module os

/// Eof error means that we reach the end of the file.
pub struct Eof {
	Error
}

// NotExpected is a generic error that means that we receave a not expecte error.
pub struct NotExpected {
	cause string
	code  int
}

fn (err NotExpected) msg() string {
	return err.cause
}

fn (err NotExpected) code() int {
	return err.code
}

pub struct File {
mut:
	cfile voidptr // Using void* instead of FILE*
pub:
	fd int
pub mut:
	is_opened bool
}

struct FileInfo {
	name string
	size int
}

fn C.fseeko(&C.FILE, u64, int) int

fn C._fseeki64(&C.FILE, u64, int) int

fn C.getc(&C.FILE) int

fn C.freopen(&char, &char, &C.FILE) &C.FILE

fn C._wfreopen(&u16, &u16, &C.FILE) &C.FILE

fn fix_windows_path(path string) string {
	mut p := path
	$if windows {
		p = path.replace('/', '\\')
	}
	return p
}

// open_file can be used to open or create a file with custom flags and permissions and returns a `File` object.
pub fn open_file(path string, mode string, options ...int) !File {
	mut flags := 0
	mut seek_to_end := false
	for m in mode {
		match m {
			`w` {
				flags |= o_create | o_trunc | o_wronly
			}
			`a` {
				flags |= o_create | o_append | o_wronly
				seek_to_end = true
			}
			`r` {
				flags |= o_rdonly
			}
			`b` {
				flags |= o_binary
			}
			`s` {
				flags |= o_sync
			}
			`n` {
				flags |= o_nonblock
			}
			`c` {
				flags |= o_noctty
			}
			`+` {
				flags &= ~o_wronly
				flags |= o_rdwr
			}
			else {}
		}
	}
	if mode == 'r+' {
		flags = o_rdwr
	}
	mut permission := 0o666
	if options.len > 0 {
		permission = options[0]
	}
	$if windows {
		if permission < 0o600 {
			permission = 0x0100
		} else {
			permission = 0x0100 | 0x0080
		}
	}
	p := fix_windows_path(path)
	fd := C.open(&char(p.str), flags, permission)
	if fd == -1 {
		return error(posix_get_error_msg(C.errno))
	}
	fdopen_mode := mode.replace('b', '')
	cfile := C.fdopen(fd, &char(fdopen_mode.str))
	if isnil(cfile) {
		return error('Failed to open or create file "${path}"')
	}
	if seek_to_end {
		// ensure appending will work, even on bsd/macos systems:
		$if windows {
			C._fseeki64(cfile, 0, C.SEEK_END)
		} $else {
			C.fseeko(cfile, 0, C.SEEK_END)
		}
	}
	return File{
		cfile: cfile
		fd: fd
		is_opened: true
	}
}

// open tries to open a file for reading and returns back a read-only `File` object.
pub fn open(path string) !File {
	/*
	$if linux {
		$if !android {
			fd := C.syscall(sys_open, path.str, 511)
			if fd == -1 {
				return error('failed to open file "$path"')
			}
			return File{
				fd: fd
				is_opened: true
			}
		}
	}
	*/
	cfile := vfopen(path, 'rb')!
	fd := fileno(cfile)
	return File{
		cfile: cfile
		fd: fd
		is_opened: true
	}
}

// create creates or opens a file at a specified location and returns a write-only `File` object.
pub fn create(path string) !File {
	/*
	// Note: android/termux/bionic is also a kind of linux,
	// but linux syscalls there sometimes fail,
	// while the libc version should work.
	$if linux {
		$if !android {
			//$if macos {
			//	fd = C.syscall(398, path.str, 0x601, 0x1b6)
			//}
			//$if linux {
			fd = C.syscall(sys_creat, path.str, 511)
			//}
			if fd == -1 {
				return error('failed to create file "$path"')
			}
			file = File{
				fd: fd
				is_opened: true
			}
			return file
		}
	}
	*/
	cfile := vfopen(path, 'wb')!
	fd := fileno(cfile)
	return File{
		cfile: cfile
		fd: fd
		is_opened: true
	}
}

