rust-too-many-lists-book/src/first.rs
2019-03-13 09:33:34 -04:00

90 lines
1.9 KiB
Rust

//! # A Bad Stack
use std::mem;
pub struct List {
head: Link,
}
enum Link {
Empty,
More(Box<Node>),
}
struct Node {
elem: i32,
next: Link,
}
impl List {
pub fn new() -> Self {
List { head: Link::Empty }
}
pub fn push(&mut self, elem: i32) {
let new_node = Box::new(Node {
elem,
next: mem::replace(&mut self.head, Link::Empty),
});
self.head = Link::More(new_node)
}
pub fn pop(&mut self) -> Option<i32> {
match mem::replace(&mut self.head, Link::Empty) {
Link::Empty => None,
Link::More(boxed_node) => {
let node = *boxed_node;
self.head = node.next;
Some(node.elem)
}
}
}
}
impl Drop for List {
fn drop(&mut self) {
let mut cur_link = mem::replace(&mut self.head, Link::Empty);
while let Link::More(mut boxed_node) = cur_link {
cur_link = mem::replace(&mut boxed_node.next, Link::Empty);
// boxed_node goes out of scope and gets dropped here;
// but its Node's `next` field has been set to Link::Empty
// so no unbounded recursion occurs.
}
}
}
#[cfg(test)]
mod test {
use super::List;
#[test]
fn basics() {
let mut list = List::new();
// Check empty list behaves right
assert_eq!(list.pop(), None);
// Populate list
list.push(1);
list.push(2);
list.push(3);
// Check normal removal
assert_eq!(list.pop(), Some(3));
assert_eq!(list.pop(), Some(2));
// Push some more just to make sure nothing's corrupted
list.push(4);
list.push(5);
// Check normal removal
assert_eq!(list.pop(), Some(5));
assert_eq!(list.pop(), Some(4));
// Check exhaustion
assert_eq!(list.pop(), Some(1));
assert_eq!(list.pop(), None);
}
}