module;

#include <algorithm>
#include <termios.h>
#include <unistd.h>

#include <cerrno>
#include <system_error>
#include <utility>

export module ttwhy.terminal:scoped_attributes;

namespace ttwhy::terminal
{

  auto static read_attributes(int file_descriptor) -> termios
  {
    auto active_attributes = termios{};
    if (::tcgetattr(file_descriptor, &active_attributes))
    {
      throw std::system_error(errno, std::system_category(), "failed to read termios attributes");
    }
    return active_attributes;
  }

  auto static get_line_discipline_flag(int file_descriptor, int flag) -> bool
  {
    auto active_attributes = read_attributes(file_descriptor);
    return active_attributes.c_lflag & flag;
  }

  auto static set_line_discipline_flag(int file_descriptor, int flag, bool enabled) -> void
  {
    auto active_attributes = read_attributes(file_descriptor);

    if ((active_attributes.c_lflag & flag) == enabled)
    {
      return;
    }

    if (enabled)
    {
      active_attributes.c_lflag = active_attributes.c_lflag | flag;
    }
    else
    {
      active_attributes.c_lflag = active_attributes.c_lflag & ~flag;
    }

    if (::tcsetattr(file_descriptor, TCSANOW, &active_attributes))
    {
      throw std::system_error(errno, std::system_category(), "failed to write termios attributes");
    }
  }

  //! RAII wrapper storing the current terminal attributes and restoring them on destruction.
  export struct scoped_attributes
  {
    //! Construct and capture the current attributes of a terminal file descriptor.
    //!
    //! @param file_descriptor The file descriptor of the terminal.
    explicit scoped_attributes(int file_descriptor)
        : m_file_descriptor{-1}
        , m_attributes{}
    {
      if (isatty(file_descriptor))
      {
        m_file_descriptor = dup(file_descriptor);
        m_attributes = read_attributes(m_file_descriptor);
      }
    }

    scoped_attributes(scoped_attributes const &) = delete;

    scoped_attributes(scoped_attributes && other)
        : m_file_descriptor{std::exchange(other.m_file_descriptor, -1)}
        , m_attributes{std::move(other.m_attributes)}
    {}

    ~scoped_attributes()
    {
      if (m_file_descriptor > 0)
      {
        ::tcsetattr(m_file_descriptor, TCSANOW, &m_attributes);
        ::close(m_file_descriptor);
      }
    }

    auto operator=(scoped_attributes const &) -> scoped_attributes & = delete;

    auto operator=(scoped_attributes && other) -> scoped_attributes &
    {
      std::ranges::swap(m_file_descriptor, other.m_file_descriptor);
      std::ranges::swap(m_attributes, other.m_attributes);
      return *this;
    }

    //! Check if canonical mode is currently active.
    //!
    //! @return True if ICANON is set, false otherwise.
    [[nodiscard]] auto canonical_mode() const noexcept -> bool
    {
      return get_line_discipline_flag(m_file_descriptor, ICANON);
    }

    //! Set whether canonical mode is active or not on an lvalue reference.
    //!
    //! @param enabled True to enable canonical mode, false to disable.
    //! @return Reference to this object for chaining.
    auto canonical_mode(bool enabled) & -> scoped_attributes &
    {
      set_line_discipline_flag(m_file_descriptor, ICANON, enabled);
      return *this;
    }

    //! Set whether canonical mode is active or not on an rvalue.
    //!
    //! @param enabled True to enable canonical mode, false to disable.
    //! @return The modified scoped_attributes object by value.
    auto canonical_mode(bool enabled) && -> scoped_attributes
    {
      set_line_discipline_flag(m_file_descriptor, ICANON, enabled);
      return std::move(*this);
    }

    //! Check if echo is currently active.
    //!
    //! @return True if ECHO is set, false otherwise.
    [[nodiscard]] auto echo() const noexcept -> bool
    {
      return get_line_discipline_flag(m_file_descriptor, ECHO);
    }

    //! Set whether echo is active or not on an lvalue reference.
    //!
    //! @param enabled True to enable echo, false to disable.
    //! @return Reference to this object for chaining.
    auto echo(bool enabled) & -> scoped_attributes &
    {
      set_line_discipline_flag(m_file_descriptor, ECHO, enabled);
      return *this;
    }

    //! Set whether echo is active or not on an rvalue.
    //!
    //! @param enabled True to enable echo, false to disable.
    //! @return The modified scoped_attributes object by value.
    auto echo(bool enabled) && -> scoped_attributes
    {
      set_line_discipline_flag(m_file_descriptor, ECHO, enabled);
      return std::move(*this);
    }

    //! Capture the currently active terminal mode and mark the attributes as suspended.
    //!
    //! Must be called before the terminal mode is changed while the process is being stopped.
    auto suspend() & -> scoped_attributes &
    {
      if (m_file_descriptor > 0)
      {
        m_suspended = true;
        m_suspended_canonical = canonical_mode();
        m_suspended_echo = echo();
      }
      return *this;
    }

    //! Restore the terminal mode captured before suspension.
    auto resume() & -> scoped_attributes &
    {
      if (m_suspended)
      {
        canonical_mode(m_suspended_canonical);
        echo(m_suspended_echo);
        m_suspended = false;
      }
      return *this;
    }

  private:
    int m_file_descriptor;
    termios m_attributes;
    bool m_suspended{false};
    bool m_suspended_canonical{false};
    bool m_suspended_echo{false};
  };

}  // namespace ttwhy::terminal