Genesis_Script.cpp

/*
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MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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*/

// This code was originally created by Jason Turner aka @lefticus
// Please examine, modify, and in general use this as an example for
// how scripting + C++ integration works.
//
// The TODOs are intentionally left FOR YOU!
//
// Please consider leaving some mention of the origin of this code
// when you use it for your own purposes.
//
// https://youtube.com/@cppweekly

// https://compiler-explorer.com/z/86558dW56
// https://gist.github.com/lefticus/5d94357725413dce5005b0b1b7f77836

#include <any>
#include <array>
#include <charconv>
#include <functional>
#include <iostream>
#include <map>
#include <ranges>
#include <span>
#include <string>

struct Genesis_Script {
  // TODO: attempt to make this use variant instead of `any` and see what changes
  struct Function {
    std::function<std::any(std::span<std::any>)> callable;
    std::size_t arity;
  };

  // TODO: Transparent comparator for std::string_view?
  // TODO: `constexpr` friendly types?
  std::map<std::string, Function> functions;
  std::map<std::string, std::any> variables;
  std::vector<std::any> stack;

  [[nodiscard]] auto get_stack_span(const std::size_t size) {
    if (stack.size() < size) { throw std::runtime_error("not enough stack elements"); }
    return std::span(stack).subspan(stack.size() - size);
  }

  void pop(std::span<std::any> elements) {
    if (elements.size() > stack.size()) { throw std::runtime_error("pop size mismatch"); }
    stack.resize(stack.size() - elements.size());
  }

  // TODO: add overload for member functions
  // TODO: add overload that knows how to handle any callable
  template <typename Ret, typename... Param>
  void add(std::string name, Ret (*func)(Param...)) {
    functions.try_emplace(
        std::move(name), // map key

        [func](std::span<std::any> params) {
          if (params.size() != sizeof...(Param)) {
            throw std::runtime_error("Wrong number of params sent to function");
          }
          // need the index sequence to unpack the param in the proper order
          return [&]<std::size_t... Index>(std::index_sequence<Index...>) -> std::any {
            if constexpr (!std::is_same_v<void, Ret>) {
              // if return type is not void, directly return it into the std::any
              return func(std::any_cast<std::remove_cvref_t<Param>>(params[Index])...);
            } else {
              // if it is void, we have to handle it separately
              func(std::any_cast<std::remove_cvref_t<Param>>(params[Index])...);
              return {};
            }
          }(std::make_index_sequence<sizeof...(Param)>()); // immediatedly invoked
                                                           // and using function template argument
                                                           // deduction to generate indices
        }, // Function that knows how to unbox span of parameters

        sizeof...(Param)); // function arity
  }

  void exec(const std::string &func_name) {
    const auto func = functions.at(func_name);
    const auto span = get_stack_span(func.arity);
    auto result = func.callable(span);
    // TODO: can we rearrange this to avoid the move? I don't think so...
    pop(span);
    if (result.has_value()) {
      stack.push_back(std::move(result));
    }
  }

  [[nodiscard]] bool lookup_id(const auto &map, const auto &id) {
    // If it start swith a '.' we call it an identifier
    if (id.starts_with('.') && map.contains(id.substr(1))) {
      stack.emplace_back(map.at(id.substr(1)));
      return true;
    }
    return false;
  }

  [[nodiscard]] bool exec_function(const auto &id) {
    if (functions.contains(id)) {
      exec(id);
      return true;
    }
    return false;
  }

  [[nodiscard]] bool exec_builtins(const auto &id) {
    // TODO: "load" function (alias for . shortcut)
    // TODO: "if"
    // TODO: "for"
    // TODO: Define function?
    // TODO: Swap? Duplicate? There are things that builtins can do
    //       that no regular function can do, because they have access
    //       to the entire system state
    if (id == "store") {
      const auto args = get_stack_span(2);
      variables[std::any_cast<std::string>(args[1])] = args[0];
      pop(args);
      return true;
    }
    return false;
  }

  template<typename Numeric>
  [[nodiscard]] bool parse_numeric(const std::string &input) {
    const auto begin = input.data();
    const auto end = std::next(input.data(), std::ssize(input));
    Numeric value = 0;
    if (auto result = std::from_chars(begin, end, value); result.ptr == end) {
      stack.emplace_back(value);
      return true;
    }
    return false;
  }

  void parse(std::string input) {
    // Observe that we try things in a particular priority order
    // and rely on short circuiting to stop evaluation as soon as
    // something succeeds.
    if (input.empty()
        || exec_builtins(input)
        || exec_function(input)
        || lookup_id(functions, input) 
        || lookup_id(variables, input)
        // order matters here, an int will parse as a double, but not the other way
        || parse_numeric<int>(input)
        || parse_numeric<double>(input)) {
      return;
    }
    // Fallthrough is just treating the thing as a string literal
    stack.emplace_back(std::move(input));
  }
};

constexpr inline std::string_view script = R"(
The Value Was: 
9
2
4
+
*
12
-
the_sum
store
.the_sum
.the_sum
+
to_string
append
print
)";

[[nodiscard]] std::string to_string(int x) { return std::to_string(x); }
void print(const std::string &s) { std::puts(s.c_str()); }

int main() {
  Genesis_Script se;

  // add functions to engine
  se.add("+", +[](int x, int y) { return x + y; });
  se.add("*", +[](int x, int y) { return x * y; });
  se.add("-", +[](int x, int y) { return x - y; });
  se.add(
      "append", +[](const std::string &lhs, const std::string &rhs) {
        return lhs + rhs;
      });
  se.add("print", print);
  se.add("to_string", to_string);

  for (const auto line : std::views::split(script, std::string_view{"\n"})) {
    se.parse(std::string{line.begin(), line.end()});
  }
}