Sync affine-cipher (exercism#841)

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diff --git a/exercises/practice/affine-cipher/.docs/instructions.md b/exercises/practice/affine-cipher/.docs/instructions.md
@@ -1,70 +1,74 @@
 # Instructions
 
-Create an implementation of the affine cipher,
-an ancient encryption system created in the Middle East.
+Create an implementation of the affine cipher, an ancient encryption system created in the Middle East.
 
 The affine cipher is a type of monoalphabetic substitution cipher.
-Each character is mapped to its numeric equivalent, encrypted with
-a mathematical function and then converted to the letter relating to
-its new numeric value. Although all monoalphabetic ciphers are weak,
-the affine cypher is much stronger than the atbash cipher,
-because it has many more keys.
+Each character is mapped to its numeric equivalent, encrypted with a mathematical function and then converted to the letter relating to its new numeric value.
+Although all monoalphabetic ciphers are weak, the affine cipher is much stronger than the atbash cipher, because it has many more keys.
+
+[//]: # " monoalphabetic as spelled by Merriam-Webster, compare to polyalphabetic "
+
+## Encryption
 
 The encryption function is:
 
-  `E(x) = (ax + b) mod m`
-  -  where `x` is the letter's index from 0 - length of alphabet - 1
-  -  `m` is the length of the alphabet. For the roman alphabet `m == 26`.
-  -  and `a` and `b` make the key
+```text
+E(x) = (ai + b) mod m
+```
 
-The decryption function is:
+Where:
+
+- `i` is the letter's index from `0` to the length of the alphabet - 1.
+- `m` is the length of the alphabet.
+  For the Roman alphabet `m` is `26`.
+- `a` and `b` are integers which make up the encryption key.
+
+Values `a` and `m` must be _coprime_ (or, _relatively prime_) for automatic decryption to succeed, i.e., they have number `1` as their only common factor (more information can be found in the [Wikipedia article about coprime integers][coprime-integers]).
+In case `a` is not coprime to `m`, your program should indicate that this is an error.
+Otherwise it should encrypt or decrypt with the provided key.
+
+For the purpose of this exercise, digits are valid input but they are not encrypted.
+Spaces and punctuation characters are excluded.
+Ciphertext is written out in groups of fixed length separated by space, the traditional group size being `5` letters.
+This is to make it harder to guess encrypted text based on word boundaries.
 
-  `D(y) = a^-1(y - b) mod m`
-  -  where `y` is the numeric value of an encrypted letter, ie. `y = E(x)`
-  -  it is important to note that `a^-1` is the modular multiplicative inverse
-     of `a mod m`
-  -  the modular multiplicative inverse of `a` only exists if `a` and `m` are
-     coprime.
+## Decryption
 
-To find the MMI of `a`:
+The decryption function is:
+
+```text
+D(y) = (a^-1)(y - b) mod m
+```
 
-  `an mod m = 1`
-  -  where `n` is the modular multiplicative inverse of `a mod m`
+Where:
 
-More information regarding how to find a Modular Multiplicative Inverse
-and what it means can be found [here.](https://en.wikipedia.org/wiki/Modular_multiplicative_inverse)
+- `y` is the numeric value of an encrypted letter, i.e., `y = E(x)`
+- it is important to note that `a^-1` is the modular multiplicative inverse (MMI) of `a mod m`
+- the modular multiplicative inverse only exists if `a` and `m` are coprime.
 
-Because automatic decryption fails if `a` is not coprime to `m` your
-program should return status 1 and `"Error: a and m must be coprime."`
-if they are not.  Otherwise it should encode or decode with the
-provided key.
+The MMI of `a` is `x` such that the remainder after dividing `ax` by `m` is `1`:
 
-The Caesar (shift) cipher is a simple affine cipher where `a` is 1 and
-`b` as the magnitude results in a static displacement of the letters.
-This is much less secure than a full implementation of the affine cipher.
+```text
+ax mod m = 1
+```
 
-Ciphertext is written out in groups of fixed length, the traditional group
-size being 5 letters, and punctuation is excluded. This is to make it
-harder to guess things based on word boundaries.
+More information regarding how to find a Modular Multiplicative Inverse and what it means can be found in the [related Wikipedia article][mmi].
 