// stdin - return an os.File for stdin
pub fn stdin() File {
	return File{
		fd: 0
		cfile: C.stdin
		is_opened: true
	}
}

// stdout - return an os.File for stdout
pub fn stdout() File {
	return File{
		fd: 1
		cfile: C.stdout
		is_opened: true
	}
}

// stderr - return an os.File for stderr
pub fn stderr() File {
	return File{
		fd: 2
		cfile: C.stderr
		is_opened: true
	}
}

// eof returns true, when the end of file has been reached
pub fn (f &File) eof() bool {
	cfile := &C.FILE(f.cfile)
	return C.feof(cfile) != 0
}

// reopen allows a `File` to be reused. It is mostly useful for reopening standard input and output.
pub fn (mut f File) reopen(path string, mode string) ! {
	p := fix_windows_path(path)
	mut cfile := &C.FILE(0)
	$if windows {
		cfile = C._wfreopen(p.to_wide(), mode.to_wide(), f.cfile)
	} $else {
		cfile = C.freopen(&char(p.str), &char(mode.str), f.cfile)
	}
	if isnil(cfile) {
		return error('Failed to reopen file "${path}"')
	}
	f.cfile = cfile
}

// read implements the Reader interface.
pub fn (f &File) read(mut buf []u8) !int {
	if buf.len == 0 {
		return Eof{}
	}
	// the following is needed, because on FreeBSD, C.feof is a macro:
	nbytes := int(C.fread(buf.data, 1, buf.len, &C.FILE(f.cfile)))
	// if no bytes were read, check for errors and end-of-file.
	if nbytes <= 0 {
		if C.feof(&C.FILE(f.cfile)) != 0 {
			return Eof{}
		}
		if C.ferror(&C.FILE(f.cfile)) != 0 {
			return NotExpected{
				cause: 'unexpected error from fread'
				code: -1
			}
		}
	}
	return nbytes
}

// **************************** Write ops  ***************************
// write implements the Writer interface.
// It returns how many bytes were actually written.
pub fn (mut f File) write(buf []u8) !int {
	if !f.is_opened {
		return error_file_not_opened()
	}
	/*
	$if linux {
		$if !android {
			res := C.syscall(sys_write, f.fd, s.str, s.len)
			return res
		}
	}
	*/
	written := int(C.fwrite(buf.data, 1, buf.len, f.cfile))
	if written == 0 && buf.len != 0 {
		return error('0 bytes written')
	}
	return written
}

// writeln writes the string `s` into the file, and appends a \n character.
// It returns how many bytes were written, including the \n character.
pub fn (mut f File) writeln(s string) !int {
	if !f.is_opened {
		return error_file_not_opened()
	}
	/*
	$if linux {
		$if !android {
			snl := s + '\n'
			C.syscall(sys_write, f.fd, snl.str, snl.len)
			return
		}
	}
	*/
	// TODO perf
	written := int(C.fwrite(s.str, 1, s.len, f.cfile))
	if written == 0 && s.len != 0 {
		return error('0 bytes written')
	}
	x := C.fputs(c'\n', f.cfile)
	if x < 0 {
		return error('could not add newline')
	}
	return written + 1
}

// write_string writes the string `s` into the file
// It returns how many bytes were actually written.
pub fn (mut f File) write_string(s string) !int {
	unsafe { f.write_full_buffer(s.str, usize(s.len))! }
	return s.len
}

// write_to implements the RandomWriter interface.
// It returns how many bytes were actually written.
// It resets the seek position to the end of the file.
pub fn (mut f File) write_to(pos u64, buf []u8) !int {
	if !f.is_opened {
		return error_file_not_opened()
	}
	$if x64 {
		$if windows {
			C._fseeki64(f.cfile, pos, C.SEEK_SET)
			res := int(C.fwrite(buf.data, 1, buf.len, f.cfile))
			if res == 0 && buf.len != 0 {
				return error('0 bytes written')
			}
			C._fseeki64(f.cfile, 0, C.SEEK_END)
			return res
		} $else {
			C.fseeko(f.cfile, pos, C.SEEK_SET)
			res := int(C.fwrite(buf.data, 1, buf.len, f.cfile))
			if res == 0 && buf.len != 0 {
				return error('0 bytes written')
			}
			C.fseeko(f.cfile, 0, C.SEEK_END)
			return res
		}
	}
	$if x32 {
		C.fseek(f.cfile, pos, C.SEEK_SET)
		res := int(C.fwrite(buf.data, 1, buf.len, f.cfile))
		if res == 0 && buf.len != 0 {
			return error('0 bytes written')
		}
		C.fseek(f.cfile, 0, C.SEEK_END)
		return res
	}
	return error('Could not write to file')
}

