mirror of https://github.com/smol-rs/vec-arena
708 lines
18 KiB
Rust
708 lines
18 KiB
Rust
//! A simple object arena.
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//!
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//! `Arena<T>` is basically just a `Vec<Option<T>>`, which allows you to:
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//!
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//! * Insert an object (reuse an existing [`None`] element, or append to the end).
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//! * Remove object at a specified index.
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//! * Access object at a specified index.
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//!
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//! # Examples
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//!
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//! Some data structures built using `Arena<T>`:
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//!
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//! * [Doubly linked list](https://github.com/stjepang/vec-arena/blob/master/examples/linked_list.rs)
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//! * [Splay tree](https://github.com/stjepang/vec-arena/blob/master/examples/splay_tree.rs)
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#![forbid(unsafe_code)]
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#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
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use std::fmt;
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use std::iter;
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use std::mem;
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use std::ops::{Index, IndexMut};
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use std::slice;
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use std::vec;
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/// A slot, which is either vacant or occupied.
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///
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/// Vacant slots in arena are linked together into a singly linked list. This allows the arena to
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/// efficiently find a vacant slot before inserting a new object, or reclaiming a slot after
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/// removing an object.
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#[derive(Clone)]
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enum Slot<T> {
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/// Vacant slot, containing index to the next slot in the linked list.
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Vacant(usize),
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/// Occupied slot, containing a value.
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Occupied(T),
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}
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impl<T> Slot<T> {
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/// Returns `true` if the slot is vacant.
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fn is_occupied(&self) -> bool {
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match self {
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Slot::Vacant(_) => false,
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Slot::Occupied(_) => true,
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}
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}
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}
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/// An object arena.
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///
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/// `Arena<T>` holds an array of slots for storing objects.
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/// Every slot is always in one of two states: occupied or vacant.
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///
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/// Essentially, this is equivalent to `Vec<Option<T>>`.
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///
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/// # Insert and remove
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///
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/// When inserting a new object into arena, a vacant slot is found and then the object is placed
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/// into the slot. If there are no vacant slots, the array is reallocated with bigger capacity.
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/// The cost of insertion is amortized `O(1)`.
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///
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/// When removing an object, the slot containing it is marked as vacant and the object is returned.
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/// The cost of removal is `O(1)`.
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// let a = arena.insert(10);
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/// let b = arena.insert(20);
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///
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/// assert_eq!(a, 0); // 10 was placed at index 0
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/// assert_eq!(b, 1); // 20 was placed at index 1
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///
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/// assert_eq!(arena.remove(a), Some(10));
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/// assert_eq!(arena.get(a), None); // slot at index 0 is now vacant
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///
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/// assert_eq!(arena.insert(30), 0); // slot at index 0 is reused
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/// ```
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///
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/// # Indexing
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///
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/// You can also access objects in an arena by index, just like you would in a [`Vec`].
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/// However, accessing a vacant slot by index or using an out-of-bounds index will result in panic.
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// let a = arena.insert(10);
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/// let b = arena.insert(20);
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///
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/// assert_eq!(arena[a], 10);
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/// assert_eq!(arena[b], 20);
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///
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/// arena[a] += arena[b];
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/// assert_eq!(arena[a], 30);
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/// ```
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///
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/// To access slots without fear of panicking, use [`get()`][`Arena::get()`] and
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/// [`get_mut()`][`Arena::get_mut()`], which return [`Option`]s.
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pub struct Arena<T> {
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/// Slots in which objects are stored.
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slots: Vec<Slot<T>>,
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/// Number of occupied slots in the arena.
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len: usize,
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/// Index of the first vacant slot in the linked list.
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head: usize,
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}
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impl<T> Arena<T> {
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/// Constructs a new, empty arena.
