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use std::io::{self, IoSlice, IoSliceMut, Read, Write};
pub(super) struct RingBuffer {
storage: Box<[u8; Self::LEN]>,
// The start index of the non-empty section of the buffer.
start: usize,
// The length of the non-empty section of the buffer.
len: usize,
}
impl RingBuffer {
/// The size of the internal storage of the ring buffer.
pub(super) const LEN: usize = 8 * 1024;
/// Create a new, empty buffer.
pub(super) fn new() -> Self {
Self {
storage: Box::new([0; Self::LEN]),
start: 0,
len: 0,
}
}
pub(super) fn is_full(&self) -> bool {
self.len == self.storage.len()
}
pub(super) fn insert<R: Read>(&mut self, read: &mut R) -> io::Result<usize> {
let inserted_len = if self.is_empty() {
// Case 1.1. The buffer is empty, meaning that there are two unfilled slices in
// `storage`:`start..` and `..start`.
let (second_slice, first_slice) = self.storage.split_at_mut(self.start);
read.read_vectored(&mut [first_slice, second_slice].map(IoSliceMut::new))?
} else {
let &mut Self { start, len, .. } = self;
let end = start + len;
if end >= self.storage.len() {
// Case 1.2. The buffer is not empty and the filled section wraps around `storage`.
// Meaning that there is only one unfilled slice in `storage`: `end..start`.
let end = end % self.storage.len();
read.read(&mut self.storage[end..start])?
} else {
// Case 1.3. The buffer is not empty and the filled section is a contiguous slice
// of `storage`. Meaning that there are two unfilled slices in `storage`: `..start`
// and `end..`.
let (mid, first_slice) = self.storage.split_at_mut(end);
let second_slice = &mut mid[..start];
read.read_vectored(&mut [first_slice, second_slice].map(IoSliceMut::new))?
}
};
self.len += inserted_len;
debug_assert!(self.start < Self::LEN);
debug_assert!(self.len <= Self::LEN);
Ok(inserted_len)
}
pub(super) fn is_empty(&self) -> bool {
self.len == 0
}
pub(super) fn remove<W: Write>(&mut self, write: &mut W) -> io::Result<usize> {
let removed_len = if self.is_full() {
// Case 2.1. The buffer is full, meaning that there are two filled slices in `storage`:
// `start..` and `..start`.
let (second_slice, first_slice) = self.storage.split_at(self.start);
write.write_vectored(&[first_slice, second_slice].map(IoSlice::new))?
} else {
let end = self.start + self.len;
if end >= self.storage.len() {
// Case 2.2. The buffer is not full and the filled section wraps around `storage`.
// Meaning that there are two non-empty slices in `storage`: `start..` and `..end`.
let end = end % self.storage.len();
let first_slice = &self.storage[self.start..];
let second_slice = &self.storage[..end];
write.write_vectored(&[first_slice, second_slice].map(IoSlice::new))?
} else {
// Case 2.3. The buffer is not full and the filled section is a contiguous slice
// of `storage.` Meaning that there is only one filled slice in `storage`:
// `start..end`.
write.write(&self.storage[self.start..end])?
}
};
self.start += removed_len;
self.start %= Self::LEN;
self.len -= removed_len;
debug_assert!(self.start < Self::LEN);
debug_assert!(self.len <= Self::LEN);
Ok(removed_len)
}
}
#[cfg(test)]
mod tests {
use super::RingBuffer;
#[test]
fn empty_buffer_is_empty() {
let buf = RingBuffer::new();
assert!(buf.is_empty());
}
#[test]
fn full_buffer_is_full() {
let mut buf = RingBuffer::new();
let inserted_len = buf.insert(&mut [0x45; RingBuffer::LEN].as_slice()).unwrap();
assert_eq!(inserted_len, RingBuffer::LEN);
assert!(buf.is_full());
}
#[test]
fn buffer_is_fifo() {
let mut buf = RingBuffer::new();
let expected = (0..=u8::MAX).collect::<Vec<u8>>();
let inserted_len = buf.insert(&mut expected.as_slice()).unwrap();
assert_eq!(inserted_len, expected.len());
let mut found = vec![];
let removed_len = buf.remove(&mut found).unwrap();
assert_eq!(removed_len, expected.len());
assert_eq!(expected, found);
}
#[test]
fn insert_into_empty_buffer_with_offset() {
const HALF_LEN: usize = RingBuffer::LEN / 2;
let mut buf = RingBuffer::new();
// This should leave the buffer empty but with the start field pointing to the middle of
// the buffer.
