/*
 * This source code is released into the public domain.
 */

module;

#include <fcntl.h>
#include <unistd.h>

#include <expected>
#include <format>
#include <stdexcept>
#include <system_error>

export module nihil:fd;

import :generic_error;

namespace nihil {

/*
 * Exception thrown when an internal fd error occurs.  This is not supposed
 * to be caught, since it indicates an internal logic error in the caller.
 */
export struct fd_logic_error final : std::logic_error {
	fd_logic_error(std::string what)
		: std::logic_error(std::move(what))
	{}
};

/*
 * fd: a file descriptor.
 */

export struct fd final {
	// Construct an empty (invalid) fd.
	fd() noexcept = default;

	// Construct an fd from an exising file destrictor, taking ownership.
	fd(int fd_) noexcept : _fd(fd_) {}

	// Destructor.  Close the fd, discarding any errors.
	~fd()
	{
		if (*this)
			this->close();
	}

	// Move from another fd, leaving the moved-from fd in an invalid state.
	fd(fd &&other) noexcept
		: _fd(std::exchange(other._fd, _invalid_fd))
	{}

	// Move assign from another fd.
	auto operator=(fd &&other) noexcept -> fd &
	{
		if (this != &other)
			_fd = std::exchange(other._fd, _invalid_fd);
		return *this;
	}

	// Not copyable.
	fd(fd const &) = delete;
	fd& operator=(fd const &) = delete;

	// Return true if this fd is valid (open).
	explicit operator bool(this fd const &self) noexcept
	{
		return self._fd != _invalid_fd;
	}

	// Close the wrapped fd.
	auto close(this fd &self) -> std::expected<void, std::error_code>
	{
		auto const ret = ::close(self.get());
		self._fd = _invalid_fd;

		if (ret == 0)
			return {};

		return std::unexpected(std::make_error_code(std::errc(errno)));
	}

	// Return the stored fd.
	auto get(this fd const &self) -> int {
		if (self)
			return self._fd;
		throw fd_logic_error("Attempt to call get() on invalid fd");
	}


	// Release the stored fd and return it.  The caller must close it.
	auto release(this fd &&self) -> int {
		if (self)
			return std::exchange(self._fd, self._invalid_fd);
		throw fd_logic_error("Attempt to release an invalid fd");
	}

private:
	static constexpr int _invalid_fd = -1;

	int _fd = _invalid_fd;
};

// Create a copy of this fd by calling dup().
export auto dup(fd const &self) -> std::expected<fd, std::error_code>
{
	auto thisfd = self.get();

	auto const newfd = ::dup(thisfd);
	if (newfd != -1)
		return {newfd};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Create a copy of this fd by calling dup2().  Note that because this results
// in the existing fd and the new fd both being managed by an fd instance,
// there are two potential cases that can cause problems:
//
// - dup()ing an fd to itself (a no-op)
// - dup()ing an fd to an fd which is already managed by an fd instance
//
// In both of these cases, either use raw_dup() instead, or immediately call
// release() on the returned fd to prevent the fd instance from closing it.
export auto dup(fd const &self, int newfd)
	-> std::expected<fd, std::error_code>
{
	auto thisfd = self.get();

	auto const ret = ::dup2(thisfd, newfd);
	if (ret != -1)
		return {newfd};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Create a copy of this fd by calling dup().
export auto raw_dup(fd const &self) -> std::expected<int, std::error_code>
{
	auto thisfd = self.get();

	auto const newfd = ::dup(thisfd);
	if (newfd != -1)
		return {newfd};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Create a copy of this fd by calling dup2().
export auto raw_dup(fd const &self, int newfd)
	-> std::expected<int, std::error_code>
{
	auto thisfd = self.get();

	auto const ret = ::dup2(thisfd, newfd);
	if (ret != -1)
		return {newfd};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Return the fnctl flags for this fd.
export auto getflags(fd const &self) -> std::expected<int, std::error_code>
{
	auto const flags = ::fcntl(self.get(), F_GETFL);
	if (flags != -1)
		return {flags};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Replace the fnctl flags for this fd.
export auto replaceflags(fd &self, int newflags)
	-> std::expected<void, std::error_code>
{
	auto const ret = ::fcntl(self.get(), F_SETFL, newflags);
	if (ret == 0)
		return {};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Add bits to the fcntl flags for this fd.  Returns the new flags.
export auto setflags(fd &self, int newflags)
	-> std::expected<int, std::error_code>
{
	auto flags = getflags(self);
	if (!flags)
		return flags;

