Replace app library with Turtle OS abstraction

.vscode/settings.json

@@ -50,4 +50,6 @@
 	"editor.insertSpaces": false,
 	"C_Cpp.errorSquiggles": "disabled",
 	"editor.rulers": [120],
+	"files.trimTrailingWhitespace": true,
+	"files.insertFinalNewline": true,
 }

build.py

@@ -30,6 +30,6 @@ def compile_package(root_module_name: str) -> None:
 
 compile_package("coral")
 compile_package("oar")
-compile_package("app")
+compile_package("turtle")
 compile_package("runtime")
 subprocess.run(f"{compile_command} {' '.join(object_file_paths)} -o ./runtime -lSDL2", shell=True, check=True)

source/app.cpp

@@ -1,317 +0,0 @@
-module;
-
-#include <SDL2/SDL.h>
-
-export module app;
-
-import coral;
-import coral.files;
-import coral.image;
-import coral.io;
-import coral.math;
-
-import oar;
-
-using native_path = coral::fixed_buffer<4096>;
-
-struct native_file : public coral::file_reader, public coral::file_writer {
-	enum class open_mode {
-		read_only,
-		overwrite,
-	};
-
-	enum class [[nodiscard]] close_result {
-		ok,
-		io_unavailable,
-	};
-
-	enum class [[nodiscard]] open_result {
-		ok,
-		io_unavailable,
-		access_denied,
-		not_found,
-	};
-
-	native_file() = default;
-
-	close_result close() {
-		if (SDL_RWclose(this->rw_ops) != 0) return close_result::io_unavailable;
-
-		this->rw_ops = nullptr;
-
-		return close_result::ok;
-	}
-
-	bool is_open() const {
-		return this->rw_ops != nullptr;
-	}
-
-	open_result open(native_path const & file_path, open_mode mode) {
-		if (this->is_open()) switch (this->close()) {
-			case close_result::ok: break;
-			case close_result::io_unavailable: return open_result::io_unavailable;
-			default: coral::unreachable();
-		}
-
-		// No room for zero terminator.
-		if (file_path.is_full()) return open_result::not_found;
-
-		switch (mode) {
-			case open_mode::read_only: {
-				this->rw_ops = SDL_RWFromFile(file_path.as_slice().as_chars().begin(), "rb");
-
-				break;
-			}
-
-			case open_mode::overwrite: {
-				this->rw_ops = SDL_RWFromFile(file_path.as_slice().as_chars().begin(), "wb");
-
-				break;
-			}
-
-			default: coral::unreachable();
-		}
-
-		if (this->rw_ops == nullptr) return open_result::not_found;
-
-		return open_result::ok;
-	}
-
-	coral::expected<coral::usize, coral::io_error> read(coral::slice<coral::u8> const & data) override {
-		if (!this->is_open()) return coral::io_error::unavailable;
-
-		coral::usize const data_read{SDL_RWread(this->rw_ops, data.pointer, sizeof(uint8_t), data.length)};
-
-		if ((data_read == 0) && (SDL_GetError() != nullptr)) return coral::io_error::unavailable;
-
-		return data_read;
-	}
-
-	coral::expected<coral::u64, coral::io_error> seek(coral::u64 offset) override {
-		if (!this->is_open()) return coral::io_error::unavailable;
-
-		// TODO: Fix cast.
-		coral::i64 const byte_position{
-			SDL_RWseek(this->rw_ops, static_cast<coral::i64>(offset), RW_SEEK_SET)};
-
-		if (byte_position == -1) return coral::io_error::unavailable;
-
-		return static_cast<coral::u64>(byte_position);
-	}
-
-	coral::expected<coral::u64, coral::io_error> tell() override {
-		if (!this->is_open()) return coral::io_error::unavailable;
-
-		coral::i64 const byte_position{SDL_RWseek(this->rw_ops, 0, RW_SEEK_SET)};
-
-		if (byte_position == -1) return coral::io_error::unavailable;
-
-		return static_cast<coral::u64>(byte_position);
-	}
-
-	coral::expected<coral::usize, coral::io_error> write(coral::slice<coral::u8 const> const & data) override {
-		if (!this->is_open()) return coral::io_error::unavailable;
-
-		coral::usize const data_written{SDL_RWwrite(this->rw_ops, data.pointer, sizeof(uint8_t), data.length)};
-
-		if ((data_written == 0) && (SDL_GetError() != nullptr)) return coral::io_error::unavailable;
-
-		return data_written;
-	}
-
-	private:
-	SDL_RWops * rw_ops{nullptr};
-};
-
-struct sandboxed_fs : public coral::fs {
-	sandboxed_fs() {
-		char * const path{SDL_GetBasePath()};
-
-		if (path == nullptr) return;
-
-		for (coral::usize index = 0; path[index] != 0; index += 1)
-			this->sandbox_path.put(path[index]);
-
-		SDL_free(path);
-
-		this->access_rules.can_read = true;
-	}
-
-	sandboxed_fs(coral::path const & organization_name, coral::path const & app_name) {
-		char * const path{SDL_GetPrefPath(organization_name.begin(), app_name.begin())};
-
-		if (path == nullptr) return;
-
-		for (coral::usize index = 0; path[index] != 0; index += 1)
