/// The OS-specific file descriptor or file handle.
handle: Handle,

pub const Handle = posix.fd_t;
pub const Mode = posix.mode_t;
pub const INode = posix.ino_t;
pub const Uid = posix.uid_t;
pub const Gid = posix.gid_t;

pub const Kind = enum {
    block_device,
    character_device,
    directory,
    named_pipe,
    sym_link,
    file,
    unix_domain_socket,
    whiteout,
    door,
    event_port,
    unknown,
};

/// This is the default mode given to POSIX operating systems for creating
/// files. `0o666` is "-rw-rw-rw-" which is counter-intuitive at first,
/// since most people would expect "-rw-r--r--", for example, when using
/// the `touch` command, which would correspond to `0o644`. However, POSIX
/// libc implementations use `0o666` inside `fopen` and then rely on the
/// process-scoped "umask" setting to adjust this number for file creation.
pub const default_mode = switch (builtin.os.tag) {
    .windows => 0,
    .wasi => 0,
    else => 0o666,
};

pub const OpenError = error{
    SharingViolation,
    PathAlreadyExists,
    FileNotFound,
    AccessDenied,
    PipeBusy,
    NameTooLong,
    /// WASI-only; file paths must be valid UTF-8.
    InvalidUtf8,
    /// Windows-only; file paths provided by the user must be valid WTF-8.
    /// https://simonsapin.github.io/wtf-8/
    InvalidWtf8,
    /// On Windows, file paths cannot contain these characters:
    /// '/', '*', '?', '"', '<', '>', '|'
    BadPathName,
    Unexpected,
    /// On Windows, `\\server` or `\\server\share` was not found.
    NetworkNotFound,
    /// On Windows, antivirus software is enabled by default. It can be
    /// disabled, but Windows Update sometimes ignores the user's preference
    /// and re-enables it. When enabled, antivirus software on Windows
    /// intercepts file system operations and makes them significantly slower
    /// in addition to possibly failing with this error code.
    AntivirusInterference,
} || posix.OpenError || posix.FlockError;

pub const OpenMode = enum {
    read_only,
    write_only,
    read_write,
};

pub const Lock = enum {
    none,
    shared,
    exclusive,
};

pub const OpenFlags = struct {
    mode: OpenMode = .read_only,

    /// Open the file with an advisory lock to coordinate with other processes
    /// accessing it at the same time. An exclusive lock will prevent other
    /// processes from acquiring a lock. A shared lock will prevent other
    /// processes from acquiring a exclusive lock, but does not prevent
    /// other process from getting their own shared locks.
    ///
    /// The lock is advisory, except on Linux in very specific circumstances[1].
    /// This means that a process that does not respect the locking API can still get access
    /// to the file, despite the lock.
    ///
    /// On these operating systems, the lock is acquired atomically with
    /// opening the file:
    /// * Darwin
    /// * DragonFlyBSD
    /// * FreeBSD
    /// * Haiku
    /// * NetBSD
    /// * OpenBSD
    /// On these operating systems, the lock is acquired via a separate syscall
    /// after opening the file:
    /// * Linux
    /// * Windows
    ///
    /// [1]: https://www.kernel.org/doc/Documentation/filesystems/mandatory-locking.txt
    lock: Lock = .none,

    /// Sets whether or not to wait until the file is locked to return. If set to true,
    /// `error.WouldBlock` will be returned. Otherwise, the file will wait until the file
    /// is available to proceed.
    lock_nonblocking: bool = false,

    /// Set this to allow the opened file to automatically become the
    /// controlling TTY for the current process.
    allow_ctty: bool = false,

    pub fn isRead(self: OpenFlags) bool {
        return self.mode != .write_only;
    }

    pub fn isWrite(self: OpenFlags) bool {
        return self.mode != .read_only;
    }
};

pub const CreateFlags = struct {
    /// Whether the file will be created with read access.
    read: bool = false,

    /// If the file already exists, and is a regular file, and the access
    /// mode allows writing, it will be truncated to length 0.
    truncate: bool = true,

    /// Ensures that this open call creates the file, otherwise causes
    /// `error.PathAlreadyExists` to be returned.
    exclusive: bool = false,

    /// Open the file with an advisory lock to coordinate with other processes
    /// accessing it at the same time. An exclusive lock will prevent other
    /// processes from acquiring a lock. A shared lock will prevent other
    /// processes from acquiring a exclusive lock, but does not prevent
    /// other process from getting their own shared locks.
    ///
    /// The lock is advisory, except on Linux in very specific circumstances[1].
    /// This means that a process that does not respect the locking API can still get access
    /// to the file, despite the lock.
    ///
    /// On these operating systems, the lock is acquired atomically with
    /// opening the file:
    /// * Darwin
    /// * DragonFlyBSD
    /// * FreeBSD
    /// * Haiku
    /// * NetBSD
    /// * OpenBSD
    /// On these operating systems, the lock is acquired via a separate syscall
    /// after opening the file:
    /// * Linux
    /// * Windows
    ///
    /// [1]: https://www.kernel.org/doc/Documentation/filesystems/mandatory-locking.txt
    lock: Lock = .none,

    /// Sets whether or not to wait until the file is locked to return. If set to true,
    /// `error.WouldBlock` will be returned. Otherwise, the file will wait until the file
    /// is available to proceed.
    lock_nonblocking: bool = false,

    /// For POSIX systems this is the file system mode the file will
    /// be created with. On other systems this is always 0.
    mode: Mode = default_mode,
};

/// Upon success, the stream is in an uninitialized state. To continue using it,
/// you must use the open() function.
pub fn close(self: File) void {
    if (is_windows) {
        windows.CloseHandle(self.handle);
    } else {
        posix.close(self.handle);
    }
}

pub const SyncError = posix.SyncError;

/// Blocks until all pending file contents and metadata modifications
/// for the file have been synchronized with the underlying filesystem.
///
/// Note that this does not ensure that metadata for the
/// directory containing the file has also reached disk.
pub fn sync(self: File) SyncError!void {
    return posix.fsync(self.handle);
}

/// Test whether the file refers to a terminal.
/// See also `supportsAnsiEscapeCodes`.
pub fn isTty(self: File) bool {
    return posix.isatty(self.handle);
}

pub fn isCygwinPty(file: File) bool {
    if (builtin.os.tag != .windows) return false;

    const handle = file.handle;

    // If this is a MSYS2/cygwin pty, then it will be a named pipe with a name in one of these formats:
    //   msys-[...]-ptyN-[...]
    //   cygwin-[...]-ptyN-[...]
    //
    // Example: msys-1888ae32e00d56aa-pty0-to-master

