Add affine-cipher exercise (#166)

config.json

@@ -547,6 +547,14 @@
         "prerequisites": [],
         "difficulty": 3
       },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "54da632d-ab6a-4a6f-8a06-cb40b84e60fe",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 4
+      },
       {
         "slug": "anagram",
         "name": "Anagram",

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/Example.roc

@@ -0,0 +1,72 @@
+module [encode, decode]
+
+alphabetSize : U64
+alphabetSize = 26
+
+groupLength : U64
+groupLength = 5
+
+encode : Str, { a : U64, b : U64 } -> Result Str [InvalidKey, BadUtf8 _ _]
+encode = \phrase, key ->
+    alphabet = encodedAlphabet? key
+    phrase
+    |> Str.toUtf8
+    |> List.joinMap \char ->
+        if char >= '0' && char <= '9' then
+            [char]
+            else
+
+        charLower = if char >= 'A' && char <= 'Z' then char - 'A' + 'a' else char
+        if charLower >= 'a' && charLower <= 'z' then
+            index = charLower - 'a' |> Num.toU64
+            when alphabet |> List.get index is
+                Ok encodedChar -> [encodedChar]
+                Err OutOfBounds -> crash "Unreachable: index cannot be out of bounds here"
+        else
+            []
+    |> List.chunksOf groupLength
+    |> List.intersperse [' ']
+    |> List.join
+    |> Str.fromUtf8
+
+encodedAlphabet : { a : U64, b : U64 } -> Result (List U8) [InvalidKey]
+encodedAlphabet = \{ a, b } ->
+    encoded =
+        List.range { start: At 'a', end: At 'z' }
+        |> List.map \char ->
+            num = (char - 'a') |> Num.toU64
+            index = (a * num + b) % alphabetSize
+            'a' + Num.toU8 index
+    if (encoded |> Set.fromList |> Set.len) < alphabetSize then
+        Err InvalidKey
+    else
+        Ok encoded
+
+decodedAlphabet : { a : U64, b : U64 } -> Result (List U8) [InvalidKey]
+decodedAlphabet = \key ->
+    encodedAlphabet? key
+        |> List.mapWithIndex \encoded, decodedIndex -> { encoded, decodedIndex }
+        |> List.sortWith \{ encoded: encoded1 }, { encoded: encoded2 } ->
+            Num.compare encoded1 encoded2
+        |> List.map \pair -> Num.toU8 pair.decodedIndex + 'a'
+        |> Ok
+
+decode : Str, { a : U64, b : U64 } -> Result Str [InvalidKey, BadUtf8 _ _, InvalidCharacter]
+decode = \phrase, key ->
+    alphabet = decodedAlphabet? key
+    phrase
+        |> Str.toUtf8
+        |> List.mapTry? \char ->
+            if char == ' ' then
+                Ok []
+            else if char >= '0' && char <= '9' then
+                Ok [char]
+            else if char >= 'a' && char <= 'z' then
+                index = char - 'a' |> Num.toU64
+                when alphabet |> List.get index is
+                    Ok decodedChar -> Ok [decodedChar]
+                    Err OutOfBounds -> crash "Unreachable: index cannot be out of bounds here"
+            else
+                Err InvalidCharacter
+        |> List.join
+        |> Str.fromUtf8

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

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ageron"
+  ],
+  "files": {
+    "solution": [
+      "AffineCipher.roc"
+    ],
+    "test": [
+      "affine-cipher-test.roc"
+    ],
+    "example": [
+      ".meta/Example.roc"
+    ]
+  },
+  "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/template.j2

@@ -0,0 +1,26 @@
+{%- import "generator_macros.j2" as macros with context -%}
+{{ macros.canonical_ref() }}
+{{ macros.header() }}
+
+import {{ exercise | to_pascal }} exposing [encode, decode]
+
+{% for supercase in cases %}
+##
+## {{ supercase["description"] }}
+##
+
+{% for case in supercase["cases"] -%}
+# {{ case["description"] }}
+expect
+    phrase = {{ case["input"]["phrase"] | to_roc }}
+    key = {a: {{ case["input"]["key"]["a"] }}, b: {{ case["input"]["key"]["b"] }}}
+    result = {{ case["property"] | to_camel }} phrase key
+    {%- if case["expected"]["error"] %}
+    result |> Result.isErr
+    {%- else %}
+    expected = Ok {{ case["expected"] | to_roc }} 
+    result == expected
+    {%- endif %}
+
+{% endfor %}
+{% endfor %}

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/AffineCipher.roc

@@ -0,0 +1,9 @@
+module [encode, decode]
+
+encode : Str, { a : U64, b : U64 } -> Result Str _
+encode = \phrase, key ->
+    crash "Please implement the 'encode' function"
+
+decode : Str, { a : U64, b : U64 } -> Result Str _
+decode = \phrase, key ->
+    crash "Please implement the 'decode' function"