import re
import pandas as pd

def preprocess(data):
    """
    Preprocesses raw WhatsApp chat data into a structured pandas DataFrame.
    """
    print("Preprocess started")

    # Robust regex to capture the timestamp pattern including optional seconds and AM/PM variations.
    pattern = r'\d{1,2}/\d{1,2}/\d{2,4},\s*(?:1[0-2]|0?[1-9]):[0-5][0-9](?::[0-5][0-9])?\s*[\s\u202f\u00a0]?(?:AM|PM)\s*-\s*'
    
    # Split the data by the timestamp pattern (case-insensitive flag re.I).
    messages = re.split(pattern, data, flags=re.I)[1:]
    
    # Find all occurrences of the timestamp pattern.
    date = re.findall(pattern, data, flags=re.I)
    
    print(f"Found {len(messages)} messages and {len(date)} dates")

    if len(messages) != len(date):
        print("Error: The number of messages and dates do not match.")
        return None

    # Clean up dates before processing by removing non-breaking spaces
    dates = [d.replace('\u202f', ' ').replace('\u00a0', ' ') for d in date]
    df = pd.DataFrame({'user_message': messages, 'message_date': dates})

    # --- Date Parsing: Try common formats ---
    known_formats = [
        '%d/%m/%y, %I:%M %p - ',        # Format without seconds (e.g., 01/01/25, 8:09 am)
        '%d/%m/%Y, %I:%M %p - ',        # Format without seconds (4-digit year)
        '%d/%m/%Y, %I:%M:%S %p - '      # Format with seconds (4-digit year)
    ]
    
    df['date'] = pd.NaT 
    for fmt in known_formats:
        converted = pd.to_datetime(df['message_date'], format=fmt, errors='coerce')
        # Fill existing NaT values with successful conversions
        df['date'] = df['date'].fillna(converted)
        
    if df['date'].isna().any():
        print("Warning: Date parsing failed for some rows. Rows without a valid date will be dropped.")
    
    df.rename(columns={'date': 'date'}, inplace=True)

    # --- Extract Users and Messages ---
    users = []
    messages_list = []
    
    # Regex to capture sender name non-greedily up to the colon separator.
    user_pattern = r'^([\w\W]+?):\s' 
    
    for message in df['user_message']:
        entry = re.split(user_pattern, message, maxsplit=1)
        
        if len(entry) > 1: # Standard message format: Sender: Message
            users.append(entry[1].strip())
            messages_list.append(entry[2].strip())
        else: # Group notification or metadata
            users.append('group_notification')
            messages_list.append(entry[0].strip())

    df['user'] = users
    df['message'] = messages_list

    # --- Final Cleanup and Feature Creation ---
    df.drop(columns=['user_message', 'message_date'], inplace=True)
    
    df.dropna(subset=['date'], inplace=True) # Drop rows where date parsing failed

    # Extract temporal features
    df['only_date'] = df['date'].dt.date
    df['year'] = df['date'].dt.year
    df['month_num'] = df['date'].dt.month
    df['month'] = df['date'].dt.month_name()
    df['day'] = df['date'].dt.day
    df['day_name'] = df['date'].dt.day_name()
    df['hour'] = df['date'].dt.hour
    df['minute'] = df['date'].dt.minute

    return df