    // First, just check that the handle is a named pipe.
    // This allows us to avoid the more costly NtQueryInformationFile call
    // for handles that aren't named pipes.
    {
        var io_status: windows.IO_STATUS_BLOCK = undefined;
        var device_info: windows.FILE_FS_DEVICE_INFORMATION = undefined;
        const rc = windows.ntdll.NtQueryVolumeInformationFile(handle, &io_status, &device_info, @sizeOf(windows.FILE_FS_DEVICE_INFORMATION), .FileFsDeviceInformation);
        switch (rc) {
            .SUCCESS => {},
            else => return false,
        }
        if (device_info.DeviceType != windows.FILE_DEVICE_NAMED_PIPE) return false;
    }

    const name_bytes_offset = @offsetOf(windows.FILE_NAME_INFO, "FileName");
    // `NAME_MAX` UTF-16 code units (2 bytes each)
    // This buffer may not be long enough to handle *all* possible paths
    // (PATH_MAX_WIDE would be necessary for that), but because we only care
    // about certain paths and we know they must be within a reasonable length,
    // we can use this smaller buffer and just return false on any error from
    // NtQueryInformationFile.
    const num_name_bytes = windows.MAX_PATH * 2;
    var name_info_bytes align(@alignOf(windows.FILE_NAME_INFO)) = [_]u8{0} ** (name_bytes_offset + num_name_bytes);

    var io_status_block: windows.IO_STATUS_BLOCK = undefined;
    const rc = windows.ntdll.NtQueryInformationFile(handle, &io_status_block, &name_info_bytes, @intCast(name_info_bytes.len), .FileNameInformation);
    switch (rc) {
        .SUCCESS => {},
        .INVALID_PARAMETER => unreachable,
        else => return false,
    }

    const name_info: *const windows.FILE_NAME_INFO = @ptrCast(&name_info_bytes);
    const name_bytes = name_info_bytes[name_bytes_offset .. name_bytes_offset + name_info.FileNameLength];
    const name_wide = std.mem.bytesAsSlice(u16, name_bytes);
    // The name we get from NtQueryInformationFile will be prefixed with a '\', e.g. \msys-1888ae32e00d56aa-pty0-to-master
    return (std.mem.startsWith(u16, name_wide, &[_]u16{ '\\', 'm', 's', 'y', 's', '-' }) or
        std.mem.startsWith(u16, name_wide, &[_]u16{ '\\', 'c', 'y', 'g', 'w', 'i', 'n', '-' })) and
        std.mem.indexOf(u16, name_wide, &[_]u16{ '-', 'p', 't', 'y' }) != null;
}

/// Test whether ANSI escape codes will be treated as such.
pub fn supportsAnsiEscapeCodes(self: File) bool {
    if (builtin.os.tag == .windows) {
        var console_mode: windows.DWORD = 0;
        if (windows.kernel32.GetConsoleMode(self.handle, &console_mode) != 0) {
            if (console_mode & windows.ENABLE_VIRTUAL_TERMINAL_PROCESSING != 0) return true;
        }

        return self.isCygwinPty();
    }
    if (builtin.os.tag == .wasi) {
        // WASI sanitizes stdout when fd is a tty so ANSI escape codes
        // will not be interpreted as actual cursor commands, and
        // stderr is always sanitized.
        return false;
    }
    if (self.isTty()) {
        if (self.handle == posix.STDOUT_FILENO or self.handle == posix.STDERR_FILENO) {
            if (posix.getenvZ("TERM")) |term| {
                if (std.mem.eql(u8, term, "dumb"))
                    return false;
            }
        }
        return true;
    }
    return false;
}

pub const SetEndPosError = posix.TruncateError;

/// Shrinks or expands the file.
/// The file offset after this call is left unchanged.
pub fn setEndPos(self: File, length: u64) SetEndPosError!void {
    try posix.ftruncate(self.handle, length);
}

pub const SeekError = posix.SeekError;

/// Repositions read/write file offset relative to the current offset.
/// TODO: integrate with async I/O
pub fn seekBy(self: File, offset: i64) SeekError!void {
    return posix.lseek_CUR(self.handle, offset);
}

/// Repositions read/write file offset relative to the end.
/// TODO: integrate with async I/O
pub fn seekFromEnd(self: File, offset: i64) SeekError!void {
    return posix.lseek_END(self.handle, offset);
}

/// Repositions read/write file offset relative to the beginning.
/// TODO: integrate with async I/O
pub fn seekTo(self: File, offset: u64) SeekError!void {
    return posix.lseek_SET(self.handle, offset);
}

pub const GetSeekPosError = posix.SeekError || posix.FStatError;

/// TODO: integrate with async I/O
pub fn getPos(self: File) GetSeekPosError!u64 {
    return posix.lseek_CUR_get(self.handle);
}

/// TODO: integrate with async I/O
pub fn getEndPos(self: File) GetSeekPosError!u64 {
    if (builtin.os.tag == .windows) {
        return windows.GetFileSizeEx(self.handle);
    }
    return (try self.stat()).size;
}

pub const ModeError = posix.FStatError;

/// TODO: integrate with async I/O
pub fn mode(self: File) ModeError!Mode {
    if (builtin.os.tag == .windows) {
        return 0;
    }
    return (try self.stat()).mode;
}

pub const Stat = struct {
    /// A number that the system uses to point to the file metadata. This
    /// number is not guaranteed to be unique across time, as some file
    /// systems may reuse an inode after its file has been deleted. Some
    /// systems may change the inode of a file over time.
    ///
    /// On Linux, the inode is a structure that stores the metadata, and
    /// the inode _number_ is what you see here: the index number of the
    /// inode.
    ///
    /// The FileIndex on Windows is similar. It is a number for a file that
    /// is unique to each filesystem.
    inode: INode,
    size: u64,
    /// This is available on POSIX systems and is always 0 otherwise.
    mode: Mode,
    kind: Kind,

    /// Last access time in nanoseconds, relative to UTC 1970-01-01.
    atime: i128,
    /// Last modification time in nanoseconds, relative to UTC 1970-01-01.
    mtime: i128,
    /// Last status/metadata change time in nanoseconds, relative to UTC 1970-01-01.
    ctime: i128,

    pub fn fromSystem(st: posix.Stat) Stat {
        const atime = st.atime();
        const mtime = st.mtime();
        const ctime = st.ctime();
        return .{
            .inode = st.ino,
            .size = @bitCast(st.size),
            .mode = st.mode,
            .kind = k: {
                const m = st.mode & posix.S.IFMT;
                switch (m) {
                    posix.S.IFBLK => break :k .block_device,
                    posix.S.IFCHR => break :k .character_device,
                    posix.S.IFDIR => break :k .directory,
                    posix.S.IFIFO => break :k .named_pipe,
                    posix.S.IFLNK => break :k .sym_link,
                    posix.S.IFREG => break :k .file,
                    posix.S.IFSOCK => break :k .unix_domain_socket,
                    else => {},
                }
                if (builtin.os.tag.isSolarish()) switch (m) {
                    posix.S.IFDOOR => break :k .door,
                    posix.S.IFPORT => break :k .event_port,
                    else => {},
                };

                break :k .unknown;
            },
            .atime = @as(i128, atime.tv_sec) * std.time.ns_per_s + atime.tv_nsec,
            .mtime = @as(i128, mtime.tv_sec) * std.time.ns_per_s + mtime.tv_nsec,
            .ctime = @as(i128, ctime.tv_sec) * std.time.ns_per_s + ctime.tv_nsec,
        };
    }

    pub fn fromWasi(st: std.os.wasi.filestat_t) Stat {
        return .{
            .inode = st.ino,
            .size = @bitCast(st.size),
            .mode = 0,
            .kind = switch (st.filetype) {
                .BLOCK_DEVICE => .block_device,
                .CHARACTER_DEVICE => .character_device,
                .DIRECTORY => .directory,
                .SYMBOLIC_LINK => .sym_link,
                .REGULAR_FILE => .file,
                .SOCKET_STREAM, .SOCKET_DGRAM => .unix_domain_socket,
                else => .unknown,
            },
            .atime = st.atim,
            .mtime = st.mtim,
            .ctime = st.ctim,
        };
    }
};

pub const StatError = posix.FStatError;

