# Cactus

This library provides an immutable cactus stack (also called a spaghetti stack
or parent pointer tree). A cactus stack is a (possibly empty) node with a
(possibly null) pointer to a parent node. Any given node has a unique path back
to the root node. Rather than mutably updating the stack, one creates and
obtains access to immutable nodes (when a node becomes unreachable its memory is
automatically reclaimed). A new child node pointing to a parent can be created
via the `child` function (analogous to the normal `push`) and a parent can be
retrieved via the `parent` function (analogous to the normal `pop`).

```rust
use cactus::Cactus;
let c = Cactus::new();
assert!(c.is_empty());
let c2 = c.child(1);
assert_eq!(c2.len(), 1);
assert_eq!(*c2.val().unwrap(), 1);
let c3 = c2.parent().unwrap();
assert!(c3.is_empty());
```

From a given node one can create multiple sub-stacks:

```rust
use cactus::Cactus;
let c = Cactus::new().child(1);
let c2 = c.child(2);
let c3 = c.child(3);
assert!(c2 != c3);
assert_eq!(c2.vals().cloned().collect::<Vec<_>>(), [2, 1]);
assert_eq!(c3.vals().cloned().collect::<Vec<_>>(), [3, 1]);
```