cpp-json/include/json/binder/json_polymorphic_binder.hpp

//
//  json_polymorphic_binder.hpp
//  json
//
//  Created by Sam Jaffe on 11/15/18.
//  Copyright © 2018 Sam Jaffe. All rights reserved.
//

#pragma once

#include "json_binder.hpp"

#include <functional>
#include <map>
#include <string>

namespace json { namespace binder {
  template <typename PtrBase, typename Derived>
  class polymorphic_pointer_binder : public binder_impl<PtrBase> {
  public:
    polymorphic_pointer_binder(binder<Derived> impl) : m_impl(impl) {}
    virtual binder_impl<PtrBase> * clone() const {
      return new polymorphic_pointer_binder(*this);
    }

    virtual void parse(PtrBase & v, char const *& data,
                       parser::options opts) const {
      if (!strncmp(data, "null", 4))
        v = nullptr;
      else {
        v = PtrBase(new Derived);
        Derived & actual = dynamic_cast<Derived &>(*v);
        m_impl.parse(actual, data, opts);
      }
    }

    void write(PtrBase const & v, std::ostream & os) const {
      if (v)
        m_impl.write(dynamic_cast<Derived const &>(*v), os);
      else
        os << "null";
    }

  private:
    binder<Derived> m_impl;
  };

  template <typename Ptr> class polymorphic_binder : public binder_impl<Ptr> {
  private:
    // Maybe a reference type
    using BaseR = decltype(*std::declval<Ptr>());
    using Base = typename std::remove_reference<BaseR>::type;
    static_assert(std::is_polymorphic<Base>::value,
                  "Must use a polymorphic type");

  public:
    polymorphic_binder() = default;
    virtual binder_impl<Ptr> * clone() const override {
      return new polymorphic_binder(*this);
    }

    template <typename Derived>
    polymorphic_binder & operator()(std::string const & k,
                                    binder_impl<Derived> const & v) {
      mapping.emplace(k, polymorphic_pointer_binder<Ptr, Derived>(v));
      reverse_lookup.emplace(
          k, [](Base const * p) { return dynamic_cast<Derived const *>(p); });
      return *this;
    }

    virtual void parse(Ptr & object, char const *& data,
                       parser::options opts) const override {
      const char ch = json::helper::get_next_element(data);
      if (ch == '{') {
        parse_object(object, ++data, opts);
      } else {
        throw not_an_object_exception(typeid(Base).name());
      }
    }
    virtual void write(Ptr const & val, std::ostream & data) const override {
      data << '{';
      using pair_t = typename decltype(reverse_lookup)::value_type;
      Base const * p = std::addressof(*val);
      auto it =
          std::find_if(reverse_lookup.begin(), reverse_lookup.end(),
                       [p](pair_t const & pair) { return pair.second(p); });
      if (it == reverse_lookup.end()) {
        throw std::domain_error("Unknown JSON binding object");
      }
      data << "\"@id\":\"" << it->first << "\",";
      data << "\"@value\":";
      mapping.find(it->first)->second.write(val, data);
      data << '}';
    }

    void parse_object(Ptr & object, char const *& data,
                      parser::options opts) const {
      std::string key;
      fetch_expected_key("@id", data);
      if (json::helper::get_next_element(++data) != '"') {
        throw json::malformed_json_exception{
            std::string("Expected polymorphic id starting with '\"', got '") +
            *data + "' instead"};
      }
      json::helper::parse_string(key, data);
      auto it = mapping.find(key);
      if (it == mapping.end()) {
        throw json::malformed_json_exception{"Unknown polymorphic type-id: '" +
                                             key + "'"};
      }
      fetch_expected_key("@value", ++data);
      it->second.parse(object, ++data, opts);
      if (!*data)
        throw unterminated_json_object();
      else if (*data != '}')
        throw json::malformed_json_exception{
            std::string("Unexpected non-terminated object '") + *data + "'"};
      else
        ++data;
    }

    void fetch_expected_key(std::string const & expected,
                            char const *& data) const {
      std::string key;
      if (json::helper::get_next_element(data) != '"') {
        throw malformed_object_key(*data);
      }
      json::helper::parse_string(key, data);
      if (key != expected) {
        throw json::malformed_json_exception{"Expecting '" + expected +
                                             "' polymorphic token, got '" +
                                             key + "' instead"};
      }
      if (json::helper::get_next_element(data) != ':') {
        throw malformed_object_association(*data);
      }
    }

  private:
    std::map<std::string, std::function<bool(Base const *)>> reverse_lookup;
    std::map<std::string, binder<Ptr>> mapping;
  };
}}