module;

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

export module turtle;

import coral;
import coral.files;
import coral.io;

using coral::closure;
using coral::io_error;
using coral::slice;
using coral::unreachable;
using coral::usize;

export namespace turtle {
	/**
	 * Path to a native I/O resource that is big enough for every supported platform.
	 */
	struct native_path {
		/**
		 * Errors that may occur during a path joining operation.
		 *
		 * [join_error::overflow] signals that the given path join exceeds the maximum valid length of a native path.
		 */
		enum class join_error {
			overflow,
		};

		/**
		 * Maximum number of bytes in a native path.
		 */
		static usize const max = 4095;

		native_path() = default;

		/**
		 * Constructs a native path from `text`, raising a static assertion if it is larger than [max].
		 */
		template<usize text_size> constexpr native_path(char const(&text)[text_size]) {
			static_assert(text_size <= max);

			for (usize i = 0; i < text_size; i += 1) this->buffer[i] = text[i];

			this->buffer[text_size] = 0;
		}

		/**
		 * Returns a weak reference to the native path as a [coral::slice].
		 *
		 * *Note*: this is an `O(N)` time function, where `N` is the path length.
		 */
		constexpr slice<char const> as_slice() const {
			return {this->buffer, this->filled()};
		}

		/**
		 * Returns the number of bytes composing the native path.
		 *
		 * *Note*: this is an `O(N)` time function, where `N` is the path length.
		 */
		constexpr usize filled() const {
			usize length {0};

			while (this->buffer[length]) length += 1;

			return length;
		}

		/**
		 * Attempts to create a new native path from the current native path joined with `text`, returning it or a
		 * [join_error].
		 *
		 * *Note*: this is an `O(N)` time function, where `N` is the path length.
		 */
		constexpr coral::expected<native_path, join_error> joined(slice<char const> const & text) const {
			usize buffer_filled {this->filled()};

			if (text.length >= (max - buffer_filled)) return join_error::overflow;

			native_path joined_path {*this};

			for (char const c : text) {
				joined_path.buffer[buffer_filled] = c;
				buffer_filled += 1;
			}

			return joined_path;
		}

		private:
		char buffer[max + 1] {0};
	};

	/**
	 * Results from a native I/O resource closing operation.
	 *
	 * [close_result::ok] means that no errors occured and the resource has been successfully closed if it was
	 * open.
	 *
	 * [close_result::io_unavailable] is a generic error to communicate that something between the hardware and the
	 * operating system layer failed while closing the resource.
	 *
	 * [close_result::access_denied] reports that the process does not have the required permissions to open the
	 * resource. This is a rare but possible error that is only possible by a system changing the underlying
	 * resource permissions after the native resource has already opened it.
	 */
	enum class [[nodiscard]] close_result {
		ok,
		io_unavailable,
		access_denied,
	};

	/**
	 * Results from a native I/O resource opening operation.
	 *
	 * [open_result::ok] means that no errors occured and the resource has been successfully opened.
	 *
	 * [open_result::io_unavailable] is a generic error to communicate that something between the hardware and the
	 * operating system layer failed while opening the resource.
	 *
	 * [open_result::access_denied] reports that the process does not have the required permissions to open the
	 * resource. While all platforms implement some form of file permissions, the specifics of this error are
	 * opaque to the caller.
	 *
	 * [open_result::not_found] indicates that no resource matching the opening query was found.
	 *
	 * [open_result::too_many] signals that there are too many files open in the current process and / or the
	 * wider operating system at the moment to open the resource.
	 *
	 * [open_result::too_big] signals that the resource is too big to open. The usual cause of this is error is
	 * attempting to open a file bigger than the addressable file range supported by the compiled application.
	 *
	 * [open_result::out_of_memory] signals that the system does not have enough memory remaining to open the resource.
	 */
	enum class [[nodiscard]] open_result {
		ok,
		io_unavailable,
		access_denied,
		not_found,
		too_many,
		too_big,
		out_of_memory,
	};

	/**
	 * Maps a [close_result] to the equivalent [open_result] and returns the value.
	 */
	open_result close_to_open_result(close_result result) {
		switch (result) {
			case close_result::ok: return open_result::ok;
			case close_result::io_unavailable: return open_result::io_unavailable;
			case close_result::access_denied: return open_result::access_denied;
			default: unreachable();
		}
	}

