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

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

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

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

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

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

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

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

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

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

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

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# This is an auto-generated file.
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#
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# Regenerating this file via `configlet sync` will:
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# - Recreate every `description` key/value pair
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# - Recreate every `reimplements` key/value pair, where they exist in problem-specifications
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# - Remove any `include = true` key/value pair (an omitted `include` key implies inclusion)
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# - Preserve any other key/value pair
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#
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# As user-added comments (using the # character) will be removed when this file
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# is regenerated, comments can be added via a `comment` key.
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[46dc5c50-5538-401d-93a5-41102680d068]
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description = "encode -> encode with two rails"
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[25691697-fbd8-4278-8c38-b84068b7bc29]
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description = "encode -> encode with three rails"
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[384f0fea-1442-4f1a-a7c4-5cbc2044002c]
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description = "encode -> encode with ending in the middle"
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[cd525b17-ec34-45ef-8f0e-4f27c24a7127]
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description = "decode -> decode with three rails"
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[dd7b4a98-1a52-4e5c-9499-cbb117833507]
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description = "decode -> decode with five rails"
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[93e1ecf4-fac9-45d9-9cd2-591f47d3b8d3]
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description = "decode -> decode with six rails"

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

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

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

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

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

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

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

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

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