draconisplusplus/include/rfl/xml/Reader.hpp

#ifndef RFL_XML_READER_HPP_
#define RFL_XML_READER_HPP_

#include <array>
#include <exception>
#include <map>
#include <memory>
#include <optional>
#include <pugixml.hpp>
#include <sstream>
#include <stdexcept>
#include <string>
#include <string_view>
#include <type_traits>
#include <unordered_map>
#include <variant>
#include <vector>

#include "../Result.hpp"
#include "../always_false.hpp"
#include "../parsing/is_view_reader.hpp"

namespace rfl {
  namespace xml {

    struct Reader {
      struct XMLInputArray {
        XMLInputArray(pugi::xml_node _node) : node_(_node) {}
        pugi::xml_node node_;
      };

      struct XMLInputObject {
        XMLInputObject(pugi::xml_node _node) : node_(_node) {}
        pugi::xml_node node_;
      };

      struct XMLInputVar {
        XMLInputVar() : node_or_attribute_(pugi::xml_node()) {}
        XMLInputVar(pugi::xml_attribute _attr) : node_or_attribute_(_attr) {}
        XMLInputVar(pugi::xml_node _node) : node_or_attribute_(_node) {}
        std::variant<pugi::xml_node, pugi::xml_attribute> node_or_attribute_;
      };

      using InputArrayType  = XMLInputArray;
      using InputObjectType = XMLInputObject;
      using InputVarType    = XMLInputVar;

      // TODO
      template <class T>
      static constexpr bool has_custom_constructor = false;

      /// XML-only helper function. This is needed because XML distinguishes
      /// between nodes and attributes.
      static rfl::Result<pugi::xml_node> cast_as_node(
          const std::variant<pugi::xml_node, pugi::xml_attribute>&
              _node_or_attribute
      ) {
        const auto cast = [](const auto& _n) -> Result<pugi::xml_node> {
          using Type = std::remove_cvref_t<decltype(_n)>;
          if constexpr (std::is_same<Type, pugi::xml_node>()) {
            return _n;
          } else {
            return Error(
                "Field '" + std::string(_n.name()) + "' is an attribute."
            );
          }
        };
        return std::visit(cast, _node_or_attribute);
      }

      rfl::Result<InputVarType> get_field(
          const std::string&    _name,
          const InputObjectType _obj
      ) const noexcept {
        const auto node = _obj.node_.child(_name.c_str());
        if (!node) {
          return rfl::Error("Object contains no field named '" + _name + "'.");
        }
        return InputVarType(node);
      }

      bool is_empty(const InputVarType _var) const noexcept {
        const auto wrap = [](const auto& _node) { return !_node; };
        return std::visit(cast_as_node, _var.node_or_attribute_)
            .transform(wrap)
            .value_or(false);
      }

      template <class T>
      rfl::Result<T> to_basic_type(const InputVarType _var) const noexcept {
        const auto get_value = [](const auto& _n) -> std::string {
          using Type = std::remove_cvref_t<decltype(_n)>;
          if constexpr (std::is_same<Type, pugi::xml_node>()) {
            return std::string(_n.child_value());
          } else {
            return std::string(_n.value());
          }
        };

        if constexpr (std::is_same<std::remove_cvref_t<T>, std::string>()) {
          return std::visit(get_value, _var.node_or_attribute_);
        } else if constexpr (std::is_same<std::remove_cvref_t<T>, bool>()) {
          return std::visit(get_value, _var.node_or_attribute_) == "true";
        } else if constexpr (std::is_floating_point<std::remove_cvref_t<T>>()) {
          const auto str = std::visit(get_value, _var.node_or_attribute_);
          try {
            return static_cast<T>(std::stod(str));
          } catch (std::exception& e) {
            return Error(
                "Could not cast '" + std::string(str) +
                "' to floating point value."
            );
          }
        } else if constexpr (std::is_integral<std::remove_cvref_t<T>>()) {
          const auto str = std::visit(get_value, _var.node_or_attribute_);
          try {
            return static_cast<T>(std::stoi(str));
          } catch (std::exception& e) {
            return Error(
                "Could not cast '" + std::string(str) + "' to integer."
            );
          }
        } else {
          static_assert(rfl::always_false_v<T>, "Unsupported type.");
        }
      }

      rfl::Result<InputArrayType> to_array(const InputVarType _var
      ) const noexcept {
        const auto wrap = [](const auto& _node) {
          return InputArrayType(_node);
        };
        return std::visit(cast_as_node, _var.node_or_attribute_)
            .transform(wrap);
      }

      template <class ArrayReader>
      std::optional<Error> read_array(
          const ArrayReader&    _array_reader,
          const InputArrayType& _arr
      ) const noexcept {
        const auto name = _arr.node_.name();
        for (auto node = _arr.node_; node; node = node.next_sibling(name)) {
          const auto err = _array_reader.read(InputVarType(node));
          if (err) {
            return err;
          }
        }
        return std::nullopt;
      }

      template <class ObjectReader>
      std::optional<Error> read_object(
          const ObjectReader&    _object_reader,
          const InputObjectType& _obj
      ) const noexcept {
        for (auto child = _obj.node_.first_child(); child;
             child      = child.next_sibling()) {
          _object_reader.read(
              std::string_view(child.name()), InputVarType(child)
          );
        }

        for (auto attr = _obj.node_.first_attribute(); attr;
             attr      = attr.next_attribute()) {
          _object_reader.read(
              std::string_view(attr.name()), InputVarType(attr)
          );
        }

        if constexpr (parsing::is_view_reader_v<ObjectReader>) {
          _object_reader.read(
              std::string_view("xml_content"), InputVarType(_obj.node_)
          );
        }

        return std::nullopt;
      }

      rfl::Result<InputObjectType> to_object(const InputVarType _var
      ) const noexcept {
        const auto wrap = [](const auto& _node) {
          return InputObjectType(_node);
        };
        return std::visit(cast_as_node, _var.node_or_attribute_)
            .transform(wrap);
      }

      template <class T>
      rfl::Result<T> use_custom_constructor(const InputVarType _var
      ) const noexcept {
        return rfl::Error("TODO");
      }
    };

  } // namespace xml
} // namespace rfl

#endif