/// Returns `Stat` containing basic information about the `File`.
/// Use `metadata` to retrieve more detailed information (e.g. creation time, permissions).
/// TODO: integrate with async I/O
pub fn stat(self: File) StatError!Stat {
    if (builtin.os.tag == .windows) {
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        var info: windows.FILE_ALL_INFORMATION = undefined;
        const rc = windows.ntdll.NtQueryInformationFile(self.handle, &io_status_block, &info, @sizeOf(windows.FILE_ALL_INFORMATION), .FileAllInformation);
        switch (rc) {
            .SUCCESS => {},
            // Buffer overflow here indicates that there is more information available than was able to be stored in the buffer
            // size provided. This is treated as success because the type of variable-length information that this would be relevant for
            // (name, volume name, etc) we don't care about.
            .BUFFER_OVERFLOW => {},
            .INVALID_PARAMETER => unreachable,
            .ACCESS_DENIED => return error.AccessDenied,
            else => return windows.unexpectedStatus(rc),
        }
        return .{
            .inode = info.InternalInformation.IndexNumber,
            .size = @as(u64, @bitCast(info.StandardInformation.EndOfFile)),
            .mode = 0,
            .kind = if (info.BasicInformation.FileAttributes & windows.FILE_ATTRIBUTE_REPARSE_POINT != 0) reparse_point: {
                var tag_info: windows.FILE_ATTRIBUTE_TAG_INFO = undefined;
                const tag_rc = windows.ntdll.NtQueryInformationFile(self.handle, &io_status_block, &tag_info, @sizeOf(windows.FILE_ATTRIBUTE_TAG_INFO), .FileAttributeTagInformation);
                switch (tag_rc) {
                    .SUCCESS => {},
                    // INFO_LENGTH_MISMATCH and ACCESS_DENIED are the only documented possible errors
                    // https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-fscc/d295752f-ce89-4b98-8553-266d37c84f0e
                    .INFO_LENGTH_MISMATCH => unreachable,
                    .ACCESS_DENIED => return error.AccessDenied,
                    else => return windows.unexpectedStatus(rc),
                }
                if (tag_info.ReparseTag & windows.reparse_tag_name_surrogate_bit != 0) {
                    break :reparse_point .sym_link;
                }
                // Unknown reparse point
                break :reparse_point .unknown;
            } else if (info.BasicInformation.FileAttributes & windows.FILE_ATTRIBUTE_DIRECTORY != 0)
                .directory
            else
                .file,
            .atime = windows.fromSysTime(info.BasicInformation.LastAccessTime),
            .mtime = windows.fromSysTime(info.BasicInformation.LastWriteTime),
            .ctime = windows.fromSysTime(info.BasicInformation.ChangeTime),
        };
    }

    if (builtin.os.tag == .wasi and !builtin.link_libc) {
        const st = try std.os.fstat_wasi(self.handle);
        return Stat.fromWasi(st);
    }

    const st = try posix.fstat(self.handle);
    return Stat.fromSystem(st);
}

pub const ChmodError = posix.FChmodError;

/// Changes the mode of the file.
/// The process must have the correct privileges in order to do this
/// successfully, or must have the effective user ID matching the owner
/// of the file.
pub fn chmod(self: File, new_mode: Mode) ChmodError!void {
    try posix.fchmod(self.handle, new_mode);
}

pub const ChownError = posix.FChownError;

/// Changes the owner and group of the file.
/// The process must have the correct privileges in order to do this
/// successfully. The group may be changed by the owner of the file to
/// any group of which the owner is a member. If the owner or group is
/// specified as `null`, the ID is not changed.
pub fn chown(self: File, owner: ?Uid, group: ?Gid) ChownError!void {
    try posix.fchown(self.handle, owner, group);
}

/// Cross-platform representation of permissions on a file.
/// The `readonly` and `setReadonly` are the only methods available across all platforms.
/// Platform-specific functionality is available through the `inner` field.
pub const Permissions = struct {
    /// You may use the `inner` field to use platform-specific functionality
    inner: switch (builtin.os.tag) {
        .windows => PermissionsWindows,
        else => PermissionsUnix,
    },

    const Self = @This();

    /// Returns `true` if permissions represent an unwritable file.
    /// On Unix, `true` is returned only if no class has write permissions.
    pub fn readOnly(self: Self) bool {
        return self.inner.readOnly();
    }

    /// Sets whether write permissions are provided.
    /// On Unix, this affects *all* classes. If this is undesired, use `unixSet`.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn setReadOnly(self: *Self, read_only: bool) void {
        self.inner.setReadOnly(read_only);
    }
};

pub const PermissionsWindows = struct {
    attributes: windows.DWORD,

    const Self = @This();

    /// Returns `true` if permissions represent an unwritable file.
    pub fn readOnly(self: Self) bool {
        return self.attributes & windows.FILE_ATTRIBUTE_READONLY != 0;
    }

    /// Sets whether write permissions are provided.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn setReadOnly(self: *Self, read_only: bool) void {
        if (read_only) {
            self.attributes |= windows.FILE_ATTRIBUTE_READONLY;
        } else {
            self.attributes &= ~@as(windows.DWORD, windows.FILE_ATTRIBUTE_READONLY);
        }
    }
};

pub const PermissionsUnix = struct {
    mode: Mode,

    const Self = @This();

    /// Returns `true` if permissions represent an unwritable file.
    /// `true` is returned only if no class has write permissions.
    pub fn readOnly(self: Self) bool {
        return self.mode & 0o222 == 0;
    }

    /// Sets whether write permissions are provided.
    /// This affects *all* classes. If this is undesired, use `unixSet`.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn setReadOnly(self: *Self, read_only: bool) void {
        if (read_only) {
            self.mode &= ~@as(Mode, 0o222);
        } else {
            self.mode |= @as(Mode, 0o222);
        }
    }

    pub const Class = enum(u2) {
        user = 2,
        group = 1,
        other = 0,
    };

    pub const Permission = enum(u3) {
        read = 0o4,
        write = 0o2,
        execute = 0o1,
    };

    /// Returns `true` if the chosen class has the selected permission.
    /// This method is only available on Unix platforms.
    pub fn unixHas(self: Self, class: Class, permission: Permission) bool {
        const mask = @as(Mode, @intFromEnum(permission)) << @as(u3, @intFromEnum(class)) * 3;
        return self.mode & mask != 0;
    }

    /// Sets the permissions for the chosen class. Any permissions set to `null` are left unchanged.
    /// This method *DOES NOT* set permissions on the filesystem: use `File.setPermissions(permissions)`
    pub fn unixSet(self: *Self, class: Class, permissions: struct {
        read: ?bool = null,
        write: ?bool = null,
        execute: ?bool = null,
    }) void {
        const shift = @as(u3, @intFromEnum(class)) * 3;
        if (permissions.read) |r| {
            if (r) {
                self.mode |= @as(Mode, 0o4) << shift;
            } else {
                self.mode &= ~(@as(Mode, 0o4) << shift);
            }
        }
        if (permissions.write) |w| {
            if (w) {
                self.mode |= @as(Mode, 0o2) << shift;
            } else {
                self.mode &= ~(@as(Mode, 0o2) << shift);
            }
        }
        if (permissions.execute) |x| {
            if (x) {
                self.mode |= @as(Mode, 0o1) << shift;
            } else {
                self.mode &= ~(@as(Mode, 0o1) << shift);
            }
        }
    }

    /// Returns a `Permissions` struct representing the permissions from the passed mode.
    pub fn unixNew(new_mode: Mode) Self {
        return Self{
            .mode = new_mode,
        };
    }
};

pub const SetPermissionsError = ChmodError;