	/**
	 * Provides unmanaged access to a native file.
	 */
	struct native_file final : public coral::file_reader, public coral::file_writer {
		/**
		 * Opening modes for files.
		 *
		 * [open_mode::read_only] requests read-only access to an existing file. Specifying a path to an invalid file
		 * will result in the open request failing.
		 *
		 * [open_mode::overwrite] requests write-only access to a new file. Specifying a path to an existing file will
		 * result in it being wiped and overwritten (assuming it is not protected by the underlying operating system).
		 *
		 * [open_mode::append] requests write-only access to an existing file. Specifying a path to an invalid file
		 * will result in the open request failing. The write cursor begins at the end of any existing data in the file.
		 */
		enum class open_mode {
			read_only,
			overwrite,
			append,
		};

		native_file() = default;

		/**
		 * Attempts to close any currently open file.
		 *
		 * A [close_result] is returned containing either [close_result::ok] to indicate success or any other value to
		 * indicate an error. See [close_result] for more details.
		 *
		 * *Note*: failing to close should not be treated as a reason to retry the closing operation, and should instead
		 * be used to inform the end-user that the operation failed or that the process should exit.
		 */
		close_result close() {
			errno = 0;

			if (::close(this->fd) != 0) switch (errno) {
				case EINTR: case EIO: case ENOSPC: return close_result::io_unavailable;
				case EDQUOT: return close_result::access_denied;
				default: unreachable();
			}

			this->fd = 0;

			return close_result::ok;
		}

		/**
		 * Returns `true` if a file is currently open, otherwise `false`.
		 */
		bool is_open() const {
			return this->fd > 0;
		}

		/**
		 * Attempts to open a native file at `file_path` using `file_open_mode` for the access policy. See [open_mode]
		 * for more information on how files may be opened.
		 *
		 * An [open_result] is returned containing either [open_result::ok] to indicate success or any other value to
		 * indicate an error. See [open_result] for more details.
		 *
		 * *Note*: the opened file must be closed using [close] once no longer needed or the process will leak file
		 * handles.
		 *
		 * *Note*: it is recommended to prefer performing file I/O via [sandboxed_fs] unless direct file access is
		 * required.
		 */
		open_result open(native_path const & file_path, open_mode file_open_mode) {
			if (this->is_open()) {
				open_result const result {close_to_open_result(this->close())};

				if (result != open_result::ok) return result;
			}

			constexpr int perms {S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH};

			errno = 0;

			switch (file_open_mode) {
				case open_mode::read_only: {
					this->fd = ::open(file_path.as_slice().as_chars().pointer, O_RDONLY, perms);

					break;
				}

				case open_mode::overwrite: {
					this->fd = ::open(file_path.as_slice().as_chars().pointer, O_WRONLY | O_CREAT | O_TRUNC, perms);

					break;
				}

				case open_mode::append: {
					this->fd = ::open(file_path.as_slice().as_chars().pointer, O_WRONLY | O_APPEND | O_CREAT, perms);

					break;
				}

				default: unreachable();
			}

			if (!this->is_open()) switch (errno) {
				case EACCES: case EDQUOT: case ENXIO: case EPERM:
				case EROFS: case ETXTBSY: return open_result::access_denied;

				case EINTR: case ENOSPC: return open_result::io_unavailable;

				case EINVAL: case EISDIR: case ELOOP: case ENAMETOOLONG:
				case ENOENT: case ENOTDIR: return open_result::not_found;

				case EOVERFLOW: case EFBIG: return open_result::too_big;
				case EMFILE: case ENFILE: return open_result::too_many;
				case ENOMEM: return open_result::out_of_memory;
				default: unreachable();
			}

			return open_result::ok;
		}

		/**
		 * See [coral::file_reader::read].
		 */
		coral::expected<usize, io_error> read(slice<coral::u8> const & data) override {
			if (!this->is_open()) return io_error::unavailable;

			coral::size const data_read {::read(this->fd, data.pointer, sizeof(coral::u8) * data.length)};

			if (data_read < 0) return io_error::unavailable;

			return data_read;
		}

		/**
		 * See [coral::file_reader::seek] and [coral::file_writer::seek].
		 */
		coral::expected<coral::u64, io_error> seek(coral::u64 offset) override {
			if (!this->is_open()) return io_error::unavailable;

			coral::i64 const data_position {::lseek(this->fd,
				static_cast<coral::i64>(coral::min(offset, coral::i64_max)), SEEK_SET)};

			if (data_position == -1) return io_error::unavailable;

			return static_cast<coral::u64>(data_position);
		}

		/**
		 * See [coral::file_reader::tell] and [coral::file_writer::tell].
		 */
		coral::expected<coral::u64, io_error> tell() override {
			if (!this->is_open()) return io_error::unavailable;

			coral::i64 const data_position {::lseek(this->fd, 0, SEEK_SET)};

			if (data_position == -1) return io_error::unavailable;

			return static_cast<coral::u64>(data_position);
		}

		/**
		 * See [coral::file_writer::write].
		 */
		coral::expected<usize, io_error> write(slice<coral::u8 const> const & data) override {
			if (!this->is_open()) return io_error::unavailable;

			coral::size const data_written {::write(this->fd, data.pointer, sizeof(coral::u8) * data.length)};

			if (data_written < 0) return io_error::unavailable;

			return data_written;
		}

		private:
		int fd {0};
	};

