feat: add practice exercise Rail Fence Cipher (#881)

config.json

@@ -1206,6 +1206,22 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 4
+      },
+      {
+        "slug": "rail-fence-cipher",
+        "name": "Rail Fence Cipher",
+        "uuid": "8a7f6f8c-ff36-4afd-9ab3-6dbd355d0263",
+        "practices": [],
+        "prerequisites": [
+            "basics",
+            "namespaces",
+            "conditionals",
+            "lists",
+            "loops",
+            "numbers",
+            "strings"
+        ],
+        "difficulty": 4
       }
     ],
     "foregone": [

exercises/practice/rail-fence-cipher/.docs/instructions.md

@@ -0,0 +1,57 @@
+# Instructions
+
+Implement encoding and decoding for the rail fence cipher.
+
+The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded.
+It was already used by the ancient Greeks.
+
+In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag).
+Finally the message is then read off in rows.
+
+For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . A . . . I . . . V . . . D . . . E . . . N . .
+```
+
+Then reads off:
+
+```text
+WECRLTEERDSOEEFEAOCAIVDEN
+```
+
+To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.
+
+```text
+? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+The first row has seven spots that can be filled with "WECRLTE".
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+Now the 2nd row takes "ERDSOEEFEAOC".
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .
+```
+
+Leaving "AIVDEN" for the last row.
+
+```text
+W . . . E . . . C . . . R . . . L . . . T . . . E
+. E . R . D . S . O . E . E . F . E . A . O . C .
+. . A . . . I . . . V . . . D . . . E . . . N . .
+```
+
+If you now read along the zig-zag shape you can read the original message.

exercises/practice/rail-fence-cipher/.meta/config.json

@@ -0,0 +1,21 @@
+{
+  "authors": [
+    "klimkin"
+  ],
+  "files": {
+    "solution": [
+      "rail_fence_cipher.cpp",
+      "rail_fence_cipher.h"
+    ],
+    "test": [
+      "rail_fence_cipher_test.cpp"
+    ],
+    "example": [
+      ".meta/example.cpp",
+      ".meta/example.h"
+    ]
+  },
+  "blurb": "Implement encoding and decoding for the rail fence cipher.",
+  "source": "Wikipedia",
+  "source_url": "https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher"
+}

exercises/practice/rail-fence-cipher/.meta/example.cpp

@@ -0,0 +1,49 @@
+#include "rail_fence_cipher.h"
+
+namespace rail_fence_cipher {
+
+std::string encode(const std::string& plaintext, int num_rails) {
+    if (num_rails <= 1) return plaintext;
+
+    const auto stride = num_rails * 2 - 2;
+    const auto length = plaintext.size();
+
+    std::string result(length, ' ');
+    auto dst = result.begin();
+
+    for (int rail = 0; rail < num_rails; ++rail) {
+        for (size_t pos = rail; pos < length; pos += stride) {
+            *dst++ = plaintext[pos];
+            if (0 < rail && rail < num_rails - 1) {
+                size_t next_pos = pos + stride - rail * 2;
+                if (next_pos < length) *dst++ = plaintext[next_pos];
+            }
+        }
+    }
+
+    return result;
+}
+
+std::string decode(const std::string& ciphertext, int num_rails) {
+    if (num_rails <= 1) return ciphertext;
+
+    const auto stride = num_rails * 2 - 2;
+    const auto length = ciphertext.size();
+
+    std::string result(length, ' ');
+    auto src = ciphertext.cbegin();
+
+    for (int rail = 0; rail < num_rails; ++rail) {
+        for (size_t pos = rail; pos < length; pos += stride) {
+            result[pos] = *src++;
+            if (0 < rail && rail < num_rails - 1) {
+                size_t next_pos = pos + stride - rail * 2;
+                if (next_pos < length) result[next_pos] = *src++;
+            }
+        }
+    }
+
+    return result;
+}
+
+}  // namespace rail_fence_cipher

exercises/practice/rail-fence-cipher/.meta/example.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <string>
+
+namespace rail_fence_cipher {
+
+std::string encode(const std::string& plaintext, int num_rails);
+std::string decode(const std::string& ciphertext, int num_rails);
+
+}  // namespace rail_fence_cipher

