fffl 1.2.0

A contiguous First-fit Freelist
Documentation 
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A contiguous, growable, first-fit freelist. 

Freelist has *O*(1) indexing and push (to the first available free slot) and *O*(1) removal.

# Examples
```
use fffl::Freelist;

let mut fl = Freelist::new();
let idx1 = fl.push(1);
let idx2 = fl.push(2);
let idx3 = fl.push(3);

assert_eq!(idx2, 1);

assert_eq!(fl.filled(), 3);
assert_eq!(fl[0], 1);

assert_eq!(fl.remove(1), Some(2));
assert_eq!(fl.remove(1), None);

assert_eq!(fl.filled(), 2);
assert_eq!(fl.free(), 1);
assert_eq!(fl.size(), 3);

let first_free_idx = fl.push(7);
assert_eq!(first_free_idx, 1);

fl[1] = 5;
assert_eq!(fl[1], 5);
```
# Indexing

The `Freelist` type allows access to values by index. 

```
use fffl::Freelist;

let fl = Freelist::from([1, 3, 5, 7]);
println!("{}", fl[1]); // displays '3'
```
*Note:* if you try to access an index which has been previously freed or isn't in the `Freelist`, the software will panic! Example:

```should_panic
use fffl::Freelist;

let mut fl = Freelist::from([1, 3, 5, 7]);
fl.remove(2);
println!("{}", fl[2]); // PANIC
```
Use [`get`] and [`get_mut`] if you want to check whether the index contains a value.

# Slicing
`Freelist` cannot be sliced.  Doing so would require internally building a slice that skips freed slots, meaning the returned slice may not be of the same length.  Moreover the apparent indices would be unordered. 

You may iterate over the entire `Freelist` via [`iter`], [`iter_mut`], or [`into_iter`], all of which will skip over empty slots.

# Guarantees
[`push`] and [`remove`] are always *O*(1), maintain index order, and offer similar performance to [`Vec`]


[`Option`]: std::option::Option
['Freelist::new`]: Freelist::new
[`Freelist`]: Freelist
[`new`]: Freelist::new
[`push`]: Freelist::push
[`next_available`]: Freelist::next_available
[`remove`]: Freelist::remove
[`size`]: Freelist::size
[`free`]: Freelist::free
[`clear`]: Freelist::clear
[`reserve`]: Freelist::reserve
[`get`]: Freelist::get
[`get_mut`]: Freelist::get_mut
[`get_unchecked`]: Freelist::get_unchecked
[`get_unchecked_mut`]: Freelist::get_unchecked_mut
[`iter`]: Freelist::iter
[`iter_mut`]: Freelist::iter_mut
[`into_iter`]: Freelist::into_iter