Community Health Index (CHI) Generator, United Kingdom, Scotland

generate_chi.py

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import random

def generate_chi(dob: str, is_odd: bool) -> str:
    """
    Generate a valid CHI (Community Health Index) number.

    Parameters:
        dob (str): Date of birth in DDMMYY format (e.g., '010203')
        is_odd (bool): True: Male, or False: Female

    Returns:
        str: A valid 10-digit CHI number, or raises ValueError if invalid.
    """

    if len(dob) != 6 or not dob.isdigit():
        raise ValueError("Date of birth must be a 6-digit string in DDMMYY format.")

    # Generate a 3-digit sequence number based on is_odd.
    # Odd = Male, Even = Female.
    while True:
        seq = random.randint(0, 999)
        if seq % 2 == (1 if is_odd else 0):  
            break
    seq_str = f"{seq:03d}"
    first_nine = dob + seq_str

    # Calculate check digit using Modulus 11.
    weights = [10, 9, 8, 7, 6, 5, 4, 3, 2]
    total = sum(int(d) * w for d, w in zip(first_nine, weights))
    remainder = total % 11
    check_digit = 11 - remainder

    if check_digit == 11:
        check_digit = 0
    elif check_digit == 10:
        # Invalid combination — regenerate sequence number.
        return generate_chi(dob, is_odd)

    return first_nine + str(check_digit)