The Luhn formula is a simple checksum formula used to validate a variety
of identification numbers, such as credit card numbers and Canadian
Social Insurance Numbers.

The formula verifies a number against its included check digit, which is
usually appended to a partial number to generate the full number. This
number must pass the following test:

- Counting from rightmost digit (which is the check digit) and moving
  left, double the value of every second digit.
- For any digits that thus become 10 or more, subtract 9 from the
  result.
  - 1111 becomes 2121.
  - 8763 becomes 7733 (from 2×6=12 → 12-9=3 and 2×8=16 → 16-9=7).
- Add all these digits together.
  - 1111 becomes 2121 sums as 2+1+2+1 to give a check digit of 6.
  - 8763 becomes 7733, and 7+7+3+3 is 20.

If the total ends in 0 (put another way, if the total modulus 10 is
congruent to 0), then the number is valid according to the Luhn formula;
else it is not valid. So, 1111 is not valid (as shown above, it comes
out to 6), while 8763 is valid (as shown above, it comes out to 20).

Write a program that, given a number

- Can check if it is valid per the Luhn formula. This should treat, for
  example, "2323 2005 7766 3554" as valid.
- Can return the checksum, or the remainder from using the Luhn method.
- Can add a check digit to make the number valid per the Luhn formula and
  return the original number plus that digit. This should give "2323 2005 7766
  3554" in response to "2323 2005 7766 355".

## About Checksums

A checksum has to do with error-detection. There are a number of different
ways in which a checksum could be calculated.

When transmitting data, you might also send along a checksum that says how
many bytes of data are being sent. That means that when the data arrives on
the other side, you can count the bytes and compare it to the checksum. If
these are different, then the data has been garbled in transmission.

In the Luhn problem the final digit acts as a sanity check for all the prior
digits. Running those prior digits through a particular algorithm should give
you that final digit.

It doesn't actually tell you if it's a real credit card number, only that it's
a plausible one. It's the same thing with the bytes that get transmitted --
you could have the right number of bytes and still have a garbled message. So
checksums are a simple sanity-check, not a real in-depth verification of the
authenticity of some data. It's often a cheap first pass, and can be used to
quickly discard obviously invalid things.