#include <vamp_zero/page_control.hpp>

#include <algorithm>
#include <cstdint>
#include <expected>
#include <optional>
#include <sstream>
#include <string>
#include <string_view>
#include <utility>

#include <boost/json/array.hpp>
#include <boost/json/object.hpp>
#include <boost/json/value.hpp>

#include <vamp_zero/errors.hpp>
#include <vamp_zero/navigation.hpp>

namespace {

constexpr auto main_menu_count = std::size_t{2};

auto wrap_next(std::size_t value, std::size_t count) -> std::size_t {
  if (count == 0U) {
    return 0U;
  }
  return (value + 1U) % count;
}

auto wrap_previous(std::size_t value, std::size_t count) -> std::size_t {
  if (count == 0U) {
    return 0U;
  }
  return value == 0U ? count - 1U : value - 1U;
}

auto menu_row_count(std::size_t item_count) -> std::size_t {
  return item_count + 1U;
}

auto clamp_state(vamp_zero::app_state state) -> vamp_zero::app_state {
  state.selected_main_menu_index = std::min(state.selected_main_menu_index, main_menu_count - 1U);
  state.selected_song_index = std::min(state.selected_song_index, menu_row_count(state.performance.groups.size()) - 1U);
  if (auto const* group = selected_group(state); group != nullptr) {
    state.selected_part_index = std::min(state.selected_part_index, menu_row_count(group->parts.size()) - 1U);
  } else {
    state.selected_part_index = 0U;
  }

  state.selected_practice_type_index = std::min(state.selected_practice_type_index, menu_row_count(state.practice.types.size()) - 1U);
  if (auto const* type = selected_practice_type(state); type != nullptr) {
    state.selected_practice_lesson_index = std::min(state.selected_practice_lesson_index, menu_row_count(type->lesson_groups.size()) - 1U);
  } else {
    state.selected_practice_lesson_index = 0U;
  }
  if (auto const* lesson = selected_practice_lesson(state); lesson != nullptr) {
    state.selected_practice_exercise_index = std::min(state.selected_practice_exercise_index, menu_row_count(lesson->exercises.size()) - 1U);
  } else {
    state.selected_practice_exercise_index = 0U;
  }
  state.selected_practice_key_index = std::min(state.selected_practice_key_index, practice_key_row_count(state) - 1U);

  if (state.mode == vamp_zero::app_mode::PERFORMANCE_PART_MENU && selected_group(state) == nullptr) {
    state.mode = vamp_zero::app_mode::PERFORMANCE_SONG_MENU;
  }
  if (state.mode == vamp_zero::app_mode::PRACTICE_LESSON_MENU && selected_practice_type(state) == nullptr) {
    state.mode = vamp_zero::app_mode::PRACTICE_TYPE_MENU;
  }
  if (state.mode == vamp_zero::app_mode::PRACTICE_EXERCISE_MENU && selected_practice_lesson(state) == nullptr) {
    state.mode = selected_practice_type(state) == nullptr ? vamp_zero::app_mode::PRACTICE_TYPE_MENU : vamp_zero::app_mode::PRACTICE_LESSON_MENU;
  }
  if (state.mode == vamp_zero::app_mode::PRACTICE_KEY_MENU) {
    auto const* exercise = selected_practice_exercise(state);
    if (exercise == nullptr) {
      state.mode = practice_fallback_menu(state);
    } else if (!supports_key_selection(*exercise) || exercise->variants.empty()) {
      state.mode = vamp_zero::app_mode::PRACTICE_EXERCISE_MENU;
      state.selected_practice_key_index = 0U;
    }
  }

  if (state.mode == vamp_zero::app_mode::PERFORMANCE_SCORE && active_part(state) == nullptr) {
    state.mode = selected_group(state) == nullptr ? vamp_zero::app_mode::PERFORMANCE_SONG_MENU : vamp_zero::app_mode::PERFORMANCE_PART_MENU;
    state.active_part_id.clear();
    state.page_index = 0;
  } else if (state.mode == vamp_zero::app_mode::PRACTICE_SCORE) {
    auto const* exercise = active_practice(state);
    if (exercise == nullptr) {
      state.mode = practice_fallback_menu(state);
      state.active_practice_id.clear();
      state.page_index = 0;
    } else if (selected_practice_variant(state, *exercise) == nullptr) {
      state.mode = vamp_zero::app_mode::PRACTICE_KEY_MENU;
      state.active_practice_id.clear();
      state.page_index = 0;
    }
  }

  if (auto pages = active_pages(state); pages != 0U) {
    state.page_index = std::min(state.page_index, pages - 1U);
  } else if (state.mode != vamp_zero::app_mode::PERFORMANCE_SCORE && state.mode != vamp_zero::app_mode::PRACTICE_SCORE) {
    state.page_index = 0;
  }
  return state;
}

auto command_json(vamp_zero::app_state const& state) -> boost::json::object {
  auto commands = boost::json::object{};
  commands["last"] = state.last_command;
  commands["next_count"] = state.next_count;
  commands["previous_count"] = state.previous_count;
  commands["select_count"] = state.select_count;
  commands["heartbeat_count"] = state.heartbeat_count;
  return commands;
}

auto part_json(vamp_zero::library_part const& part) -> boost::json::object {
  auto result = boost::json::object{};
  result["id"] = part.id;
  result["title"] = part.title;
  result["composer"] = part.composer;
  result["key_signature"] = part.key_signature;
  result["part"] = part.part;
  result["source_path"] = part.source_path;
  result["measures"] = static_cast<std::uint64_t>(part.measure_count);
  result["pages"] = static_cast<std::uint64_t>(part.total_pages);
  return result;
}

auto exercise_json(vamp_zero::practice_exercise const& exercise) -> boost::json::object {
  auto variants = boost::json::array{};
  for (auto const& variant : exercise.variants) {
    auto item = boost::json::object{};
    item["id"] = variant.id;
    item["key_signature"] = variant.key_signature;
    item["source_path"] = variant.source_path;
    item["measures"] = static_cast<std::uint64_t>(variant.measure_count);
    item["pages"] = static_cast<std::uint64_t>(variant.total_pages);
    variants.push_back(std::move(item));
  }
  auto result = boost::json::object{};
  result["id"] = exercise.id;
  result["practice_type_id"] = exercise.practice_type_id;
  result["practice_type_title"] = exercise.practice_type_title;
  result["lesson_group_id"] = exercise.lesson_group_id;
  result["lesson_group_title"] = exercise.lesson_group_title;
  result["supports_key_selection"] = supports_key_selection(exercise);
  result["title"] = exercise.title;
  result["composer"] = exercise.composer;
  result["key_signature"] = exercise.key_signature;
  result["source_path"] = exercise.source_path;
  result["measures"] = static_cast<std::uint64_t>(exercise.measure_count);
  result["pages"] = static_cast<std::uint64_t>(exercise.total_pages);
  result["variant_count"] = static_cast<std::uint64_t>(exercise.variants.size());
  result["variants"] = std::move(variants);
  return result;
}

auto lesson_json(vamp_zero::practice_lesson_group const& lesson) -> boost::json::object {
  auto exercises = boost::json::array{};
  for (auto const& exercise : lesson.exercises) {
    exercises.push_back(exercise_json(exercise));
  }
  auto result = boost::json::object{};
  result["id"] = lesson.id;
  result["title"] = lesson.title;
  result["exercise_count"] = static_cast<std::uint64_t>(lesson.exercises.size());
  result["exercises"] = std::move(exercises);
  return result;
}

auto practice_type_exercise_count(vamp_zero::practice_type_group const& type) -> std::size_t {
  auto count = std::size_t{};
  for (auto const& lesson : type.lesson_groups) {
    count += lesson.exercises.size();
  }
  return count;
}

auto type_json(vamp_zero::practice_type_group const& type) -> boost::json::object {
  auto lessons = boost::json::array{};
  for (auto const& lesson : type.lesson_groups) {
    lessons.push_back(lesson_json(lesson));
  }
  auto result = boost::json::object{};
  result["id"] = type.id;
  result["title"] = type.title;
  result["lesson_group_count"] = static_cast<std::uint64_t>(type.lesson_groups.size());
  result["exercise_count"] = static_cast<std::uint64_t>(practice_type_exercise_count(type));
  result["lesson_groups"] = std::move(lessons);
  return result;
}

auto performance_json(vamp_zero::app_state const& state) -> boost::json::object {
  auto groups = boost::json::array{};
  for (auto const& group : state.performance.groups) {
    auto parts = boost::json::array{};
    for (auto const& part : group.parts) {
      parts.push_back(part_json(part));
    }
    auto item = boost::json::object{};
    item["title"] = group.title;
    item["composer"] = group.composer;
    item["parts"] = std::move(parts);
    groups.push_back(std::move(item));
  }

  auto library = boost::json::object{};
  library["song_count"] = static_cast<std::uint64_t>(state.performance.groups.size());
  library["part_count"] = static_cast<std::uint64_t>(state.performance.parts.size());
  library["groups"] = std::move(groups);
  return library;
}

auto practice_json(vamp_zero::app_state const& state) -> boost::json::object {
  auto types = boost::json::array{};
  for (auto const& type : state.practice.types) {
    types.push_back(type_json(type));
  }
  auto exercises = boost::json::array{};
  for (auto const& exercise : state.practice.exercises) {
    exercises.push_back(exercise_json(exercise));
  }
  auto keys = boost::json::array{};
  for (auto key : vamp_zero::practice_key_names()) {
    keys.push_back(boost::json::value{std::string{key}});
  }

  auto selection = boost::json::object{};
  selection["type_row_index"] = static_cast<std::uint64_t>(state.selected_practice_type_index);
  selection["lesson_row_index"] = static_cast<std::uint64_t>(state.selected_practice_lesson_index);
  selection["exercise_row_index"] = static_cast<std::uint64_t>(state.selected_practice_exercise_index);
  selection["key_row_index"] = static_cast<std::uint64_t>(state.selected_practice_key_index);
  if (auto const* type = selected_practice_type(state); type != nullptr) {
    selection["type"] = type_json(*type);
  }
  if (auto const* lesson = selected_practice_lesson(state); lesson != nullptr) {
    selection["lesson_group"] = lesson_json(*lesson);
  }
  if (auto const* exercise = selected_practice_exercise(state); exercise != nullptr) {
    selection["exercise"] = exercise_json(*exercise);
    auto variant = selected_practice_variant(state, *exercise);
    if (variant != nullptr) {
      auto item = boost::json::object{};
      item["id"] = variant->id;
      item["key_signature"] = variant->key_signature;
      item["source_path"] = variant->source_path;
      item["measures"] = static_cast<std::uint64_t>(variant->measure_count);
      item["pages"] = static_cast<std::uint64_t>(variant->total_pages);
      selection["variant"] = std::move(item);
    }
  }
  if (auto const* exercise = selected_practice_exercise(state); exercise != nullptr) {
    if (auto key = selected_practice_key(state, *exercise)) {
      selection["key"] = std::string{*key};
    }
  }

  auto practice = boost::json::object{};
  practice["type_count"] = static_cast<std::uint64_t>(state.practice.types.size());
  practice["exercise_count"] = static_cast<std::uint64_t>(state.practice.exercises.size());
  practice["types"] = std::move(types);
  practice["exercises"] = std::move(exercises);
  practice["key_count"] = static_cast<std::uint64_t>(vamp_zero::practice_key_names().size());
  practice["keys"] = std::move(keys);
  practice["selection"] = std::move(selection);
  return practice;
}

auto sanitize_line_value(std::string value) -> std::string {
  for (auto& ch : value) {
    if (ch == '\r' || ch == '\n') {
      ch = ' ';
    }
  }
  return value;
}

}

auto vamp_zero::apply_app_command(vamp_zero::app_state state, vamp_protocol::page_command command) -> vamp_zero::app_state {
  state = clamp_state(std::move(state));
  if (command == vamp_protocol::page_command::NEXT) {
    ++state.next_count;
  } else if (command == vamp_protocol::page_command::PREVIOUS) {
    ++state.previous_count;
  } else {
    ++state.select_count;
  }
  state.last_command = vamp_protocol::page_command_name(command);

  switch (state.mode) {
  case vamp_zero::app_mode::MAIN_MENU:
    if (command == vamp_protocol::page_command::NEXT) {
      state.selected_main_menu_index = wrap_next(state.selected_main_menu_index, main_menu_count);
    } else if (command == vamp_protocol::page_command::PREVIOUS) {
      state.selected_main_menu_index = wrap_previous(state.selected_main_menu_index, main_menu_count);
    } else if (state.selected_main_menu_index == 0U) {
      state.mode = vamp_zero::app_mode::PERFORMANCE_SONG_MENU;
      state.selected_song_index = 0;
      state.selected_part_index = 0;
      state.active_part_id.clear();
      state.page_index = 0;
    } else {
      state.mode = vamp_zero::app_mode::PRACTICE_TYPE_MENU;
      state.selected_practice_type_index = 0;
      state.selected_practice_lesson_index = 0;
      state.selected_practice_exercise_index = 0;
      state.selected_practice_key_index = 0;
      state.active_practice_id.clear();
      state.page_index = 0;
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PERFORMANCE_SONG_MENU:
    if (command == vamp_protocol::page_command::NEXT) {
      state.selected_song_index = wrap_next(state.selected_song_index, menu_row_count(state.performance.groups.size()));
      state.selected_part_index = 0;
    } else if (command == vamp_protocol::page_command::PREVIOUS) {
      state.selected_song_index = wrap_previous(state.selected_song_index, menu_row_count(state.performance.groups.size()));
      state.selected_part_index = 0;
    } else if (state.selected_song_index == 0U) {
      state.mode = vamp_zero::app_mode::MAIN_MENU;
    } else {
      state.mode = vamp_zero::app_mode::PERFORMANCE_PART_MENU;
      state.selected_part_index = 0;
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PERFORMANCE_PART_MENU:
    if (auto const* group = selected_group(state); group != nullptr) {
      if (command == vamp_protocol::page_command::NEXT) {
        state.selected_part_index = wrap_next(state.selected_part_index, menu_row_count(group->parts.size()));
      } else if (command == vamp_protocol::page_command::PREVIOUS) {
        state.selected_part_index = wrap_previous(state.selected_part_index, menu_row_count(group->parts.size()));
      } else if (state.selected_part_index == 0U) {
        state.mode = vamp_zero::app_mode::PERFORMANCE_SONG_MENU;
        state.active_part_id.clear();
        state.page_index = 0;
      } else if (auto const* part = selected_part(state); part != nullptr) {
        state.active_part_id = part->id;
        state.active_practice_id.clear();
        state.page_index = 0;
        state.mode = vamp_zero::app_mode::PERFORMANCE_SCORE;
      }
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PERFORMANCE_SCORE:
    if (auto const* part = active_part(state); part != nullptr) {
      if (command == vamp_protocol::page_command::NEXT) {
        state.page_index = std::min(state.page_index + 1U, part->total_pages == 0U ? 0U : part->total_pages - 1U);
      } else if (command == vamp_protocol::page_command::PREVIOUS) {
        state.page_index = state.page_index == 0U ? 0U : state.page_index - 1U;
      } else {
        state.mode = vamp_zero::app_mode::PERFORMANCE_PART_MENU;
        state.active_part_id.clear();
        state.page_index = 0;
      }
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PRACTICE_TYPE_MENU:
    if (command == vamp_protocol::page_command::NEXT) {
      state.selected_practice_type_index = wrap_next(state.selected_practice_type_index, menu_row_count(state.practice.types.size()));
      state.selected_practice_lesson_index = 0;
      state.selected_practice_exercise_index = 0;
      state.selected_practice_key_index = 0;
    } else if (command == vamp_protocol::page_command::PREVIOUS) {
      state.selected_practice_type_index = wrap_previous(state.selected_practice_type_index, menu_row_count(state.practice.types.size()));
      state.selected_practice_lesson_index = 0;
      state.selected_practice_exercise_index = 0;
      state.selected_practice_key_index = 0;
    } else if (state.selected_practice_type_index == 0U) {
      state.mode = vamp_zero::app_mode::MAIN_MENU;
    } else {
      state.mode = vamp_zero::app_mode::PRACTICE_LESSON_MENU;
      state.selected_practice_lesson_index = 0;
      state.selected_practice_exercise_index = 0;
      state.selected_practice_key_index = 0;
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PRACTICE_LESSON_MENU:
    if (auto const* type = selected_practice_type(state); type != nullptr) {
      if (command == vamp_protocol::page_command::NEXT) {
        state.selected_practice_lesson_index = wrap_next(state.selected_practice_lesson_index, menu_row_count(type->lesson_groups.size()));
        state.selected_practice_exercise_index = 0;
        state.selected_practice_key_index = 0;
      } else if (command == vamp_protocol::page_command::PREVIOUS) {
        state.selected_practice_lesson_index = wrap_previous(state.selected_practice_lesson_index, menu_row_count(type->lesson_groups.size()));
        state.selected_practice_exercise_index = 0;
        state.selected_practice_key_index = 0;
      } else if (state.selected_practice_lesson_index == 0U) {
        state.mode = vamp_zero::app_mode::PRACTICE_TYPE_MENU;
        state.active_practice_id.clear();
        state.page_index = 0;
      } else {
        state.mode = vamp_zero::app_mode::PRACTICE_EXERCISE_MENU;
        state.selected_practice_exercise_index = 0;
        state.selected_practice_key_index = 0;
      }
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PRACTICE_EXERCISE_MENU:
    if (auto const* lesson = selected_practice_lesson(state); lesson != nullptr) {
      if (command == vamp_protocol::page_command::NEXT) {
        state.selected_practice_exercise_index = wrap_next(state.selected_practice_exercise_index, menu_row_count(lesson->exercises.size()));
        state.selected_practice_key_index = 0;
      } else if (command == vamp_protocol::page_command::PREVIOUS) {
        state.selected_practice_exercise_index = wrap_previous(state.selected_practice_exercise_index, menu_row_count(lesson->exercises.size()));
        state.selected_practice_key_index = 0;
      } else if (state.selected_practice_exercise_index == 0U) {
        state.mode = vamp_zero::app_mode::PRACTICE_LESSON_MENU;
        state.active_practice_id.clear();
        state.page_index = 0;
      } else if (auto const* exercise = selected_practice_exercise(state); exercise != nullptr) {
        state.active_part_id.clear();
        state.active_practice_id.clear();
        state.page_index = 0;
        if (supports_key_selection(*exercise) && !exercise->variants.empty()) {
          state.mode = vamp_zero::app_mode::PRACTICE_KEY_MENU;
          state.selected_practice_key_index = 0;
        } else {
          state.active_practice_id = exercise->id;
          state.selected_practice_key_index = 0;
          state.mode = vamp_zero::app_mode::PRACTICE_SCORE;
        }
      }
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PRACTICE_KEY_MENU:
    if (auto const* exercise = selected_practice_exercise(state); exercise != nullptr && supports_key_selection(*exercise)) {
      if (command == vamp_protocol::page_command::NEXT) {
        state.selected_practice_key_index = wrap_next(state.selected_practice_key_index, menu_row_count(exercise->variants.size()));
      } else if (command == vamp_protocol::page_command::PREVIOUS) {
        state.selected_practice_key_index = wrap_previous(state.selected_practice_key_index, menu_row_count(exercise->variants.size()));
      } else if (state.selected_practice_key_index == 0U) {
        state.mode = vamp_zero::app_mode::PRACTICE_EXERCISE_MENU;
        state.active_practice_id.clear();
        state.page_index = 0;
      } else if (selected_practice_variant(state, *exercise) != nullptr) {
        state.active_practice_id = exercise->id;
        state.active_part_id.clear();
        state.page_index = 0;
        state.mode = vamp_zero::app_mode::PRACTICE_SCORE;
      }
    }
    return clamp_state(std::move(state));

  case vamp_zero::app_mode::PRACTICE_SCORE:
    if (auto const* exercise = active_practice(state); exercise != nullptr) {
      auto const* variant = selected_practice_variant(state, *exercise);
      if (command == vamp_protocol::page_command::NEXT) {
        auto pages = variant == nullptr ? 0U : variant->total_pages;
        state.page_index = std::min(state.page_index + 1U, pages == 0U ? 0U : pages - 1U);
      } else if (command == vamp_protocol::page_command::PREVIOUS) {
        state.page_index = state.page_index == 0U ? 0U : state.page_index - 1U;
      } else {
        state.mode = supports_key_selection(*exercise) ? vamp_zero::app_mode::PRACTICE_KEY_MENU : vamp_zero::app_mode::PRACTICE_EXERCISE_MENU;
        state.active_practice_id.clear();
        state.page_index = 0;
      }
    }
    return clamp_state(std::move(state));
  }
  return clamp_state(std::move(state));
}

auto vamp_zero::app_status_json(vamp_zero::app_state const& state) -> boost::json::value {
  auto selection = boost::json::object{};
  selection["main_menu_index"] = static_cast<std::uint64_t>(state.selected_main_menu_index);
  selection["song_row_index"] = static_cast<std::uint64_t>(state.selected_song_index);
  selection["part_row_index"] = static_cast<std::uint64_t>(state.selected_part_index);
  if (auto const* group = selected_group(state); group != nullptr) {
    selection["title"] = group->title;
    selection["composer"] = group->composer;
  }
  if (auto const* part = selected_part(state); part != nullptr) {
    selection["part"] = part_json(*part);
  }

  auto active = boost::json::object{};
  if (state.mode == vamp_zero::app_mode::PERFORMANCE_SCORE) {
    if (auto const* part = active_part(state); part != nullptr) {
      active["selected"] = true;
      active["kind"] = "performance";
      active["part"] = part_json(*part);
      active["page_index"] = static_cast<std::uint64_t>(state.page_index);
      active["page_number"] = static_cast<std::uint64_t>(state.page_index + 1U);
      active["page_total"] = static_cast<std::uint64_t>(part->total_pages);
    }
  } else if (state.mode == vamp_zero::app_mode::PRACTICE_SCORE) {
    if (auto const* exercise = active_practice(state); exercise != nullptr) {
      auto const* variant = selected_practice_variant(state, *exercise);
      active["selected"] = true;
      active["kind"] = "practice";
      active["exercise"] = exercise_json(*exercise);
      active["key"] = effective_practice_key(state, *exercise);
      active["page_index"] = static_cast<std::uint64_t>(state.page_index);
      active["page_number"] = static_cast<std::uint64_t>(state.page_index + 1U);
      active["page_total"] = static_cast<std::uint64_t>(variant == nullptr ? 0U : variant->total_pages);
    }
  }
  if (active.empty()) {
    active["selected"] = false;
    active["page_index"] = 0;
    active["page_number"] = 0;
    active["page_total"] = 0;
  }

  auto remote = boost::json::object{};
  remote["pico_connected"] = state.pico_connected;
  remote["heartbeat_count"] = state.heartbeat_count;

  auto performance = performance_json(state);
  auto root = boost::json::object{};
  root["mode"] = vamp_zero::to_string(state.mode);
  root["library"] = performance;
  root["performance"] = std::move(performance);
  root["practice"] = practice_json(state);
  root["selection"] = std::move(selection);
  root["active"] = std::move(active);
  root["commands"] = command_json(state);
  root["remote"] = std::move(remote);
  return root;
}

auto vamp_zero::remote_display_text(vamp_zero::app_state const& state) -> std::string {
  auto title = std::string{};
  auto part = std::string{};
  auto page = std::size_t{};
  auto pages = std::size_t{};
  auto active = false;
  if (state.mode == vamp_zero::app_mode::PERFORMANCE_SCORE) {
    if (auto const* selected = active_part(state); selected != nullptr) {
      active = true;
      title = selected->title;
      part = selected->part;
      page = state.page_index + 1U;
      pages = selected->total_pages;
    }
  } else if (state.mode == vamp_zero::app_mode::PRACTICE_SCORE) {
    if (auto const* selected = active_practice(state); selected != nullptr) {
      auto const* variant = selected_practice_variant(state, *selected);
      active = true;
      title = selected->title;
      part = "Key: " + effective_practice_key(state, *selected);
      page = state.page_index + 1U;
      pages = variant == nullptr ? 0U : variant->total_pages;
    }
  }

  auto output = std::ostringstream{};
  output << "version=1\n";
  output << "mode=" << vamp_zero::to_string(state.mode) << "\n";
  output << "active=" << (active ? "1" : "0") << "\n";
  output << "title=" << sanitize_line_value(title) << "\n";
  output << "part=" << sanitize_line_value(part) << "\n";
  output << "page=" << page << "\n";
  output << "pages=" << pages << "\n";
  return output.str();
}

auto vamp_zero::validate_app_state(vamp_zero::app_state const& state) -> std::expected<void, modulus::error> {
  if (state.selected_main_menu_index >= main_menu_count) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "selected main menu index is out of range"));
  }
  if (state.selected_song_index >= menu_row_count(state.performance.groups.size())) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "selected performance song row is out of range"));
  }
  if (auto const* group = selected_group(state); group != nullptr && state.selected_part_index >= menu_row_count(group->parts.size())) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "selected performance part row is out of range"));
  }
  if (state.selected_practice_type_index >= menu_row_count(state.practice.types.size())) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "selected practice type row is out of range"));
  }
  if (auto const* type = selected_practice_type(state); type != nullptr && state.selected_practice_lesson_index >= menu_row_count(type->lesson_groups.size())) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "selected practice lesson row is out of range"));
  }
  if (auto const* lesson = selected_practice_lesson(state); lesson != nullptr && state.selected_practice_exercise_index >= menu_row_count(lesson->exercises.size())) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "selected practice exercise row is out of range"));
  }
  if (state.selected_practice_key_index >= practice_key_row_count(state)) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "selected practice key row is out of range"));
  }
  if (state.mode == vamp_zero::app_mode::PRACTICE_LESSON_MENU && selected_practice_type(state) == nullptr) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "practice lesson mode requires a selected practice type"));
  }
  if (state.mode == vamp_zero::app_mode::PRACTICE_EXERCISE_MENU && selected_practice_lesson(state) == nullptr) {
    return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "practice exercise mode requires a selected lesson group"));
  }
  if (state.mode == vamp_zero::app_mode::PRACTICE_KEY_MENU) {
    auto const* exercise = selected_practice_exercise(state);
    if (exercise == nullptr || !supports_key_selection(*exercise)) {
      return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "practice key mode requires a melody exercise"));
    }
  }
  if (state.mode == vamp_zero::app_mode::PERFORMANCE_SCORE) {
    auto const* part = active_part(state);
    if (part == nullptr) {
      return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "performance score mode requires an active part"));
    }
    if (part->total_pages == 0U || state.page_index >= part->total_pages) {
      return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "performance score page index is out of range"));
    }
  }
  if (state.mode == vamp_zero::app_mode::PRACTICE_SCORE) {
    auto const* exercise = active_practice(state);
    if (exercise == nullptr) {
      return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "practice score mode requires an active exercise"));
    }
    auto const* variant = selected_practice_variant(state, *exercise);
    if (variant == nullptr) {
      return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "practice score mode requires a selected generated variant"));
    }
    if (variant->total_pages == 0U || state.page_index >= variant->total_pages) {
      return std::unexpected(vamp_zero::make_error(vamp_zero::error_code::INVALID_ARGUMENT, "practice score page index is out of range"));
    }
  }
  return {};
}