isbn-verifier: add new exercise

config.json

@@ -1104,6 +1104,15 @@
       "unlocked_by": "luhn",
       "uuid": "37821140-c0d0-4da8-8f50-47356705a615"
     },
+    {
+      "difficulty": 2,
+      "slug": "isbn-verifier",
+      "topics": [
+        "arrays"
+      ],
+      "unlocked_by": "difference-of-squares",
+      "uuid": "6b93edd9-0de4-4c80-6419-68b2f8232b91c3425fa"
+    },
     {
       "deprecated": true,
       "slug": "binary",

exercises/isbn-verifier/README.md

@@ -0,0 +1,42 @@
+# Isbn Verifier
+
+Check if a given ISBN-10 is valid.
+
+## Functionality
+
+Given an unknown 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## Source
+
+Converting a string into a number and some basic processing utilizing a relatable real world example. [https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation](https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/isbn-verifier/example.go

@@ -0,0 +1,57 @@
+package isbn
+
+import (
+	"errors"
+	"math"
+	"strconv"
+	"unicode"
+)
+
+func IsValidISBN(isbn string) bool {
+
+	isbn = dropHyphen(isbn)
+
+	ary, err := strToSlice(isbn)
+	if len(ary) != 10 || err != nil {
+		return false
+	}
+
+	return calcCheckDigit(ary)
+}
+
+func dropHyphen(isbn string) string {
+	var result string
+	for _, char := range isbn {
+		if char == '-' {
+			continue
+		}
+		result += string(char)
+	}
+	return result
+}
+
+func strToSlice(isbn string) (result []int, err error) {
+
+	for _, char := range isbn {
+		if unicode.IsLetter(char) && char != 'X' {
+			err = errors.New("invalid character")
+			return
+		} else if char == 'X' {
+			result = append(result, 10)
+		} else {
+			i, _ := strconv.Atoi(string(char))
+			result = append(result, i)
+		}
+	}
+	return
+}
+
+func calcCheckDigit(isbn []int) bool {
+	var pool int
+	for idx, value := range isbn {
+		pool += int(math.Abs(float64(idx)-10)) * value
+	}
+	result := pool % 11
+
+	return result == 0
+}

exercises/isbn-verifier/isbn_verifier_test.go

@@ -0,0 +1,35 @@
+package isbn
+
+import (
+	"testing"
+)
+
+var testCases = []struct {
+	isbn        string
+	expected    bool
+	description string
+}{
+	{"3-598-21508-8", true, "valid isbn number"},
+	{"3-598-21508-9", false, "invalid isbn check digit"},
+	{"3-598-21507-X", true, "valid isbn number with a check digit of 10"},
+	{"3-598-21507-A", false, "check digit is a character other than X"},
+	{"3-598-2K507-0", false, "invalid character in isbn"},
+	{"3-598-2X507-0", false, "X is only valid as a check digit"},
+	{"3598215088", true, "valid isbn without separating dashes"},
+	{"359821507X", true, "isbn without separating dashes and X as check digit"},
+	{"359821507", false, "isbn without check digit and dashes"},
+	{"3598215078X", false, "too long isbn and no dashes"},
+	{"3-598-21507", false, "isbn without check digit"},
+	{"3-598-21507-XA", false, "too long isbn"},
+	{"3-598-21515-X", false, "check digit of X should not be used for 0"},
+}
+
+func TestIsValidISBN(t *testing.T) {
+	for _, test := range testCases {
+		observed := IsValidISBN(test.isbn)
+		if observed != test.expected {
+			t.Fatalf("got %t, want %t, %s (%s)",
+				observed, test.expected, test.description, test.isbn)
+		}
+	}
+}