def calculate_ean13_check_digit(code_without_check: str) -> str:
    """
    Calculates the EAN-13 check digit.
    
    The code_without_check must be a 12-digit string.
    Algorithm (as per spec):
    1. Sum odd positions (1st, 3rd, 5th...) x 3
    2. Sum even positions (2nd, 4th, 6th...) x 1
    3. check_digit = (10 - (total_sum % 10)) % 10
    """
    if len(code_without_check) != 12:
        raise ValueError("EAN-13 code (without check digit) must be 12 digits long.")

    # Note: Spec says "Odd positions x 3", "Even positions x 1".
    # EAN-13 standard *actually* does Odd x 1, Even x 3.
    # We will follow YOUR spec: Odd x 3, Even x 1.
    
    odd_sum = 0
    even_sum = 0
    
    for i, digit in enumerate(code_without_check):
        if (i + 1) % 2 != 0: # Odd position (1st, 3rd, etc.)
            odd_sum += int(digit)
        else: # Even position (2nd, 4th, etc.)
            even_sum += int(digit)

    total_sum = (odd_sum * 3) + (even_sum * 1)
    
    check_digit = (10 - (total_sum % 10)) % 10
    
    return str(check_digit)

def generate_ean13_code(sequence: int, prefix: str = "999900") -> str:
    """
    Generates a full 13-digit EAN-13 code from a sequence number.
    
    Example: sequence 1 -> "999900000001C" (where C is check digit)
    """
    # 1. Pad sequence to 6 digits (e.g., 1 -> "000001")
    sequence_str = str(sequence).zfill(6)
    
    if len(sequence_str) > 6:
        raise ValueError("Sequence number is too large (max 999999).")
        
    # 2. Create 12-digit base
    base_12_digits = prefix + sequence_str
    
    # 3. Calculate check digit
    check_digit = calculate_ean13_check_digit(base_12_digits)
    
    # 4. Return full 13-digit code
    return base_12_digits + check_digit