Add isbn-verifier exercise (#370)

config.json

@@ -607,6 +607,14 @@
           "filtering",
           "strings"
         ]
+      },
+      {
+        "slug": "isbn-verifier",
+        "name": "Isbn Verifier",
+        "uuid": "b0cbc3b5-1d6c-40b8-bdd1-5ba9089024aa",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

exercises/practice/isbn-verifier/.docs/instructions.md

@@ -0,0 +1,42 @@
+# Instructions
+
+The [ISBN-10 verification process][isbn-verification] 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:
+
+```text
+(d₁ * 10 + d₂ * 9 + d₃ * 8 + d₄ * 7 + d₅ * 6 + d₆ * 5 + d₇ * 4 + d₈ * 3 + d₉ * 2 + d₁₀ * 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:
+
+```text
+(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.
+
+[isbn-verification]: https://en.wikipedia.org/wiki/International_Standard_Book_Number

exercises/practice/isbn-verifier/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "isbn-verifier.el"
+    ],
+    "test": [
+      "isbn-verifier-test.el"
+    ],
+    "example": [
+      ".meta/example.el"
+    ]
+  },
+  "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"
+}

exercises/practice/isbn-verifier/.meta/example.el

@@ -0,0 +1,23 @@
+;;; isbn-verifier.el --- ISBN Verifier (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'cl-lib)
+
+(defun digit-value (c)
+  (if (= ?X c) 10 (- c ?0)))
+
+(defun validp (isbn)
+    (let ((total 0)
+          (weight 10)
+          (characters (remq ?- (cl-coerce isbn 'list))))
+      (cl-loop for c in characters
+        always (or (cl-digit-char-p c) (and (= ?X c) (= weight 1)))
+        do (setq total (+ total (* weight (digit-value c))))
+        do (setq weight (- weight 1))
+        finally return (and (= 0 weight) (= 0 (% total 11))))))
+
+(provide 'isbn-verifier)
+;;; isbn-verifier.el ends here

exercises/practice/isbn-verifier/.meta/tests.toml

@@ -0,0 +1,67 @@
+# This is an auto-generated file.
+#
+# Regenerating this file via `configlet sync` will:
+# - Recreate every `description` key/value pair
+# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
+# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
+# - Preserve any other key/value pair
+#
+# As user-added comments (using the # character) will be removed when this file
+# is regenerated, comments can be added via a `comment` key.
+
+[0caa3eac-d2e3-4c29-8df8-b188bc8c9292]
+description = "valid isbn"
+
+[19f76b53-7c24-45f8-87b8-4604d0ccd248]
+description = "invalid isbn check digit"
+
+[4164bfee-fb0a-4a1c-9f70-64c6a1903dcd]
+description = "valid isbn with a check digit of 10"
+
+[3ed50db1-8982-4423-a993-93174a20825c]
+description = "check digit is a character other than X"
+
+[9416f4a5-fe01-4b61-a07b-eb75892ef562]
+description = "invalid check digit in isbn is not treated as zero"
+
+[c19ba0c4-014f-4dc3-a63f-ff9aefc9b5ec]
+description = "invalid character in isbn is not treated as zero"
+
+[28025280-2c39-4092-9719-f3234b89c627]
+description = "X is only valid as a check digit"
+
+[f6294e61-7e79-46b3-977b-f48789a4945b]
+description = "valid isbn without separating dashes"
+
+[185ab99b-3a1b-45f3-aeec-b80d80b07f0b]
+description = "isbn without separating dashes and X as check digit"
+
+[7725a837-ec8e-4528-a92a-d981dd8cf3e2]
+description = "isbn without check digit and dashes"
+
+[47e4dfba-9c20-46ed-9958-4d3190630bdf]
+description = "too long isbn and no dashes"
+
+[737f4e91-cbba-4175-95bf-ae630b41fb60]
+description = "too short isbn"
+
+[5458a128-a9b6-4ff8-8afb-674e74567cef]
+description = "isbn without check digit"
+
+[70b6ad83-d0a2-4ca7-a4d5-a9ab731800f7]
+description = "check digit of X should not be used for 0"
+
+[94610459-55ab-4c35-9b93-ff6ea1a8e562]
+description = "empty isbn"
+
+[7bff28d4-d770-48cc-80d6-b20b3a0fb46c]
+description = "input is 9 characters"
+
+[ed6e8d1b-382c-4081-8326-8b772c581fec]
+description = "invalid characters are not ignored after checking length"
+
+[daad3e58-ce00-4395-8a8e-e3eded1cdc86]
+description = "invalid characters are not ignored before checking length"
+
+[fb5e48d8-7c03-4bfb-a088-b101df16fdc3]
+description = "input is too long but contains a valid isbn"

