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use magic_buffer::MagicBuffer;
use std::cell::Cell;
/// Buffer is a FIFO queue for bytes, built on a ring buffer.
/// It always provides contiguous slices for both the readable and writable parts,
/// using an underlying buffer that is "mirrored" in virtual memory.
pub struct Buffer {
buffer: MagicBuffer,
/// first readable byte
head: Cell<usize>,
/// first writable byte
tail: usize,
}
impl Buffer {
/// Allocate a fresh buffer, with the specified capacity.
/// The buffer can contain at most `capacity - 1` bytes.
/// The capacity must be a power of two, and at least [Buffer::min_len].
pub fn new(capacity: usize) -> Buffer {
Buffer {
// MagicBuffer::new verifies that `capacity` is a power of two,
// and at least MagicBuffer::min_len().
buffer: MagicBuffer::new(capacity).unwrap(),
// `head == tail` means the buffer is empty.
// In order to ensure that this remains unambiguous,
// the buffer can only be filled with capacity-1 bytes.
head: Cell::new(0),
tail: 0,
}
}
/// Returns the minimum buffer capacity.
/// This depends on the operating system and architecture.
pub fn min_capacity() -> usize {
MagicBuffer::min_len()
}
/// Return the capacity of the buffer.
/// This is equal to `self.data().len() + self.space().len() + 1`.
pub fn capacity(&self) -> usize {
self.buffer.len()
}
/// Return the valid, readable data in the buffer.
pub fn data(&self) -> &[u8] {
let len = self.buffer.len();
let head = self.head.get();
if head <= self.tail {
&self.buffer[head..self.tail]
} else {
&self.buffer[head..self.tail + len]
}
}
/// Mark `read_len` bytes of the readable data as consumed, freeing the space.
pub fn consume(&self, read_len: usize) {
debug_assert!(read_len <= self.data().len());
let mut head = self.head.get();
head += read_len;
head &= self.buffer.len() - 1;
self.head.set(head);
}
/// Return the empty, writable space in the buffer.
pub fn space(&mut self) -> &mut [u8] {
let len = self.buffer.len();
let head = self.head.get();
if head <= self.tail {
&mut self.buffer[self.tail..head + len - 1]
} else {
&mut self.buffer[self.tail..head - 1]
}
}
/// Mark `written_len` bytes of the writable space as valid, readable data.
pub fn commit(&mut self, written_len: usize) {
debug_assert!(written_len <= self.space().len());
self.tail += written_len;
self.tail &= self.buffer.len() - 1;
}
}
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