Add affine-cipher exercise (#291)

config.json

@@ -682,6 +682,14 @@
         "prerequisites": [],
         "difficulty": 2
       },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "19d44506-07c6-4ae0-88c8-1450266c4e7b",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "state-of-tic-tac-toe",
         "name": "State of Tic-Tac-Toe",

exercises/practice/affine-cipher/.docs/instructions.md

@@ -0,0 +1,74 @@
+# Instructions
+
+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 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:
+
+```text
+E(x) = (ai + b) mod m
+```
+
+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.
+
+## Decryption
+
+The decryption function is:
+
+```text
+D(y) = (a^-1)(y - b) mod m
+```
+
+Where:
+
+- `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.
+
+The MMI of `a` is `x` such that the remainder after dividing `ax` by `m` is `1`:
+
+```text
+ax mod m = 1
+```
+
+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
+
+- 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

exercises/practice/affine-cipher/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "source/affine_cipher.d"
+    ],
+    "test": [
+      "source/affine_cipher.d"
+    ],
+    "example": [
+      "example/affine_cipher.d"
+    ]
+  },
+  "blurb": "Create an implementation of the Affine cipher, an ancient encryption algorithm from the Middle East.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Affine_cipher"
+}

exercises/practice/affine-cipher/.meta/tests.toml

@@ -0,0 +1,58 @@
+# 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.
+
+[2ee1d9af-1c43-416c-b41b-cefd7d4d2b2a]
+description = "encode -> encode yes"
+
+[785bade9-e98b-4d4f-a5b0-087ba3d7de4b]
+description = "encode -> encode no"
+
+[2854851c-48fb-40d8-9bf6-8f192ed25054]
+description = "encode -> encode OMG"
+
+[bc0c1244-b544-49dd-9777-13a770be1bad]
+description = "encode -> encode O M G"
+
+[381a1a20-b74a-46ce-9277-3778625c9e27]
+description = "encode -> encode mindblowingly"
+
+[6686f4e2-753b-47d4-9715-876fdc59029d]
+description = "encode -> encode numbers"
+
+[ae23d5bd-30a8-44b6-afbe-23c8c0c7faa3]
+description = "encode -> encode deep thought"
+
+[c93a8a4d-426c-42ef-9610-76ded6f7ef57]
+description = "encode -> encode all the letters"
+
+[0673638a-4375-40bd-871c-fb6a2c28effb]
+description = "encode -> encode with a not coprime to m"
+
+[3f0ac7e2-ec0e-4a79-949e-95e414953438]
+description = "decode -> decode exercism"
+
+[241ee64d-5a47-4092-a5d7-7939d259e077]
+description = "decode -> decode a sentence"
+
+[33fb16a1-765a-496f-907f-12e644837f5e]
+description = "decode -> decode numbers"
+
+[20bc9dce-c5ec-4db6-a3f1-845c776bcbf7]
+description = "decode -> decode all the letters"
+
+[623e78c0-922d-49c5-8702-227a3e8eaf81]
+description = "decode -> decode with no spaces in input"
+
+[58fd5c2a-1fd9-4563-a80a-71cff200f26f]
+description = "decode -> decode with too many spaces"
+
+[b004626f-c186-4af9-a3f4-58f74cdb86d5]
+description = "decode -> decode with a not coprime to m"

exercises/practice/affine-cipher/dub.sdl

@@ -0,0 +1,2 @@
+name "affine-cipher"
+buildRequirements "disallowDeprecations"

