Created
July 9, 2019 15:24
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| #include <cassert> | |
| #include <variant> | |
| #include <optional> | |
| #include <string> | |
| template <typename Child, typename Initial, typename Accepting, typename... Rest> | |
| struct StateMachine { | |
| using StateVariant = std::variant<Initial, Accepting, Rest...>; | |
| template <typename Events> | |
| static std::optional<Accepting> process(Events events, Initial init) { | |
| StateVariant current_state{std::move(init)}; | |
| for(auto& e : events) { | |
| current_state = std::visit([&](auto state) { | |
| return Child::transition(std::move(state), std::move(e)); | |
| }, std::move(current_state)); | |
| } | |
| return std::holds_alternative<Accepting>(current_state) | |
| ? std::optional(std::get<Accepting>(std::move(current_state))) | |
| : std::nullopt; | |
| } | |
| }; | |
| struct Line { std::string str; }; | |
| struct Accept { std::string str; }; | |
| struct Fail {}; | |
| struct NewLineTerminated : StateMachine<NewLineTerminated, Line, Accept, Fail> { | |
| static StateVariant transition(Line l, char c) { | |
| if(c == '\n') { | |
| return Accept{std::move(l.str)}; | |
| } else { | |
| return Line{l.str + c}; | |
| } | |
| } | |
| static StateVariant transition(Accept, char) { return Fail{}; } | |
| static StateVariant transition(Fail, char) { return Fail{}; } | |
| }; | |
| int main() { | |
| assert(NewLineTerminated::process(std::string("abcd\n"), Line{})->str == "abcd"); | |
| assert(NewLineTerminated::process(std::string("abcd\ndef"), Line{}) == std::nullopt); | |
| assert(NewLineTerminated::process(std::string("ab"), Line{"cd"}) == std::nullopt); | |
| assert(NewLineTerminated::process(std::string("ab\n"), Line{"cd"})->str == "cdab"); | |
| } |
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