## Exercism - ISBN Verifier

There are lots of different ways to approach every exercise on Exercism. Why not see how others have solved it?

### Instructions

The ISBN-10 verification process is used to validate book identification numbers. These normally contain dashes and look like: 3-598-21508-8

**ISBN**

The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only). In the case the check character is an X, this represents the value '10'. These may be communicated with or without hyphens, and can be checked for their validity by the following formula:

(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0 If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.

**Example**

Let's take the ISBN-10 3-598-21508-8. We plug it in to the formula, and get:

(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 == 0 Since the result is 0, this proves that our ISBN is valid.

**Task**

Given a string the program should check if the provided string is a valid ISBN-10. Putting this into place requires some thinking about preprocessing/parsing of the string prior to calculating the check digit for the ISBN.

The program should be able to verify ISBN-10 both with and without separating dashes.

**Caveats**

Converting from strings to numbers can be tricky in certain languages. Now, it's even trickier since the check digit of an ISBN-10 may be 'X' (representing '10'). For instance 3-598-21507-X is a valid ISBN-10.

### Solution

I really liked the solution for this one. I manage to get into some new functionality that I was not so familiar with `Thread Macros`. During the development of the solution I recognized this pattern of passing the result of the previous `expr`

to the next one. But I needed a `named`

"thingie" to be passed around. The `as->`

function just came in handy!

```
(ns isbn-verifier)
(defn isbn-formula [seq-vl seq-size]
(if (empty? seq-vl)
[]
(conj (isbn-formula (rest seq-vl) (- seq-size 1)) (* (Integer. (first seq-vl)) seq-size))))
(defn isbn? [isbn] ;; <- arglist goes here
(let [isbn-seq (re-seq #"\d|[X]$" isbn)
size (count isbn-seq)]
(as-> 0 data
(contains? (set isbn-seq) "X")
(if true
(isbn-formula (map #(if (= % "X") "10" %) isbn-seq) size)
(isbn-formula isbn-seq size))
(reduce + data)
(mod data 11)
(= data 0))))
```

Some regexp too. o/

**Link to solution at Exercism**: Link