Add affine-cipher exercise

config.json

@@ -897,6 +897,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 1
+      },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "dc1161ae-d7c3-47e8-b3d0-3fd458eb6488",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 1
       }
     ]
   },

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": [
+    "JohnMertz"
+  ],
+  "files": {
+    "solution": [
+      "lib/AffineCipher.pm",
+    ],
+    "test": [
+      "t/affine-cipher.t"
+    ],
+    "example": [
+      ".meta/solutions/lib/AffineCipher.pm",
+    ]
+  },
+  "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/solutions/lib/AffineCipher.pm

@@ -0,0 +1,47 @@
+package AffineCipher;
+
+use strict;
+use warnings;
+use experimental qw<signatures postderef postderef_qq>;
+
+use Exporter qw<import>;
+our @EXPORT_OK = qw<encode decode>;
+
+sub encode ( $phrase, $a, $b ) {
+    die "a and m must be coprime." unless _coprime($a);
+    my $ret = '';
+    foreach ( split( //, $phrase ) ) {
+        next        if $_ eq ' ';
+        $ret .= ' ' if ( $_ =~ /[[:alpha:]\d]/ && !( ( length($ret) + 1 ) % 6 ) );
+        if ( ord( lc($_) ) < 97 ) {
+            $ret .= $_ if ( $_ =~ /\d/ );
+        } else {
+            $ret .= chr( ( ( $a * ( ord( lc($_) ) - 97 ) + $b ) % 26 ) + 97 );
+        }
+    }
+    return $ret;
+}
+
+sub decode ( $phrase, $a, $b ) {
+    die "a and m must be coprime." unless _coprime($a);
+    my ( $mmi, $ret ) = ( 0, '' );
+    $mmi++ until ( ( $a * $mmi ) % 26 == 1 );
+    foreach ( split( //, $phrase ) ) {
+        next if $_ eq ' ';
+        if ( ord( lc($_) ) < 97 ) {
+            $ret .= $_ if ( $_ =~ /\d/ );
+        } else {
+            $ret .= chr( ( $mmi * ( ( ord( lc($_) ) - 97 ) - $b ) % 26 ) + 97 );
+        }
+    }
+    return $ret;
+}
+
+sub _coprime ($a) {
+    for ( 2 .. 26 ) {
+        return 0 unless ( $a % $_ + 26 % $_ );
+    }
+    return 1;
+}
+
+1;

exercises/practice/affine-cipher/.meta/solutions/t/affine-cipher.t

@@ -0,0 +1 @@
+../../../t/affine-cipher.t

exercises/practice/affine-cipher/.meta/template-data.yaml

@@ -0,0 +1,90 @@
+subs: encode decode
+
+properties:
+  encode:
+    test: |-
+      use Data::Dmp;
+      if (ref $case->{expected} eq ref {}) {
+          sprintf(<<~'END', map {dmp($_)} $case->{input}{phrase}, $case->{input}{key}{a}, $case->{input}{key}{b}, qr/$case->{expected}{error}/, $case->{description});
+          like(
+              dies( sub { encode(%s, %s, %s) } ),
+              %s,
+              %s,
+          );
+          END
+      } else {
+          sprintf(<<~'END', dmp($case->{input}{phrase}), $case->{input}{key}{a}, $case->{input}{key}{b}, dmp($case->{expected}), dmp($case->{description}));
+          is(
+              encode(%s, %s, %s),
+              %s,
+              %s,
+          );
+          END
+      }
+  decode:
+    test: |-
+      use Data::Dmp;
+      if (ref $case->{expected} eq ref {}) {
+          sprintf(<<~'END', map {dmp($_)} $case->{input}{phrase}, $case->{input}{key}{a}, $case->{input}{key}{b}, qr/$case->{expected}{error}/, $case->{description});
+          like(
+              dies( sub { decode(%s, %s, %s) } ),
+              %s,
+              %s,
+          );
+          END
+      } else {
+          sprintf(<<~'END', dmp($case->{input}{phrase}), $case->{input}{key}{a}, $case->{input}{key}{b}, dmp($case->{expected}), dmp($case->{description}));
+          is(
+              decode(%s, %s, %s),
+              %s,
+              %s,
+          );
+          END
+      }
+
+stub: |-
+  sub encode ($phrase, $a, $b) {
+      return undef;
+  }
+
+  sub decode ($phrase, $a, $b) {
+      return undef;
+  }
+
+example: |-
+  sub encode ($phrase, $a, $b) {
+      die "a and m must be coprime." unless _coprime($a);
+      my $ret = '';
+      foreach (split(//, $phrase)) {
+          next if $_ eq ' ';
+          $ret .= ' ' if ($_ =~ /[[:alpha:]\d]/ && !((length($ret)+1) % 6));
+          if (ord(lc($_)) < 97) {
+              $ret .= $_ if ($_ =~ /\d/);
+          } else {
+              $ret .= chr((($a*(ord(lc($_))-97) + $b) % 26)+97);
+          }
+      }
+      return $ret;
+  }
+
+  sub decode ($phrase, $a, $b) {
+      die "a and m must be coprime." unless _coprime($a);
+      my ($mmi, $ret) = (0, '');
+      $mmi++ until ( ($a * $mmi) % 26 == 1 );
+      foreach (split(//, $phrase)) {
+          next if $_ eq ' ';
+          if (ord(lc($_)) < 97) {
+              $ret .= $_ if ($_ =~ /\d/);
+          } else {
+              $ret .= chr(($mmi*((ord(lc($_))-97)-$b) % 26)+97);
+          }
+      }
+      return $ret;
+  }
+
+  sub _coprime ($a) {
+      for (2..26) {
+          return 0 unless ($a % $_ + 26 % $_);
+      }
+      return 1;
+  }

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/lib/AffineCipher.pm

@@ -0,0 +1,16 @@
+package AffineCipher;
+
+use v5.40;
+
+use Exporter qw<import>;
+our @EXPORT_OK = qw<encode decode>;
+
+sub encode ( $phrase, $a, $b ) {
+    return undef;
+}
+
+sub decode ( $phrase, $a, $b ) {
+    return undef;
+}
+
+1;