exercises/practice/affine-cipher/example/affine_cipher.d

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+module affine_cipher;
+
+import std.ascii : isAlphaNum, isDigit, toLower;
+
+pure string encode(immutable string phrase, uint a, uint b)
+{
+    if (!inverses[a])
+    {
+        throw new Exception("a and m must be coprime.");
+    }
+
+    return process(phrase, a, b, 5);
+}
+
+pure string decode(immutable string phrase, uint a, uint b)
+{
+    uint inverse = inverses[a];
+    if (!inverse)
+    {
+        throw new Exception("a and m must be coprime.");
+    }
+
+    return process(phrase, inverse, inverse * (26 - (b % 26)), uint.max);
+}
+
+pure string process(immutable string phrase, uint a, uint b,  uint chunk)
+{
+    size_t nextSpace = chunk;
+    string result;
+    foreach (dchar c; phrase) {
+        if (!isAlphaNum(c)) {
+            continue;
+        }
+        if (result.length == nextSpace) {
+            result ~= ' ';
+            nextSpace += chunk + 1;
+        }
+        if (!isDigit(c)) {
+            c = 'a' + ((toLower(c) - 'a') * a + b) % 26;
+        }
+        result ~= c;
+    }
+    return result;
+}
+
+private immutable uint[] inverses = [
+    0,
+    1,
+    0,
+    9,
+    0,
+    21,
+    0,
+    15,
+    0,
+    3,
+    0,
+    19,
+    0,
+    0,
+    0,
+    7,
+    0,
+    23,
+    0,
+    11,
+    0,
+    5,
+    0,
+    17,
+    0,
+    25,
+    0
+];
+
+unittest
+{
+    import std.algorithm.comparison : equal;
+    import std.exception : assertThrown;
+
+    immutable int allTestsEnabled = 0;
+
+    // Encode-encode yes
+    assert(equal("xbt", encode("yes", 5, 7)));
+
+    static if (allTestsEnabled)
+    {
+        // Encode-encode no
+        assert(equal("fu", encode("no", 15, 18)));
+
+        // Encode-encode OMG
+        assert(equal("lvz", encode("OMG", 21, 3)));
+
+        // Encode-encode O M G
+        assert(equal("hjp", encode("O M G", 25, 47)));
+
+        // Encode-encode mindblowingly
+        assert(equal("rzcwa gnxzc dgt", encode("mindblowingly", 11, 15)));
+
+        // Encode-encode numbers
+        assert(equal("jqgjc rw123 jqgjc rw", encode("Testing,1 2 3, testing.", 3, 4)));
+
+        // Encode-encode deep thought
+        assert(equal("iynia fdqfb ifje", encode("Truth is fiction.", 5, 17)));
+
+        // Encode-encode all the letters
+        assert(equal("swxtj npvyk lruol iejdc blaxk swxmh qzglf", encode("The quick brown fox jumps over the lazy dog.", 17, 33)));
+
+        // Encode-encode with a not coprime to m
+        assertThrown(encode("This is a test.", 6, 17));
+
+        // Decode-decode exercism
+        assert(equal("exercism", decode("tytgn fjr", 3, 7)));
+
+        // Decode-decode a sentence
+        assert(equal("anobstacleisoftenasteppingstone", decode("qdwju nqcro muwhn odqun oppmd aunwd o", 19, 16)));
+
+        // Decode-decode numbers
+        assert(equal("testing123testing", decode("odpoz ub123 odpoz ub", 25, 7)));
+
+        // Decode-decode all the letters
+        assert(equal("thequickbrownfoxjumpsoverthelazydog", decode("swxtj npvyk lruol iejdc blaxk swxmh qzglf", 17, 33)));
+
+        // Decode-decode with no spaces in input
+        assert(equal("thequickbrownfoxjumpsoverthelazydog", decode("swxtjnpvyklruoliejdcblaxkswxmhqzglf", 17, 33)));
+
+        // Decode-decode with too many spaces
+        assert(equal("jollygreengiant", decode("vszzm    cly   yd cg    qdp", 15, 16)));
+
+        // Decode-decode with a not coprime to m
+        assertThrown(decode("Test", 13, 5));
+    }
+}

exercises/practice/affine-cipher/source/affine_cipher.d

@@ -0,0 +1,70 @@
+module affine_cipher;
+
+pure string encode(immutable string phrase, uint a, uint b)
+{
+    // implement this function
+}
+
+pure string decode(immutable string phrase, uint a, uint b)
+{
+    // implement this function
+}
+
+unittest
+{
+    import std.algorithm.comparison : equal;
+    import std.exception : assertThrown;
+
+    immutable int allTestsEnabled = 0;
+
+    // Encode-encode yes
+    assert(equal("xbt", encode("yes", 5, 7)));
+
+    static if (allTestsEnabled)
+    {
+        // Encode-encode no
+        assert(equal("fu", encode("no", 15, 18)));
+
+        // Encode-encode OMG
+        assert(equal("lvz", encode("OMG", 21, 3)));
+
+        // Encode-encode O M G
+        assert(equal("hjp", encode("O M G", 25, 47)));
+
+        // Encode-encode mindblowingly
+        assert(equal("rzcwa gnxzc dgt", encode("mindblowingly", 11, 15)));
+
+        // Encode-encode numbers
+        assert(equal("jqgjc rw123 jqgjc rw", encode("Testing,1 2 3, testing.", 3, 4)));
+
+        // Encode-encode deep thought
+        assert(equal("iynia fdqfb ifje", encode("Truth is fiction.", 5, 17)));
+
+        // Encode-encode all the letters
+        assert(equal("swxtj npvyk lruol iejdc blaxk swxmh qzglf", encode("The quick brown fox jumps over the lazy dog.", 17, 33)));
+
+        // Encode-encode with a not coprime to m
+        assertThrown(encode("This is a test.", 6, 17));
+
+        // Decode-decode exercism
+        assert(equal("exercism", decode("tytgn fjr", 3, 7)));
+
+        // Decode-decode a sentence
+        assert(equal("anobstacleisoftenasteppingstone", decode("qdwju nqcro muwhn odqun oppmd aunwd o", 19, 16)));
+
+        // Decode-decode numbers
+        assert(equal("testing123testing", decode("odpoz ub123 odpoz ub", 25, 7)));
+
+        // Decode-decode all the letters
+        assert(equal("thequickbrownfoxjumpsoverthelazydog", decode("swxtj npvyk lruol iejdc blaxk swxmh qzglf", 17, 33)));
+
+        // Decode-decode with no spaces in input
+        assert(equal("thequickbrownfoxjumpsoverthelazydog", decode("swxtjnpvyklruoliejdcblaxkswxmhqzglf", 17, 33)));
+
+        // Decode-decode with too many spaces
+        assert(equal("jollygreengiant", decode("vszzm    cly   yd cg    qdp", 15, 16)));
+
+        // Decode-decode with a not coprime to m
+        assertThrown(decode("Test", 13, 5));
+    }
+}