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///
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/// The arena will not allocate until objects are inserted into it.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena: Arena<i32> = Arena::new();
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/// ```
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#[inline]
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pub fn new() -> Self {
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Arena {
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slots: Vec::new(),
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len: 0,
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head: !0,
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}
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}
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/// Constructs a new, empty arena with the specified capacity (number of slots).
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///
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/// The arena will be able to hold exactly `cap` objects without reallocating.
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/// If `cap` is 0, the arena will not allocate.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::with_capacity(10);
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///
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/// assert_eq!(arena.len(), 0);
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/// assert_eq!(arena.capacity(), 10);
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///
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/// // These inserts are done without reallocating...
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/// for i in 0..10 {
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/// arena.insert(i);
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/// }
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/// assert_eq!(arena.capacity(), 10);
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///
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/// // ... but this one will reallocate.
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/// arena.insert(11);
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/// assert!(arena.capacity() > 10);
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/// ```
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#[inline]
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pub fn with_capacity(cap: usize) -> Self {
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Arena {
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slots: Vec::with_capacity(cap),
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len: 0,
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head: !0,
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}
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}
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/// Returns the number of slots in the arena.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let arena: Arena<i32> = Arena::with_capacity(10);
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/// assert_eq!(arena.capacity(), 10);
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/// ```
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#[inline]
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pub fn capacity(&self) -> usize {
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self.slots.capacity()
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}
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/// Returns the number of occupied slots in the arena.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// assert_eq!(arena.len(), 0);
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///
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/// for i in 0..10 {
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/// arena.insert(());
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/// assert_eq!(arena.len(), i + 1);
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/// }
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/// ```
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#[inline]
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pub fn len(&self) -> usize {
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self.len
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}
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/// Returns `true` if all slots are vacant.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// assert!(arena.is_empty());
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///
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/// arena.insert(1);
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/// assert!(!arena.is_empty());
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/// ```
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#[inline]
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pub fn is_empty(&self) -> bool {
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self.len == 0
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}
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/// Returns the index of the slot that next [`insert`][`Arena::insert()`] will use if no
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/// mutating calls take place in between.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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///
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/// let a = arena.next_vacant();
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/// let b = arena.insert(1);
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/// assert_eq!(a, b);
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/// let c = arena.next_vacant();
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/// let d = arena.insert(2);
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/// assert_eq!(c, d);
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/// ```
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#[inline]
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pub fn next_vacant(&self) -> usize {
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if self.head == !0 {
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self.len
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} else {
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self.head
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}
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}
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/// Inserts an object into the arena and returns the slot index it was stored in.
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///
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/// The arena will reallocate if it's full.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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///
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/// let a = arena.insert(1);
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/// let b = arena.insert(2);
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/// assert!(a != b);
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/// ```
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#[inline]
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pub fn insert(&mut self, object: T) -> usize {
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self.len += 1;
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if self.head == !0 {
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self.slots.push(Slot::Occupied(object));
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self.len - 1
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} else {
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let index = self.head;
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match self.slots[index] {
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Slot::Vacant(next) => {
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self.head = next;
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self.slots[index] = Slot::Occupied(object);
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}
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Slot::Occupied(_) => unreachable!(),
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}
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index
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}
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}
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/// Removes the object stored at `index` from the arena and returns it.
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///
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/// If the slot is vacant or `index` is out of bounds, [`None`] will be returned.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// let a = arena.insert("hello");
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///
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/// assert_eq!(arena.len(), 1);
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/// assert_eq!(arena.remove(a), Some("hello"));
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///
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/// assert_eq!(arena.len(), 0);
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/// assert_eq!(arena.remove(a), None);
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/// ```
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#[inline]
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pub fn remove(&mut self, index: usize) -> Option<T> {
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match self.slots.get_mut(index) {
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None => None,
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Some(&mut Slot::Vacant(_)) => None,
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Some(slot @ &mut Slot::Occupied(_)) => {
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if let Slot::Occupied(object) = mem::replace(slot, Slot::Vacant(self.head)) {
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self.head = index;
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self.len -= 1;
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Some(object)
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} else {
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unreachable!();
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}
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}
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}
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}
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/// Clears the arena, removing and dropping all objects it holds.
