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#[cfg(test)]
mod tests;
use crate::io::prelude::*;
use crate::alloc::Allocator;
use crate::cmp;
use crate::io::{self, BorrowedCursor, ErrorKind, IoSlice, IoSliceMut, SeekFrom};
/// A `Cursor` wraps an in-memory buffer and provides it with a
/// [`Seek`] implementation.
///
/// `Cursor`s are used with in-memory buffers, anything implementing
/// <code>[AsRef]<\[u8]></code>, to allow them to implement [`Read`] and/or [`Write`],
/// allowing these buffers to be used anywhere you might use a reader or writer
/// that does actual I/O.
///
/// The standard library implements some I/O traits on various types which
/// are commonly used as a buffer, like <code>Cursor<[Vec]\<u8>></code> and
/// <code>Cursor<[&\[u8\]][bytes]></code>.
///
/// # Examples
///
/// We may want to write bytes to a [`File`] in our production
/// code, but use an in-memory buffer in our tests. We can do this with
/// `Cursor`:
///
/// [bytes]: crate::slice "slice"
/// [`File`]: crate::fs::File
///
/// ```no_run
/// use std::io::prelude::*;
/// use std::io::{self, SeekFrom};
/// use std::fs::File;
///
/// // a library function we've written
/// fn write_ten_bytes_at_end<W: Write + Seek>(mut writer: W) -> io::Result<()> {
/// writer.seek(SeekFrom::End(-10))?;
///
/// for i in 0..10 {
/// writer.write(&[i])?;
/// }
///
/// // all went well
/// Ok(())
/// }
///
/// # fn foo() -> io::Result<()> {
/// // Here's some code that uses this library function.
/// //
/// // We might want to use a BufReader here for efficiency, but let's
/// // keep this example focused.
/// let mut file = File::create("foo.txt")?;
///
/// write_ten_bytes_at_end(&mut file)?;
/// # Ok(())
/// # }
///
/// // now let's write a test
/// #[test]
/// fn test_writes_bytes() {
/// // setting up a real File is much slower than an in-memory buffer,
/// // let's use a cursor instead
/// use std::io::Cursor;
/// let mut buff = Cursor::new(vec![0; 15]);
///
/// write_ten_bytes_at_end(&mut buff).unwrap();
///
/// assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Debug, Default, Eq, PartialEq)]
pub struct Cursor<T> {
inner: T,
pos: u64,
}
impl<T> Cursor<T> {
/// Creates a new cursor wrapping the provided underlying in-memory buffer.
///
/// Cursor initial position is `0` even if underlying buffer (e.g., [`Vec`])
/// is not empty. So writing to cursor starts with overwriting [`Vec`]
/// content, not with appending to it.
///
/// # Examples
///
/// ```
/// use std::io::Cursor;
///
/// let buff = Cursor::new(Vec::new());
/// # fn force_inference(_: &Cursor<Vec<u8>>) {}
/// # force_inference(&buff);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
pub const fn new(inner: T) -> Cursor<T> {
Cursor { pos: 0, inner }
}
/// Consumes this cursor, returning the underlying value.
///
/// # Examples
///
/// ```
/// use std::io::Cursor;
///
/// let buff = Cursor::new(Vec::new());
/// # fn force_inference(_: &Cursor<Vec<u8>>) {}
/// # force_inference(&buff);
///
/// let vec = buff.into_inner();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn into_inner(self) -> T {
self.inner
}
/// Gets a reference to the underlying value in this cursor.
///
/// # Examples
///
/// ```
/// use std::io::Cursor;
///
/// let buff = Cursor::new(Vec::new());
/// # fn force_inference(_: &Cursor<Vec<u8>>) {}
/// # force_inference(&buff);
///
/// let reference = buff.get_ref();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
pub const fn get_ref(&self) -> &T {
&self.inner
}
/// Gets a mutable reference to the underlying value in this cursor.
///
/// Care should be taken to avoid modifying the internal I/O state of the
/// underlying value as it may corrupt this cursor's position.
