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///! An encapsulation of `BufReader`'s buffer management logic.
///
/// This module factors out the basic functionality of `BufReader` in order to protect two core
/// invariants:
/// * `filled` bytes of `buf` are always initialized
/// * `pos` is always <= `filled`
/// Since this module encapsulates the buffer management logic, we can ensure that the range
/// `pos..filled` is always a valid index into the initialized region of the buffer. This means
/// that user code which wants to do reads from a `BufReader` via `buffer` + `consume` can do so
/// without encountering any runtime bounds checks.
use crate::cmp;
use crate::io::{self, Read, ReadBuf};
use crate::mem::MaybeUninit;
pub struct Buffer {
// The buffer.
buf: Box<[MaybeUninit<u8>]>,
// The current seek offset into `buf`, must always be <= `filled`.
pos: usize,
// Each call to `fill_buf` sets `filled` to indicate how many bytes at the start of `buf` are
// initialized with bytes from a read.
filled: usize,
}
impl Buffer {
#[inline]
pub fn with_capacity(capacity: usize) -> Self {
let buf = Box::new_uninit_slice(capacity);
Self { buf, pos: 0, filled: 0 }
}
#[inline]
pub fn buffer(&self) -> &[u8] {
// SAFETY: self.pos and self.cap are valid, and self.cap => self.pos, and
// that region is initialized because those are all invariants of this type.
unsafe { MaybeUninit::slice_assume_init_ref(self.buf.get_unchecked(self.pos..self.filled)) }
}
#[inline]
pub fn capacity(&self) -> usize {
self.buf.len()
}
#[inline]
pub fn filled(&self) -> usize {
self.filled
}
#[inline]
pub fn pos(&self) -> usize {
self.pos
}
#[inline]
pub fn discard_buffer(&mut self) {
self.pos = 0;
self.filled = 0;
}
#[inline]
pub fn consume(&mut self, amt: usize) {
self.pos = cmp::min(self.pos + amt, self.filled);
}
/// If there are `amt` bytes available in the buffer, pass a slice containing those bytes to
/// `visitor` and return true. If there are not enough bytes available, return false.
#[inline]
pub fn consume_with<V>(&mut self, amt: usize, mut visitor: V) -> bool
where
V: FnMut(&[u8]),
{
if let Some(claimed) = self.buffer().get(..amt) {
visitor(claimed);
// If the indexing into self.buffer() succeeds, amt must be a valid increment.
self.pos += amt;
true
} else {
false
}
}
#[inline]
pub fn unconsume(&mut self, amt: usize) {
self.pos = self.pos.saturating_sub(amt);
}
#[inline]
pub fn fill_buf(&mut self, mut reader: impl Read) -> io::Result<&[u8]> {
// If we've reached the end of our internal buffer then we need to fetch
// some more data from the reader.
// Branch using `>=` instead of the more correct `==`
// to tell the compiler that the pos..cap slice is always valid.
if self.pos >= self.filled {
debug_assert!(self.pos == self.filled);
let mut readbuf = ReadBuf::uninit(&mut self.buf);
reader.read_buf(&mut readbuf)?;
self.filled = readbuf.filled_len();
self.pos = 0;
}
Ok(self.buffer())
}
}
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