-			this->sandbox_path.put(path[index]);
-
-		SDL_free(path);
-
-		this->access_rules.can_read = true;
-	}
-
-	access_rules query_access() override {
-		return this->access_rules;
-	}
-
-	void read_file(coral::path const & file_path, coral::closure<void(coral::file_reader &)> const & then) override {
-		if (!this->access_rules.can_read) return;
-
-		native_path sandbox_file_path;
-		{
-			coral::expected const written = sandbox_file_path.write(this->sandbox_path.as_slice());
-
-			if (!written.is_ok() || (written.value() != this->sandbox_path.count())) return;
-		}
-
-		{
-			coral::expected const written =
-				sandbox_file_path.write(file_path.as_slice().as_bytes());
-
-			if (!written.is_ok() || (written.value() != this->sandbox_path.count())) return;
-		}
-
-		native_file file;
-
-		if (file.open(sandbox_file_path, native_file::open_mode::read_only) !=
-			native_file::open_result::ok) return;
-
-		then(file);
-
-		if (file.close() != native_file::close_result::ok)
-			// Error orphaned file handle!
-			return;
-	}
-
-	void write_file(coral::path const & file_path, coral::closure<void(coral::file_writer &)> const & then) override {
-		if (!this->access_rules.can_write) return;
-
-		native_path sandbox_file_path;
-		{
-			coral::expected const written = sandbox_file_path.write(this->sandbox_path.as_slice());
-
-			if (!written.is_ok() || (written.value() != this->sandbox_path.count())) return;
-		}
-
-		{
-			coral::expected const written =
-				sandbox_file_path.write(file_path.as_slice().as_bytes());
-
-			if (!written.is_ok() || (written.value() != this->sandbox_path.count())) return;
-		}
-
-		native_file file;
-
-		if (file.open(sandbox_file_path, native_file::open_mode::overwrite) !=
-			native_file::open_result::ok) return;
-
-		then(file);
-
-		if (file.close() != native_file::close_result::ok)
-			// Error orphaned file handle!
-			return;
-	}
-
-	private:
-	native_path sandbox_path;
-
-	access_rules access_rules{
-		.can_read = false,
-		.can_write = false,
-	};
-};
-
-export namespace app {
-	enum class log_level {
-		notice,
-		warning,
-		error,
-	};
-
-	struct client {
-		coral::fs & base() {
-			return this->base_sandbox;
-		}
-
-		void display(coral::u16 screen_width, coral::u16 screen_height) {
-			SDL_SetWindowSize(this->window, screen_width, screen_height);
-			SDL_ShowWindow(this->window);
-		}
-
-		void log(log_level level, coral::slice<char const> const & message) {
-			coral::i32 const length{static_cast<coral::i32>(
-				coral::min(message.length, static_cast<size_t>(coral::i32_max)))};
-
-			SDL_LogMessage(SDL_LOG_CATEGORY_APPLICATION,
-				SDL_LOG_PRIORITY_INFO, "%.*s", length, message.pointer);
-		}
-
-		bool poll() {
-			while (SDL_PollEvent(&this->event) != 0) {
-				switch (this->event.type) {
-					case SDL_QUIT: return false;
-				}
-			}
-
-			return true;
-		}
-
-		coral::fs & resources() {
-			return this->resources_archive;
-		}
-
-		static int run(coral::path const & title, coral::closure<int(client &)> const & start) {
-			constexpr int windowpos {SDL_WINDOWPOS_UNDEFINED};
-			constexpr coral::u32 windowflags {SDL_WINDOW_HIDDEN};
-			constexpr int window_width {640};
-			constexpr int window_height {480};
-
-			SDL_Window * const window {SDL_CreateWindow(
-				title.begin(), windowpos, windowpos, window_width, window_height, windowflags)};
-
-			if (window == nullptr) return 0xff;
-
-			struct : public coral::allocator {
-				coral::u8 * reallocate(coral::u8 * allocation, coral::usize requested_size) override {
-					return reinterpret_cast<coral::u8 *>(SDL_realloc(allocation, requested_size));
-				}
-
-				void deallocate(void * allocation) override {
-					SDL_free(allocation);
-				}
-			} allocator;
-
-			client app_client {&allocator, window, title};
-
-			return start(app_client);
-		}
-
-		coral::allocator & thread_safe_allocator() {
-			return *this->allocator;
-		}
-
-		coral::fs & user() {
-			return this->user_sandbox;
-		}
-
-		private:
-		client(coral::allocator * allocator, SDL_Window * window,
-			coral::path const & title) : user_sandbox{"ona", title} {
-
-			this->allocator = allocator;
-			this->window = window;
-		}
-
-		coral::allocator * allocator;
-
-		SDL_Window * window;
-
-		SDL_Event event;
-
-		sandboxed_fs base_sandbox;
-
-		sandboxed_fs user_sandbox;
-
-		oar::archive resources_archive{&base_sandbox, "base.oar"};
-	};
-}

