Build a Credit Card validator using Go

Introduction 🪩

In this tutorial, we will build a credit card validator using Go and the Luhn Algorithm.

The Luhn algorithm is a mathematical formula used to validate credit card numbers. We are going to write a function that implements it and use it to validate a card number.
To achieve this, we will build a very simple server in Go which processes POST requests, extract the JSON payload with the card number, and returns a JSON response if the number is valid or not.

⚙️What you'll need

• Your IDE of choice

• Postman (alternatively you can use Curl)

Let's start coding 🚀

Project set up

1. Create a new directory for your project, for example, credit-card-validation.

2. Open a terminal and navigate to the project directory: cd path/to/credit-card-validation

3. Inside your project directory, create a Go file where we will implement the Luhn algorithm: touch luhn_algorithm.go

Implement the Luhn Algorithm

To implement the algorithm, I've translated into code the rules of the formula, taken from the internet. So, there isn't much to add there but I will post the code for the function with inline comments to give context.

package main

func luhnAlgorithm(cardNumber string) bool {
    // this function implements the luhn algorithm
    // it takes as argument a cardnumber of type string
    // and it returns a boolean (true or false) if the
    // card number is valid or not

    // initialise a variable to keep track of the total sum of digits
    total := 0
    // Initialize a flag to track whether the current digit is the second digit from the right.
    isSecondDigit := false

    // iterate through the card number digits in reverse order
    for i := len(cardNumber) - 1; i >= 0; i-- {
        // conver the digit character to an integer
        digit := int(cardNumber[i] - '0')

        if isSecondDigit {
            // double the digit for each second digit from the right
            digit *= 2
            if digit > 9 {
                // If doubling the digit results in a two-digit number,
                //subtract 9 to get the sum of digits.
                digit -= 9
            }
        }

        // Add the current digit to the total sum
        total += digit

        //Toggle the flag for the next iteration.
        isSecondDigit = !isSecondDigit
    }

    // return whether the total sum is divisible by 10
    // making it a valid luhn number
    return total%10 == 0
}

Build your server

We will now set up a simple server that you can run locally. Create a new Go file: touch main.go

We're only going to write one route, and for a bit of extra flare, we'll pass the port number as a command line argument, instead of hard-coding it.

func main() {
    args := os.Args
    port := args[1]

    // Register the creditCardValidator function to handle requests at the root ("/") path.
    http.HandleFunc("/", creditCardValidator)
    fmt.Println("Listening on port:", port) 
    // Start an HTTP server listening on the specified port.
    err := http.ListenAndServe(":"+port, nil)
    if err != nil {
        fmt.Println("Error:", err) // Print an error message if the server fails to start.
    }
}

In the code, the have put a placeholder function creditCardValidator to handle the requests that arrive at route "/" - we will now implement this function in the same file.

import (
    "encoding/json"
    "fmt"
    "net/http"
    "os"
)
type Response struct {
    Valid bool `json:"valid"`
}

// creditCardValidator handles the credit card validation logic and JSON response.
func creditCardValidator(writer http.ResponseWriter, request *http.Request) {
    // Check if the request method is POST.
    if request.Method != http.MethodPost {
        // if not, throw an error
        http.Error(writer, "Invalid request method", http.StatusMethodNotAllowed)
        return
    }
    // Create a struct to hold the incoming JSON payload.
    var cardNumber struct {
        Number string `json:"number"` // Number field holds the credit card number.
    }

    // Decode the JSON payload from the request body into the cardNumber struct.
    err := json.NewDecoder(request.Body).Decode(&cardNumber)
    if err != nil {
        http.Error(writer, "Invalid JSON payload", http.StatusBadRequest)
        return
    }
    // Validate the credit card number using the Luhn algorithm.
    isValid := luhnAlgorithm(cardNumber.Number)
    // Create a response struct with the validation result.
    response := Response{Valid: isValid}

    // Marshal the response struct into JSON format.
    jsonResponse, err := json.Marshal(response)
    if err != nil {
        http.Error(writer, "Error creating response", http.StatusInternalServerError)
        return
    }

    // Set the content type header to indicate JSON response.
    writer.Header().Set("Content-Type", "application/json")

    // Write the JSON response back to the client.
    writer.Write(jsonResponse)
}

Deep dive 🤿

The & symbol

Note that when we are decoding the JSON, we are using the symbol &before the variable cardNumber.

In the context of our code snippet, cardNumber is a struct variable that holds the credit card number extracted from the JSON payload. The Decode function of the json.NewDecoder reads JSON data from an input source (in this case, the request body r.Body) and tries to populate the fields of the provided struct (in this case, cardNumber) with the corresponding JSON values.

The &cardNumber part is passing a pointer to the cardNumber struct to the Decode function. This allows the Decode function to directly modify the fields of the cardNumber struct using the memory address of the variable, rather than making a copy of the struct. This is more memory-efficient and allows you to work with the actual struct instance rather than a copy!

To "marshal"

In Go, to "marshal" refers to the process of converting a Go data structure (such as a struct, map, or array) into its JSON representation. In other words, it's the process of encoding the Go data into a JSON format that can be sent over the network or stored in a file. That's what we are doing when using json.marshal(response).

Initialize Go modules

1. Initialize Go modules to manage dependencies:

    go mod init credit-card-validation
   

Finally, run the project

1. Run the project using the Go compiler. I am using port 8080, but you can use whichever port you prefer.

    go run main.go luhn_algorithm.go 8080
   

To test if it's working, use Postman or Curl to send a POST request to the server at route "/", adding a credit card number to the request body:

1. Set the headers: Content-Type: application/json

2. Send a POST request to http://localhost:8080/

3. Set the request body to { "number": "4003600000000014" }

If all goes well, you should receive this response {"valid": true}

Conclusion 🏁

Congratulations on building a credit card validator!

Let me know, what else should I implement? And did you enjoy this blog post? Leave a comment with your thoughts, and see you next time 👋