///
/// # Examples
///
/// ```
/// use std::io::Cursor;
///
/// let mut buff = Cursor::new(Vec::new());
/// # fn force_inference(_: &Cursor<Vec<u8>>) {}
/// # force_inference(&buff);
///
/// let reference = buff.get_mut();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn get_mut(&mut self) -> &mut T {
&mut self.inner
}
/// Returns the current position of this cursor.
///
/// # Examples
///
/// ```
/// use std::io::Cursor;
/// use std::io::prelude::*;
/// use std::io::SeekFrom;
///
/// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
///
/// assert_eq!(buff.position(), 0);
///
/// buff.seek(SeekFrom::Current(2)).unwrap();
/// assert_eq!(buff.position(), 2);
///
/// buff.seek(SeekFrom::Current(-1)).unwrap();
/// assert_eq!(buff.position(), 1);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
pub const fn position(&self) -> u64 {
self.pos
}
/// Sets the position of this cursor.
///
/// # Examples
///
/// ```
/// use std::io::Cursor;
///
/// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
///
/// assert_eq!(buff.position(), 0);
///
/// buff.set_position(2);
/// assert_eq!(buff.position(), 2);
///
/// buff.set_position(4);
/// assert_eq!(buff.position(), 4);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn set_position(&mut self, pos: u64) {
self.pos = pos;
}
}
impl<T> Cursor<T>
where
T: AsRef<[u8]>,
{
/// Returns the remaining slice.
///
/// # Examples
///
/// ```
/// #![feature(cursor_remaining)]
/// use std::io::Cursor;
///
/// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
///
/// assert_eq!(buff.remaining_slice(), &[1, 2, 3, 4, 5]);
///
/// buff.set_position(2);
/// assert_eq!(buff.remaining_slice(), &[3, 4, 5]);
///
/// buff.set_position(4);
/// assert_eq!(buff.remaining_slice(), &[5]);
///
/// buff.set_position(6);
/// assert_eq!(buff.remaining_slice(), &[]);
/// ```
#[unstable(feature = "cursor_remaining", issue = "86369")]
pub fn remaining_slice(&self) -> &[u8] {
let len = self.pos.min(self.inner.as_ref().len() as u64);
&self.inner.as_ref()[(len as usize)..]
}
/// Returns `true` if the remaining slice is empty.
///
/// # Examples
///
/// ```
/// #![feature(cursor_remaining)]
/// use std::io::Cursor;
///
/// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
///
/// buff.set_position(2);
/// assert!(!buff.is_empty());
///
/// buff.set_position(5);
/// assert!(buff.is_empty());
///
/// buff.set_position(10);
/// assert!(buff.is_empty());
/// ```
#[unstable(feature = "cursor_remaining", issue = "86369")]
pub fn is_empty(&self) -> bool {
self.pos >= self.inner.as_ref().len() as u64
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for Cursor<T>
where
T: Clone,
{
#[inline]
fn clone(&self) -> Self {
Cursor { inner: self.inner.clone(), pos: self.pos }
}
#[inline]
fn clone_from(&mut self, other: &Self) {
self.inner.clone_from(&other.inner);
self.pos = other.pos;
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> io::Seek for Cursor<T>
where
T: AsRef<[u8]>,
{
fn seek(&mut self, style: SeekFrom) -> io::Result<u64> {
let (base_pos, offset) = match style {
SeekFrom::Start(n) => {
self.pos = n;
return Ok(n);
}
SeekFrom::End(n) => (self.inner.as_ref().len() as u64, n),
SeekFrom::Current(n) => (self.pos, n),
};
match base_pos.checked_add_signed(offset) {
Some(n) => {
self.pos = n;
Ok(self.pos)
}
None => Err(io::const_io_error!(
ErrorKind::InvalidInput,
"invalid seek to a negative or overflowing position",
)),
}
}
fn stream_len(&mut self) -> io::Result<u64> {
Ok(self.inner.as_ref().len() as u64)
}
fn stream_position(&mut self) -> io::Result<u64> {
