ona/source/oar/oar.zig

const coral = @import("coral");

const ona = @import("ona");

pub const Archive = struct {
	state_table: [state_max]State = [_]State{.{}} ** state_max,
	file_accessor: ona.files.FileAccessor,
	file_path: []const u8,

	const State = struct {
		readable_file: ?*ona.files.ReadableFile = null,
		data_head: u64 = 0,
		data_size: u64 = 0,
		data_cursor: u64 = 0,

		fn cast(archived_file: *ArchivedFile) *Archive.State {
			return @ptrCast(*State, @alignCast(@alignOf(State), archived_file));
		}
	};

	const state_max = 64;

	pub fn open_archived(self: *Archive, path: Path) OpenError!*ArchivedFile {
		const state_index = find_available_state: {
			var index: usize = 0;

			while (index < self.state_table.len) : (index += 1) {
				if (self.state_table[index].readable_file == null) break :find_available_state index;
			}

			return error.TooManyFiles;
		};

		const archive_file = try self.file_accessor.open_readable(self.file_path);

		errdefer _ = archive_file.close();

		var archive_header = Header.empty;

		if ((try archive_file.read(&archive_header.bytes)) != Header.size) return error.ArchiveInvalid;

		// Read file table.
		var head: u64 = 0;
		var tail: u64 = archive_header.layout.entry_count - 1;
		const path_hash = path.hash();

		while (head <= tail) {
			const midpoint = head + ((tail - head) / 2);
			var archive_block = Block.empty;

			try archive_file.seek(Header.size + archive_header.layout.total_data_size + (Block.size * midpoint));

			if ((try archive_file.read(&archive_block.bytes)) != Block.size) return error.ArchiveInvalid;

			const path_hash_comparison = path_hash - archive_block.layout.path_hash;

			if (path_hash_comparison > 0) {
				head = (midpoint + 1);

				continue;
			}

			if (path_hash_comparison < 0) {
				tail = (midpoint - 1);

				continue;
			}

			const path_comparison = path.compare(archive_block.layout.path);

			if (path_comparison > 0) {
				head = (midpoint + 1);

				continue;
			}

			if (path_comparison < 0) {
				tail = (midpoint - 1);

				continue;
			}

			self.state_table[state_index] = .{
				.readable_file = archive_file,
				.data_head = archive_block.layout.data_head,
				.data_size = archive_block.layout.data_size,
				.data_cursor = 0,
			};

			return @ptrCast(*ArchivedFile, &(self.state_table[state_index]));
		}

		return error.FileNotFound;
	}
};

pub const ArchivedFile = opaque {
	pub fn as_reader(self: *ArchivedFile) coral.io.Reader {
		return coral.io.Reader.bind(self, ArchivedFile);
	}

	pub fn close(self: *ArchivedFile) bool {
		const state = Archive.State.cast(self);

		if (state.readable_file) |readable_file| {
			defer state.readable_file = null;

			return readable_file.close();
		}

		return true;
	}

	pub fn read(self: *ArchivedFile, buffer: []u8) coral.io.ReadError!usize {
		const state = Archive.State.cast(self);

		if (state.readable_file) |readable_file| {
			const actual_cursor = coral.math.min(u64,
				state.data_head + state.data_cursor, state.data_head + state.data_size);

			try readable_file.seek(actual_cursor);

			const buffer_read = coral.math.min(usize, buffer.len, state.data_size - actual_cursor);

			defer state.data_cursor += buffer_read;

			return readable_file.read(buffer[0..buffer_read]);
		}

		return error.IoUnavailable;
	}

	pub fn size(self: *ArchivedFile) u64 {
		return Archive.State.cast(self).data_size;
	}
};

const Block = extern union {
	layout: extern struct {
		path: Path,
		path_hash: u64,
		data_head: u64,
		data_size: u64,
	},

	bytes: [size]u8,

	const empty = Block{ .bytes = [_]u8{0} ** size };

	const size = 512;
};

const Header = extern union {
	layout: extern struct {
		signature: [signature_magic.len]u8,
		entry_count: u32,
		total_data_size: u64,
	},

	bytes: [size]u8,

	const empty = Header{ .bytes = [_]u8{0} ** size };

	const signature_magic = [_]u8{ 'o', 'a', 'r', 1 };

	const size = 16;
};

pub const OpenError = ona.files.OpenError || coral.io.ReadError || error{
	ArchiveInvalid,
};

pub const Path = extern struct {
	buffer: [maximum + 1]u8,

	pub const DataError = error{
		PathCorrupt,
	};

	pub const ParseError = error{
		TooLong,
	};

	pub fn compare(self: Path, other: Path) isize {
		return coral.io.compare(&self.buffer, &other.buffer);
	}

	pub fn data(self: Path) DataError![:0]const u8 {
		// Verify presence of zero terminator.
		if (self.buffer[self.filled()] != 0) return error.PathCorrupt;

		return @ptrCast([:0]const u8, self.buffer[0..self.filled()]);
	}

	pub fn filled(self: Path) usize {
		return maximum - self.remaining();
	}

	pub fn hash(self: Path) u64 {
		// Fowler–Noll–Vo hash function is used here as it has a lower collision rate for smaller inputs.
		const fnv_prime = 0x100000001b3;
		var hash_code = @as(u64, 0xcbf29ce484222325);

		for (self.buffer[0..self.filled()]) |byte| {
			hash_code = hash_code ^ byte;
			hash_code = hash_code *% fnv_prime;
		}

		return hash_code;
	}

	pub const maximum = 255;

	pub fn parse(bytes: []const u8) ParseError!Path {
		if (bytes.len > maximum) return error.TooLong;

		// Int cast is safe as bytes length is confirmed to be smaller than or equal to u8 maximum.
		var parsed_path = Path{ .buffer = ([_]u8{0} ** maximum) ++ [_]u8{maximum - @intCast(u8, bytes.len)} };

		coral.io.copy(&parsed_path.buffer, bytes);

		return parsed_path;
	}

	pub fn remaining(self: Path) usize {
		return self.buffer[maximum];
	}

	pub const seperator = '/';
};