// write_ptr writes `size` bytes to the file, starting from the address in `data`.
// Note: write_ptr is unsafe and should be used carefully, since if you pass invalid
// pointers to it, it will cause your programs to segfault.
[unsafe]
pub fn (mut f File) write_ptr(data voidptr, size int) int {
	return int(C.fwrite(data, 1, size, f.cfile))
}

// write_full_buffer writes a whole buffer of data to the file, starting from the
// address in `buffer`, no matter how many tries/partial writes it would take.
[unsafe]
pub fn (mut f File) write_full_buffer(buffer voidptr, buffer_len usize) ! {
	if buffer_len <= usize(0) {
		return
	}
	if !f.is_opened {
		return error_file_not_opened()
	}
	mut ptr := &u8(buffer)
	mut remaining_bytes := i64(buffer_len)
	for remaining_bytes > 0 {
		unsafe {
			x := i64(C.fwrite(ptr, 1, remaining_bytes, f.cfile))
			ptr += x
			remaining_bytes -= x
			if x <= 0 {
				return error('C.fwrite returned 0')
			}
		}
	}
}

// write_ptr_at writes `size` bytes to the file, starting from the address in `data`,
// at byte offset `pos`, counting from the start of the file (pos 0).
// Note: write_ptr_at is unsafe and should be used carefully, since if you pass invalid
// pointers to it, it will cause your programs to segfault.
[unsafe]
pub fn (mut f File) write_ptr_at(data voidptr, size int, pos u64) int {
	$if x64 {
		$if windows {
			C._fseeki64(f.cfile, pos, C.SEEK_SET)
			res := int(C.fwrite(data, 1, size, f.cfile))
			C._fseeki64(f.cfile, 0, C.SEEK_END)
			return res
		} $else {
			C.fseeko(f.cfile, pos, C.SEEK_SET)
			res := int(C.fwrite(data, 1, size, f.cfile))
			C.fseeko(f.cfile, 0, C.SEEK_END)
			return res
		}
	}
	$if x32 {
		C.fseek(f.cfile, pos, C.SEEK_SET)
		res := int(C.fwrite(data, 1, size, f.cfile))
		C.fseek(f.cfile, 0, C.SEEK_END)
		return res
	}
	return 0
}

// **************************** Read ops  ***************************

// fread wraps C.fread and handles error and end-of-file detection.
fn fread(ptr voidptr, item_size int, items int, stream &C.FILE) !int {
	nbytes := int(C.fread(ptr, item_size, items, stream))
	// If no bytes were read, check for errors and end-of-file.
	if nbytes <= 0 {
		// If fread encountered end-of-file return the none error. Note that fread
		// may read data and encounter the end-of-file, but we shouldn't return none
		// in that case which is why we only check for end-of-file if no data was
		// read. The caller will get none on their next call because there will be
		// no data available and the end-of-file will be encountered again.
		if C.feof(stream) != 0 {
			return Eof{}
		}
		// If fread encountered an error, return it. Note that fread and ferror do
		// not tell us what the error was, so we can't return anything more specific
		// than there was an error. This is because fread and ferror do not set
		// errno.
		if C.ferror(stream) != 0 {
			return error('file read error')
		}
	}
	return nbytes
}

// read_bytes reads bytes from the beginning of the file.
// Utility method, same as .read_bytes_at(size, 0).
pub fn (f &File) read_bytes(size int) []u8 {
	return f.read_bytes_at(size, 0)
}