// ┌───────────────────┐
// │ │
// └───────────────────┘
// ▲
// │
// start
buf.insert(&mut [0u8; HALF_LEN].as_slice()).unwrap();
buf.remove(&mut vec![]).unwrap();
assert_eq!(buf.start, HALF_LEN);
assert_eq!(buf.len, 0);
// Then we fill the first half of the buffer with ones and the second one with twos in a
// single insertion. This tests case 1.1.
// ┌─────────┬─────────┐
// │ 2 │ 1 │
// └─────────┴─────────┘
// ▲
// │
// start
let mut expected = vec![1; HALF_LEN];
expected.extend_from_slice(&[2; HALF_LEN]);
buf.insert(&mut expected.as_slice()).unwrap();
assert_eq!(buf.start, HALF_LEN);
assert_eq!(buf.len, RingBuffer::LEN);
// When we remove all the elements of the buffer we should find them in the same order we
// inserted them. This tests case 2.1.
let mut found = vec![];
let removed_len = buf.remove(&mut found).unwrap();
assert_eq!(removed_len, expected.len());
assert_eq!(expected, found);
assert_eq!(buf.start, HALF_LEN);
assert_eq!(buf.len, 0);
}
#[test]
fn insert_into_non_empty_wrapping_buffer() {
const QUARTER_LEN: usize = RingBuffer::LEN / 4;
let mut buf = RingBuffer::new();
// This should leave the buffer empty but with the start field pointing to the middle of
// the buffer.
// ┌───────────────────────┐
// │ │
// └───────────────────────┘
// ▲
// │
// start
buf.insert(&mut [0; 2 * QUARTER_LEN].as_slice()).unwrap();
buf.remove(&mut vec![]).unwrap();
assert_eq!(buf.start, 2 * QUARTER_LEN);
assert_eq!(buf.len, 0);
// Then we fill one quarter of the buffer with ones. This gives us a non-empty buffer whose
// empty section is not contiguous.
// ┌───────────┬─────┬─────┐
// │ │ 1 │ │
// └───────────┴─────┴─────┘
// ▲
// │
// start
let mut expected = vec![1; QUARTER_LEN];
buf.insert(&mut expected.as_slice()).unwrap();
assert_eq!(buf.start, 2 * QUARTER_LEN);
assert_eq!(buf.len, QUARTER_LEN);
// Then we fill one quarter of the buffer with twos and another quarter of the buffer with
// threes in a single insertion. This tests case 1.2.
// ┌─────┬─────┬─────┬─────┐
// │ 3 │ │ 1 │ 2 │
// └─────┴─────┴─────┴─────┘
// ▲
// │
// start
let mut second_half = vec![2; QUARTER_LEN];
second_half.extend_from_slice(&[3; QUARTER_LEN]);
buf.insert(&mut second_half.as_slice()).unwrap();
expected.extend(second_half);
assert_eq!(buf.start, 2 * QUARTER_LEN);
assert_eq!(buf.len, 3 * QUARTER_LEN);
// When we remove all the elements of the buffer we should find them in the same order we
// inserted them. This tests case 2.2.
let mut found = vec![];
let removed_len = buf.remove(&mut found).unwrap();
assert_eq!(removed_len, expected.len());
assert_eq!(expected, found);
assert_eq!(buf.start, QUARTER_LEN);
assert_eq!(buf.len, 0);
}
#[test]
fn insert_into_non_empty_non_wrapping_buffer() {
const QUARTER_LEN: usize = RingBuffer::LEN / 4;
let mut buf = RingBuffer::new();
// We fill one quarter of the buffer with ones. This gives us a non-empty buffer whose
// empty section is contiguous.
// ┌─────┬────────────────┐
// │ 1 │ │
// └─────┴────────────────┘
// ▲
// │
// └ start
let mut expected = vec![1; QUARTER_LEN];
buf.insert(&mut expected.as_slice()).unwrap();
assert_eq!(buf.start, 0);
assert_eq!(buf.len, QUARTER_LEN);
// Then we fill one quarter of the buffer with twos. This tests case 1.3.
// ┌─────┬─────┬──────────┐
// │ 1 │ 2 │ │
// └─────┴─────┴──────────┘
// ▲
// │
// └ start
let second_half = vec![2; QUARTER_LEN];
buf.insert(&mut second_half.as_slice()).unwrap();
expected.extend(second_half);
assert_eq!(buf.start, 0);
assert_eq!(buf.len, 2 * QUARTER_LEN);
// When we remove all the elements of the buffer we should find them in the same order we
// inserted them. This tests case 2.3.
let mut found = vec![];
let removed_len = buf.remove(&mut found).unwrap();
assert_eq!(removed_len, expected.len());
assert_eq!(expected, found);
assert_eq!(buf.start, 2 * QUARTER_LEN);
assert_eq!(buf.len, 0);
}
}