Add affine-cipher practice exercise (#515)

config.json

@@ -1367,6 +1367,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 3
+      },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "e3a84895-fb1b-4d03-b56a-84cb00b3cf93",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

exercises/practice/affine-cipher/.busted

@@ -0,0 +1,5 @@
+return {
+  default = {
+    ROOT = { '.' }
+  }
+}

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": [
+    "ryanplusplus"
+  ],
+  "files": {
+    "solution": [
+      "affine-cipher.lua"
+    ],
+    "test": [
+      "affine-cipher_spec.lua"
+    ],
+    "example": [
+      ".meta/example.lua"
+    ]
+  },
+  "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.lua

@@ -0,0 +1,48 @@
+local M = 26
+
+local function coprime(x, y)
+  while y ~= 0 do
+    x, y = y, x % y
+  end
+  return x == 1
+end
+
+local function partition(s)
+  return s:gsub('.....', '%0 '):gsub(' $', '')
+end
+
+local function mmi(x, mod)
+  local mmi = 1
+  while (x * mmi) % mod ~= 1 do
+    mmi = mmi + 1
+  end
+  return mmi
+end
+
+local function encode_char(char, key)
+  local x = char:byte() - ('a'):byte()
+  return string.char((key.a * x + key.b) % M + ('a'):byte())
+end
+
+local function decode_char(char, key)
+  local x = char:byte() - ('a'):byte()
+  return string.char((mmi(key.a, M) * (x - key.b)) % M + ('a'):byte())
+end
+
+local function encode(phrase, key)
+  assert(coprime(key.a, M), 'a and m must be coprime.')
+
+  return partition(phrase:gsub('%W', ''):gsub('%a', function(char)
+    return encode_char(char:lower(), key)
+  end))
+end
+
+local function decode(phrase, key)
+  assert(coprime(key.a, M), 'a and m must be coprime.')
+
+  return phrase:gsub('%s', ''):gsub('%a', function(char)
+    return decode_char(char:lower(), key)
+  end)
+end
+
+return { encode = encode, decode = decode }

exercises/practice/affine-cipher/.meta/spec_generator.lua

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+return {
+  module_name = 'affine_cipher',
+
+  generate_test = function(case)
+    if type(case.expected) == 'string' then
+      local template = [[
+      local phrase = '%s'
+      local expected = '%s'
+      assert.are.same(expected, affine_cipher.%s(phrase, { a = %d, b = %d }))]]
+      return template:format(case.input.phrase, case.expected, case.property, case.input.key.a, case.input.key.b)
+    else
+      local template = [[
+        local phrase = '%s'
+        assert.has.error(function()
+          affine_cipher.%s(phrase, { a = %d, b = %d })
+        end, '%s')]]
+      return template:format(case.input.phrase, case.property, case.input.key.a, case.input.key.b, case.expected.error)
+    end
+  end
+}

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.lua

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+local function encode(phrase, key)
+
+end
+
+local function decode(phrase, key)
+
+end
+
+return { encode = encode, decode = decode }

exercises/practice/affine-cipher/affine-cipher_spec.lua

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+local affine_cipher = require('affine-cipher')
+
+describe('affine-cipher', function()
+  describe('encode', function()
+    it('encode yes', function()
+      local phrase = 'yes'
+      local expected = 'xbt'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 5, b = 7 }))
+    end)
+
+    it('encode no', function()
+      local phrase = 'no'
+      local expected = 'fu'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 15, b = 18 }))
+    end)
+
+    it('encode omg', function()
+      local phrase = 'OMG'
+      local expected = 'lvz'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 21, b = 3 }))
+    end)
+
+    it('encode o m g', function()
+      local phrase = 'O M G'
+      local expected = 'hjp'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 25, b = 47 }))
+    end)
+
+    it('encode mindblowingly', function()
+      local phrase = 'mindblowingly'
+      local expected = 'rzcwa gnxzc dgt'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 11, b = 15 }))
+    end)
+
+    it('encode numbers', function()
+      local phrase = 'Testing,1 2 3, testing.'
+      local expected = 'jqgjc rw123 jqgjc rw'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 3, b = 4 }))
+    end)
+
+    it('encode deep thought', function()
+      local phrase = 'Truth is fiction.'
+      local expected = 'iynia fdqfb ifje'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 5, b = 17 }))
+    end)
+
+    it('encode all the letters', function()
+      local phrase = 'The quick brown fox jumps over the lazy dog.'
+      local expected = 'swxtj npvyk lruol iejdc blaxk swxmh qzglf'
+      assert.are.same(expected, affine_cipher.encode(phrase, { a = 17, b = 33 }))
+    end)
+
+    it('encode with a not coprime to m', function()
+      local phrase = 'This is a test.'
+      assert.has.error(function()
+        affine_cipher.encode(phrase, { a = 6, b = 17 })
+      end, 'a and m must be coprime.')
+    end)
+  end)
+
+  describe('decode', function()
+    it('decode exercism', function()
+      local phrase = 'tytgn fjr'
+      local expected = 'exercism'
+      assert.are.same(expected, affine_cipher.decode(phrase, { a = 3, b = 7 }))
+    end)
+
+    it('decode a sentence', function()
+      local phrase = 'qdwju nqcro muwhn odqun oppmd aunwd o'
+      local expected = 'anobstacleisoftenasteppingstone'
+      assert.are.same(expected, affine_cipher.decode(phrase, { a = 19, b = 16 }))
+    end)
+
+    it('decode numbers', function()
+      local phrase = 'odpoz ub123 odpoz ub'
+      local expected = 'testing123testing'
+      assert.are.same(expected, affine_cipher.decode(phrase, { a = 25, b = 7 }))
+    end)
+
+    it('decode all the letters', function()
+      local phrase = 'swxtj npvyk lruol iejdc blaxk swxmh qzglf'
+      local expected = 'thequickbrownfoxjumpsoverthelazydog'
+      assert.are.same(expected, affine_cipher.decode(phrase, { a = 17, b = 33 }))
+    end)
+
+    it('decode with no spaces in input', function()
+      local phrase = 'swxtjnpvyklruoliejdcblaxkswxmhqzglf'
+      local expected = 'thequickbrownfoxjumpsoverthelazydog'
+      assert.are.same(expected, affine_cipher.decode(phrase, { a = 17, b = 33 }))
+    end)
+
+    it('decode with too many spaces', function()
+      local phrase = 'vszzm    cly   yd cg    qdp'
+      local expected = 'jollygreengiant'
+      assert.are.same(expected, affine_cipher.decode(phrase, { a = 15, b = 16 }))
+    end)
+
+    it('decode with a not coprime to m', function()
+      local phrase = 'Test'
+      assert.has.error(function()
+        affine_cipher.decode(phrase, { a = 13, b = 5 })
+      end, 'a and m must be coprime.')
+    end)
+  end)
+end)