Add affine-cipher exercise (#114)

config.json

@@ -546,6 +546,14 @@
         "prerequisites": [],
         "difficulty": 6
       },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "3ecb43fc-179d-4510-84a3-63859f72c9df",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 6
+      },
       {
         "slug": "parallel-letter-frequency",
         "name": "Parallel Letter Frequency",

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": [
+    "erikschierboom"
+  ],
+  "files": {
+    "solution": [
+      "affine-cipher.ua"
+    ],
+    "test": [
+      "tests.ua"
+    ],
+    "example": [
+      ".meta/example.ua"
+    ]
+  },
+  "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/example.ua

@@ -0,0 +1,9 @@
+Coprime  ← ¬/↥≡⌕ ∩(°/×)
+MMI      ← ⊗1◿:×⇡,
+Validate ← ⍤"a and m must be coprime" (Coprime :26)
+Groups   ← ⊜(□∘)\+=0◿5°⊏
+Sanitize ← ▽+⊃(⌵±|×⊃(≥@0|≤@9)).¯⌵
+Encrypt  ← ⍜(-@a▽)(◿26+×)⌵±.
+Encode   ← /$"_ _" Groups Encrypt Sanitize ⊃(⊙Validate|⋅∘)
+Decrypt  ← ◿26×⊃(⋅(MMI:26)|-:⊙⋅∘)
+Decode   ← ⍜(-@a▽|Decrypt)⌵±. ▽≠@\s. ⊃(⊙Validate|⋅∘)

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/affine-cipher.ua

@@ -0,0 +1,7 @@
+# Encode plaintext using the Affine Cipher
+# Ciphertext ? Key Plaintext
+Encode ← |3 ⊙⋅⋅(⍤ "Please implement Encode" 0)
+
+# Decode plaintext using the Affine Cipher
+# Plaintext ? Key Ciphertext
+Decode ← |3 ⊙⋅⋅(⍤ "Please implement Decode" 0)

exercises/practice/affine-cipher/tests.ua

@@ -0,0 +1,49 @@
+~ "affine-cipher.ua" ~ Decode Encode
+
+# encode yes
+⍤⤙≍ "xbt" Encode "yes" 5 7
+
+# encode no
+⍤⤙≍ "fu" Encode "no" 15 18
+
+# encode OMG
+⍤⤙≍ "lvz" Encode "OMG" 21 3
+
+# encode O M G
+⍤⤙≍ "hjp" Encode "O M G" 25 47
+
+# encode mindblowingly
+⍤⤙≍ "rzcwa gnxzc dgt" Encode "mindblowingly" 11 15
+
+# encode numbers
+⍤⤙≍ "jqgjc rw123 jqgjc rw" Encode "Testing,1 2 3, testing." 3 4
+
+# encode deep thought
+⍤⤙≍ "iynia fdqfb ifje" Encode "Truth is fiction." 5 17
+
+# encode all the letters
+⍤⤙≍ "swxtj npvyk lruol iejdc blaxk swxmh qzglf" Encode "The quick brown fox jumps over the lazy dog." 17 33
+
+# encode with a not coprime to m
+⍤⤙≍ "a and m must be coprime" ⍣(Encode "This is a test." 6 17)
+
+# decode exercism
+⍤⤙≍ "exercism" Decode "tytgn fjr" 3 7
+
+# decode a sentence
+⍤⤙≍ "anobstacleisoftenasteppingstone" Decode "qdwju nqcro muwhn odqun oppmd aunwd o" 19 16
+
+# decode numbers
+⍤⤙≍ "testing123testing" Decode "odpoz ub123 odpoz ub" 25 7
+
+# decode all the letters
+⍤⤙≍ "thequickbrownfoxjumpsoverthelazydog" Decode "swxtj npvyk lruol iejdc blaxk swxmh qzglf" 17 33
+
+# decode with no spaces in input
+⍤⤙≍ "thequickbrownfoxjumpsoverthelazydog" Decode "swxtjnpvyklruoliejdcblaxkswxmhqzglf" 17 33
+
+# decode with too many spaces
+⍤⤙≍ "jollygreengiant" Decode "vszzm    cly   yd cg    qdp" 15 16
+
+# decode with a not coprime to m
+⍤⤙≍ "a and m must be coprime" ⍣(Decode "Test" 13 5)