source/coral.cpp

@@ -50,6 +50,8 @@ export namespace coral {
 
 	using i64 = int64_t;
 
+	usize const i64_max = 0xffffffffffffffff;
+
 	using f32 = float;
 
 	using f64 = double;
@@ -317,20 +319,22 @@ export namespace coral {
 	 * lifetime of any functor assigned to it.
 	 */
 	template<typename result, typename... arguments> struct closure<result(arguments...)> {
-		template<typename callable> closure(callable call) requires function_pointer<callable, arguments...> {
+		template<typename callable> closure(callable && call)
+			requires (functor<callable, arguments...> || function_pointer<callable, arguments...>) {
+
+			if constexpr (functor<callable, arguments...>) {
+				this->dispatch = [](void * context, arguments... dispatch_arguments) -> result {
+					return (*reinterpret_cast<callable *>(context))(dispatch_arguments...);
+				};
+
+				this->context = &call;
+			} else if constexpr (function_pointer<callable, arguments...>) {
 				this->dispatch = [](void * context, arguments... dispatch_arguments) -> result {
 					return (reinterpret_cast<callable>(context))(dispatch_arguments...);
 				};
 
 				this->context = reinterpret_cast<void *>(call);
 			}
-
-		template<typename callable> closure(callable && call) requires functor<callable, arguments...> {
-			this->dispatch = [](void * context, arguments... dispatch_arguments) -> result {
-				return (*reinterpret_cast<callable *>(context))(dispatch_arguments...);
-			};
-
-			this->context = &call;
 		}
 
 		template<typename callable> closure(callable & call) requires functor<callable, arguments...> {
@@ -446,20 +450,27 @@ export namespace coral {
 		template<typename value> using rebound = expected<value, errors>;
 
 		/**
-		 * Constructs from `value`, creating an [expected] that contains the expected type.
+		 * Constructs from `value`, creating an [expected] with the expected type.
 		 */
-		expected(expects const & value) {
+		constexpr expected(expects const & value) {
 			(*reinterpret_cast<expects *>(this->buffer)) = value;
 			this->buffer[buffer_size] = 1;
 		}
 
 		/**
-		 * Constructs from `error`, creating an [expected] that does not contain the expected type.
+		 * Constructs from `error`, creating an [expected] with an error.
 		 */
-		expected(errors const & error) {
+		constexpr expected(errors const & error) {
 			(*reinterpret_cast<errors *>(this->buffer)) = error;
 		}
 