exercises/practice/rail-fence-cipher/.meta/tests.toml

@@ -0,0 +1,28 @@
+# 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.
+
+[46dc5c50-5538-401d-93a5-41102680d068]
+description = "encode -> encode with two rails"
+
+[25691697-fbd8-4278-8c38-b84068b7bc29]
+description = "encode -> encode with three rails"
+
+[384f0fea-1442-4f1a-a7c4-5cbc2044002c]
+description = "encode -> encode with ending in the middle"
+
+[cd525b17-ec34-45ef-8f0e-4f27c24a7127]
+description = "decode -> decode with three rails"
+
+[dd7b4a98-1a52-4e5c-9499-cbb117833507]
+description = "decode -> decode with five rails"
+
+[93e1ecf4-fac9-45d9-9cd2-591f47d3b8d3]
+description = "decode -> decode with six rails"

exercises/practice/rail-fence-cipher/CMakeLists.txt

@@ -0,0 +1,64 @@
+# 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
+if(${MSVC})
+    set_target_properties(${exercise} PROPERTIES
+        COMPILE_DEFINITIONS_DEBUG _SCL_SECURE_NO_WARNINGS)
+endif()
+
+# Run the tests on every build
+add_custom_target(test_${exercise} ALL DEPENDS ${exercise} COMMAND ${exercise})

exercises/practice/rail-fence-cipher/rail_fence_cipher.cpp

@@ -0,0 +1,9 @@
+#include "rail_fence_cipher.h"
+
+namespace rail_fence_cipher {
+
+std::string encode(const std::string& plaintext, int num_rails) {}
+
+std::string decode(const std::string& ciphertext, int num_rails) {}
+
+}  // namespace rail_fence_cipher

exercises/practice/rail-fence-cipher/rail_fence_cipher.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <string>
+
+namespace rail_fence_cipher {
+
+std::string encode(const std::string& plaintext, int num_rails);
+std::string decode(const std::string& ciphertext, int num_rails);
+
+}  // namespace rail_fence_cipher

exercises/practice/rail-fence-cipher/rail_fence_cipher_test.cpp

@@ -0,0 +1,42 @@
+#include "rail_fence_cipher.h"
+#ifdef EXERCISM_TEST_SUITE
+#include <catch2/catch.hpp>
+#else
+#include "test/catch.hpp"
+#endif
+
+TEST_CASE("encode with two rails", "[46dc5c50-5538-401d-93a5-41102680d068]") {
+    REQUIRE(rail_fence_cipher::encode("XOXOXOXOXOXOXOXOXO", 2) ==
+            "XXXXXXXXXOOOOOOOOO");
+}
+
+#if defined(EXERCISM_RUN_ALL_TESTS)
+
+TEST_CASE("encode with three rails", "[25691697-fbd8-4278-8c38-b84068b7bc29]") {
+    REQUIRE(rail_fence_cipher::encode("WEAREDISCOVEREDFLEEATONCE", 3) ==
+            "WECRLTEERDSOEEFEAOCAIVDEN");
+}
+
+TEST_CASE("encode with ending in the middle",
+          "[384f0fea-1442-4f1a-a7c4-5cbc2044002c]") {
+    REQUIRE(rail_fence_cipher::encode("EXERCISES", 4) == "ESXIEECSR");
+}
+
+TEST_CASE("decode_with_three_rails", "[cd525b17-ec34-45ef-8f0e-4f27c24a7127]") {
+    REQUIRE(rail_fence_cipher::decode("TEITELHDVLSNHDTISEIIEA", 3) ==
+            "THEDEVILISINTHEDETAILS");
+}
+
+TEST_CASE("decode_with_five_rails", "[dd7b4a98-1a52-4e5c-9499-cbb117833507]") {
+    REQUIRE(rail_fence_cipher::decode("EIEXMSMESAORIWSCE", 5) ==
+            "EXERCISMISAWESOME");
+}
+
+TEST_CASE("decode_with_six_rails", "[93e1ecf4-fac9-45d9-9cd2-591f47d3b8d3]") {
+    REQUIRE(rail_fence_cipher::decode(
+                "133714114238148966225439541018335470986172518171757571896261",
+                6) ==
+            "112358132134558914423337761098715972584418167651094617711286");
+}
+
+#endif