Added "affine-cipher" exercise (#973)

* Added alphametics exercise to the track

* Added affine-cipher exercise to the track

* Changed difficulty in config.json

* Minor indent change in example.cpp

* Initial state of main and changed guard

* Removed alphametic for new branch

* Added prerequisites and practices

* Changed prerequisites and practices

* Run pre-commit locally

---------

Co-authored-by: Christian Willner <34183939+vaeng@users.noreply.github.com>

Author: marcelweikum

config.json

@@ -1245,6 +1245,22 @@
         ],
         "difficulty": 4
       },
+      {
+        "slug": "affine-cipher",
+        "name": "Affine Cipher",
+        "uuid": "61374c16-0a99-4616-9244-a38919eafbe1",
+        "practices": [
+          "math",
+          "exceptions"
+        ],
+        "prerequisites": [
+          "functions",
+          "comparisons",
+          "loops",
+          "booleans"
+        ],
+        "difficulty": 6
+      },
       {
         "slug": "alphametics",
         "name": "Alphametics",

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 Latin 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 +1,21 @@
+{
+  "authors": [
+    "marcelweikum"
+  ],
+  "files": {
+    "solution": [
+      "affine_cipher.cpp",
+      "affine_cipher.h"
+    ],
+    "test": [
+      "affine_cipher_test.cpp"
+    ],
+    "example": [
+      ".meta/example.cpp",
+      ".meta/example.h"
+    ]
+  },
+  "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.cpp

@@ -0,0 +1,65 @@
+#include <cctype>
+#include <stdexcept>
+
+#include "affine_cipher.h"
+
+namespace affine_cipher {
+
+static int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
+
+static int mod(int x, int m) {
+    int r = x % m;
+    return r < 0 ? r + m : r;
+}
+
+static int modinv(int a, int m) {
+    int t = 0, newt = 1;
+    int r = m, newr = a;
+    while (newr != 0) {
+        int q = r / newr;
+        t -= q * newt;
+        std::swap(t, newt);
+        r -= q * newr;
+        std::swap(r, newr);
+    }
+    if (r != 1) throw std::invalid_argument("a and m must be coprime.");
+    return t < 0 ? t + m : t;
+}
+
+std::string encode(const std::string& text, int a, int b) {
+    const int m = 26;
+    if (gcd(a, m) != 1) throw std::invalid_argument("a and m must be coprime.");
+    std::string out;
+    out.reserve(text.size());
+    for (char ch : text) {
+        if (std::isalpha(ch)) {
+            int x = std::tolower(ch) - 'a';
+            out += char('a' + mod(a * x + b, m));
+        } else if (std::isdigit(ch)) {
+            out += ch;
+        }
+    }
+    for (int i = 5; i < (int)out.size(); i += 6) {
+        out.insert(out.begin() + i, ' ');
+    }
+    return out;
+}
+
+std::string decode(const std::string& text, int a, int b) {
+    const int m = 26;
+    if (gcd(a, m) != 1) throw std::invalid_argument("a and m must be coprime.");
+    int inv = modinv(a, m);
+    std::string out;
+    out.reserve(text.size());
+    for (char ch : text) {
+        if (std::isalpha(ch)) {
+            int y = std::tolower(ch) - 'a';
+            out += char('a' + mod(inv * (y - b), m));
+        } else if (std::isdigit(ch)) {
+            out += ch;
+        }
+    }
+    return out;
+}
+
+}  // namespace affine_cipher

exercises/practice/affine-cipher/.meta/example.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <string>
+
+namespace affine_cipher {
+
+std::string encode(const std::string& input, int a, int b);
+std::string decode(const std::string& input, int a, int b);
+
+}  // namespace 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/CMakeLists.txt

@@ -0,0 +1,67 @@
+# Get the exercise name from the current directory
+get_filename_component(exercise ${CMAKE_CURRENT_SOURCE_DIR} NAME)
+
+# Basic CMake project
+cmake_minimum_required(VERSION 3.5.1)
+
+# Name the project after the exercise
+project(${exercise} CXX)
+
+# Get a source filename from the exercise name by replacing -'s with _'s
+string(REPLACE "-" "_" file ${exercise})
+
+# Implementation could be only a header
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${file}.cpp)
+    set(exercise_cpp ${file}.cpp)
+else()
+    set(exercise_cpp "")
+endif()
+
+# Use the common Catch library?
+if(EXERCISM_COMMON_CATCH)
+    # For Exercism track development only
+    add_executable(${exercise} ${file}_test.cpp ${exercise_cpp} ${file}.h $<TARGET_OBJECTS:catchlib>)
+elseif(EXERCISM_TEST_SUITE)
+    # The Exercism test suite is being run, the Docker image already
+    # includes a pre-built version of Catch.
+    find_package(Catch2 REQUIRED)
+    add_executable(${exercise} ${file}_test.cpp ${exercise_cpp} ${file}.h)
+    target_link_libraries(${exercise} PRIVATE Catch2::Catch2WithMain)
+    # When Catch is installed system wide we need to include a different
+    # header, we need this define to use the correct one.
+    target_compile_definitions(${exercise} PRIVATE EXERCISM_TEST_SUITE)
+else()
+    # Build executable from sources and headers
+    add_executable(${exercise} ${file}_test.cpp ${exercise_cpp} ${file}.h test/tests-main.cpp)
+endif()
+
+set_target_properties(${exercise} PROPERTIES
+    CXX_STANDARD 17
+    CXX_STANDARD_REQUIRED OFF
+    CXX_EXTENSIONS OFF
+)
+
+set(CMAKE_BUILD_TYPE Debug)
+
+if("${CMAKE_CXX_COMPILER_ID}" MATCHES "(GNU|Clang)")
+    set_target_properties(${exercise} PROPERTIES
+        COMPILE_FLAGS "-Wall -Wextra -Wpedantic -Werror"
+    )
+endif()
+
+# Configure to run all the tests?
+if(${EXERCISM_RUN_ALL_TESTS})
+    target_compile_definitions(${exercise} PRIVATE EXERCISM_RUN_ALL_TESTS)
+endif()
+
+# Tell MSVC not to warn us about unchecked iterators in debug builds
+# Treat warnings as errors
+# Treat type conversion warnings C4244 and C4267 as level 4 warnings, i.e. ignore them in level 3
+if(${MSVC})
+    set_target_properties(${exercise} PROPERTIES
+        COMPILE_DEFINITIONS_DEBUG _SCL_SECURE_NO_WARNINGS
+        COMPILE_FLAGS "/WX /w44244 /w44267")
+endif()
+
+# Run the tests on every build
+add_custom_target(test_${exercise} ALL DEPENDS ${exercise} COMMAND ${exercise})

exercises/practice/affine-cipher/affine_cipher.cpp

@@ -0,0 +1,7 @@
+#include "affine_cipher.h"
+
+namespace affine_cipher {
+
+// TODO: add your solution here
+
+}  // namespace affine_cipher

exercises/practice/affine-cipher/affine_cipher.h

@@ -0,0 +1,10 @@
+#ifndef AFFINE_CIPHER_H
+#define AFFINE_CIPHER_H
+
+namespace affine_cipher {
+
+// TODO: add your solution here
+
+}  // namespace affine_cipher
+
+#endif  // AFFINE_CIPHER_H