Python Critical path calculation

cpm.py

import json
data = [
    {
        'activity': 'a',
        "duration": 3,
        "predecessors": []
    },
    {
        'activity': 'b',
        "duration": 4,
        "predecessors": ['a']
    },
    {
        'activity': 'c',
        "duration": 2,
        "predecessors": ['a']
    },
    {
        'activity': 'd',
        "duration": 5,
        "predecessors": ['b']
    },
    {
        'activity': 'e',
        "duration": 1,
        "predecessors": ['c']
    },
    {
        'activity': 'f',
        "duration": 2,
        "predecessors": ['c']
    },
    {
        'activity': 'g',
        "duration": 4,
        "predecessors": ['d', 'e']
    },
    {
        'activity': 'h',
        "duration": 3,
        "predecessors": ['f', 'g']
    }
]


def calculate_cpm(data):
    """
    Optimized Critical Path Method (CPM) calculation.
    """
    # Build quick lookup for activities
    activity_lookup = {activity['activity']: activity for activity in data}
    
    # Step 1: Forward Pass
    for activity in data:
        es = 0
        for pred in activity['predecessors']:
            pred_ef = activity_lookup[pred]['ef']
            es = max(es, pred_ef)
        activity['es'] = es
        activity['ef'] = es + activity['duration']
    
    # Step 2: Build successors
    successors = {activity['activity']: [] for activity in data}
    for activity in data:
        for pred in activity['predecessors']:
            successors[pred].append(activity['activity'])
    
    # Step 3: Backward Pass
    for activity in reversed(data):
        if not successors[activity['activity']]:
            activity['lf'] = activity['ef']
        else:
            successor_ls = [activity_lookup[succ]['ls'] for succ in successors[activity['activity']]]
            activity['lf'] = min(successor_ls)
        activity['ls'] = activity['lf'] - activity['duration']
        activity['slack'] = activity['lf'] - activity['ef']
    
    # Step 4: Critical Path
    critical_path = [activity['activity'] for activity in data if activity['slack'] == 0]
    print(f"Critical Path: {critical_path}")
    
    return data

critical_path = calculate_cpm(data)
print(json.dumps(critical_path, indent=4, sort_keys=True))