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use core::fmt;
use core::mem::MaybeUninit;
use core::ops::Deref;
use core::ptr;
use core::slice;
/// A "history buffer", similar to a write-only ring buffer of fixed length.
///
/// This buffer keeps a fixed number of elements. On write, the oldest element
/// is overwritten. Thus, the buffer is useful to keep a history of values with
/// some desired depth, and for example calculate a rolling average.
///
/// # Examples
/// ```
/// use heapless::HistoryBuffer;
///
/// // Initialize a new buffer with 8 elements.
/// let mut buf = HistoryBuffer::<_, 8>::new();
///
/// // Starts with no data
/// assert_eq!(buf.recent(), None);
///
/// buf.write(3);
/// buf.write(5);
/// buf.extend(&[4, 4]);
///
/// // The most recent written element is a four.
/// assert_eq!(buf.recent(), Some(&4));
///
/// // To access all elements in an unspecified order, use `as_slice()`.
/// for el in buf.as_slice() { println!("{:?}", el); }
///
/// // Now we can prepare an average of all values, which comes out to 4.
/// let avg = buf.as_slice().iter().sum::<usize>() / buf.len();
/// assert_eq!(avg, 4);
/// ```
pub struct HistoryBuffer<T, const N: usize> {
data: [MaybeUninit<T>; N],
write_at: usize,
filled: bool,
}
impl<T, const N: usize> HistoryBuffer<T, N> {
const INIT: MaybeUninit<T> = MaybeUninit::uninit();
/// Constructs a new history buffer.
///
/// The construction of a `HistoryBuffer` works in `const` contexts.
///
/// # Examples
///
/// ```
/// use heapless::HistoryBuffer;
///
/// // Allocate a 16-element buffer on the stack
/// let x: HistoryBuffer<u8, 16> = HistoryBuffer::new();
/// assert_eq!(x.len(), 0);
/// ```
#[inline]
pub const fn new() -> Self {
// Const assert
crate::sealed::greater_than_0::<N>();
Self {
data: [Self::INIT; N],
write_at: 0,
filled: false,
}
}
/// Clears the buffer, replacing every element with the default value of
/// type `T`.
pub fn clear(&mut self) {
*self = Self::new();
}
}
impl<T, const N: usize> HistoryBuffer<T, N>
where
T: Copy + Clone,
{
/// Constructs a new history buffer, where every element is the given value.
///
/// # Examples
///
/// ```
/// use heapless::HistoryBuffer;
///
/// // Allocate a 16-element buffer on the stack
/// let mut x: HistoryBuffer<u8, 16> = HistoryBuffer::new_with(4);
/// // All elements are four
/// assert_eq!(x.as_slice(), [4; 16]);
/// ```
#[inline]
pub fn new_with(t: T) -> Self {
Self {
data: [MaybeUninit::new(t); N],
write_at: 0,
filled: true,
}
}
/// Clears the buffer, replacing every element with the given value.
pub fn clear_with(&mut self, t: T) {
*self = Self::new_with(t);
}
}
impl<T, const N: usize> HistoryBuffer<T, N> {
/// Returns the current fill level of the buffer.
#[inline]
pub fn len(&self) -> usize {
if self.filled {
N
} else {
self.write_at
}
}
/// Returns the capacity of the buffer, which is the length of the
/// underlying backing array.
#[inline]
pub fn capacity(&self) -> usize {
N
}
/// Writes an element to the buffer, overwriting the oldest value.
pub fn write(&mut self, t: T) {
if self.filled {
// Drop the old before we overwrite it.
unsafe { ptr::drop_in_place(self.data[self.write_at].as_mut_ptr()) }
}
self.data[self.write_at] = MaybeUninit::new(t);
self.write_at += 1;
if self.write_at == self.capacity() {
self.write_at = 0;
self.filled = true;
}
}
/// Clones and writes all elements in a slice to the buffer.
