Add isbn-verifier exercise

exercises/isbn-verifier/canonical-data.json

@@ -0,0 +1,88 @@
+{
+    "exercise": "isbn-verifier",
+    "version": "1.0.0",
+    "comments": [
+        "An expected value of true indicates a valid ISBN-10, ",
+        "whereas false means the ISBN-10 is invalid."
+    ],
+    "cases": [
+        {
+            "description": "valid isbn number",
+            "property": "isbn",
+            "input": "3-598-21508-8",
+            "expected": true
+        },
+        {
+            "description": "invalid isbn check digit",
+            "property": "isbn",
+            "input": "3-598-21508-9",
+            "expected": false
+        },
+        {
+            "description": "valid isbn number with a check digit of 10",
+            "property": "isbn",
+            "input": "3-598-21507-X",
+            "expected": true
+        },
+        {
+            "description": "check digit is a character other than X",
+            "property": "isbn",
+            "input": "3-598-21507-A",
+            "expected": false
+        },
+        {
+            "description": "invalid character in isbn",
+            "property": "isbn",
+            "input": "3-598-2K507-0",
+            "expected": false
+        },
+        {
+            "description": "X is only valid as a check digit",
+            "property": "isbn",
+            "input": "3-598-2X507-0",
+            "expected": false
+        },
+        {
+            "description": "valid isbn without separating dashes",
+            "property": "isbn",
+            "input": "3598215088",
+            "expected": true
+        },
+        {
+            "description": "isbn without separating dashes and X as check digit",
+            "property": "isbn",
+            "input": "359821507X",
+            "expected": true
+        },
+        {
+            "description": "isbn without check digit and dashes",
+            "property": "isbn",
+            "input": "359821507",
+            "expected": false
+        },
+        {
+            "description": "too long isbn and no dashes",
+            "property": "isbn",
+            "input": "3598215078X",
+            "expected": false
+        },
+        {
+            "description": "isbn without check digit",
+            "property": "isbn",
+            "input": "3-598-21507",
+            "expected": false
+        },
+        {
+            "description": "too long isbn",
+            "property": "isbn",
+            "input": "3-598-21507-XA",
+            "expected": false
+        },
+        {
+            "description": "check digit of X should not be used for 0",
+            "property": "isbn",
+            "input": "3-598-21515-X",
+            "expected": false
+        }
+    ]
+}

exercises/isbn-verifier/description.md

@@ -0,0 +1,35 @@
+Check if a given ISBN-10 is valid.
+
+## Functionality
+
+Given an unkown 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 allow for ISBN-10 without the separating dashes to be verified as well.
+
+## ISBN
+
+Let's take a random ISBN-10 number, say `3-598-21508-8` for this.
+The first digit block indicates the group where the ISBN belongs. Groups can consist of shared languages, geographic regions or countries. The leading '3' signals this ISBN is from a german speaking country.
+The following number block is to identify the publisher. Since this is a three digit publisher number there is a 5 digit title number for this book.
+The last digit in the ISBN is the check digit which is used to detect read errors.
+
+The first 9 digits in the ISBN have to be between 0 and 9.
+The check digit can additionally be an 'X' to allow 10 to be a valid check digit as well.
+
+A valid ISBN-10 is calculated with this formula `(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0`
+So for our example ISBN this means:
+(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 = 0
+
+Which proves that the ISBN is valid.
+
+## Caveats
+
+Converting from string to number can be tricky in certain languages.
+It's getting even trickier since the check-digit of an ISBN-10 can be 'X'.
+
+## Bonus tasks
+
+* Generate a valid ISBN-13 from the input ISBN-10 (and maybe verify it again with a derived verifier)
+
+* Generate valid ISBN, maybe even from a given starting ISBN

exercises/isbn-verifier/metadata.yml

@@ -0,0 +1,4 @@
+---
+blurb: "Check if a given string is a valid ISBN-10 number."
+source: "Converting a string into a number and some basic processing utilizing a relatable real world example."
+source_url: "https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation"