module;

#include <asio/as_tuple.hpp>
#include <asio/awaitable.hpp>
#include <asio/buffer.hpp>
#include <asio/use_awaitable.hpp>
#include <asio/write.hpp>

#include <algorithm>

#include <charconv>
#include <concepts>
#include <cstddef>
#include <cstdint>
#include <format>
#include <string>
#include <string_view>
#include <type_traits>
#include <variant>

export module ttwhy.terminal:echo;

import :events;
import ttwhy.core;

namespace ttwhy::terminal
{

  using namespace std::string_view_literals;

  constexpr auto vte_backspace_sequence = "\b \b"sv;
  constexpr auto vte_newline_sequence = "\r\n"sv;
  constexpr auto vte_horizontal_tab_sequence = "\t"sv;
  constexpr auto vte_cursor_left_sequence = "\x1b[D"sv;
  constexpr auto vte_cursor_right_sequence = "\x1b[C"sv;
  constexpr auto vte_delete_character_sequence = "\x1b[P"sv;
  constexpr auto vte_cursor_left_sequence_format = "\x1b[{}D"sv;
  constexpr auto vte_cursor_right_sequence_format = "\x1b[{}C"sv;

  namespace detail
  {
    //! Maps a function key to its CSI parameter code (F5 through F20).
    auto function_key_parameter(function_key key) -> unsigned int
    {
      switch (key)
      {
        case function_key::f5:
          return 15;
        case function_key::f6:
          return 17;
        case function_key::f7:
          return 18;
        case function_key::f8:
          return 19;
        case function_key::f9:
          return 20;
        case function_key::f10:
          return 21;
        case function_key::f11:
          return 23;
        case function_key::f12:
          return 24;
        case function_key::f13:
          return 25;
        case function_key::f14:
          return 26;
        case function_key::f15:
          return 28;
        case function_key::f16:
          return 29;
        case function_key::f17:
          return 31;
        case function_key::f18:
          return 32;
        case function_key::f19:
          return 33;
        case function_key::f20:
          return 34;
        default:
          return 0;
      }
    }

    //! Appends an xterm-style modifier parameter to a CSI sequence.
    auto apply_modifier(std::string sequence, modifier_set modifiers) -> std::string
    {
      auto const mask = static_cast<std::uint8_t>(modifiers);
      auto const modifier_param = [&mask]() -> unsigned int {
        if ((mask & static_cast<std::uint8_t>(modifier::shift)) != 0)
        {
          return 2;
        }
        if ((mask & static_cast<std::uint8_t>(modifier::alt)) != 0)
        {
          return 3;
        }
        if ((mask & static_cast<std::uint8_t>(modifier::ctrl)) != 0)
        {
          return 4;
        }
        if ((mask & static_cast<std::uint8_t>(modifier::meta)) != 0)
        {
          return 5;
        }
        return 1;
      }();

      if (modifier_param == 1)
      {
        return sequence;
      }

      auto const terminator = sequence.find_last_of('~');
      if (terminator == std::string::npos)
      {
        return sequence;
      }
      return sequence.substr(0, terminator) + std::to_string(modifier_param) + sequence.substr(terminator);
    }

    //! Builds the terminal escape sequence that represents a function key press.
    auto function_key_sequence(function_key key, modifier_set modifiers) -> std::string
    {
      switch (key)
      {
        case function_key::f1:
          return apply_modifier(std::string{"\x1bOP"}, modifiers);
        case function_key::f2:
          return apply_modifier(std::string{"\x1bOQ"}, modifiers);
        case function_key::f3:
          return apply_modifier(std::string{"\x1bOR"}, modifiers);
        case function_key::f4:
          return apply_modifier(std::string{"\x1bOS"}, modifiers);
        default:
          return apply_modifier(std::format("\x1b[{}~", function_key_parameter(key)), modifiers);
      }
    }
  }  // namespace detail

  //! An asynchronous router that echoes characters and handles terminal navigation.
  //!
  //! @tparam StreamType The output stream descriptor type.
  export template<typename StreamType>
  struct echo
  {
    //! Construct an echo router writing to an output stream.
    //!
    //! @param stream The output stream to write echoed text and escape sequences to.
    explicit echo(StreamType & stream)
        : m_output_stream{stream}
    {}

    //! Get the current cursor position within the printed line.
    //!
    //! @return The zero-based cursor position.
    [[nodiscard]] auto cursor_position() const noexcept -> std::size_t
    {
      return m_cursor_position;
    }

    //! Get the maximum cursor position reached on the current line.
    //!
    //! @return The maximum position reached.
    [[nodiscard]] auto max_position() const noexcept -> std::size_t
    {
      return m_max_position;
    }

    //! Asynchronously process an incoming terminal input event.
    //!
    //! @param event The input event to handle.
    //! @return An awaitable task that completes when processing finishes.
    auto process(input_event event) -> asio::awaitable<void>
    {
      co_await std::visit(
          [&](auto && event) -> asio::awaitable<void> {
            using event_type = std::decay_t<decltype(event)>;

            if constexpr (std::same_as<character_event, event_type>)
            {
              co_await process_character(event.value);
            }
            else if constexpr (std::same_as<control_event, event_type>)
            {
              co_await process_control_key(event.key);
            }
            else if constexpr (std::same_as<navigation_event, event_type>)
            {
              co_await process_navigation_key(event);
            }
            else if constexpr (std::same_as<function_key_event, event_type>)
            {
              co_await process_function_key(event);
            }
            else if constexpr (std::same_as<ctrl_chord_event, event_type>)
            {
              co_await process_ctrl_chord(event);
            }
          },
          event);
    }

  private:
    auto process_function_key(function_key_event event) -> asio::awaitable<void>
    {
      auto const sequence = detail::function_key_sequence(event.key, event.modifiers);
      co_await asio::async_write(m_output_stream, asio::buffer(sequence.data(), sequence.size()),
                                 asio::as_tuple(asio::use_awaitable));
    }

    auto process_ctrl_chord(ctrl_chord_event event) -> asio::awaitable<void>
    {
      auto const control_character = static_cast<char>(static_cast<unsigned char>(event.key) - 0x40u);
      co_await asio::async_write(m_output_stream, asio::buffer(&control_character, 1),
                                 asio::as_tuple(asio::use_awaitable));
    }

    auto process_character(char character) -> asio::awaitable<void>
    {
      auto [error, bytes_written] =
          co_await asio::async_write(m_output_stream, asio::buffer(&character, 1), asio::as_tuple(asio::use_awaitable));
      if (!error)
      {
        m_cursor_position += bytes_written;
        m_max_position = std::max(m_max_position, m_cursor_position);
      }
    }

    auto process_control_key(control_key key) -> asio::awaitable<void>
    {
      switch (key)
      {
        case control_key::backspace:
          if (m_cursor_position > 0)
          {
            auto [error, bytes_written] = co_await asio::async_write(
                m_output_stream, asio::buffer(vte_backspace_sequence), asio::as_tuple(asio::use_awaitable));
            if (!error)
            {
              if (m_cursor_position == m_max_position)
              {
                --m_max_position;
              }
              --m_cursor_position;
            }
          }
          break;
        case control_key::enter:
        {
          auto [error, bytes_written] = co_await asio::async_write(m_output_stream, asio::buffer(vte_newline_sequence),
                                                                   asio::as_tuple(asio::use_awaitable));
          if (!error)
          {
            m_cursor_position = 0;
            m_max_position = 0;
          }
          break;
        }
        case control_key::tab:
        {
          auto [error, bytes_written] = co_await asio::async_write(
              m_output_stream, asio::buffer(vte_horizontal_tab_sequence), asio::as_tuple(asio::use_awaitable));
          if (!error)
          {
            auto const tab_stop = 8uz - (m_cursor_position % 8uz);
            m_cursor_position += tab_stop;
            m_max_position = std::max(m_max_position, m_cursor_position);
          }
          break;
        }
        default:
          co_await asio::async_write(m_output_stream, asio::buffer(std::format("{{CTRL:{}}}", std::to_underlying(key))),
                                     asio::as_tuple(asio::use_awaitable));
          break;
      };
    }

    auto process_navigation_key(navigation_event event) -> asio::awaitable<void>
    {
      switch (event.key)
      {
        case navigation_key::left:
          if (m_cursor_position > 0)
          {
            auto [error, bytes_written] = co_await asio::async_write(
                m_output_stream, asio::buffer(vte_cursor_left_sequence), asio::as_tuple(asio::use_awaitable));
            if (!error)
            {
              --m_cursor_position;
            }
          }
          break;

        case navigation_key::right:
          if (m_cursor_position < m_max_position)
          {
            auto [error, bytes_written] = co_await asio::async_write(
                m_output_stream, asio::buffer(vte_cursor_right_sequence), asio::as_tuple(asio::use_awaitable));
            if (!error)
            {
              ++m_cursor_position;
            }
          }
          break;

        case navigation_key::home:
          if (m_cursor_position > 0)
          {
            auto sequence = std::format(vte_cursor_left_sequence_format, m_cursor_position);
            auto [error, bytes_written] = co_await asio::async_write(m_output_stream, asio::buffer(sequence),
                                                                     asio::as_tuple(asio::use_awaitable));
            if (!error)
            {
              m_cursor_position = 0;
            }
          }
          break;

        case navigation_key::end:
          if (m_cursor_position < m_max_position)
          {
            auto const distance = m_max_position - m_cursor_position;
            auto sequence = std::format(vte_cursor_right_sequence_format, distance);
            auto [error, bytes_written] = co_await asio::async_write(m_output_stream, asio::buffer(sequence),
                                                                     asio::as_tuple(asio::use_awaitable));
            if (!error)
            {
              m_cursor_position = m_max_position;
            }
          }
          break;

        case navigation_key::delete_key:
          if (m_cursor_position < m_max_position)
          {
            auto [error, bytes_written] = co_await asio::async_write(
                m_output_stream, asio::buffer(vte_delete_character_sequence), asio::as_tuple(asio::use_awaitable));
            if (!error)
            {
              --m_max_position;
            }
          }
          break;

        default:
          break;
      }
    }

    StreamType & m_output_stream;
    std::size_t m_cursor_position{};
    std::size_t m_max_position{};
  };

}  // namespace ttwhy::terminal