+		/**
+		 * Invokes the `apply` procedure if the expected is not ok, otherwise having no side-effects.
+		 */
+		void and_then(closure<void(expects &)> const & apply) {
+			if (this->is_ok()) apply(*this->ok());
+		}
+
 		/**
 		 * Returns the contained error as an [optional].
 		 */

source/coral/files.cpp

@@ -33,7 +33,7 @@ export namespace coral {
 		 * Returns a weak reference to the [path] as a [slice].
 		 */
 		constexpr slice<char const> as_slice() const {
-			return {this->buffer, this->byte_size()};
+			return {this->buffer, this->filled()};
 		}
 
 		/**
@@ -45,13 +45,6 @@ export namespace coral {
 			return reinterpret_cast<char const *>(this->buffer);
 		}
 
-		/**
-		 * Returns the number of bytes composing the path.
-		 */
-		constexpr usize byte_size() const {
-			return max - this->buffer[max];
-		}
-
 		/**
 		 * Compares the path to `that`, returning the difference between the two paths or `0` if they are identical.
 		 */
@@ -63,7 +56,7 @@ export namespace coral {
 		 * Returns the tail pointer of the path name.
 		 */
 		char const * end() const {
-			return this->buffer + this->byte_size();
+			return this->buffer + this->filled();
 		}
 
 		/**
@@ -73,6 +66,13 @@ export namespace coral {
 			return coral::equals(this->as_slice().as_bytes(), that.as_slice().as_bytes());
 		}
 
+		/**
+		 * Returns the number of characters composing the path.
+		 */
+		constexpr usize filled() const {
+			return max - this->buffer[max];
+		}
+
 		/**
 		 * Returns the path hash code.
 		 *
@@ -95,7 +95,7 @@ export namespace coral {
 			path joined_path = *this;
 
 			for (char const c : text) {
-				joined_path.buffer[joined_path.byte_size()] = c;
+				joined_path.buffer[joined_path.filled()] = c;
 				joined_path.buffer[max] -= 1;
 			}
 

source/coral/io.cpp

@@ -17,7 +17,7 @@ export namespace coral {
 		 * [fixed_buffer] is not mutated or out-of-scope.
 		 */
 		slice<u8 const> as_slice() const {
-			return {0, this->filled};
+			return {this->data, this->data_filled};
 		}
 
 		/**
@@ -27,13 +27,6 @@ export namespace coral {
 			return this->data;
 		}
 
-		/**
-		 * Returns the number of bytes in the buffer that have been filled with data.
-		 */
-		usize count() const {
-			return this->filled;
-		}
-
 		/**
 		 * Returns the tail pointer of the buffer data.
 		 */
@@ -41,18 +34,25 @@ export namespace coral {
 			return this->data + this->cursor;
 		}
 
+		/**
+		 * Returns the number of bytes in the buffer that have been filled with data.
+		 */
+		usize filled() const {
+			return this->data_filled;
+		}
+
 		/**
 		 * Returns `true` if the buffer is completely empty of data, otherwise `false`.
 		 */
 		bool is_empty() const {
-			return this->filled == capacity;
+			return this->data_filled == capacity;
 		}
 
 		/**
 		 * Returns `true` if the buffer has been completely filled with data, otherwise `false`.
 		 */
 		bool is_full() const {
-			return this->filled == capacity;
+			return this->data_filled == capacity;
 		}
 
 		/**
@@ -62,7 +62,7 @@ export namespace coral {
 		bool put(u8 data) {
 			if (this->is_full()) return false;
 
-			this->filled += 1;
+			this->data_filled += 1;
 			this->data[this->write_index] = data;
 			this->write_index = (this->write_index + 1) % capacity;
 
@@ -73,9 +73,9 @@ export namespace coral {
 		 * Reads whatever data is in the buffer into `data`, returning the number of bytes read from the buffer.
 		 */
 		expected<usize, io_error> read(slice<u8> const & data) override {
-			slice const readable_data{this->data, min(this->filled, data.length)};
+			slice const readable_data {this->data, min(this->data_filled, data.length)};
 