exercises/practice/isbn-verifier/isbn-verifier-test.el

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+;;; isbn-verifier-test.el --- Tests for Isbn Verifier (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+(load-file "isbn-verifier.el")
+(declare-function validp "isbn-verifier.el" (isbn))
+
+
+(ert-deftest valid-isbn ()
+  (should (validp "3-598-21508-8")))
+
+
+(ert-deftest invalid-isbn-check-digit ()
+  (should-not (validp "3-598-21508-9")))
+
+
+(ert-deftest valid-isbn-with-a-check-digit-of-10 ()
+  (should (validp "3-598-21507-X")))
+
+
+(ert-deftest check-digit-is-a-character-other-than-x ()
+  (should-not (validp "3-598-21507-A")))
+
+
+(ert-deftest invalid-check-digit-in-isbn-is-not-treated-as-zero ()
+  (should-not (validp "4-598-21507-B")))
+
+
+(ert-deftest invalid-character-in-isbn-is-not-treated-as-zero ()
+  (should-not (validp "3-598-P1581-X")))
+
+
+(ert-deftest x-is-only-valid-as-a-check-digit ()
+  (should-not (validp "3-598-2X507-9")))
+
+
+(ert-deftest valid-isbn-without-separating-dashes ()
+  (should (validp "3598215088")))
+
+
+(ert-deftest isbn-without-separating-dashes-and-x-as-check-digit ()
+  (should (validp "359821507X")))
+
+
+(ert-deftest isbn-without-check-digit-and-dashes ()
+  (should-not (validp "359821507")))
+
+
+(ert-deftest too-long-isbn-and-no-dashes ()
+  (should-not (validp "3598215078X")))
+
+
+(ert-deftest too-short-isbn ()
+  (should-not (validp "00")))
+
+
+(ert-deftest isbn-without-check-digit ()
+  (should-not (validp "3-598-21507")))
+
+
+(ert-deftest check-digit-of-x-should-not-be-used-for-0 ()
+  (should-not (validp "3-598-21515-X")))
+
+
+(ert-deftest empty-isbn ()
+  (should-not (validp "")))
+
+
+(ert-deftest input-is-9-characters ()
+  (should-not (validp "134456729")))
+
+
+(ert-deftest invalid-characters-are-not-ignored-after-checking-length ()
+  (should-not (validp "3132P34035")))
+
+
+(ert-deftest invalid-characters-are-not-ignored-before-checking-length ()
+  (should-not (validp "3598P215088")))
+
+
+(ert-deftest input-is-too-long-but-contains-a-valid-isbn ()
+  (should-not (validp "98245726788")))
+
+
+(provide 'isbn-verifier-test)
+;;; isbn-verifier-test.el ends here

exercises/practice/isbn-verifier/isbn-verifier.el

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+;;; isbn-verifier.el --- ISBN Verifier (exercism)  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;;; Code:
+
+
+(defun validp (isbn)
+  (error "Delete this S-Expression and write your own implementation"))
+
+
+(provide 'isbn-verifier)
+;;; isbn-verifier.el ends here