")