SJF Scheduling — Shortest Job First (Preemptive and Non-Preemptive)

import heapq

def sjf_non_preemptive(processes: list[dict]) -> list[dict]:
    procs = sorted(processes, key=lambda p: p['arrival'])
    results = []
    current_time = 0
    heap = []  # (burst, arrival, process)
    i = 0
    while i < len(procs) or heap:
        # Add all processes that have arrived
        while i < len(procs) and procs[i]['arrival'] <= current_time:
            heapq.heappush(heap, (procs[i]['burst'], procs[i]['arrival'], procs[i]))
            i += 1
        if not heap:
            current_time = procs[i]['arrival']
            continue
        burst, arrival, p = heapq.heappop(heap)
        start = current_time
        ct  = start + p['burst']
        tat = ct - p['arrival']
        wt  = tat - p['burst']
        current_time = ct
        results.append({**p, 'start': start, 'ct': ct, 'tat': tat, 'wt': wt})
    return results

processes = [
    {'pid':'P1','arrival':0,'burst':8},
    {'pid':'P2','arrival':1,'burst':4},
    {'pid':'P3','arrival':2,'burst':2},
    {'pid':'P4','arrival':3,'burst':1},
]
results = sjf_non_preemptive(processes)
for r in results:
    print(f"{r['pid']}: WT={r['wt']}, TAT={r['tat']}")
print(f"Avg WT: {sum(r['wt'] for r in results)/len(results):.2f}")

def srtf(processes: list[dict]) -> list[dict]:
    """Shortest Remaining Time First — preemptive SJF."""
    procs = [dict(p, remaining=p['burst'], start=-1) for p in processes]
    procs.sort(key=lambda p: p['arrival'])
    n = len(procs)
    time = 0
    completed = 0
    current = None
    results = {}
    while completed < n:
        # Find process with shortest remaining time among arrived
        arrived = [p for p in procs if p['arrival'] <= time and p['remaining'] > 0]
        if not arrived:
            time += 1
            continue
        current = min(arrived, key=lambda p: p['remaining'])
        if current['start'] == -1:
            current['start'] = time
        current['remaining'] -= 1
        time += 1
        if current['remaining'] == 0:
            ct  = time
            tat = ct - current['arrival']
            wt  = tat - current['burst']
            results[current['pid']] = {**current,'ct':ct,'tat':tat,'wt':wt}
            completed += 1
    return list(results.values())

results = srtf(processes)
for r in results:
    print(f"{r['pid']}: WT={r['wt']}, TAT={r['tat']}")
print(f"Avg WT: {sum(r['wt'] for r in results)/len(results):.2f}")