	*flags |= newflags;
	auto const ret = replaceflags(self, *flags);
	if (!ret)
		return std::unexpected(ret.error());

	return {*flags};
}

// Remove bits from the fcntl flags for this fd.  Returns the new flags.
export auto clearflags(fd &self, int clrflags)
	-> std::expected<int, std::error_code>
{
	auto flags = getflags(self);
	if (!flags)
		return flags;

	*flags &= ~clrflags;
	auto const ret = replaceflags(self, *flags);
	if (!ret)
		return std::unexpected(ret.error());

	return {*flags};
}

// Return the fd flags for this fd.
export auto getfdflags(fd const &self) -> std::expected<int, std::error_code>
{
	auto const flags = ::fcntl(self.get(), F_GETFD);
	if (flags != -1)
		return {flags};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Replace the fd flags for this fd.
export auto replacefdflags(fd &self, int newflags)
	-> std::expected<void, std::error_code>
{
	auto const ret = ::fcntl(self.get(), F_SETFD, newflags);
	if (ret != -1)
		return {};

	return std::unexpected(std::make_error_code(std::errc(errno)));
}

// Add bits to the fd flags for this fd.  Returns the new flags.
export auto setfdflags(fd &self, int newflags)
	-> std::expected<int, std::error_code>
{
	auto flags = getfdflags(self);
	if (!flags)
		return flags;

	*flags |= newflags;
	auto const ret = replacefdflags(self, *flags);
	if (!ret)
		return std::unexpected(ret.error());

	return {*flags};
}

// Remove bits from the fd flags for this fd.  Returns the new flags.
export auto clearfdflags(fd &self, int clrflags)
	-> std::expected<int, std::error_code>
{
	auto flags = getfdflags(self);
	if (!flags)
		return flags;

	*flags &= ~clrflags;
	auto ret = replacefdflags(self, *flags);
	if (!ret)
		return std::unexpected(ret.error());

	return {*flags};
}

// Create two fds by calling pipe() and return them.
export auto pipe() -> std::expected<std::pair<fd, fd>, std::error_code> {
	auto fds = std::array<int, 2>{};

	if (auto const ret = ::pipe(fds.data()); ret != 0)
		return std::unexpected(std::make_error_code(std::errc(errno)));

	return {{fd(fds[0]), fd(fds[1])}};
}

/*
 * Write data to a file descriptor from the provided buffer.  Returns the
 * number of bytes (not objects) written.  Incomplete writes may cause a
 * partial object to be written.
 */
export auto write(fd &file, std::ranges::contiguous_range auto &&range)
	-> std::expected<std::size_t, std::error_code>
{
	auto const ret = ::write(file.get(), std::ranges::data(range),
				 std::ranges::size(range));
	if (ret >= 0)
		return ret;
	return std::unexpected(std::make_error_code(std::errc(errno)));
}

/*
 * Read data from a file descriptor into the provided buffer.  Returns the
 * number of bytes (not objects) read.  Incomplete reads may cause a partial
 * object to be read.
 */
export auto read(fd &file, std::ranges::contiguous_range auto &&range)
	-> std::expected<std::size_t, std::error_code>
{
	auto const ret = ::read(file.get(), std::ranges::data(range),
				std::ranges::size(range));
	if (ret >= 0)
		return ret;
	return std::unexpected(std::make_error_code(std::errc(errno)));
}

} // namespace nihil