/// Sets permissions according to the provided `Permissions` struct.
/// This method is *NOT* available on WASI
pub fn setPermissions(self: File, permissions: Permissions) SetPermissionsError!void {
    switch (builtin.os.tag) {
        .windows => {
            var io_status_block: windows.IO_STATUS_BLOCK = undefined;
            var info = windows.FILE_BASIC_INFORMATION{
                .CreationTime = 0,
                .LastAccessTime = 0,
                .LastWriteTime = 0,
                .ChangeTime = 0,
                .FileAttributes = permissions.inner.attributes,
            };
            const rc = windows.ntdll.NtSetInformationFile(
                self.handle,
                &io_status_block,
                &info,
                @sizeOf(windows.FILE_BASIC_INFORMATION),
                .FileBasicInformation,
            );
            switch (rc) {
                .SUCCESS => return,
                .INVALID_HANDLE => unreachable,
                .ACCESS_DENIED => return error.AccessDenied,
                else => return windows.unexpectedStatus(rc),
            }
        },
        .wasi => @compileError("Unsupported OS"), // Wasi filesystem does not *yet* support chmod
        else => {
            try self.chmod(permissions.inner.mode);
        },
    }
}

/// Cross-platform representation of file metadata.
/// Platform-specific functionality is available through the `inner` field.
pub const Metadata = struct {
    /// Exposes platform-specific functionality.
    inner: switch (builtin.os.tag) {
        .windows => MetadataWindows,
        .linux => MetadataLinux,
        .wasi => MetadataWasi,
        else => MetadataUnix,
    },

    const Self = @This();

    /// Returns the size of the file
    pub fn size(self: Self) u64 {
        return self.inner.size();
    }

    /// Returns a `Permissions` struct, representing the permissions on the file
    pub fn permissions(self: Self) Permissions {
        return self.inner.permissions();
    }

    /// Returns the `Kind` of file.
    /// On Windows, can only return: `.file`, `.directory`, `.sym_link` or `.unknown`
    pub fn kind(self: Self) Kind {
        return self.inner.kind();
    }

    /// Returns the last time the file was accessed in nanoseconds since UTC 1970-01-01
    pub fn accessed(self: Self) i128 {
        return self.inner.accessed();
    }

    /// Returns the time the file was modified in nanoseconds since UTC 1970-01-01
    pub fn modified(self: Self) i128 {
        return self.inner.modified();
    }

    /// Returns the time the file was created in nanoseconds since UTC 1970-01-01
    /// On Windows, this cannot return null
    /// On Linux, this returns null if the filesystem does not support creation times, or if the kernel is older than 4.11
    /// On Unices, this returns null if the filesystem or OS does not support creation times
    /// On MacOS, this returns the ctime if the filesystem does not support creation times; this is insanity, and yet another reason to hate on Apple
    pub fn created(self: Self) ?i128 {
        return self.inner.created();
    }
};

pub const MetadataUnix = struct {
    stat: posix.Stat,

    const Self = @This();

    /// Returns the size of the file
    pub fn size(self: Self) u64 {
        return @intCast(self.stat.size);
    }

    /// Returns a `Permissions` struct, representing the permissions on the file
    pub fn permissions(self: Self) Permissions {
        return .{ .inner = .{ .mode = self.stat.mode } };
    }

    /// Returns the `Kind` of the file
    pub fn kind(self: Self) Kind {
        if (builtin.os.tag == .wasi and !builtin.link_libc) return switch (self.stat.filetype) {
            .BLOCK_DEVICE => .block_device,
            .CHARACTER_DEVICE => .character_device,
            .DIRECTORY => .directory,
            .SYMBOLIC_LINK => .sym_link,
            .REGULAR_FILE => .file,
            .SOCKET_STREAM, .SOCKET_DGRAM => .unix_domain_socket,
            else => .unknown,
        };

        const m = self.stat.mode & posix.S.IFMT;

        switch (m) {
            posix.S.IFBLK => return .block_device,
            posix.S.IFCHR => return .character_device,
            posix.S.IFDIR => return .directory,
            posix.S.IFIFO => return .named_pipe,
            posix.S.IFLNK => return .sym_link,
            posix.S.IFREG => return .file,
            posix.S.IFSOCK => return .unix_domain_socket,
            else => {},
        }

        if (builtin.os.tag.isSolarish()) switch (m) {
            posix.S.IFDOOR => return .door,
            posix.S.IFPORT => return .event_port,
            else => {},
        };

        return .unknown;
    }

    /// Returns the last time the file was accessed in nanoseconds since UTC 1970-01-01
    pub fn accessed(self: Self) i128 {
        const atime = self.stat.atime();
        return @as(i128, atime.tv_sec) * std.time.ns_per_s + atime.tv_nsec;
    }

    /// Returns the last time the file was modified in nanoseconds since UTC 1970-01-01
    pub fn modified(self: Self) i128 {
        const mtime = self.stat.mtime();
        return @as(i128, mtime.tv_sec) * std.time.ns_per_s + mtime.tv_nsec;
    }

    /// Returns the time the file was created in nanoseconds since UTC 1970-01-01.
    /// Returns null if this is not supported by the OS or filesystem
    pub fn created(self: Self) ?i128 {
        if (!@hasDecl(@TypeOf(self.stat), "birthtime")) return null;
        const birthtime = self.stat.birthtime();

        // If the filesystem doesn't support this the value *should* be:
        // On FreeBSD: tv_nsec = 0, tv_sec = -1
        // On NetBSD and OpenBSD: tv_nsec = 0, tv_sec = 0
        // On MacOS, it is set to ctime -- we cannot detect this!!
        switch (builtin.os.tag) {
            .freebsd => if (birthtime.tv_sec == -1 and birthtime.tv_nsec == 0) return null,
            .netbsd, .openbsd => if (birthtime.tv_sec == 0 and birthtime.tv_nsec == 0) return null,
            .macos => {},
            else => @compileError("Creation time detection not implemented for OS"),
        }

        return @as(i128, birthtime.tv_sec) * std.time.ns_per_s + birthtime.tv_nsec;
    }
};

/// `MetadataUnix`, but using Linux's `statx` syscall.
/// On Linux versions below 4.11, `statx` will be filled with data from stat.
pub const MetadataLinux = struct {
    statx: std.os.linux.Statx,

    const Self = @This();

    /// Returns the size of the file
    pub fn size(self: Self) u64 {
        return self.statx.size;
    }

    /// Returns a `Permissions` struct, representing the permissions on the file
    pub fn permissions(self: Self) Permissions {
        return Permissions{ .inner = PermissionsUnix{ .mode = self.statx.mode } };
    }

    /// Returns the `Kind` of the file
    pub fn kind(self: Self) Kind {
        const m = self.statx.mode & posix.S.IFMT;

        switch (m) {
            posix.S.IFBLK => return .block_device,
            posix.S.IFCHR => return .character_device,
            posix.S.IFDIR => return .directory,
            posix.S.IFIFO => return .named_pipe,
            posix.S.IFLNK => return .sym_link,
            posix.S.IFREG => return .file,
            posix.S.IFSOCK => return .unix_domain_socket,
            else => {},
        }

        return .unknown;
    }

    /// Returns the last time the file was accessed in nanoseconds since UTC 1970-01-01
    pub fn accessed(self: Self) i128 {
        return @as(i128, self.statx.atime.tv_sec) * std.time.ns_per_s + self.statx.atime.tv_nsec;
    }

    /// Returns the last time the file was modified in nanoseconds since UTC 1970-01-01
    pub fn modified(self: Self) i128 {
        return @as(i128, self.statx.mtime.tv_sec) * std.time.ns_per_s + self.statx.mtime.tv_nsec;
    }