///
/// If the slice is longer than the buffer, only the last `self.len()`
/// elements will actually be stored.
pub fn extend_from_slice(&mut self, other: &[T])
where
T: Clone,
{
for item in other {
self.write(item.clone());
}
}
/// Returns a reference to the most recently written value.
///
/// # Examples
///
/// ```
/// use heapless::HistoryBuffer;
///
/// let mut x: HistoryBuffer<u8, 16> = HistoryBuffer::new();
/// x.write(4);
/// x.write(10);
/// assert_eq!(x.recent(), Some(&10));
/// ```
pub fn recent(&self) -> Option<&T> {
if self.write_at == 0 {
if self.filled {
Some(unsafe { &*self.data[self.capacity() - 1].as_ptr() })
} else {
None
}
} else {
Some(unsafe { &*self.data[self.write_at - 1].as_ptr() })
}
}
/// Returns the array slice backing the buffer, without keeping track
/// of the write position. Therefore, the element order is unspecified.
pub fn as_slice(&self) -> &[T] {
unsafe { slice::from_raw_parts(self.data.as_ptr() as *const _, self.len()) }
}
/// Returns a pair of slices which contain, in order, the contents of the buffer.
///
/// # Examples
///
/// ```
/// use heapless::HistoryBuffer;
///
/// let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
/// buffer.extend([0, 0, 0]);
/// buffer.extend([1, 2, 3, 4, 5, 6]);
/// assert_eq!(buffer.as_slices(), (&[1, 2, 3][..], &[4, 5, 6][..]));
/// ```
pub fn as_slices(&self) -> (&[T], &[T]) {
let buffer = self.as_slice();
if !self.filled {
(buffer, &[])
} else {
(&buffer[self.write_at..], &buffer[..self.write_at])
}
}
/// Returns an iterator for iterating over the buffer from oldest to newest.
///
/// # Examples
///
/// ```
/// use heapless::HistoryBuffer;
///
/// let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
/// buffer.extend([0, 0, 0, 1, 2, 3, 4, 5, 6]);
/// let expected = [1, 2, 3, 4, 5, 6];
/// for (x, y) in buffer.oldest_ordered().zip(expected.iter()) {
/// assert_eq!(x, y)
/// }
///
/// ```
pub fn oldest_ordered<'a>(&'a self) -> OldestOrdered<'a, T, N> {
if self.filled {
OldestOrdered {
buf: self,
cur: self.write_at,
wrapped: false,
}
} else {
// special case: act like we wrapped already to handle empty buffer.
OldestOrdered {
buf: self,
cur: 0,
wrapped: true,
}
}
}
}
impl<T, const N: usize> Extend<T> for HistoryBuffer<T, N> {
fn extend<I>(&mut self, iter: I)
where
I: IntoIterator<Item = T>,
{
for item in iter.into_iter() {
self.write(item);
}
}
}
impl<'a, T, const N: usize> Extend<&'a T> for HistoryBuffer<T, N>
where
T: 'a + Clone,
{
fn extend<I>(&mut self, iter: I)
where
I: IntoIterator<Item = &'a T>,
{
self.extend(iter.into_iter().cloned())
}
}
impl<T, const N: usize> Clone for HistoryBuffer<T, N>
where
T: Clone,
{
fn clone(&self) -> Self {
let mut ret = Self::new();
for (new, old) in ret.data.iter_mut().zip(self.as_slice()) {
new.write(old.clone());
}
ret.filled = self.filled;
ret.write_at = self.write_at;
ret
}
}
impl<T, const N: usize> Drop for HistoryBuffer<T, N> {
fn drop(&mut self) {
unsafe {
ptr::drop_in_place(ptr::slice_from_raw_parts_mut(
self.data.as_mut_ptr() as *mut T,
self.len(),
))
}
}
}
impl<T, const N: usize> Deref for HistoryBuffer<T, N> {
type Target = [T];
fn deref(&self) -> &[T] {
self.as_slice()
}
}
impl<T, const N: usize> AsRef<[T]> for HistoryBuffer<T, N> {
#[inline]
fn as_ref(&self) -> &[T] {
self
}
}
impl<T, const N: usize> fmt::Debug for HistoryBuffer<T, N>
where
T: fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