Ok(self.pos)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Read for Cursor<T>
where
T: AsRef<[u8]>,
{
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
let n = Read::read(&mut self.remaining_slice(), buf)?;
self.pos += n as u64;
Ok(n)
}
fn read_buf(&mut self, mut cursor: BorrowedCursor<'_>) -> io::Result<()> {
let prev_written = cursor.written();
Read::read_buf(&mut self.fill_buf()?, cursor.reborrow())?;
self.pos += (cursor.written() - prev_written) as u64;
Ok(())
}
fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
let mut nread = 0;
for buf in bufs {
let n = self.read(buf)?;
nread += n;
if n < buf.len() {
break;
}
}
Ok(nread)
}
fn is_read_vectored(&self) -> bool {
true
}
fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
let n = buf.len();
Read::read_exact(&mut self.remaining_slice(), buf)?;
self.pos += n as u64;
Ok(())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> BufRead for Cursor<T>
where
T: AsRef<[u8]>,
{
fn fill_buf(&mut self) -> io::Result<&[u8]> {
Ok(self.remaining_slice())
}
fn consume(&mut self, amt: usize) {
self.pos += amt as u64;
}
}
// Non-resizing write implementation
#[inline]
fn slice_write(pos_mut: &mut u64, slice: &mut [u8], buf: &[u8]) -> io::Result<usize> {
let pos = cmp::min(*pos_mut, slice.len() as u64);
let amt = (&mut slice[(pos as usize)..]).write(buf)?;
*pos_mut += amt as u64;
Ok(amt)
}
#[inline]
fn slice_write_vectored(
pos_mut: &mut u64,
slice: &mut [u8],
bufs: &[IoSlice<'_>],
) -> io::Result<usize> {
let mut nwritten = 0;
for buf in bufs {
let n = slice_write(pos_mut, slice, buf)?;
nwritten += n;
if n < buf.len() {
break;
}
}
Ok(nwritten)
}
/// Reserves the required space, and pads the vec with 0s if necessary.
fn reserve_and_pad<A: Allocator>(
pos_mut: &mut u64,
vec: &mut Vec<u8, A>,
buf_len: usize,
) -> io::Result<usize> {
let pos: usize = (*pos_mut).try_into().map_err(|_| {
io::const_io_error!(
ErrorKind::InvalidInput,
"cursor position exceeds maximum possible vector length",
)
})?;
// For safety reasons, we don't want these numbers to overflow
// otherwise our allocation won't be enough
let desired_cap = pos.saturating_add(buf_len);
if desired_cap > vec.capacity() {
// We want our vec's total capacity
// to have room for (pos+buf_len) bytes. Reserve allocates
// based on additional elements from the length, so we need to
// reserve the difference
vec.reserve(desired_cap - vec.len());
}
// Pad if pos is above the current len.
if pos > vec.len() {
let diff = pos - vec.len();
// Unfortunately, `resize()` would suffice but the optimiser does not
// realise the `reserve` it does can be eliminated. So we do it manually
// to eliminate that extra branch
let spare = vec.spare_capacity_mut();
debug_assert!(spare.len() >= diff);
// Safety: we have allocated enough capacity for this.
// And we are only writing, not reading
unsafe {
spare.get_unchecked_mut(..diff).fill(core::mem::MaybeUninit::new(0));
vec.set_len(pos);
}
}
Ok(pos)
}
/// Writes the slice to the vec without allocating
/// # Safety: vec must have buf.len() spare capacity
unsafe fn vec_write_unchecked<A>(pos: usize, vec: &mut Vec<u8, A>, buf: &[u8]) -> usize
where
A: Allocator,
{
debug_assert!(vec.capacity() >= pos + buf.len());
vec.as_mut_ptr().add(pos).copy_from(buf.as_ptr(), buf.len());
pos + buf.len()
}
/// Resizing write implementation for [`Cursor`]
///
/// Cursor is allowed to have a pre-allocated and initialised
/// vector body, but with a position of 0. This means the [`Write`]
/// will overwrite the contents of the vec.
///
/// This also allows for the vec body to be empty, but with a position of N.