    /// Returns the time the file was created in nanoseconds since UTC 1970-01-01.
    /// Returns null if this is not supported by the filesystem, or on kernels before than version 4.11
    pub fn created(self: Self) ?i128 {
        if (self.statx.mask & std.os.linux.STATX_BTIME == 0) return null;
        return @as(i128, self.statx.btime.tv_sec) * std.time.ns_per_s + self.statx.btime.tv_nsec;
    }
};

pub const MetadataWasi = struct {
    stat: std.os.wasi.filestat_t,

    pub fn size(self: @This()) u64 {
        return self.stat.size;
    }

    pub fn permissions(self: @This()) Permissions {
        return .{ .inner = .{ .mode = self.stat.mode } };
    }

    pub fn kind(self: @This()) Kind {
        return switch (self.stat.filetype) {
            .BLOCK_DEVICE => .block_device,
            .CHARACTER_DEVICE => .character_device,
            .DIRECTORY => .directory,
            .SYMBOLIC_LINK => .sym_link,
            .REGULAR_FILE => .file,
            .SOCKET_STREAM, .SOCKET_DGRAM => .unix_domain_socket,
            else => .unknown,
        };
    }

    pub fn accessed(self: @This()) i128 {
        return self.stat.atim;
    }

    pub fn modified(self: @This()) i128 {
        return self.stat.mtim;
    }

    pub fn created(self: @This()) ?i128 {
        return self.stat.ctim;
    }
};

pub const MetadataWindows = struct {
    attributes: windows.DWORD,
    reparse_tag: windows.DWORD,
    _size: u64,
    access_time: i128,
    modified_time: i128,
    creation_time: i128,

    const Self = @This();

    /// Returns the size of the file
    pub fn size(self: Self) u64 {
        return self._size;
    }

    /// Returns a `Permissions` struct, representing the permissions on the file
    pub fn permissions(self: Self) Permissions {
        return .{ .inner = .{ .attributes = self.attributes } };
    }

    /// Returns the `Kind` of the file.
    /// Can only return: `.file`, `.directory`, `.sym_link` or `.unknown`
    pub fn kind(self: Self) Kind {
        if (self.attributes & windows.FILE_ATTRIBUTE_REPARSE_POINT != 0) {
            if (self.reparse_tag & windows.reparse_tag_name_surrogate_bit != 0) {
                return .sym_link;
            }
        } else if (self.attributes & windows.FILE_ATTRIBUTE_DIRECTORY != 0) {
            return .directory;
        } else {
            return .file;
        }
        return .unknown;
    }

    /// Returns the last time the file was accessed in nanoseconds since UTC 1970-01-01
    pub fn accessed(self: Self) i128 {
        return self.access_time;
    }

    /// Returns the time the file was modified in nanoseconds since UTC 1970-01-01
    pub fn modified(self: Self) i128 {
        return self.modified_time;
    }

    /// Returns the time the file was created in nanoseconds since UTC 1970-01-01.
    /// This never returns null, only returning an optional for compatibility with other OSes
    pub fn created(self: Self) ?i128 {
        return self.creation_time;
    }
};

pub const MetadataError = posix.FStatError;

pub fn metadata(self: File) MetadataError!Metadata {
    return .{
        .inner = switch (builtin.os.tag) {
            .windows => blk: {
                var io_status_block: windows.IO_STATUS_BLOCK = undefined;
                var info: windows.FILE_ALL_INFORMATION = undefined;

                const rc = windows.ntdll.NtQueryInformationFile(self.handle, &io_status_block, &info, @sizeOf(windows.FILE_ALL_INFORMATION), .FileAllInformation);
                switch (rc) {
                    .SUCCESS => {},
                    // Buffer overflow here indicates that there is more information available than was able to be stored in the buffer
                    // size provided. This is treated as success because the type of variable-length information that this would be relevant for
                    // (name, volume name, etc) we don't care about.
                    .BUFFER_OVERFLOW => {},
                    .INVALID_PARAMETER => unreachable,
                    .ACCESS_DENIED => return error.AccessDenied,
                    else => return windows.unexpectedStatus(rc),
                }

                const reparse_tag: windows.DWORD = reparse_blk: {
                    if (info.BasicInformation.FileAttributes & windows.FILE_ATTRIBUTE_REPARSE_POINT != 0) {
                        var tag_info: windows.FILE_ATTRIBUTE_TAG_INFO = undefined;
                        const tag_rc = windows.ntdll.NtQueryInformationFile(self.handle, &io_status_block, &tag_info, @sizeOf(windows.FILE_ATTRIBUTE_TAG_INFO), .FileAttributeTagInformation);
                        switch (tag_rc) {
                            .SUCCESS => {},
                            // INFO_LENGTH_MISMATCH and ACCESS_DENIED are the only documented possible errors
                            // https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-fscc/d295752f-ce89-4b98-8553-266d37c84f0e
                            .INFO_LENGTH_MISMATCH => unreachable,
                            .ACCESS_DENIED => return error.AccessDenied,
                            else => return windows.unexpectedStatus(rc),
                        }
                        break :reparse_blk tag_info.ReparseTag;
                    }
                    break :reparse_blk 0;
                };

                break :blk .{
                    .attributes = info.BasicInformation.FileAttributes,
                    .reparse_tag = reparse_tag,
                    ._size = @as(u64, @bitCast(info.StandardInformation.EndOfFile)),
                    .access_time = windows.fromSysTime(info.BasicInformation.LastAccessTime),
                    .modified_time = windows.fromSysTime(info.BasicInformation.LastWriteTime),
                    .creation_time = windows.fromSysTime(info.BasicInformation.CreationTime),
                };
            },
            .linux => blk: {
                const l = std.os.linux;
                var stx = std.mem.zeroes(l.Statx);
                const rcx = l.statx(self.handle, "\x00", l.AT.EMPTY_PATH, l.STATX_TYPE |
                    l.STATX_MODE | l.STATX_ATIME | l.STATX_MTIME | l.STATX_BTIME, &stx);

                switch (posix.errno(rcx)) {
                    .SUCCESS => {},
                    // NOSYS happens when `statx` is unsupported, which is the case on kernel versions before 4.11
                    // Here, we call `fstat` and fill `stx` with the data we need
                    .NOSYS => {
                        const st = try posix.fstat(self.handle);

                        stx.mode = @as(u16, @intCast(st.mode));

                        // Hacky conversion from timespec to statx_timestamp
                        stx.atime = std.mem.zeroes(l.statx_timestamp);
                        stx.atime.tv_sec = st.atim.tv_sec;
                        stx.atime.tv_nsec = @as(u32, @intCast(st.atim.tv_nsec)); // Guaranteed to succeed (tv_nsec is always below 10^9)

                        stx.mtime = std.mem.zeroes(l.statx_timestamp);
                        stx.mtime.tv_sec = st.mtim.tv_sec;
                        stx.mtime.tv_nsec = @as(u32, @intCast(st.mtim.tv_nsec));

                        stx.mask = l.STATX_BASIC_STATS | l.STATX_MTIME;
                    },
                    .BADF => unreachable,
                    .FAULT => unreachable,
                    .NOMEM => return error.SystemResources,
                    else => |err| return posix.unexpectedErrno(err),
                }

                break :blk .{
                    .statx = stx,
                };
            },
            .wasi => .{ .stat = try std.os.fstat_wasi(self.handle) },
            else => .{ .stat = try posix.fstat(self.handle) },
        },
    };
}

pub const UpdateTimesError = posix.FutimensError || windows.SetFileTimeError;