<[T] as fmt::Debug>::fmt(self, f)
}
}
impl<T, const N: usize> Default for HistoryBuffer<T, N> {
fn default() -> Self {
Self::new()
}
}
impl<T, const N: usize> PartialEq for HistoryBuffer<T, N>
where
T: PartialEq,
{
fn eq(&self, other: &Self) -> bool {
self.oldest_ordered().eq(other.oldest_ordered())
}
}
/// An iterator on the underlying buffer ordered from oldest data to newest
#[derive(Clone)]
pub struct OldestOrdered<'a, T, const N: usize> {
buf: &'a HistoryBuffer<T, N>,
cur: usize,
wrapped: bool,
}
impl<'a, T, const N: usize> Iterator for OldestOrdered<'a, T, N> {
type Item = &'a T;
fn next(&mut self) -> Option<&'a T> {
if self.cur == self.buf.len() && self.buf.filled {
// roll-over
self.cur = 0;
self.wrapped = true;
}
if self.cur == self.buf.write_at && self.wrapped {
return None;
}
let item = &self.buf[self.cur];
self.cur += 1;
Some(item)
}
}
#[cfg(test)]
mod tests {
use crate::HistoryBuffer;
use core::fmt::Debug;
use core::sync::atomic::{AtomicUsize, Ordering};
#[test]
fn new() {
let x: HistoryBuffer<u8, 4> = HistoryBuffer::new_with(1);
assert_eq!(x.len(), 4);
assert_eq!(x.as_slice(), [1; 4]);
assert_eq!(*x, [1; 4]);
let x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
assert_eq!(x.as_slice(), []);
}
#[test]
fn write() {
let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
x.write(1);
x.write(4);
assert_eq!(x.as_slice(), [1, 4]);
x.write(5);
x.write(6);
x.write(10);
assert_eq!(x.as_slice(), [10, 4, 5, 6]);
x.extend([11, 12].iter());
assert_eq!(x.as_slice(), [10, 11, 12, 6]);
}
#[test]
fn clear() {
let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new_with(1);
x.clear();
assert_eq!(x.as_slice(), []);
let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
x.clear_with(1);
assert_eq!(x.as_slice(), [1; 4]);
}
#[test]
fn clone() {
let mut x: HistoryBuffer<u8, 3> = HistoryBuffer::new();
for i in 0..10 {
assert_eq!(x.as_slice(), x.clone().as_slice());
x.write(i);
}
// Records number of clones locally and globally.
static GLOBAL: AtomicUsize = AtomicUsize::new(0);
#[derive(Default, PartialEq, Debug)]
struct InstrumentedClone(usize);
impl Clone for InstrumentedClone {
fn clone(&self) -> Self {
GLOBAL.fetch_add(1, Ordering::Relaxed);
Self(self.0 + 1)
}
}
let mut y: HistoryBuffer<InstrumentedClone, 2> = HistoryBuffer::new();
let _ = y.clone();
assert_eq!(GLOBAL.load(Ordering::Relaxed), 0);
y.write(InstrumentedClone(0));
assert_eq!(GLOBAL.load(Ordering::Relaxed), 0);
assert_eq!(y.clone().as_slice(), [InstrumentedClone(1)]);
assert_eq!(GLOBAL.load(Ordering::Relaxed), 1);
y.write(InstrumentedClone(0));
assert_eq!(GLOBAL.load(Ordering::Relaxed), 1);
assert_eq!(
y.clone().as_slice(),
[InstrumentedClone(1), InstrumentedClone(1)]
);
assert_eq!(GLOBAL.load(Ordering::Relaxed), 3);
assert_eq!(
y.clone().clone().clone().as_slice(),
[InstrumentedClone(3), InstrumentedClone(3)]
);
assert_eq!(GLOBAL.load(Ordering::Relaxed), 9);
}
#[test]
fn recent() {
let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
assert_eq!(x.recent(), None);
x.write(1);
x.write(4);
assert_eq!(x.recent(), Some(&4));
x.write(5);
x.write(6);
x.write(10);
assert_eq!(x.recent(), Some(&10));
}
#[test]
fn as_slice() {
let mut x: HistoryBuffer<u8, 4> = HistoryBuffer::new();
assert_eq!(x.as_slice(), []);
x.extend([1, 2, 3, 4, 5].iter());
assert_eq!(x.as_slice(), [5, 2, 3, 4]);
}
/// Test whether .as_slices() behaves as expected.
