It Gets Easier. You Have To Do It Every Day. That's The Hard Part But It Does Get Easier

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2019-08-05 04:20 UTC

It Gets Easier. You Have To Do It Every Day. That's The Hard Part But It Does Get Easier

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2019-08-12 04:20 UTC
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Problem B
Luhn's Checksum Algorithm

/problems/luhnchecksum/file/statement/en/img-0001.jpg
Image by simpson33 (iStock), Used under license

In 1954, Hans Peter Luhn, a researcher at IBM, filed a patent describing a simple checksum algorithm for numbers written as strings of base-$10$ digits. If a number is chosen according to Luhn’s technique, the algorithm provides a basic integrity check. This means that with reasonably high probability it can detect whether one or more digits have been accidentally modified. (On the other hand, it provides essentially no protection against intentional modifications.) Most credit card and bank card numbers can be validated using Luhn’s checksum algorithm, as can the national identification numbers of several countries (including Canada).

Given a number $n = d_ k d_{k-1} \ldots d_2 d_1$, where each $d_ i$ is a base-$10$ digit, here is how to apply Luhn’s checksum test:

  1. Starting at the right end of $n$, transform every second digit $d_ i$ (i.e., $d_2, d_4, d_6, \ldots $) as follows:

    • multiply $d_ i$ by $2$

    • if $2 \cdot d_ i$ consists of more than one digit, i.e., is greater than 9, add these digits together; this will always produce a single-digit number

  2. Add up all the digits of $n$ after the transformation step. If the resulting sum is divisible by $10$, $n$ passes the Luhn checksum test. Otherwise, $n$ fails the Luhn checksum test.

For example, consider the number $n = 1234567890123411$ from Sample Input 1. The first row of Figure 1 gives the original digits of $n$, and the second row contains the digits of $n$ after the transformation step, with transformed digits shown in bold. The sum of the digits in the second row is

\[ 2+2+6+4+1+6+5+8+9+0+2+2+6+4+2+1=60 \]

and since $60$ is divisible by $10$, $n$ passes the Luhn checksum test.

$1$

$2$

$3$

$4$

$5$

$6$

$7$

$8$

$9$

$0$

$1$

$2$

$3$

$4$

$1$

$1$

$\mathbf{2}$

$2$

$\mathbf{6}$

$4$

$\mathbf{1}$

$6$

$\mathbf{5}$

$8$

$\mathbf{9}$

$0$

$\mathbf{2}$

$2$

$\mathbf{6}$

$4$

$\mathbf{2}$

$1$

Figure 1: Application of Luhn’s algorithm to $n = 1234567890123411$

Input

The first line of input contains a single integer $T$ $(1 \leq T \leq 100)$, the number of test cases. Each of the following $T$ lines contains a single test case consisting of a number given as a string of base-$10$ digits (09). The length of each string is between $2$ and $50$, inclusive, and numbers may have leading (leftmost) zeros.

Output

For each test case, output a single line containing “PASS” if the number passes the Luhn checksum test, or “FAIL” if the number fails the Luhn checksum test.

Sample Input 1 Sample Output 1
3
00554
999
1234567890123411
PASS
FAIL
PASS