// read_bytes_at reads `size` bytes at the given position in the file.
pub fn (f &File) read_bytes_at(size int, pos u64) []u8 {
	mut arr := []u8{len: size}
	nreadbytes := f.read_bytes_into(pos, mut arr) or {
		// return err
		return []
	}
	return arr[0..nreadbytes]
}

// read_bytes_into_newline reads from the beginning of the file into the provided buffer.
// Each consecutive call on the same file continues reading where it previously ended.
// A read call is either stopped, if the buffer is full, a newline was read or EOF.
pub fn (f &File) read_bytes_into_newline(mut buf []u8) !int {
	if buf.len == 0 {
		return error(@FN + ': `buf.len` == 0')
	}
	newline := 10
	mut c := 0
	mut buf_ptr := 0
	mut nbytes := 0

	stream := &C.FILE(f.cfile)
	for (buf_ptr < buf.len) {
		c = C.getc(stream)
		match c {
			C.EOF {
				if C.feof(stream) != 0 {
					return nbytes
				}
				if C.ferror(stream) != 0 {
					return error('file read error')
				}
			}
			newline {
				buf[buf_ptr] = u8(c)
				nbytes++
				return nbytes
			}
			else {
				buf[buf_ptr] = u8(c)
				buf_ptr++
				nbytes++
			}
		}
	}
	return nbytes
}

// read_bytes_into fills `buf` with bytes at the given position in the file.
// `buf` *must* have length greater than zero.
// Returns the number of read bytes, or an error.
pub fn (f &File) read_bytes_into(pos u64, mut buf []u8) !int {
	if buf.len == 0 {
		return error(@FN + ': `buf.len` == 0')
	}
	$if x64 {
		$if windows {
			// Note: fseek errors if pos == os.file_size, which we accept
			C._fseeki64(f.cfile, pos, C.SEEK_SET)
			nbytes := fread(buf.data, 1, buf.len, f.cfile)!
			$if debug {
				C._fseeki64(f.cfile, 0, C.SEEK_SET)
			}
			return nbytes
		} $else {
			C.fseeko(f.cfile, pos, C.SEEK_SET)
			nbytes := fread(buf.data, 1, buf.len, f.cfile)!
			$if debug {
				C.fseeko(f.cfile, 0, C.SEEK_SET)
			}
			return nbytes
		}
	}
	$if x32 {
		C.fseek(f.cfile, pos, C.SEEK_SET)
		nbytes := fread(buf.data, 1, buf.len, f.cfile)!
		$if debug {
			C.fseek(f.cfile, 0, C.SEEK_SET)
		}
		return nbytes
	}
	return error('Could not read file')
}

// read_from implements the RandomReader interface.
pub fn (f &File) read_from(pos u64, mut buf []u8) !int {
	if buf.len == 0 {
		return 0
	}
	$if x64 {
		$if windows {
			C._fseeki64(f.cfile, pos, C.SEEK_SET)
		} $else {
			C.fseeko(f.cfile, pos, C.SEEK_SET)
		}

		nbytes := fread(buf.data, 1, buf.len, f.cfile)!
		return nbytes
	}
	$if x32 {
		C.fseek(f.cfile, pos, C.SEEK_SET)
		nbytes := fread(buf.data, 1, buf.len, f.cfile)!
		return nbytes
	}
	return error('Could not read file')
}

// read_into_ptr reads at most max_size bytes from the file and writes it into ptr.
// Returns the amount of bytes read or an error.
pub fn (f &File) read_into_ptr(ptr &u8, max_size int) !int {
	return fread(ptr, 1, max_size, f.cfile)
}

// **************************** Utility  ops ***********************
// flush writes any buffered unwritten data left in the file stream.
pub fn (mut f File) flush() {
	if !f.is_opened {
		return
	}
	C.fflush(f.cfile)
}

pub struct FileNotOpenedError {
	Error
}

pub fn (err FileNotOpenedError) msg() string {
	return 'os: file not opened'
}

pub struct SizeOfTypeIs0Error {
	Error
}

pub fn (err SizeOfTypeIs0Error) msg() string {
	return 'os: size of type is 0'
}

fn error_file_not_opened() IError {
	return &FileNotOpenedError{}
}

fn error_size_of_type_0() IError {
	return &SizeOfTypeIs0Error{}
}

// read_struct reads a single struct of type `T`
pub fn (mut f File) read_struct<T>(mut t T) ! {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(*t))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	nbytes := fread(t, 1, tsize, f.cfile)!
	if nbytes != tsize {
		return error_with_code('incomplete struct read', nbytes)
	}
}