/// The underlying file system may have a different granularity than nanoseconds,
/// and therefore this function cannot guarantee any precision will be stored.
/// Further, the maximum value is limited by the system ABI. When a value is provided
/// that exceeds this range, the value is clamped to the maximum.
/// TODO: integrate with async I/O
pub fn updateTimes(
    self: File,
    /// access timestamp in nanoseconds
    atime: i128,
    /// last modification timestamp in nanoseconds
    mtime: i128,
) UpdateTimesError!void {
    if (builtin.os.tag == .windows) {
        const atime_ft = windows.nanoSecondsToFileTime(atime);
        const mtime_ft = windows.nanoSecondsToFileTime(mtime);
        return windows.SetFileTime(self.handle, null, &atime_ft, &mtime_ft);
    }
    const times = [2]posix.timespec{
        posix.timespec{
            .tv_sec = math.cast(isize, @divFloor(atime, std.time.ns_per_s)) orelse maxInt(isize),
            .tv_nsec = math.cast(isize, @mod(atime, std.time.ns_per_s)) orelse maxInt(isize),
        },
        posix.timespec{
            .tv_sec = math.cast(isize, @divFloor(mtime, std.time.ns_per_s)) orelse maxInt(isize),
            .tv_nsec = math.cast(isize, @mod(mtime, std.time.ns_per_s)) orelse maxInt(isize),
        },
    };
    try posix.futimens(self.handle, &times);
}

/// Reads all the bytes from the current position to the end of the file.
/// On success, caller owns returned buffer.
/// If the file is larger than `max_bytes`, returns `error.FileTooBig`.
pub fn readToEndAlloc(self: File, allocator: Allocator, max_bytes: usize) ![]u8 {
    return self.readToEndAllocOptions(allocator, max_bytes, null, @alignOf(u8), null);
}

/// Reads all the bytes from the current position to the end of the file.
/// On success, caller owns returned buffer.
/// If the file is larger than `max_bytes`, returns `error.FileTooBig`.
/// If `size_hint` is specified the initial buffer size is calculated using
/// that value, otherwise an arbitrary value is used instead.
/// Allows specifying alignment and a sentinel value.
pub fn readToEndAllocOptions(
    self: File,
    allocator: Allocator,
    max_bytes: usize,
    size_hint: ?usize,
    comptime alignment: u29,
    comptime optional_sentinel: ?u8,
) !(if (optional_sentinel) |s| [:s]align(alignment) u8 else []align(alignment) u8) {
    // If no size hint is provided fall back to the size=0 code path
    const size = size_hint orelse 0;

    // The file size returned by stat is used as hint to set the buffer
    // size. If the reported size is zero, as it happens on Linux for files
    // in /proc, a small buffer is allocated instead.
    const initial_cap = (if (size > 0) size else 1024) + @intFromBool(optional_sentinel != null);
    var array_list = try std.ArrayListAligned(u8, alignment).initCapacity(allocator, initial_cap);
    defer array_list.deinit();

    self.reader().readAllArrayListAligned(alignment, &array_list, max_bytes) catch |err| switch (err) {
        error.StreamTooLong => return error.FileTooBig,
        else => |e| return e,
    };

    if (optional_sentinel) |sentinel| {
        return try array_list.toOwnedSliceSentinel(sentinel);
    } else {
        return try array_list.toOwnedSlice();
    }
}

pub const ReadError = posix.ReadError;
pub const PReadError = posix.PReadError;

pub fn read(self: File, buffer: []u8) ReadError!usize {
    if (is_windows) {
        return windows.ReadFile(self.handle, buffer, null);
    }

    return posix.read(self.handle, buffer);
}

/// Returns the number of bytes read. If the number read is smaller than `buffer.len`, it
/// means the file reached the end. Reaching the end of a file is not an error condition.
pub fn readAll(self: File, buffer: []u8) ReadError!usize {
    var index: usize = 0;
    while (index != buffer.len) {
        const amt = try self.read(buffer[index..]);
        if (amt == 0) break;
        index += amt;
    }
    return index;
}

/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pread(self: File, buffer: []u8, offset: u64) PReadError!usize {
    if (is_windows) {
        return windows.ReadFile(self.handle, buffer, offset);
    }

    return posix.pread(self.handle, buffer, offset);
}

/// Returns the number of bytes read. If the number read is smaller than `buffer.len`, it
/// means the file reached the end. Reaching the end of a file is not an error condition.
/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn preadAll(self: File, buffer: []u8, offset: u64) PReadError!usize {
    var index: usize = 0;
    while (index != buffer.len) {
        const amt = try self.pread(buffer[index..], offset + index);
        if (amt == 0) break;
        index += amt;
    }
    return index;
}

/// See https://github.com/ziglang/zig/issues/7699
pub fn readv(self: File, iovecs: []const posix.iovec) ReadError!usize {
    if (is_windows) {
        // TODO improve this to use ReadFileScatter
        if (iovecs.len == 0) return @as(usize, 0);
        const first = iovecs[0];
        return windows.ReadFile(self.handle, first.iov_base[0..first.iov_len], null);
    }

    return posix.readv(self.handle, iovecs);
}

/// Returns the number of bytes read. If the number read is smaller than the total bytes
/// from all the buffers, it means the file reached the end. Reaching the end of a file
/// is not an error condition.
///
/// The `iovecs` parameter is mutable because:
/// * This function needs to mutate the fields in order to handle partial
///   reads from the underlying OS layer.
/// * The OS layer expects pointer addresses to be inside the application's address space
///   even if the length is zero. Meanwhile, in Zig, slices may have undefined pointer
///   addresses when the length is zero. So this function modifies the iov_base fields
///   when the length is zero.
///
/// Related open issue: https://github.com/ziglang/zig/issues/7699
pub fn readvAll(self: File, iovecs: []posix.iovec) ReadError!usize {
    if (iovecs.len == 0) return 0;

    // We use the address of this local variable for all zero-length
    // vectors so that the OS does not complain that we are giving it
    // addresses outside the application's address space.
    var garbage: [1]u8 = undefined;
    for (iovecs) |*v| {
        if (v.iov_len == 0) v.iov_base = &garbage;
    }

    var i: usize = 0;
    var off: usize = 0;
    while (true) {
        var amt = try self.readv(iovecs[i..]);
        var eof = amt == 0;
        off += amt;
        while (amt >= iovecs[i].iov_len) {
            amt -= iovecs[i].iov_len;
            i += 1;
            if (i >= iovecs.len) return off;
            eof = false;
        }
        if (eof) return off;
        iovecs[i].iov_base += amt;
        iovecs[i].iov_len -= amt;
    }
}

/// See https://github.com/ziglang/zig/issues/7699
/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn preadv(self: File, iovecs: []const posix.iovec, offset: u64) PReadError!usize {
    if (is_windows) {
        // TODO improve this to use ReadFileScatter
        if (iovecs.len == 0) return @as(usize, 0);
        const first = iovecs[0];
        return windows.ReadFile(self.handle, first.iov_base[0..first.iov_len], offset);
    }

    return posix.preadv(self.handle, iovecs, offset);
}

/// Returns the number of bytes read. If the number read is smaller than the total bytes
/// from all the buffers, it means the file reached the end. Reaching the end of a file
/// is not an error condition.
/// The `iovecs` parameter is mutable because this function needs to mutate the fields in
/// order to handle partial reads from the underlying OS layer.
/// See https://github.com/ziglang/zig/issues/7699
/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn preadvAll(self: File, iovecs: []posix.iovec, offset: u64) PReadError!usize {
    if (iovecs.len == 0) return 0;

    var i: usize = 0;
    var off: usize = 0;
    while (true) {
        var amt = try self.preadv(iovecs[i..], offset + off);
        var eof = amt == 0;
        off += amt;
        while (amt >= iovecs[i].iov_len) {
            amt -= iovecs[i].iov_len;
            i += 1;
            if (i >= iovecs.len) return off;
            eof = false;
        }
        if (eof) return off;
        iovecs[i].iov_base += amt;
        iovecs[i].iov_len -= amt;
    }
}

pub const WriteError = posix.WriteError;
pub const PWriteError = posix.PWriteError;

pub fn write(self: File, bytes: []const u8) WriteError!usize {
    if (is_windows) {
        return windows.WriteFile(self.handle, bytes, null);
    }

    return posix.write(self.handle, bytes);
}

pub fn writeAll(self: File, bytes: []const u8) WriteError!void {
    var index: usize = 0;
    while (index < bytes.len) {
        index += try self.write(bytes[index..]);
    }
}

