module;

#include <boost/json.hpp>
#include <boost/system/system_error.hpp>

module routemon:config$impl;

import std;
import :config;
import :log;
import :util;

namespace json = boost::json;

namespace routemon::config {

class location
{
  std::optional<std::pair<std::string_view, util::not_null<location const*>>>
      next_;

  auto append_to(std::string& s) const -> void
  {
    if (next_)
    {
      next_->second->append_to(s);
      s += next_->first;
      s += ".";
    }
  }

public:
  location() = default;

  explicit location(location const& next, std::string_view entry)
    : next_{std::make_pair(entry, util::not_null{&next})}
  {
  }

  [[nodiscard]] auto to_string() const -> std::string
  {
    if (!next_)
      return "";
    auto s = std::string{};
    next_->second->append_to(s);
    s += next_->first;
    return s;
  }

  auto sub(std::string_view entry) const& -> location
  {
    return location{*this, entry};
  }
};

class object_reader
{
  location loc_;
  json::object const& obj_;
  std::unordered_set<std::string> visited_;

public:
  object_reader(json::value const& jv, location loc)
  try : loc_{std::move(loc)}, obj_{jv.as_object()} {}
  catch (boost::system::system_error const&)
  {
    throw std::runtime_error{std::format(
        "expected an object at {}", loc.to_string())};
  }

  auto check_unused() const -> void
  {
    for (auto const& kv : obj_)
    {
      if (!visited_.contains(std::string{kv.key()}))
      {
        throw std::runtime_error{std::format(
            "unexpected key {}", loc_.sub(kv.key()).to_string())};
      }
    }
  }

  template <class T>
  auto allow_at(std::string_view key) -> std::optional<T>
  {
    visited_.emplace(key);
    auto const loc = loc_.sub(key);
    if (auto const jv = obj_.try_at(key); jv && !jv->is_null())
    {
      try
      {
        return json::value_to<T>(*jv, loc);
      }
      catch (boost::system::system_error const& e)
      {
        throw std::runtime_error{std::format(
            "failed to read {}: {}", loc.to_string(), e.code().message())};
      }
    }
    else
    {
      return std::nullopt;
    }
  }

  template <class T>
  auto expect_at(std::string_view key) -> T
  {
    if (auto&& mv = allow_at<T>(key); mv)
    {
      return std::forward<T>(*mv);
    }
    else
    {
      throw std::runtime_error{std::format(
          "did not find expected key {}", loc_.sub(key).to_string())};
    }
  }
};

auto as_checked_object(
    json::value const& jv, location const& loc,
    std::invocable<object_reader&> auto const& f)
    -> decltype(f(std::declval<object_reader&>()))
{
  auto r = object_reader{jv, loc};
  auto&& v = f(r);
  r.check_unused();
  return std::forward<decltype(f(r))>(v);
}

auto tag_invoke(
    json::value_to_tag<rwgps> const&, json::value const& jv,
    location const& loc) -> rwgps
{
  return as_checked_object(
      jv, loc,
      [](object_reader& r) -> rwgps
      {
        return {
          .api_key_filename = r.expect_at<std::string>("api_key_filename"),
          .auth_token_filename =
              r.expect_at<std::string>("auth_token_filename"),
        };
      });
}

auto tag_invoke(
    json::value_to_tag<situations> const&, json::value const& jv,
    location const& loc) -> situations
{
  return as_checked_object(
      jv, loc,
      [](object_reader& r) -> situations
      {
        return {
          .datex2_filename = r.expect_at<std::string>("datex2_filename"),
        };
      });
}

auto tag_invoke(
    json::value_to_tag<database> const&, json::value const& jv,
    location const& loc) -> database
{
  return as_checked_object(
      jv, loc,
      [](object_reader& r) -> database
      {
        return {
          .sqlite3_filename = r.expect_at<std::string>("sqlite3_filename"),
        };
      });
}

auto tag_invoke(
    json::value_to_tag<http_server> const&, json::value const& jv,
    location const& loc) -> http_server
{
  return as_checked_object(
      jv, loc,
      [](object_reader& r) -> http_server
      {
        return {
          .lax_cors = r.expect_at<bool>("lax_cors"),
        };
      });
}

auto tag_invoke(
    json::value_to_tag<logger> const&, json::value const& jv,
    location const& loc) -> logger
{
  return as_checked_object(
      jv, loc,
      [obj_loc = loc](object_reader& r) -> logger
      {
        auto const level_str = r.expect_at<std::string>("level");
        auto level = log::level{};
        if (level_str == "debug")
          level = log::level::debug;
        else if (level_str == "info")
          level = log::level::info;
        else if (level_str == "warn")
          level = log::level::warn;
        else if (level_str == "error")
          level = log::level::error;
        else
          throw std::runtime_error{std::format(
              "unable to parse log level {:?} at {}: expected one "
              "of {{debug, info, warn, error}}",
              level_str, obj_loc.sub("level").to_string())};
        return logger{level};
      });
}

auto json_value_to_app(json::value const& jv) -> app
{
  return as_checked_object(
      jv, location{},
      [](object_reader& r) -> app
      {
        return {
          .rwgps = r.allow_at<rwgps>("rwgps"),
          .situations = r.expect_at<situations>("situations"),
          .database = r.expect_at<database>("database"),
          .http_server = r.expect_at<http_server>("http_server"),
          .logger = r.expect_at<logger>("logger"),
        };
      });
}

auto load_file(std::string const& filename) -> app
{
  // TODO: migrate away from json::value_to w/ tag_invoke.
  // Boost.JSON does not properly propagate custom exceptions accross
  // json::value_to calls when certain containers are involved
  // (std::{optional,variant,vector}).

  auto f = std::ifstream{filename, std::ios::binary};
  if (!f.is_open())
    throw std::runtime_error{std::format("failed to open {}", filename)};
  auto jv = json::value{};
  try
  {
    jv = json::parse(f);
  }
  catch (boost::system::system_error const& e)
  {
    throw std::runtime_error{std::format(
        "failed to parse: {}", e.code().message())};
  }
  return json_value_to_app(jv);
}

} // namespace routemon::config