-			this->filled -= readable_data.length;
+			this->data_filled -= readable_data.length;
 
 			for (usize index = 0; index < readable_data.length; index += 1) {
 				data[index] = this->data[this->read_index];
@@ -85,6 +85,13 @@ export namespace coral {
 			return readable_data.length;
 		}
 
+		/**
+		 * Returns the remaining unfilled buffer space in bytes.
+		 */
+		usize remaining() const {
+			return capacity - this->data_filled;
+		}
+
 		/**
 		 * Attempts to write `data` to the buffer, returning the number of bytes written or [io_error::unavailable] if
 		 * it has been completely filled and no more bytes can be written.
@@ -92,9 +99,9 @@ export namespace coral {
 		expected<usize, io_error> write(slice<u8 const> const & data) override {
 			if (this->is_full()) return io_error::unavailable;
 
-			slice const writable_data{data.sliced(0, min(data.length, this->filled))};
+			slice const writable_data {data.sliced(0, min(data.length, this->remaining()))};
 
-			this->filled += writable_data.length;
+			this->data_filled += writable_data.length;
 
 			for (usize index = 0; index < writable_data.length; index += 1) {
 				this->data[this->write_index] = data[index];
@@ -105,7 +112,7 @@ export namespace coral {
 		}
 
 		private:
-		usize filled {0};
+		usize data_filled {0};
 
 		usize read_index {0};
 

source/coral/stack.cpp

@@ -73,7 +73,7 @@ export namespace coral {
 	 *
 	 * *Note*: the [allocator] referenced in the stack must remain valid for the duration of the stack lifetime.
 	 */
-	template<typename element, usize init_capacity = 1> struct small_stack : public stack<element> {
+	template<typename element, usize init_capacity = 1> struct small_stack final : public stack<element> {
 		small_stack(allocator & dynamic_allocator) : dynamic_allocator{dynamic_allocator} {}
 
 		~small_stack() override {
@@ -255,9 +255,8 @@ export namespace coral {
 	 * Readable type for streaming data from a [stack] containing [u8] values.
 	 */
 	struct stack_reader : public reader {
-		stack_reader(byte_stack const * stack) {
+		stack_reader(byte_stack const & stack) : stack{stack} {
 			this->cursor = 0;
-			this->stack = stack;
 		}
 
 		/**
@@ -266,7 +265,7 @@ export namespace coral {
 		expected<usize, io_error> read(slice<u8> const & buffer) override {
 			usize data_written = 0;
 
-			this->stack->every([&](u8 byte) -> bool {
+			this->stack.every([&](u8 byte) -> bool {
 				buffer[data_written] = byte;
 				data_written += 1;
 
@@ -281,23 +280,21 @@ export namespace coral {
 		private:
 		usize cursor {0};
 
-		byte_stack const * stack {nullptr};
+		byte_stack const & stack;
 	};
 
 	/**
 	 * Writable type for appending data to a [contiguous_range] containing [u8] values.
 	 */
 	struct stack_writer : public writer {
-		stack_writer(byte_stack * stack) {
-			this->stack = stack;
-		}
+		stack_writer(byte_stack & stack) : stack{stack} {}
 
 		/**
 		 * Attempts to write `buffer` to the target stack, returning the number of bytes written or an [io_error] if it
 		 * failed to commit `buffer` to the stack memory.
 		 */
 		expected<usize, io_error> write(slice<u8 const> const & buffer) override {
-			switch (this->stack->push_all(buffer)) {
+			switch (this->stack.push_all(buffer)) {
 				case push_result::ok: return buffer.length;
 				case push_result::out_of_memory: return io_error::unavailable;
 				default: unreachable();
@@ -305,6 +302,6 @@ export namespace coral {
 		}
 
 		private:
-		byte_stack * stack {nullptr};
+		byte_stack & stack;
 	};
 }

source/oar.cpp

@@ -260,8 +260,7 @@ struct walker final : public file_walker {
 
 export namespace oar {
 	struct archive : public fs {
-		archive(fs * backing_fs, path const & archive_path) {
-			this->backing_fs = backing_fs;
+		archive(fs & backing_fs, path const & archive_path) : backing_fs{backing_fs} {
 			this->archive_path = archive_path;
 		}
 