/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pwrite(self: File, bytes: []const u8, offset: u64) PWriteError!usize {
    if (is_windows) {
        return windows.WriteFile(self.handle, bytes, offset);
    }

    return posix.pwrite(self.handle, bytes, offset);
}

/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pwriteAll(self: File, bytes: []const u8, offset: u64) PWriteError!void {
    var index: usize = 0;
    while (index < bytes.len) {
        index += try self.pwrite(bytes[index..], offset + index);
    }
}

/// See https://github.com/ziglang/zig/issues/7699
/// See equivalent function: `std.net.Stream.writev`.
pub fn writev(self: File, iovecs: []const posix.iovec_const) WriteError!usize {
    if (is_windows) {
        // TODO improve this to use WriteFileScatter
        if (iovecs.len == 0) return @as(usize, 0);
        const first = iovecs[0];
        return windows.WriteFile(self.handle, first.iov_base[0..first.iov_len], null);
    }

    return posix.writev(self.handle, iovecs);
}

/// The `iovecs` parameter is mutable because:
/// * This function needs to mutate the fields in order to handle partial
///   writes from the underlying OS layer.
/// * The OS layer expects pointer addresses to be inside the application's address space
///   even if the length is zero. Meanwhile, in Zig, slices may have undefined pointer
///   addresses when the length is zero. So this function modifies the iov_base fields
///   when the length is zero.
/// See https://github.com/ziglang/zig/issues/7699
/// See equivalent function: `std.net.Stream.writevAll`.
pub fn writevAll(self: File, iovecs: []posix.iovec_const) WriteError!void {
    if (iovecs.len == 0) return;

    // We use the address of this local variable for all zero-length
    // vectors so that the OS does not complain that we are giving it
    // addresses outside the application's address space.
    var garbage: [1]u8 = undefined;
    for (iovecs) |*v| {
        if (v.iov_len == 0) v.iov_base = &garbage;
    }

    var i: usize = 0;
    while (true) {
        var amt = try self.writev(iovecs[i..]);
        while (amt >= iovecs[i].iov_len) {
            amt -= iovecs[i].iov_len;
            i += 1;
            if (i >= iovecs.len) return;
        }
        iovecs[i].iov_base += amt;
        iovecs[i].iov_len -= amt;
    }
}

/// See https://github.com/ziglang/zig/issues/7699
/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pwritev(self: File, iovecs: []posix.iovec_const, offset: u64) PWriteError!usize {
    if (is_windows) {
        // TODO improve this to use WriteFileScatter
        if (iovecs.len == 0) return @as(usize, 0);
        const first = iovecs[0];
        return windows.WriteFile(self.handle, first.iov_base[0..first.iov_len], offset);
    }

    return posix.pwritev(self.handle, iovecs, offset);
}

/// The `iovecs` parameter is mutable because this function needs to mutate the fields in
/// order to handle partial writes from the underlying OS layer.
/// See https://github.com/ziglang/zig/issues/7699
/// On Windows, this function currently does alter the file pointer.
/// https://github.com/ziglang/zig/issues/12783
pub fn pwritevAll(self: File, iovecs: []posix.iovec_const, offset: u64) PWriteError!void {
    if (iovecs.len == 0) return;

    var i: usize = 0;
    var off: u64 = 0;
    while (true) {
        var amt = try self.pwritev(iovecs[i..], offset + off);
        off += amt;
        while (amt >= iovecs[i].iov_len) {
            amt -= iovecs[i].iov_len;
            i += 1;
            if (i >= iovecs.len) return;
        }
        iovecs[i].iov_base += amt;
        iovecs[i].iov_len -= amt;
    }
}

pub const CopyRangeError = posix.CopyFileRangeError;

pub fn copyRange(in: File, in_offset: u64, out: File, out_offset: u64, len: u64) CopyRangeError!u64 {
    const adjusted_len = math.cast(usize, len) orelse maxInt(usize);
    const result = try posix.copy_file_range(in.handle, in_offset, out.handle, out_offset, adjusted_len, 0);
    return result;
}

/// Returns the number of bytes copied. If the number read is smaller than `buffer.len`, it
/// means the in file reached the end. Reaching the end of a file is not an error condition.
pub fn copyRangeAll(in: File, in_offset: u64, out: File, out_offset: u64, len: u64) CopyRangeError!u64 {
    var total_bytes_copied: u64 = 0;
    var in_off = in_offset;
    var out_off = out_offset;
    while (total_bytes_copied < len) {
        const amt_copied = try copyRange(in, in_off, out, out_off, len - total_bytes_copied);
        if (amt_copied == 0) return total_bytes_copied;
        total_bytes_copied += amt_copied;
        in_off += amt_copied;
        out_off += amt_copied;
    }
    return total_bytes_copied;
}

pub const WriteFileOptions = struct {
    in_offset: u64 = 0,

    /// `null` means the entire file. `0` means no bytes from the file.
    /// When this is `null`, trailers must be sent in a separate writev() call
    /// due to a flaw in the BSD sendfile API. Other operating systems, such as
    /// Linux, already do this anyway due to API limitations.
    /// If the size of the source file is known, passing the size here will save one syscall.
    in_len: ?u64 = null,

    headers_and_trailers: []posix.iovec_const = &[0]posix.iovec_const{},

    /// The trailer count is inferred from `headers_and_trailers.len - header_count`
    header_count: usize = 0,
};

pub const WriteFileError = ReadError || error{EndOfStream} || WriteError;

pub fn writeFileAll(self: File, in_file: File, args: WriteFileOptions) WriteFileError!void {
    return self.writeFileAllSendfile(in_file, args) catch |err| switch (err) {
        error.Unseekable,
        error.FastOpenAlreadyInProgress,
        error.MessageTooBig,
        error.FileDescriptorNotASocket,
        error.NetworkUnreachable,
        error.NetworkSubsystemFailed,
        => return self.writeFileAllUnseekable(in_file, args),

        else => |e| return e,
    };
}

/// Does not try seeking in either of the File parameters.
/// See `writeFileAll` as an alternative to calling this.
pub fn writeFileAllUnseekable(self: File, in_file: File, args: WriteFileOptions) WriteFileError!void {
    const headers = args.headers_and_trailers[0..args.header_count];
    const trailers = args.headers_and_trailers[args.header_count..];

    try self.writevAll(headers);

    try in_file.reader().skipBytes(args.in_offset, .{ .buf_size = 4096 });

    var fifo = std.fifo.LinearFifo(u8, .{ .Static = 4096 }).init();
    if (args.in_len) |len| {
        var stream = std.io.limitedReader(in_file.reader(), len);
        try fifo.pump(stream.reader(), self.writer());
    } else {
        try fifo.pump(in_file.reader(), self.writer());
    }

