## The challenge#

Given a credit card number we can determine who the issuer/vendor is with a few basic knowns.

Complete the function `getIssuer()` that will use the values shown below to determine the card issuer for a given card number. If the number cannot be matched then the function should return the string `Unknown`.

``````| Card Type  | Begins With          | Number Length |
|------------|----------------------|---------------|
| AMEX       | 34 or 37             | 15            |
| Discover   | 6011                 | 16            |
| Mastercard | 51, 52, 53, 54 or 55 | 16            |
| VISA       | 4                    | 13 or 16      |
``````

Examples

``````getIssuer(4111111111111111) == "VISA"
getIssuer(4111111111111) == "VISA"
getIssuer(4012888888881881) == "VISA"
getIssuer(378282246310005) == "AMEX"
getIssuer(6011111111111117) == "Discover"
getIssuer(5105105105105100) == "Mastercard"
getIssuer(5105105105105106) == "Mastercard"
getIssuer(9111111111111111) == "Unknown"
``````

## The solution in Java code#

Option 1:

``````public class CreditCardChecker {
public static String getIssuer(String cardNumber) {
if ((cardNumber.startsWith("34") || cardNumber.startsWith("37")) && cardNumber.length()==15)
return "AMEX";
if (cardNumber.startsWith("6011") && cardNumber.length()==16)
return "Discover";
if ((cardNumber.startsWith("51") || cardNumber.startsWith("52") || cardNumber.startsWith("53") || cardNumber.startsWith("54") || cardNumber.startsWith("55")) && cardNumber.length()==16)
return "Mastercard";
if (cardNumber.startsWith("4") && (cardNumber.length()==13 || cardNumber.length()==16))
return "VISA";
return "Unknown";
}
}
``````

Option 2:

``````import static java.util.regex.Pattern.matches;
public class CreditCardChecker {
public static String getIssuer(String cn) {
if (matches("^3[4|7].{13}\$", cn)) return "AMEX";
if (matches("^6011.{12}\$", cn)) return "Discover";
if (matches("^5[1-5].{14}\$", cn)) return "Mastercard";
if (matches("^4.{12}(...)?\$", cn)) return "VISA";
return "Unknown";
}
}
``````

Option 3:

``````public class CreditCardChecker {
public static String getIssuer(String cardNumber) {
String visa = "4+(\\d{15}|\\d{12})";
String amex = "(34|37)+\\d{13}";
String discover = "6011+\\d{12}";
String mastercard = "(51|52|53|54|55)+\\d{14}";
String result = "Unknown";
if (cardNumber.matches(visa)) return "VISA";
if (cardNumber.matches(amex)) return "AMEX";
if (cardNumber.matches(discover)) return "Discover";
if (cardNumber.matches(mastercard)) return "Mastercard";
return result;
}
}
``````

## Test cases to validate our solution#

``````import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;
public class SolutionTest {
@Test
public void issuerTests() {
assertEquals("VISA",       CreditCardChecker.getIssuer("4111111111111111"));
assertEquals("AMEX",       CreditCardChecker.getIssuer("378282246310005"));
assertEquals("Unknown",    CreditCardChecker.getIssuer("9111111111111111"));
assertEquals("Mastercard", CreditCardChecker.getIssuer("5105105105105100"));
}
}
``````

``````import org.junit.Test;
import static org.junit.Assert.assertEquals;
import org.junit.runners.JUnit4;

public class SolutionTest {
@Test
public void issuerTests() {
assertEquals("VISA",       CreditCardChecker.getIssuer("4111111111111111"));
assertEquals("VISA",       CreditCardChecker.getIssuer("4111111111111"));
assertEquals("VISA",       CreditCardChecker.getIssuer("4012888888881881"));
assertEquals("Unknown",    CreditCardChecker.getIssuer("41111111111111"));
assertEquals("Unknown",    CreditCardChecker.getIssuer("411111111111111"));

assertEquals("AMEX",       CreditCardChecker.getIssuer("378282246310005"));
assertEquals("AMEX",       CreditCardChecker.getIssuer("348282246310005"));

assertEquals("Discover",   CreditCardChecker.getIssuer("6011111111111117"));

assertEquals("Mastercard", CreditCardChecker.getIssuer("5105105105105100"));
assertEquals("Mastercard", CreditCardChecker.getIssuer("5105105105105106"));
assertEquals("Mastercard", CreditCardChecker.getIssuer("5205105105105106"));
assertEquals("Mastercard", CreditCardChecker.getIssuer("5305105105105106"));
assertEquals("Mastercard", CreditCardChecker.getIssuer("5405105105105106"));
assertEquals("Mastercard", CreditCardChecker.getIssuer("5505105105105106"));

assertEquals("Unknown",    CreditCardChecker.getIssuer("9111111111111111"));
assertEquals("Unknown",    CreditCardChecker.getIssuer("5605105105105106"));
}
@Test
public void randomTests() {

}
}
``````