@@ -269,7 +268,7 @@ export namespace oar {
 		 * See [fs::walk_files].
 		 */
 		void walk_files(path const & target_path, closure<void(file_walker &)> const & then) override {
-			this->backing_fs->read_file(this->archive_path, [&](file_reader & archive_reader) {
+			this->backing_fs.read_file(this->archive_path, [&](file_reader & archive_reader) {
 				entry archive_entry{&archive_reader};
 
 				if (archive_entry.find(entry_kind::directory, target_path) != entry::find_result::ok) return;
@@ -284,9 +283,7 @@ export namespace oar {
 		 * See [fs::read_file].
 		 */
 		void read_file(path const & file_path, closure<void(file_reader &)> const & then) override {
-			if ((this->backing_fs == nullptr) || (this->archive_path.byte_size() == 0)) return;
-
-			this->backing_fs->read_file(this->archive_path, [&](file_reader & archive_reader) {
+			this->backing_fs.read_file(this->archive_path, [&](file_reader & archive_reader) {
 				entry archive_entry {&archive_reader};
 
 				if (archive_entry.find(entry_kind::file, file_path) != entry::find_result::ok) return;
@@ -296,7 +293,7 @@ export namespace oar {
 		}
 
 		private:
-		fs * backing_fs;
+		fs & backing_fs;
 
 		path archive_path;
 	};