    try self.writevAll(trailers);
}

/// Low level function which can fail for OS-specific reasons.
/// See `writeFileAll` as an alternative to calling this.
/// TODO integrate with async I/O
fn writeFileAllSendfile(self: File, in_file: File, args: WriteFileOptions) posix.SendFileError!void {
    const count = blk: {
        if (args.in_len) |l| {
            if (l == 0) {
                return self.writevAll(args.headers_and_trailers);
            } else {
                break :blk l;
            }
        } else {
            break :blk 0;
        }
    };
    const headers = args.headers_and_trailers[0..args.header_count];
    const trailers = args.headers_and_trailers[args.header_count..];
    const zero_iovec = &[0]posix.iovec_const{};
    // When reading the whole file, we cannot put the trailers in the sendfile() syscall,
    // because we have no way to determine whether a partial write is past the end of the file or not.
    const trls = if (count == 0) zero_iovec else trailers;
    const offset = args.in_offset;
    const out_fd = self.handle;
    const in_fd = in_file.handle;
    const flags = 0;
    var amt: usize = 0;
    hdrs: {
        var i: usize = 0;
        while (i < headers.len) {
            amt = try posix.sendfile(out_fd, in_fd, offset, count, headers[i..], trls, flags);
            while (amt >= headers[i].iov_len) {
                amt -= headers[i].iov_len;
                i += 1;
                if (i >= headers.len) break :hdrs;
            }
            headers[i].iov_base += amt;
            headers[i].iov_len -= amt;
        }
    }
    if (count == 0) {
        var off: u64 = amt;
        while (true) {
            amt = try posix.sendfile(out_fd, in_fd, offset + off, 0, zero_iovec, zero_iovec, flags);
            if (amt == 0) break;
            off += amt;
        }
    } else {
        var off: u64 = amt;
        while (off < count) {
            amt = try posix.sendfile(out_fd, in_fd, offset + off, count - off, zero_iovec, trailers, flags);
            off += amt;
        }
        amt = @as(usize, @intCast(off - count));
    }
    var i: usize = 0;
    while (i < trailers.len) {
        while (amt >= trailers[i].iov_len) {
            amt -= trailers[i].iov_len;
            i += 1;
            if (i >= trailers.len) return;
        }
        trailers[i].iov_base += amt;
        trailers[i].iov_len -= amt;
        amt = try posix.writev(self.handle, trailers[i..]);
    }
}

pub const Reader = io.Reader(File, ReadError, read);

pub fn reader(file: File) Reader {
    return .{ .context = file };
}

pub const Writer = io.Writer(File, WriteError, write);

pub fn writer(file: File) Writer {
    return .{ .context = file };
}

pub const SeekableStream = io.SeekableStream(
    File,
    SeekError,
    GetSeekPosError,
    seekTo,
    seekBy,
    getPos,
    getEndPos,
);

pub fn seekableStream(file: File) SeekableStream {
    return .{ .context = file };
}

const range_off: windows.LARGE_INTEGER = 0;
const range_len: windows.LARGE_INTEGER = 1;

pub const LockError = error{
    SystemResources,
    FileLocksNotSupported,
} || posix.UnexpectedError;

/// Blocks when an incompatible lock is held by another process.
/// A process may hold only one type of lock (shared or exclusive) on
/// a file. When a process terminates in any way, the lock is released.
///
/// Assumes the file is unlocked.
///
/// TODO: integrate with async I/O
pub fn lock(file: File, l: Lock) LockError!void {
    if (is_windows) {
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        const exclusive = switch (l) {
            .none => return,
            .shared => false,
            .exclusive => true,
        };
        return windows.LockFile(
            file.handle,
            null,
            null,
            null,
            &io_status_block,
            &range_off,
            &range_len,
            null,
            windows.FALSE, // non-blocking=false
            @intFromBool(exclusive),
        ) catch |err| switch (err) {
            error.WouldBlock => unreachable, // non-blocking=false
            else => |e| return e,
        };
    } else {
        return posix.flock(file.handle, switch (l) {
            .none => posix.LOCK.UN,
            .shared => posix.LOCK.SH,
            .exclusive => posix.LOCK.EX,
        }) catch |err| switch (err) {
            error.WouldBlock => unreachable, // non-blocking=false
            else => |e| return e,
        };
    }
}

/// Assumes the file is locked.
pub fn unlock(file: File) void {
    if (is_windows) {
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        return windows.UnlockFile(
            file.handle,
            &io_status_block,
            &range_off,
            &range_len,
            null,
        ) catch |err| switch (err) {
            error.RangeNotLocked => unreachable, // Function assumes unlocked.
            error.Unexpected => unreachable, // Resource deallocation must succeed.
        };
    } else {
        return posix.flock(file.handle, posix.LOCK.UN) catch |err| switch (err) {
            error.WouldBlock => unreachable, // unlocking can't block
            error.SystemResources => unreachable, // We are deallocating resources.
            error.FileLocksNotSupported => unreachable, // We already got the lock.
            error.Unexpected => unreachable, // Resource deallocation must succeed.
        };
    }
}

/// Attempts to obtain a lock, returning `true` if the lock is
/// obtained, and `false` if there was an existing incompatible lock held.
/// A process may hold only one type of lock (shared or exclusive) on
/// a file. When a process terminates in any way, the lock is released.
///
/// Assumes the file is unlocked.
///
/// TODO: integrate with async I/O
pub fn tryLock(file: File, l: Lock) LockError!bool {
    if (is_windows) {
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        const exclusive = switch (l) {
            .none => return,
            .shared => false,
            .exclusive => true,
        };
        windows.LockFile(
            file.handle,
            null,
            null,
            null,
            &io_status_block,
            &range_off,
            &range_len,
            null,
            windows.TRUE, // non-blocking=true
            @intFromBool(exclusive),
        ) catch |err| switch (err) {
            error.WouldBlock => return false,
            else => |e| return e,
        };
    } else {
        posix.flock(file.handle, switch (l) {
            .none => posix.LOCK.UN,
            .shared => posix.LOCK.SH | posix.LOCK.NB,
            .exclusive => posix.LOCK.EX | posix.LOCK.NB,
        }) catch |err| switch (err) {
            error.WouldBlock => return false,
            else => |e| return e,
        };
    }
    return true;
}

/// Assumes the file is already locked in exclusive mode.
/// Atomically modifies the lock to be in shared mode, without releasing it.
///
/// TODO: integrate with async I/O
pub fn downgradeLock(file: File) LockError!void {
    if (is_windows) {
        // On Windows it works like a semaphore + exclusivity flag. To implement this
        // function, we first obtain another lock in shared mode. This changes the
        // exclusivity flag, but increments the semaphore to 2. So we follow up with
        // an NtUnlockFile which decrements the semaphore but does not modify the
        // exclusivity flag.
        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
        windows.LockFile(
            file.handle,
            null,
            null,
            null,
            &io_status_block,
            &range_off,
            &range_len,
            null,
            windows.TRUE, // non-blocking=true
            windows.FALSE, // exclusive=false
        ) catch |err| switch (err) {
            error.WouldBlock => unreachable, // File was not locked in exclusive mode.
            else => |e| return e,
        };
        return windows.UnlockFile(
            file.handle,
            &io_status_block,
            &range_off,
            &range_len,
            null,
        ) catch |err| switch (err) {
            error.RangeNotLocked => unreachable, // File was not locked.
            error.Unexpected => unreachable, // Resource deallocation must succeed.
        };
    } else {
        return posix.flock(file.handle, posix.LOCK.SH | posix.LOCK.NB) catch |err| switch (err) {
            error.WouldBlock => unreachable, // File was not locked in exclusive mode.
            else => |e| return e,
        };
    }
}

const File = @This();
const std = @import("../std.zig");
const builtin = @import("builtin");
const Allocator = std.mem.Allocator;
const posix = std.posix;
const io = std.io;
const math = std.math;
const assert = std.debug.assert;
const windows = std.os.windows;
const Os = std.builtin.Os;
const maxInt = std.math.maxInt;
const is_windows = builtin.os.tag == .windows;