 ## General Examples
 
- - Encoding `test` gives `ybty` with the key a=5 b=7
- - Decoding `ybty` gives `test` with the key a=5 b=7
- - Decoding `ybty` gives `lqul` with the wrong key a=11 b=7
- - Decoding `kqlfd jzvgy tpaet icdhm rtwly kqlon ubstx`
-   - gives `thequickbrownfoxjumpsoverthelazydog` with the key a=19 b=13
- - Encoding `test` with the key a=18 b=13
-   - gives `Error: a and m must be coprime.`
-   - because a and m are not relatively prime
-
-## Examples of finding a Modular Multiplicative Inverse (MMI)
-
-  - simple example:
-    - `9 mod 26 = 9`
-    - `9 * 3 mod 26 = 27 mod 26 = 1`
-    - `3` is the MMI of `9 mod 26`
-  - a more complicated example:
-    - `15 mod 26 = 15`
-    - `15 * 7 mod 26 = 105 mod 26 = 1`
-    - `7` is the MMI of `15 mod 26`
+- Encrypting `"test"` gives `"ybty"` with the key `a = 5`, `b = 7`
+- Decrypting `"ybty"` gives `"test"` with the key `a = 5`, `b = 7`
+- Decrypting `"ybty"` gives `"lqul"` with the wrong key `a = 11`, `b = 7`
+- Decrypting `"kqlfd jzvgy tpaet icdhm rtwly kqlon ubstx"` gives `"thequickbrownfoxjumpsoverthelazydog"` with the key `a = 19`, `b = 13`
+- Encrypting `"test"` with the key `a = 18`, `b = 13` is an error because `18` and `26` are not coprime
+
+## Example of finding a Modular Multiplicative Inverse (MMI)
+
+Finding MMI for `a = 15`:
+
+- `(15 * x) mod 26 = 1`
+- `(15 * 7) mod 26 = 1`, ie. `105 mod 26 = 1`
+- `7` is the MMI of `15 mod 26`
+
+[mmi]: https://en.wikipedia.org/wiki/Modular_multiplicative_inverse
+[coprime-integers]: https://en.wikipedia.org/wiki/Coprime_integers
diff --git a/exercises/practice/affine-cipher/.meta/example.php b/exercises/practice/affine-cipher/.meta/example.php
@@ -1,27 +1,5 @@
 <?php
 
-/*
- * By adding type hints and enabling strict type checking, code can become
- * easier to read, self-documenting and reduce the number of potential bugs.
- * By default, type declarations are non-strict, which means they will attempt
- * to change the original type to match the type specified by the
- * type-declaration.
- *
- * In other words, if you pass a string to a function requiring a float,
- * it will attempt to convert the string value to a float.
- *
- * To enable strict mode, a single declare directive must be placed at the top
- * of the file.
- * This means that the strictness of typing is configured on a per-file basis.
- * This directive not only affects the type declarations of parameters, but also
- * a function's return type.
- *
- * For more info review the Concept on strict type checking in the PHP track
- * <link>.
- *
- * To disable strict typing, comment out the directive below.
- */
-
 declare(strict_types=1);
 
 function encode($text, $a, $b)

diff --git a/exercises/practice/affine-cipher/AffineCipherTest.php b/exercises/practice/affine-cipher/AffineCipherTest.php
@@ -1,27 +1,5 @@
 <?php
 
-/*
- * By adding type hints and enabling strict type checking, code can become
- * easier to read, self-documenting and reduce the number of potential bugs.
- * By default, type declarations are non-strict, which means they will attempt
- * to change the original type to match the type specified by the
- * type-declaration.
- *
- * In other words, if you pass a string to a function requiring a float,
- * it will attempt to convert the string value to a float.
- *
- * To enable strict mode, a single declare directive must be placed at the top
- * of the file.
- * This means that the strictness of typing is configured on a per-file basis.
- * This directive not only affects the type declarations of parameters, but also
- * a function's return type.
- *
- * For more info review the Concept on strict type checking in the PHP track
- * <link>.
- *
- * To disable strict typing, comment out the directive below.
- */
-
 declare(strict_types=1);
 
 /**
@@ -38,34 +16,58 @@ public static function setUpBeforeClass(): void
     }
 
     /**
-     * Tests for encoding English to ciphertext with keys.
+     * Encoding from English to affine cipher
      */
 
+    /**
+     * uuid 2ee1d9af-1c43-416c-b41b-cefd7d4d2b2a
+     * @testdox encode yes
+     */
     public function testEncodeYes(): void
     {
         $this->assertEquals('xbt', encode('yes', 5, 7));
     }
 
+    /**
+     * uuid 785bade9-e98b-4d4f-a5b0-087ba3d7de4b
+     * @testdox encode no
+     */
     public function testEncodeNo(): void
     {
         $this->assertEquals('fu', encode('no', 15, 18));
     }
 