source/turtle.cpp

@@ -0,0 +1,604 @@
+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::path;
+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};
+	};
+
+	/**
+	 * Provides unmanaged access to a native directory.
+	 */
+	struct native_directory final : public coral::file_walker {
+		native_directory() = default;
+
+		/**
+		 * Attempts to close any currently open directory.
+		 *
+		 * 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() {
+			if (closedir(this->dir) == 0) return close_result::io_unavailable;
+
+			return close_result::ok;
+		}
+
+		/**
+		 * See [coral::file_walker::has_next].
+		 */
+		bool has_next() override {
+			return this->entry != nullptr;
+		}
+
+		/**
+		 * Returns `true` if a directory is currently open, otherwise `false`.
+		 */
+		bool is_open() const {
+			return this->dir != nullptr;
+		}
+
+		/**
+		 * See [coral::file_walker::next].
+		 */
+		coral::expected<path, io_error> next() override {
+			usize name_length {0};
+			constexpr usize name_max {sizeof(dirent::d_name) / sizeof(char)};
+
+			while ((name_length < name_max) && (this->entry->d_name[name_length] != 0)) name_length += 1;
+
+			path current_path {path{}.joined(slice{this->entry->d_name, name_length})};
+
+			errno = 0;
+			this->entry = readdir(this->dir);
+
+			if (this->entry == nullptr) switch (errno) {
+				case EBADF: return io_error::unavailable;
+				default: unreachable();
+			}
+
+			return current_path;
+		}
+
+		/**
+		 * Attempts to open a native directory at `directory_path`.
+		 *
+		 * 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 directory must be closed using [close] once no longer needed or the process will leak
+		 * directory streams.
+		 *
+		 * *Note*: if a directory is currently open under the native directory, it will attempt to close it before
+		 * proceeding with opening the next. This means that [open] is safe to call without first calling [close].
+		 *
+		 * *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 & directory_path) {
+			if (this->is_open()) {
+				open_result const result {close_to_open_result(this->close())};
+
+				if (result != open_result::ok) return result;
+			}
+
+			// No room for zero terminator.
+			errno = 0;
+			this->dir = opendir(directory_path.as_slice().as_chars().pointer);
+
+			if (!this->is_open()) switch (errno) {
+				case EACCES: return open_result::access_denied;
+				case EMFILE: case ENFILE: return open_result::too_many;
+				case ENOENT: case ENOTDIR: return open_result::not_found;
+				case ENOMEM: return open_result::out_of_memory;
+			}
+
+			errno = 0;
+			this->entry = readdir(this->dir);
+
+			if (this->entry == nullptr) switch (errno) {
+				case EBADF: {
+					if (this->is_open()) {
+						open_result const result {close_to_open_result(this->close())};
+
+						if (result != open_result::ok) return result;
+					}
+
+					return open_result::io_unavailable;
+				}
+
+				default: unreachable();
+			}
+
+			return open_result::ok;
+		}
+
+		private:
+		DIR * dir {nullptr};
+
+		dirent * entry {nullptr};
+	};
+
+	/**
+	 * [coral::fs] wrapper around native file system access to provide a managed and system-agnostic environment for
+	 * performing file I/O.
+	 */
+	struct sandboxed_fs : public coral::fs {
+		/**
+		 * 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 `sandbox_path` with `sandbox_permissions` as the permissions given to users
+		 * of it.
+		 */
+		sandboxed_fs(native_path const & sandbox_path, permissions const & access_permissions) {
+			this->path = sandbox_path;
+			this->access_permissions = access_permissions;
+		}
+
+		/**
+		 * Returns a reference to a [sandboxed_fs] 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 sandboxed_fs & base() {
+			static sandboxed_fs base_fs {"./", {
+				.can_read = true,
+				.can_walk = true
+			}};
+
+			return base_fs;
+		}
+
+		/**
+		 * Returns a reference to a [sandboxed_fs] 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 sandboxed_fs & temp() {
+			static sandboxed_fs base_fs {"/tmp", {
+				.can_read = true,
+				.can_write = false,
+			}};
+
+			return base_fs;
+		}
+
+		/**
+		 * See [coral::fs::read_file].
+		 *
+		 * *Note*: this function will only work on sandboxes with the [permissions::can_read] flag enabled.
+		 */
+		void read_file(path const & target_path, closure<void(coral::file_reader &)> const & then) override {
+			if (!this->access_permissions.can_read) return;
+
+			this->path.joined(target_path.as_slice()).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);
+
+				// TODO: Error orphaned file handle!
+				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(path const & target_path, closure<void(coral::file_walker &)> const & then) override {
+			if (!this->access_permissions.can_walk) return;
+
+			this->path.joined(target_path.as_slice()).and_then([&](native_path const & native_directory_path) -> void {
+				native_directory directory;
+
+				if (directory.open(native_directory_path) == open_result::ok) return;
+
+				then(directory);
+
+				// TODO: Error orphaned file handle!
+				if (directory.close() != close_result::ok) return;
+			});
+		}
+
+		/**
+		 * See [coral::fs::write_file].
+		 *
+		 * *Note*: this function will only work on sandboxes with the [permissions::can_write] flag enabled.
+		 */
+		void write_file(path const & target_path, closure<void(coral::file_writer &)> const & then) override {
+			if (!this->access_permissions.can_write) return;
+
+			this->path.joined(target_path.as_slice()).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);
+
+				// TODO: Error orphaned file handle!
+				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 output device used for writing data out from to the wider system.
+	 *
+	 * This [coral::writer] is particularly useful for command-line tools which require communicating with another
+	 * process via pipes or an end-user via the shell.
+	 */
+	coral::writer & output() {
+		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;
+	}
+}

source/turtle/io.cpp

@@ -0,0 +1,54 @@
+export module turtle.io;
+
+import coral;
+import coral.files;
+
+import turtle;
+
+export namespace turtle {
+	enum class log_level {
+		notice,
+		warning,
+		error,
+	};
+
+	struct event_loop {
+		void log(log_level level, coral::slice<char const> const & message) {
+			static_cast<void>(output().write(message.as_chars().as_bytes()));
+			static_cast<void>(output().write(coral::slice{"\n"}.as_bytes()));
+		}
+
+		bool poll() {
+			return false;
+		}
+
+		static int run(coral::path const & title, coral::closure<int(event_loop &)> execute) {
+			event_loop loop{title};
+
+			return execute(loop);
+		}
+
+		private:
+		coral::path title;
+
+		event_loop(coral::path const & title) {
+			this->title = title;
+		}
+	};
+
+	struct system_allocator : public coral::allocator {
+		system_allocator() = default;
+
+		// TODO: implement thread-safety.
+
+		coral::u8 * reallocate(coral::u8 * maybe_allocation, coral::usize requested_size) override {
+			if (maybe_allocation != nullptr) coral::unreachable();
+
+			return nullptr;
+		}
+
+		void deallocate(void * allocation) override {
+			if (allocation != nullptr) coral::unreachable();
+		}
+	};
+};