cursor.rs - source
Description: Source of the Rust file `library/std/src/io/cursor.rs`.
Keywords: rust, rustlang, rust-lang

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    #[cfg(test)]
    mod tests;

    use crate::io::prelude::*;

    use crate::cmp;
    use crate::io::{self, Error, ErrorKind, Initializer, IoSlice, IoSliceMut, SeekFrom};

    use core::convert::TryInto;

    /// A `Cursor` wraps an in-memory buffer and provides it with a
    /// [`Seek`] implementation.
    ///
    /// `Cursor`s are used with in-memory buffers, anything implementing
    /// [`AsRef`]`<[u8]>`, 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 `Cursor<`[`Vec`]`<u8>>` and
    /// `Cursor<`[`&[u8]`][bytes]`>`.
    ///
    /// # 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
    /// [`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>(writer: &mut 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(Clone, 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;
        }
    }

    #[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),
            };
            let new_pos = if offset >= 0 {
                base_pos.checked_add(offset as u64)
            } else {
                base_pos.checked_sub((offset.wrapping_neg()) as u64)
            };
            match new_pos {
                Some(n) => {
                    self.pos = n;
                    Ok(self.pos)
                }
                None => Err(Error::new(
                    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.fill_buf()?, buf)?;
            self.pos += n as u64;
            Ok(n)
        }

        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.fill_buf()?, buf)?;
            self.pos += n as u64;
            Ok(())
        }

        #[inline]
        unsafe fn initializer(&self) -> Initializer {
            Initializer::nop()
        }
    }

    #[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]> {
            let amt = cmp::min(self.pos, self.inner.as_ref().len() as u64);
            Ok(&self.inner.as_ref()[(amt as usize)..])
        }
        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)
    }

    // Resizing write implementation
    fn vec_write(pos_mut: &mut u64, vec: &mut Vec<u8>, buf: &[u8]) -> io::Result<usize> {
        let pos: usize = (*pos_mut).try_into().map_err(|_| {
            Error::new(
                ErrorKind::InvalidInput,
                "cursor position exceeds maximum possible vector length",
            )
        })?;
        // Make sure the internal buffer is as least as big as where we
        // currently are
        let len = vec.len();
        if len < pos {
            // use `resize` so that the zero filling is as efficient as possible
            vec.resize(pos, 0);
        }
        // Figure out what bytes will be used to overwrite what's currently
        // there (left), and what will be appended on the end (right)
        {
            let space = vec.len() - pos;
            let (left, right) = buf.split_at(cmp::min(space, buf.len()));
            vec[pos..pos + left.len()].copy_from_slice(left);
            vec.extend_from_slice(right);
        }

        // Bump us forward
        *pos_mut = (pos + buf.len()) as u64;
        Ok(buf.len())
    }

    fn vec_write_vectored(
        pos_mut: &mut u64,
        vec: &mut Vec<u8>,
        bufs: &[IoSlice<'_>],
    ) -> io::Result<usize> {
        let mut nwritten = 0;
        for buf in bufs {
            nwritten += vec_write(pos_mut, vec, buf)?;
        }
        Ok(nwritten)
    }

    #[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 Write for Cursor<&mut Vec<u8>> {
        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 Write for Cursor<Vec<u8>> {
        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 Write for Cursor<Box<[u8]>> {
        #[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(())
        }
    }