+    /**
+     * uuid 2854851c-48fb-40d8-9bf6-8f192ed25054
+     * @testdox encode OMG
+     */
     public function testEncodeOMG(): void
     {
         $this->assertEquals('lvz', encode('OMG', 21, 3));
     }
 
+    /**
+     * uuid bc0c1244-b544-49dd-9777-13a770be1bad
+     * @testdox encode O M G
+     */
     public function testEncodeOMGWithSpaces(): void
     {
         $this->assertEquals('hjp', encode('O M G', 25, 47));
     }
 
+    /**
+     * uuid 381a1a20-b74a-46ce-9277-3778625c9e27
+     * @testdox encode mindblowingly
+     */
     public function testEncodemindblowingly(): void
     {
         $this->assertEquals('rzcwa gnxzc dgt', encode('mindblowingly', 11, 15));
     }
 
+    /**
+     * uuid 6686f4e2-753b-47d4-9715-876fdc59029d
+     * @testdox encode numbers
+     */
     public function testEncodenumbers(): void
     {
         $this->assertEquals(
@@ -74,11 +76,19 @@ public function testEncodenumbers(): void
         );
     }
 
+    /**
+     * uuid ae23d5bd-30a8-44b6-afbe-23c8c0c7faa3
+     * @testdox encode deep thought
+     */
     public function testEncodeDeepThought(): void
     {
         $this->assertEquals('iynia fdqfb ifje', encode('Truth is fiction.', 5, 17));
     }
 
+    /**
+     * uuid c93a8a4d-426c-42ef-9610-76ded6f7ef57
+     * @testdox encode all the letters
+     */
     public function testEncodeAllTheLetters(): void
     {
         $this->assertEquals(
@@ -87,21 +97,33 @@ public function testEncodeAllTheLetters(): void
         );
     }
 
+    /**
+     * uuid 0673638a-4375-40bd-871c-fb6a2c28effb
+     * @testdox encode with a not coprime to m
+     */
     public function testEncodeWithANotCoprimeToM(): void
     {
         $this->expectException(Exception::class);
         encode('This is a test', 6, 17);
     }
 
     /**
-     * Test decoding from ciphertext to English with keys
+     * Decoding from affine cipher to all-lowercase-mashed-together English
      */
 
+    /**
+     * uuid 3f0ac7e2-ec0e-4a79-949e-95e414953438
+     * @testdox decode exercism
+     */
     public function testDecodeExercism(): void
     {
         $this->assertEquals('exercism', decode('tytgn fjr', 3, 7));
     }
 
+    /**
+     * uuid 241ee64d-5a47-4092-a5d7-7939d259e077
+     * @testdox decode a sentence
+     */
     public function testDecodeASentence(): void
     {
         $this->assertEquals(
@@ -110,6 +132,10 @@ public function testDecodeASentence(): void
         );
     }
 
+    /**
+     * uuid 33fb16a1-765a-496f-907f-12e644837f5e
+     * @testdox decode numbers
+     */
     public function testDecodeNumbers(): void
     {
         $this->assertEquals(
@@ -118,6 +144,10 @@ public function testDecodeNumbers(): void
         );
     }
 
+    /**
+     * uuid 20bc9dce-c5ec-4db6-a3f1-845c776bcbf7
+     * @testdox decode all the letters
+     */
     public function testDecodeAllTheLetters(): void
     {
         $this->assertEquals(
@@ -126,6 +156,10 @@ public function testDecodeAllTheLetters(): void
         );
     }
 
+    /**
+     * uuid 623e78c0-922d-49c5-8702-227a3e8eaf81
+     * @testdox decode with no spaces in input
+     */
     public function testDecodeWithNoSpacesInInput(): void
     {
         $this->assertEquals(
@@ -134,6 +168,10 @@ public function testDecodeWithNoSpacesInInput(): void
         );
     }
 
+    /**
+     * uuid 58fd5c2a-1fd9-4563-a80a-71cff200f26f
+     * @testdox decode with too many spaces
+     */
     public function testDecodeWithTooManySpaces(): void
     {
         $this->assertEquals(
@@ -142,6 +180,10 @@ public function testDecodeWithTooManySpaces(): void
         );
     }
 
+    /**
+     * uuid b004626f-c186-4af9-a3f4-58f74cdb86d5
+     * @testdox decode with a not coprime to m
+     */
     public function testDecodeWithANotCoprimeToM(): void
     {
         $this->expectException(Exception::class);