Luhn Algorithm for Credit Card Number Validation

LuhnAlgorithm.py

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+"""
+Luhn Algorithm for Credit Card Number Validation
+
+Luhn algorithm is a checksum formula used to validate identification numbers, 
+        including credit card numbers.
+
+How it works:
+1. Traverse digits from right to left
+2. Double every second digit
+3. If doubling result > 9, subtract 9
+4. Sum all digits
+5. If sum % 10 == 0, the number is valid
+
+Author: matei
+Date: 2025
+"""
+
+def luhn_checksum(card_number):
+    def digits_of(n):
+        return [int(d) for d in str(n)]
+
+    digits = digits_of(card_number)
+    odd_digits = digits[-1::-2]
+    even_digits = digits[-2::-2]
+
+    checksum = sum(odd_digits)
+    for digit in even_digits:
+        checksum += sum(digits_of(digit * 2))
+
+    return checksum % 10
+
+
+def is_luhn_valid(card_number):
+    """
+    Check if a card number is valid according to the Luhn algorithm.
+
+    """
+    card_str = str(card_number).replace(" ", "").replace("-", "")
+
+    if not card_str.isdigit():
+        return False
+
+    if len(card_str) < 13 or len(card_str) > 19:
+        return False
+
+    return luhn_checksum(card_str) == 0
+
+
+def identify_card_type(card_number):
+    """
+    Identify card type based on prefix and length.
+
+    """
+    card_str = str(card_number).replace(" ", "").replace("-", "")
+
+    if not card_str.isdigit():
+        return "Invalid format"
+
+    length = len(card_str)
+
+    if card_str.startswith('4') and length in [13, 16, 19]:
+        return "Visa"
+    elif (card_str.startswith(('51', '52', '53', '54', '55')) or 
+          (card_str.startswith('2') and 2221 <= int(card_str[:4]) <= 2720)) and length == 16:
+        return "MasterCard"
+    elif card_str.startswith(('34', '37')) and length == 15:
+        return "American Express"
+    else:
+        return "Unknown card type"
+
+
+def generate_luhn_valid_number(prefix, length):
+    """
+    Generate a Luhn-valid number with given prefix.
+
+    """
+    import random
+
+    if len(prefix) >= length:
+        return "Prefix too long for desired length"
+
+    number = prefix
+    for _ in range(length - len(prefix) - 1):
+        number += str(random.randint(0, 9))
+
+    for check_digit in range(10):
+        test_number = number + str(check_digit)
+        if is_luhn_valid(test_number):
+            return test_number
+
+    return "Could not generate valid number"
+
+
+def format_card_number(card_number, hide_middle=True):
+    """
+    Format card number for display.
+
+    Args:
+        card_number (str): Card number
+        hide_middle (bool): Whether to hide middle digits
+
+    """
+    card_str = str(card_number).replace(" ", "").replace("-", "")
+
+    if hide_middle and len(card_str) >= 8:
+        return card_str[:4] + " **** **** " + card_str[-4:]
+    else:
+        return ' '.join([card_str[i:i+4] for i in range(0, len(card_str), 4)])
+
+
+def validate_and_analyze(card_number):
+    """
+    Validate and provide detailed analysis of a card number.
