Validate Credit Card Number

Create a function that takes a number as an argument and returns true if the number is a valid credit card number, false otherwise.

Credit card numbers must be between 14-19 digits in length, and pass the Luhn test, described below:

1 Remove the last digit (this is the "check digit").
2 Reverse the number.
3 Double the value of each digit in odd-numbered positions. If the doubled value has more than 1 digit, add the digits together (e.g. 8 x 2 = 16 ➞ 1 + 6 = 7).
4 Add all digits.
5 Subtract the last digit of the sum (from step 4) from 10. The result should be equal to the check digit from step 1.

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Examples

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validateCard(1234567890123456) ➞ false

// Step 1: check digit = 6, num = 123456789012345
// Step 2: num reversed = 543210987654321
// Step 3: digit array after selective doubling: [1, 4, 6, 2, 2, 0, 9, 8, 5, 6, 1, 4, 6, 2, 2]
// Step 4: sum = 58
// Step 5: 10 - 8 = 2 (not equal to 6) ➞ false

validateCard(1234567890123452) ➞ true

// Same as above, but check digit checks out.

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Tests

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import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class ChallengeTests {
  @Test
  public void test1() {
    assertEquals(false, Challenge.validateCard(79927398714L));
  }
   
    @Test
  public void test2() {
        System.out.println("Passes Luhn test, but too short.");
    assertEquals(false, Challenge.validateCard(79927398713L));
  }
   
    @Test
  public void test3() {
    assertEquals(true, Challenge.validateCard(709092739800713L));
  }
   
    @Test
  public void test4() {
    assertEquals(false, Challenge.validateCard(1234567890123456L));
  }
   
    @Test
  public void test5() {
    assertEquals(true, Challenge.validateCard(12345678901237L));
  }
   
    @Test
  public void test6() {
    assertEquals(true, Challenge.validateCard(5496683867445267L));
  }
   
    @Test
  public void test7() {
    assertEquals(false, Challenge.validateCard(4508793361140566L));
  }
   
    @Test
  public void test8() {
    assertEquals(true, Challenge.validateCard(376785877526048L));
  }
   
    @Test
  public void test9() {
    assertEquals(false, Challenge.validateCard(36717601781975L));
  }
}

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