Credit

Credit is a program written in C that validates credit card numbers using Luhn's Algorithm. The program determines whether a card number is valid and identifies the type of card (American Express, MasterCard, Visa, or INVALID) based on card number structure and checksum verification.

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📖 Background

Credit card numbers are long decimal numbers that include a checksum to verify their validity. Each type of card has unique properties:

• American Express (AMEX): 15 digits, starting with 34 or 37.
• MasterCard: 16 digits, starting with 51, 52, 53, 54, or 55.
• Visa: 13 or 16 digits, starting with 4.

Luhn's Algorithm

The program uses Luhn's Algorithm to validate the card number:

1. Multiply every second-to-last digit by 2.
2. Add the digits of the resulting products together.
3. Add this sum to the sum of the digits that weren’t multiplied by 2.
4. If the total modulo 10 is 0, the card number is valid.

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🚀 How It Works

Input and Output

1. The program prompts the user for a credit card number.
2. It validates the card using Luhn’s Algorithm.
3. If valid, it identifies the type of card:
   • AMEX for American Express.
   • MASTERCARD for MasterCard.
   • VISA for Visa.
4. If invalid, it outputs INVALID.

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🛠 Installation

This program requires the CS50 library. Follow these instructions to install it:

Install the CS50 Library:

Ubuntu

curl -s https://packagecloud.io/install/repositories/cs50/repo/script.deb.sh | sudo bash
sudo apt install libcs50

Fedora

curl -s https://packagecloud.io/install/repositories/cs50/repo/script.rpm.sh | sudo bash
sudo dnf install libcs50

From Source (Linux and macOS)

1. Download the latest release from CS50 Library Releases.
2. Extract the downloaded file:

   tar -xvf libcs50-*.tar.gz
   cd libcs50-*
   sudo make install

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💻 Usage

Compile the Program

Use the following command to compile the program with the CS50 library:

gcc -o credit credit.c -lcs50

Run the Program

Run the program using:

./credit

Example Input and Output

Valid Visa Card

Input:

Number: 4003600000000014

Output:

VISA

Invalid Card

Input:

Number: 123456789012345

Output:

INVALID

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🧪 Example Scenarios

Scenario 1: Valid American Express Card

• Input: 378282246310005
• Output: AMEX

Scenario 2: Valid MasterCard

• Input: 5105105105105100
• Output: MASTERCARD

Scenario 3: Valid Visa (13 Digits)

• Input: 4111111111111
• Output: VISA

Scenario 4: Invalid Number

• Input: 6176292929
• Output: INVALID

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🔍 Code Overview

Key Steps in the Code:

1. Input Validation:

   • Prompts the user for a numeric input using get_long from the CS50 library to handle large numbers.

2. Checksum Calculation:

   • Implements Luhn’s Algorithm:
     • Multiplies every second-to-last digit by 2.
     • Sums the digits of the products and the remaining digits.
     • Checks if the total modulo 10 equals 0.

3. Card Type Identification:

   • Identifies the card type based on the length and starting digits:
     • AMEX for 15 digits starting with 34 or 37.
     • MASTERCARD for 16 digits starting with 51–55.
     • VISA for 13 or 16 digits starting with 4.

4. Output:

   • Prints the card type or INVALID.

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🛡️ Edge Cases

1. Invalid Length:

   • Rejects card numbers with lengths other than 13, 15, or 16.

2. Invalid Checksum:

   • Outputs INVALID if the checksum fails.

3. Unsupported Starting Digits:

   • Outputs INVALID for valid-length numbers that don’t match known card types.

4. Non-Numeric Input:

   • The program ensures only numeric input is processed.

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📜 Licensing

This project was developed as part of a CS50 assignment and adheres to its guidelines.

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