/// This means that [`Write`] will pad the vec with 0 initially,
/// before writing anything from that point
fn vec_write<A>(pos_mut: &mut u64, vec: &mut Vec<u8, A>, buf: &[u8]) -> io::Result<usize>
where
A: Allocator,
{
let buf_len = buf.len();
let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
// Write the buf then progress the vec forward if necessary
// Safety: we have ensured that the capacity is available
// and that all bytes get written up to pos
unsafe {
pos = vec_write_unchecked(pos, vec, buf);
if pos > vec.len() {
vec.set_len(pos);
}
};
// Bump us forward
*pos_mut += buf_len as u64;
Ok(buf_len)
}
/// Resizing write_vectored implementation for [`Cursor`]
///
/// Cursor is allowed to have a pre-allocated and initialised
/// vector body, but with a position of 0. This means the [`Write`]
/// will overwrite the contents of the vec.
///
/// This also allows for the vec body to be empty, but with a position of N.
/// This means that [`Write`] will pad the vec with 0 initially,
/// before writing anything from that point
fn vec_write_vectored<A>(
pos_mut: &mut u64,
vec: &mut Vec<u8, A>,
bufs: &[IoSlice<'_>],
) -> io::Result<usize>
where
A: Allocator,
{
// For safety reasons, we don't want this sum to overflow ever.
// If this saturates, the reserve should panic to avoid any unsound writing.
let buf_len = bufs.iter().fold(0usize, |a, b| a.saturating_add(b.len()));
let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
// Write the buf then progress the vec forward if necessary
// Safety: we have ensured that the capacity is available
// and that all bytes get written up to the last pos
unsafe {
for buf in bufs {
pos = vec_write_unchecked(pos, vec, buf);
}
if pos > vec.len() {
vec.set_len(pos);
}
}
// Bump us forward
*pos_mut += buf_len as u64;
Ok(buf_len)
}
#[stable(feature = "rust1", since = "1.0.0")]
impl Write for Cursor<&mut [u8]> {
#[inline]
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
slice_write(&mut self.pos, self.inner, buf)
}
#[inline]
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
slice_write_vectored(&mut self.pos, self.inner, bufs)
}
#[inline]
fn is_write_vectored(&self) -> bool {
true
}
#[inline]
fn flush(&mut self) -> io::Result<()> {
Ok(())
}
}
#[stable(feature = "cursor_mut_vec", since = "1.25.0")]
impl<A> Write for Cursor<&mut Vec<u8, A>>
where
A: Allocator,
{
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
vec_write(&mut self.pos, self.inner, buf)
}
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
vec_write_vectored(&mut self.pos, self.inner, bufs)
}
#[inline]
fn is_write_vectored(&self) -> bool {
true
}
#[inline]
fn flush(&mut self) -> io::Result<()> {
Ok(())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<A> Write for Cursor<Vec<u8, A>>
where
A: Allocator,
{
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
vec_write(&mut self.pos, &mut self.inner, buf)
}
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
vec_write_vectored(&mut self.pos, &mut self.inner, bufs)
}
#[inline]
fn is_write_vectored(&self) -> bool {
true
}
#[inline]
fn flush(&mut self) -> io::Result<()> {
Ok(())
}
}
#[stable(feature = "cursor_box_slice", since = "1.5.0")]
impl<A> Write for Cursor<Box<[u8], A>>
where
A: Allocator,
{
#[inline]
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
slice_write(&mut self.pos, &mut self.inner, buf)
}
#[inline]
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
}
#[inline]
fn is_write_vectored(&self) -> bool {
true
}
#[inline]
fn flush(&mut self) -> io::Result<()> {
Ok(())
}
}
#[stable(feature = "cursor_array", since = "1.61.0")]
impl<const N: usize> Write for Cursor<[u8; N]> {
#[inline]
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
slice_write(&mut self.pos, &mut self.inner, buf)
}
#[inline]
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
}
#[inline]
fn is_write_vectored(&self) -> bool {
true
}
#[inline]
fn flush(&mut self) -> io::Result<()> {
Ok(())
}
}