sgx_tstd/src/io/cursor.rs

// Copyright (C) 2017-2018 Baidu, Inc. All Rights Reserved.
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use io::prelude::*;
use io::{self, Initializer, SeekFrom, Error, ErrorKind};
use core::cmp;

/// A `Cursor` wraps another type and provides it with a
/// [`Seek`] implementation.
///
/// `Cursor`s are typically used with in-memory buffers 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]`>`.
///

#[derive(Clone, Debug)]
pub struct Cursor<T> {
    inner: T,
    pos: u64,
}

impl<T> Cursor<T> {
    /// Creates a new cursor wrapping the provided underlying I/O object.
    ///
    /// Cursor initial position is `0` even if underlying object (e.
    /// g. `Vec`) is not empty. So writing to cursor starts with
    /// overwriting `Vec` content, not with appending to it.
    ///
    pub fn new(inner: T) -> Cursor<T> {
        Cursor { pos: 0, inner: inner }
    }

    /// Consumes this cursor, returning the underlying value.
    pub fn into_inner(self) -> T { self.inner }

    /// Gets a reference to the underlying value in this cursor.
    pub 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.
    pub fn get_mut(&mut self) -> &mut T { &mut self.inner }

    /// Returns the current position of this cursor.
    pub fn position(&self) -> u64 { self.pos }

    /// Sets the position of this cursor.
    pub fn set_position(&mut self, pos: u64) { self.pos = pos; }
}

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"))
        }
    }
}

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_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()
    }
}

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
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)
}

/// Compensate removal of some impls per
#[cfg(any(target_pointer_width = "16",
          target_pointer_width = "32"))]
fn try_into(n: u64) -> Result<usize, ()> {
    if n <= (<usize>::max_value() as u64) {
        Ok(n as usize)
    } else {
        Err(())
    }
}

#[cfg(any(target_pointer_width = "64"))]
fn try_into(n: u64) -> Result<usize, ()> {
    Ok(n as usize)
}

// Resizing write implementation
fn vec_write(pos_mut: &mut u64, vec: &mut Vec<u8>, buf: &[u8]) -> io::Result<usize> {
    let pos: usize = try_into(*pos_mut).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())
}

impl<'a> Write for Cursor<&'a mut [u8]> {
    #[inline]
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        slice_write(&mut self.pos, self.inner, buf)
    }
    fn flush(&mut self) -> io::Result<()> { Ok(()) }
}

impl<'a> Write for Cursor<&'a mut Vec<u8>> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        vec_write(&mut self.pos, self.inner, buf)
    }
    fn flush(&mut self) -> io::Result<()> { Ok(()) }
}

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 flush(&mut self) -> io::Result<()> { Ok(()) }
}

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)
    }
    fn flush(&mut self) -> io::Result<()> { Ok(()) }
}