#[test]
fn as_slices() {
let mut buffer: HistoryBuffer<u8, 4> = HistoryBuffer::new();
let mut extend_then_assert = |extend: &[u8], assert: (&[u8], &[u8])| {
buffer.extend(extend);
assert_eq!(buffer.as_slices(), assert);
};
extend_then_assert(b"a", (b"a", b""));
extend_then_assert(b"bcd", (b"abcd", b""));
extend_then_assert(b"efg", (b"d", b"efg"));
extend_then_assert(b"h", (b"efgh", b""));
extend_then_assert(b"123456", (b"34", b"56"));
}
/// Test whether .as_slices() and .oldest_ordered() produce elements in the same order.
#[test]
fn as_slices_equals_ordered() {
let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
for n in 0..20 {
buffer.write(n);
let (head, tail) = buffer.as_slices();
assert_eq_iter(
[head, tail].iter().copied().flatten(),
buffer.oldest_ordered(),
)
}
}
#[test]
fn ordered() {
// test on an empty buffer
let buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
let mut iter = buffer.oldest_ordered();
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
// test on a un-filled buffer
let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
buffer.extend([1, 2, 3]);
assert_eq!(buffer.len(), 3);
assert_eq_iter(buffer.oldest_ordered(), &[1, 2, 3]);
// test on a filled buffer
let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
buffer.extend([0, 0, 0, 1, 2, 3, 4, 5, 6]);
assert_eq!(buffer.len(), 6);
assert_eq_iter(buffer.oldest_ordered(), &[1, 2, 3, 4, 5, 6]);
// comprehensive test all cases
for n in 0..50 {
const N: usize = 7;
let mut buffer: HistoryBuffer<u8, N> = HistoryBuffer::new();
buffer.extend(0..n);
assert_eq_iter(
buffer.oldest_ordered().copied(),
n.saturating_sub(N as u8)..n,
);
}
}
/// Compares two iterators item by item, making sure they stop at the same time.
fn assert_eq_iter<I: Eq + Debug>(
a: impl IntoIterator<Item = I>,
b: impl IntoIterator<Item = I>,
) {
let mut a = a.into_iter();
let mut b = b.into_iter();
let mut i = 0;
loop {
let a_item = a.next();
let b_item = b.next();
assert_eq!(a_item, b_item, "{}", i);
i += 1;
if b_item.is_none() {
break;
}
}
}
#[test]
fn partial_eq() {
let mut x: HistoryBuffer<u8, 3> = HistoryBuffer::new();
let mut y: HistoryBuffer<u8, 3> = HistoryBuffer::new();
assert_eq!(x, y);
x.write(1);
assert_ne!(x, y);
y.write(1);
assert_eq!(x, y);
for _ in 0..4 {
x.write(2);
assert_ne!(x, y);
for i in 0..5 {
x.write(i);
y.write(i);
}
assert_eq!(
x,
y,
"{:?} {:?}",
x.iter().collect::<Vec<_>>(),
y.iter().collect::<Vec<_>>()
);
}
}
}