// read_struct_at reads a single struct of type `T` at position specified in file
pub fn (mut f File) read_struct_at<T>(mut t T, pos u64) ! {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(*t))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	mut nbytes := 0
	$if x64 {
		$if windows {
			C._fseeki64(f.cfile, pos, C.SEEK_SET)
			nbytes = fread(t, 1, tsize, f.cfile)!
			C._fseeki64(f.cfile, 0, C.SEEK_END)
		} $else {
			C.fseeko(f.cfile, pos, C.SEEK_SET)
			nbytes = fread(t, 1, tsize, f.cfile)!
			C.fseeko(f.cfile, 0, C.SEEK_END)
		}
	}
	$if x32 {
		C.fseek(f.cfile, pos, C.SEEK_SET)
		nbytes = fread(t, 1, tsize, f.cfile)!
		C.fseek(f.cfile, 0, C.SEEK_END)
	}
	if nbytes != tsize {
		return error_with_code('incomplete struct read', nbytes)
	}
}

// read_raw reads and returns a single instance of type `T`
pub fn (mut f File) read_raw<T>() !T {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(T))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	mut t := T{}
	nbytes := fread(&t, 1, tsize, f.cfile)!
	if nbytes != tsize {
		return error_with_code('incomplete struct read', nbytes)
	}
	return t
}

// read_raw_at reads and returns a single instance of type `T` starting at file byte offset `pos`
pub fn (mut f File) read_raw_at<T>(pos u64) !T {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(T))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	mut nbytes := 0
	mut t := T{}
	$if x64 {
		$if windows {
			if C._fseeki64(f.cfile, pos, C.SEEK_SET) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
			nbytes = fread(&t, 1, tsize, f.cfile)!
			if C._fseeki64(f.cfile, 0, C.SEEK_END) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
		} $else {
			if C.fseeko(f.cfile, pos, C.SEEK_SET) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
			nbytes = fread(&t, 1, tsize, f.cfile)!
			if C.fseeko(f.cfile, 0, C.SEEK_END) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
		}
	}
	$if x32 {
		if C.fseek(f.cfile, pos, C.SEEK_SET) != 0 {
			return error(posix_get_error_msg(C.errno))
		}
		nbytes = fread(&t, 1, tsize, f.cfile)!
		if C.fseek(f.cfile, 0, C.SEEK_END) != 0 {
			return error(posix_get_error_msg(C.errno))
		}
	}

	if nbytes != tsize {
		return error_with_code('incomplete struct read', nbytes)
	}
	return t
}

// write_struct writes a single struct of type `T`
pub fn (mut f File) write_struct<T>(t &T) ! {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(T))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	C.errno = 0
	nbytes := int(C.fwrite(t, 1, tsize, f.cfile))
	if C.errno != 0 {
		return error(posix_get_error_msg(C.errno))
	}
	if nbytes != tsize {
		return error_with_code('incomplete struct write', nbytes)
	}
}

// write_struct_at writes a single struct of type `T` at position specified in file
pub fn (mut f File) write_struct_at<T>(t &T, pos u64) ! {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(T))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	C.errno = 0
	mut nbytes := 0
	$if x64 {
		$if windows {
			C._fseeki64(f.cfile, pos, C.SEEK_SET)
			nbytes = int(C.fwrite(t, 1, tsize, f.cfile))
			C._fseeki64(f.cfile, 0, C.SEEK_END)
		} $else {
			C.fseeko(f.cfile, pos, C.SEEK_SET)
			nbytes = int(C.fwrite(t, 1, tsize, f.cfile))
			C.fseeko(f.cfile, 0, C.SEEK_END)
		}
	}
	$if x32 {
		C.fseek(f.cfile, pos, C.SEEK_SET)
		nbytes = int(C.fwrite(t, 1, tsize, f.cfile))
		C.fseek(f.cfile, 0, C.SEEK_END)
	}
	if C.errno != 0 {
		return error(posix_get_error_msg(C.errno))
	}
	if nbytes != tsize {
		return error_with_code('incomplete struct write', nbytes)
	}
}