	/**
	 * [coral::file_system] wrapper around native file system access to provide a managed and system-agnostic
	 * environment for performing file I/O.
	 */
	struct file_sandbox : public coral::file_system {
		/**
		 * Permission flags that a [sandboxed_fs] may specify for restricting access to it.
		 */
		struct permissions {
			bool can_read;

			bool can_write;

			bool can_walk;
		};

		/**
		 * Constructs a sandbox located at `path` with `sandbox_permissions` as the permissions given to users of it.
		 */
		file_sandbox(native_path const & path, permissions const & access_permissions) {
			this->path = path;
			this->access_permissions = access_permissions;
		}

		/**
		 * Returns a reference to a [file_sandbox] that provides access to the base directory which, on most systems, is
		 * the current working directory.
		 *
		 * The base directory may be used to access things loose files created outside of the application, such as user-
		 * generated files and modifications.
		 *
		 * *Note*: The base file system does not permit being written to.
		 */
		static file_sandbox & base() {
			static file_sandbox base_sandbox {"./", {
				.can_read = true,
				.can_walk = true
			}};

			return base_sandbox;
		}

		/**
		 * Returns a reference to a [file_sandbox] that operates as a temporary file store.
		 *
		 * As the name implies, the existence of files that exist here are not guaranteed beyond the duration of an
		 * application run lifetime. The purpose of the temporary file store is to support transactional I/O
		 * operations that only want to replace files if new ones has been successfully constructed already.
		 *
		 * *Note*: The temp file system does not permit being walked.
		 */
		static file_sandbox & temp() {
			static file_sandbox temp_sandbox {"/tmp", {
				.can_read = true,
				.can_write = false,
			}};

			return temp_sandbox;
		}

		/**
		 * See [coral::fs::read_file].
		 *
		 * *Note*: this function will only work on sandboxes with the [permissions::can_read] flag enabled.
		 */
		void read_file(slice<char const> const & path, closure<void(coral::file_reader &)> const & then) override {
			if (!this->access_permissions.can_read) return;

			this->path.joined(path).ok().and_then([&](native_path const & native_file_path) -> void {
				native_file file;

				if (file.open(native_file_path, native_file::open_mode::read_only) != open_result::ok) return;

				then(file);

				if (file.close() != close_result::ok) return;
			});
		}

		/**
		 * See [coral::fs::walk_files].
		 *
		 * *Note*: this function will only work on sandboxes with the [permissions::can_walk] flag enabled.
		 */
		void walk_files(slice<char const> const & path, closure<void(coral::file_walker &)> const & then) override {
			if (!this->access_permissions.can_walk) return;

			this->path.joined(path).ok().and_then([&](native_path const & native_directory_path) -> void {
				// TODO: Implement.
			});
		}

		/**
		 * See [coral::fs::write_file].
		 *
		 * *Note*: this function will only work on sandboxes with the [permissions::can_write] flag enabled.
		 */
		void write_file(slice<char const> const & path, closure<void(coral::file_writer &)> const & then) override {
			if (!this->access_permissions.can_write) return;

			this->path.joined(path).ok().and_then([&](native_path const & native_file_path) -> void {
				native_file file;

				if (file.open(native_file_path, native_file::open_mode::overwrite) != open_result::ok) return;

				then(file);

				if (file.close() != close_result::ok) return;
			});
		}

		private:
		native_path path;

		permissions access_permissions {
			.can_read = false,
			.can_write = false,
			.can_walk = false,
		};
	};

	/**
	 * Returns a reference to the process-wide error logging [coral::writer] used for recording errors.
	 */
	coral::writer & error_log() {
		static struct : public coral::writer {
			coral::expected<usize, io_error> write(slice<coral::u8 const> const & data) override {
				coral::size const data_written {::write(STDOUT_FILENO, data.pointer, sizeof(coral::u8) * data.length)};

				if (data_written == -1) return io_error::unavailable;

				return data_written;
			};
		} output_writer;

		return output_writer;
	}
}