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use std::iter::{ExactSizeIterator, FusedIterator};
use crate::arena::{Arena, Index};
/// See [`Arena::drain`][Arena::drain].
pub struct Drain<'a, T> {
pub(crate) arena: &'a mut Arena<T>,
pub(crate) slot: u32,
}
impl<'a, T> Iterator for Drain<'a, T> {
type Item = (Index, T);
fn next(&mut self) -> Option<Self::Item> {
loop {
// If there are no entries remaining in the arena, we should always
// return None. Using this check instead of comparing with the
// arena's size allows us to skip any trailing empty entries.
if self.arena.is_empty() {
return None;
}
// slot may overflow if the arena's underlying storage contains more
// than 2^32 elements, but its internal length value was not
// changed, as it overflowing would panic before reaching this code.
let slot = self.slot;
self.slot = self
.slot
.checked_add(1)
.unwrap_or_else(|| panic!("Overflowed u32 trying to drain Arena"));
// If this entry is occupied, this method will mark it as an empty.
// Otherwise, we'll continue looping until we've drained all
// occupied entries from the arena.
if let Some((index, value)) = self.arena.remove_entry_by_slot(slot) {
return Some((index, value));
}
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
(self.arena.len(), Some(self.arena.len()))
}
}
impl<'a, T> FusedIterator for Drain<'a, T> {}
impl<'a, T> ExactSizeIterator for Drain<'a, T> {}
impl<'a, T> Drop for Drain<'a, T> {
// Continue iterating/dropping if there are any elements left.
fn drop(&mut self) {
self.for_each(drop);
}
}
#[cfg(test)]
mod test {
use crate::Arena;
use std::collections::HashSet;
#[test]
fn drain() {
let mut arena = Arena::with_capacity(2);
let one = arena.insert(1);
let two = arena.insert(2);
let mut drained_pairs = HashSet::new();
{
let mut drain = arena.drain();
assert_eq!(drain.size_hint(), (2, Some(2)));
drained_pairs.insert(drain.next().unwrap());
assert_eq!(drain.size_hint(), (1, Some(1)));
// Do not fully drain so we can ensure everything is dropped when the
// `Drain` is dropped.
assert_eq!(drain.size_hint(), (1, Some(1)));
}
assert_eq!(arena.len(), 0);
assert_eq!(arena.capacity(), 2);
assert_eq!(drained_pairs.len(), 1);
// We should still be able to use the arena after this.
let one_prime = arena.insert(1);
let two_prime = arena.insert(2);
assert_eq!(arena.len(), 2);
assert_eq!(arena.capacity(), 2);
assert_eq!(arena.get(one_prime), Some(&1));
assert_eq!(arena.get(two_prime), Some(&2));
assert_eq!(arena.get(one), None);
assert_eq!(arena.get(two), None);
}
}
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