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// Copyright (c) 2024-present, fjall-rs
// This source code is licensed under both the Apache 2.0 and MIT License
// (found in the LICENSE-* files in the repository)
//! A fork of <https://github.com/dodomorandi/double-ended-peekable>
//! to allow accessing the inner type
//!
//! Also changes the generics a bit so it plays well with `self_cell`.
use core::{fmt::Debug, hash::Hash, hint::unreachable_unchecked, mem};
/// An _extension trait_ to create [`DoubleEndedPeekable`].
///
/// This has a blanket implementation for all types that implement [`Iterator`].
pub trait DoubleEndedPeekableExt<T, I: Iterator<Item = T>> {
/// Creates an iterator which works similarly to [`Peekable`], but also provides additional
/// functions if the underlying type implements [`DoubleEndedIterator`].
///
/// See [`DoubleEndedPeekable`] for more information.
///
/// [`Peekable`]: core::iter::Peekable
fn double_ended_peekable(self) -> DoubleEndedPeekable<T, I>;
}
impl<T, I> DoubleEndedPeekableExt<T, I> for I
where
I: Iterator<Item = T>,
{
#[inline]
fn double_ended_peekable(self) -> DoubleEndedPeekable<T, I> {
DoubleEndedPeekable {
iter: self,
front: MaybePeeked::Unpeeked,
back: MaybePeeked::Unpeeked,
}
}
}
/// An advanced version of [`Peekable`] that works well with double-ended iterators.
///
/// This `struct` is created by the [`double_ended_peekable`] method on [`DoubleEndedPeekableExt`].
///
/// [`Peekable`]: core::iter::Peekable
/// [`double_ended_peekable`]: DoubleEndedPeekableExt::double_ended_peekable
pub struct DoubleEndedPeekable<T, I: Iterator<Item = T>> {
iter: I,
front: MaybePeeked<T>,
back: MaybePeeked<T>,
}
impl<T, I> DoubleEndedPeekable<T, I>
where
I: Iterator<Item = T>,
{
pub(crate) fn reset_front_peeked(&mut self) {
self.front = MaybePeeked::Unpeeked;
}
pub(crate) fn reset_back_peeked(&mut self) {
self.back = MaybePeeked::Unpeeked;
}
pub fn inner_mut(&mut self) -> &mut I {
&mut self.iter
}
/// Returns a reference to the `next()` value without advancing the iterator.
///
/// See [`Peekable::peek`] for more information.
///
/// [`Peekable::peek`]: core::iter::Peekable::peek
#[inline]
pub fn peek(&mut self) -> Option<&I::Item> {
self.front
.get_peeked_or_insert_with(|| self.iter.next())
.as_ref()
.or_else(|| self.back.peeked_value_ref())
}
/// Consumes and returns the next value of this iterator if a condition is true.
///
/// See [`Peekable::next_if`] for more information.
///
/// [`Peekable::next_if`]: core::iter::Peekable::next_if
#[inline]
pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item> {
match self.next() {
Some(item) if func(&item) => Some(item),
other => {
debug_assert!(self.front.is_unpeeked());
self.front = MaybePeeked::Peeked(other);
None
}
}
}
}
impl<T, I> DoubleEndedPeekable<T, I>
where
I: DoubleEndedIterator<Item = T>,
{
/// Returns a reference to the `next_back()` value without advancing the _back_ of the iterator.
///
/// Like [`next_back`], if there is a value, it is wrapped in a `Some(T)`.
/// But if the iteration is over, `None` is returned.
///
/// [`next_back`]: DoubleEndedIterator::next_back
///
/// Because `peek_back()` returns a reference, and many iterators iterate over references,
/// there can be a possibly confusing situation where the return value is a double reference.
#[inline]
pub fn peek_back(&mut self) -> Option<&I::Item> {
self.back
.get_peeked_or_insert_with(|| self.iter.next_back())
.as_ref()
.or_else(|| self.front.peeked_value_ref())
}
}
impl<T, I> Iterator for DoubleEndedPeekable<T, I>
where
I: Iterator<Item = T>,
{
type Item = I::Item;
#[inline]
fn next(&mut self) -> Option<Self::Item> {
match self.front.take() {
MaybePeeked::Peeked(out @ Some(_)) => out,
MaybePeeked::Peeked(None) => self.back.take().into_peeked_value(),
MaybePeeked::Unpeeked => match self.iter.next() {
item @ Some(_) => item,
None => self.back.take().into_peeked_value(),
},
}
}
}
impl<T, I> DoubleEndedIterator for DoubleEndedPeekable<T, I>
where
I: DoubleEndedIterator<Item = T>,
{
#[inline]
fn next_back(&mut self) -> Option<Self::Item> {
match self.back.take() {
MaybePeeked::Peeked(out @ Some(_)) => out,
MaybePeeked::Peeked(None) => self.front.take().into_peeked_value(),
MaybePeeked::Unpeeked => match self.iter.next_back() {
out @ Some(_) => out,
None => self.front.take().into_peeked_value(),
},
}
}
}
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
enum MaybePeeked<T> {
#[default]
Unpeeked,
Peeked(Option<T>),
}
impl<T> MaybePeeked<T> {
fn get_peeked_or_insert_with<F>(&mut self, f: F) -> &mut Option<T>
where
F: FnOnce() -> Option<T>,
{
if matches!(self, Self::Unpeeked) {
*self = Self::Peeked(f());
}
let Self::Peeked(peeked) = self else {
// SAFETY: it cannot be `Unpeeked` because that case has been just replaced with
// `Peeked`, and we only have two possible states.
#[expect(unsafe_code, reason = "see safety")]
unsafe {
unreachable_unchecked()
}
};
peeked
}
const fn peeked_value_ref(&self) -> Option<&T> {
match self {
Self::Unpeeked | Self::Peeked(None) => None,
Self::Peeked(Some(peeked)) => Some(peeked),
}
}
fn take(&mut self) -> Self {
mem::replace(self, Self::Unpeeked)
}
fn into_peeked_value(self) -> Option<T> {
match self {
Self::Unpeeked | Self::Peeked(None) => None,
Self::Peeked(Some(peeked)) => Some(peeked),
}
}
const fn is_unpeeked(&self) -> bool {
matches!(self, Self::Unpeeked)
}
}