use std::mem;
pub enum Slot<T> {
Empty,
Filled(T),
}
pub struct FreeList<T> {
entries: Vec<Slot<T>>,
split: usize,
capacity: usize,
}
impl<T> FreeList<T> {
pub fn with_capacity(capacity: usize) -> FreeList<T> {
let entries = (0..capacity).map(|_| Slot::Empty).collect::<Vec<_>>();
FreeList {
entries,
split: 0,
capacity,
}
}
pub fn pop(&mut self) -> Option<T> {
let idx = self.split;
if idx == 0 {
None
} else {
match mem::replace(&mut self.entries[idx - 1], Slot::Empty) {
Slot::Filled(v) => {
self.split = idx - 1;
Some(v)
}
_ => panic!("FreeList is corrupt"),
}
}
}
pub fn insert(&mut self, val: T) -> Option<T> {
let next = self.split + 1;
if next < self.capacity {
self.entries[self.split] = Slot::Filled(val);
self.split = next;
None
} else {
Some(val)
}
}
}