+
+    """
+    card_str = str(card_number).replace(" ", "").replace("-", "")
+
+    result = {
+        'original': card_number,
+        'cleaned': card_str,
+        'formatted': format_card_number(card_str, hide_middle=False),
+        'is_valid': False,
+        'card_type': 'Unknown',
+        'length': len(card_str),
+        'checksum': None,
+        'errors': []
+    }
+
+    if not card_str.isdigit():
+        result['errors'].append("Contains non-digit characters")
+        return result
+
+    if len(card_str) < 13 or len(card_str) > 19:
+        result['errors'].append(f"Invalid length: {len(card_str)} (should be 13-19)")
+        return result
+
+    result['checksum'] = luhn_checksum(card_str)
+    result['is_valid'] = result['checksum'] == 0
+
+    if result['is_valid']:
+        result['card_type'] = identify_card_type(card_str)
+    else:
+        result['errors'].append(f"Failed Luhn check (checksum: {result['checksum']})")
+
+    return result
+
+
+def main():
+
+    print("=" * 65)
+    print("       CREDIT CARD VALIDATOR - LUHN ALGORITHM")
+    print("=" * 65)
+
+    test_cards = [
+        ("4532015112830366", "Visa - Valid"),
+        ("4532015112830367", "Visa - Invalid"),
+        ("5555555555554444", "MasterCard - Valid"),
+        ("5555555555554443", "MasterCard - Invalid"),
+        ("378282246310005", "American Express - Valid"),
+        ("371449635398431", "American Express - Valid"),
+        ("1234567890123456", "Invalid number"),
+    ]
+
+    print("\n TESTING CARD NUMBERS:")
+    print("-" * 65)
+    print(f"{'Card Number':<20} | {'Status':<10} | {'Type':<15} | {'Note'}")
+    print("-" * 65)
+
+    for card, note in test_cards:
+        analysis = validate_and_analyze(card)
+        formatted_card = format_card_number(card, hide_middle=True)
+
+        status = "VALID, we good bro" if analysis['is_valid'] else "INVALID, not good bro"
+        card_type = analysis['card_type'] if analysis['is_valid'] else "N/A"
+
+        print(f"{formatted_card:<20} | {status:<10} | {card_type:<15} | {note}")
+
+    print("\n" + "=" * 65)
+
+    print("\n GENERATING VALID CARD NUMBERS:")
+    print("-" * 65)
+
+    examples = [
+        ("4532", 16, "Visa"),
+        ("5555", 16, "MasterCard"),
+        ("3782", 15, "American Express"),
+    ]
+
+    for prefix, length, card_type in examples:
+        generated = generate_luhn_valid_number(prefix, length)
+        formatted = format_card_number(generated, hide_middle=False)
+        is_valid = is_luhn_valid(generated)
+
+        print(f"{card_type}: {formatted} {'hehe' if is_valid else 'no'}")
+
+    print("\n" + "=" * 65)
+    print("\n DETAILED ANALYSIS EXAMPLE:")
+    print("-" * 65)
+
+    example_card = "378282246310005"
+    analysis = validate_and_analyze(example_card)
+
+    print(f"Original input: {analysis['original']}")
+    print(f"Cleaned number: {analysis['cleaned']}")
+    print(f"Formatted: {analysis['formatted']}")
+    print(f"Length: {analysis['length']} digits")
+    print(f"Luhn checksum: {analysis['checksum']}")
+    print(f"Is valid: {'YES, we good' if analysis['is_valid'] else 'NO, :('}")
+    print(f"Card type: {analysis['card_type']}")
+
+    if analysis['errors']:
+        print(f"Errors: {', '.join(analysis['errors'])}")
+
+    print("\n" + "=" * 65)
+    print("\n INTERACTIVE TESTING:")
+    print("Enter a card number to validate (or 'exit' to quit)")
+
+    while True:
+        user_input = input("\nCard number: ").strip()
+
+        if user_input.lower() == 'exit':
+            print("bye bye")
+            break
+
+        if not user_input:
+            continue
+
+        analysis = validate_and_analyze(user_input)
+
+        print(f"\nAnalysis for: {format_card_number(user_input, hide_middle=True)}")
+        print("-" * 40)
+
+        if analysis['is_valid']:
+            print(f"Status: VALID")
+            print(f"Type: {analysis['card_type']}")
+            print(f"Length: {analysis['length']} digits")
+        else:
+            print(f"Status: INVALID")
+            if analysis['errors']:
+                print(f"Issues: {', '.join(analysis['errors'])}")
+
+
+if __name__ == "__main__":
+    main()