// TODO `write_raw[_at]` implementations are copy-pasted from `write_struct[_at]`

// write_raw writes a single instance of type `T`
pub fn (mut f File) write_raw<T>(t &T) ! {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(T))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	C.errno = 0
	nbytes := int(C.fwrite(t, 1, tsize, f.cfile))
	if C.errno != 0 {
		return error(posix_get_error_msg(C.errno))
	}
	if nbytes != tsize {
		return error_with_code('incomplete struct write', nbytes)
	}
}

// write_raw_at writes a single instance of type `T` starting at file byte offset `pos`
pub fn (mut f File) write_raw_at<T>(t &T, pos u64) ! {
	if !f.is_opened {
		return error_file_not_opened()
	}
	tsize := int(sizeof(T))
	if tsize == 0 {
		return error_size_of_type_0()
	}
	mut nbytes := 0

	$if x64 {
		$if windows {
			if C._fseeki64(f.cfile, pos, C.SEEK_SET) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
			nbytes = int(C.fwrite(t, 1, tsize, f.cfile))
			if C.errno != 0 {
				return error(posix_get_error_msg(C.errno))
			}
			if C._fseeki64(f.cfile, 0, C.SEEK_END) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
		} $else {
			if C.fseeko(f.cfile, pos, C.SEEK_SET) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
			nbytes = int(C.fwrite(t, 1, tsize, f.cfile))
			if C.errno != 0 {
				return error(posix_get_error_msg(C.errno))
			}
			if C.fseeko(f.cfile, 0, C.SEEK_END) != 0 {
				return error(posix_get_error_msg(C.errno))
			}
		}
	}
	$if x32 {
		if C.fseek(f.cfile, pos, C.SEEK_SET) != 0 {
			return error(posix_get_error_msg(C.errno))
		}
		nbytes = int(C.fwrite(t, 1, tsize, f.cfile))
		if C.errno != 0 {
			return error(posix_get_error_msg(C.errno))
		}
		if C.fseek(f.cfile, 0, C.SEEK_END) != 0 {
			return error(posix_get_error_msg(C.errno))
		}
	}

	if nbytes != tsize {
		return error_with_code('incomplete struct write', nbytes)
	}
}

pub enum SeekMode {
	start
	current
	end
}

// seek moves the file cursor (if any) associated with a file
// to a new location, offset `pos` bytes from the origin. The origin
// is dependent on the `mode` and can be:
//   .start   -> the origin is the start of the file
//   .current -> the current position/cursor in the file
//   .end     -> the end of the file
// If the file is not seek-able, or an error occures, the error will
// be returned to the caller.
// A successful call to the fseek() function clears the end-of-file
// indicator for the file.
pub fn (mut f File) seek(pos i64, mode SeekMode) ! {
	if !f.is_opened {
		return error_file_not_opened()
	}
	whence := int(mode)
	mut res := 0
	$if x64 {
		$if windows {
			res = C._fseeki64(f.cfile, pos, whence)
		} $else {
			res = C.fseeko(f.cfile, pos, whence)
		}
	}
	$if x32 {
		res = C.fseek(f.cfile, pos, whence)
	}
	if res == -1 {
		return error(posix_get_error_msg(C.errno))
	}
}

// tell will return the current offset of the file cursor measured from
// the start of the file, in bytes. It is complementary to seek, i.e.
// you can use the return value as the `pos` parameter to .seek( pos, .start ),
// so that your next read will happen from the same place.
pub fn (f &File) tell() !i64 {
	if !f.is_opened {
		return error_file_not_opened()
	}
	pos := C.ftell(f.cfile)
	if pos == -1 {
		return error(posix_get_error_msg(C.errno))
	}
	return pos
}