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///
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/// Keeps the allocated memory for reuse.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// for i in 0..10 {
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/// arena.insert(i);
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/// }
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///
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/// assert_eq!(arena.len(), 10);
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/// arena.clear();
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/// assert_eq!(arena.len(), 0);
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/// ```
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#[inline]
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pub fn clear(&mut self) {
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self.slots.clear();
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self.len = 0;
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self.head = !0;
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}
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/// Returns a reference to the object stored at `index`.
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///
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/// If the slot is vacant or `index` is out of bounds, [`None`] will be returned.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// let index = arena.insert("hello");
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///
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/// assert_eq!(arena.get(index), Some(&"hello"));
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/// arena.remove(index);
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/// assert_eq!(arena.get(index), None);
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/// ```
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#[inline]
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pub fn get(&self, index: usize) -> Option<&T> {
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match self.slots.get(index) {
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None => None,
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Some(&Slot::Vacant(_)) => None,
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Some(&Slot::Occupied(ref object)) => Some(object),
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}
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}
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/// Returns a mutable reference to the object stored at `index`.
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///
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/// If the slot is vacant or `index` is out of bounds, [`None`] will be returned.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// let index = arena.insert(7);
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///
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/// assert_eq!(arena.get_mut(index), Some(&mut 7));
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/// *arena.get_mut(index).unwrap() *= 10;
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/// assert_eq!(arena.get_mut(index), Some(&mut 70));
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/// ```
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#[inline]
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pub fn get_mut(&mut self, index: usize) -> Option<&mut T> {
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match self.slots.get_mut(index) {
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None => None,
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Some(&mut Slot::Vacant(_)) => None,
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Some(&mut Slot::Occupied(ref mut object)) => Some(object),
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}
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}
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/// Swaps two objects in the arena.
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///
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/// The two indices are `a` and `b`.
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///
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/// # Panics
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///
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/// Panics if any of the indices is out of bounds or any of the slots is vacant.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// let a = arena.insert(7);
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/// let b = arena.insert(8);
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///
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/// arena.swap(a, b);
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/// assert_eq!(arena.get(a), Some(&8));
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/// assert_eq!(arena.get(b), Some(&7));
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/// ```
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#[inline]
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pub fn swap(&mut self, a: usize, b: usize) {
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assert!(self.slots[a].is_occupied(), "invalid object ID");
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assert!(self.slots[b].is_occupied(), "invalid object ID");
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if a != b {
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let (a, b) = (a.min(b), a.max(b));
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let (l, r) = self.slots.split_at_mut(b);
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mem::swap(&mut l[a], &mut r[0]);
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}
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}
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/// Reserves capacity for at least `additional` more objects to be inserted.
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///
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/// The arena may reserve more space to avoid frequent reallocations.
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///
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/// # Panics
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///
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/// Panics if the new capacity overflows `usize`.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// arena.insert("hello");
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///
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/// arena.reserve(10);
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/// assert!(arena.capacity() >= 11);
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/// ```
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pub fn reserve(&mut self, additional: usize) {
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let vacant = self.slots.len() - self.len;
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if additional > vacant {
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self.slots.reserve(additional - vacant);
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}
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}
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/// Reserves the minimum capacity for exactly `additional` more objects to be inserted.
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///
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/// Note that the allocator may give the arena more space than it requests.
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///
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/// # Panics
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///
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/// Panics if the new capacity overflows `usize`.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// arena.insert("hello");
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///
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/// arena.reserve_exact(10);
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/// assert!(arena.capacity() >= 11);
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/// ```
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pub fn reserve_exact(&mut self, additional: usize) {
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let vacant = self.slots.len() - self.len;
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if additional > vacant {
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self.slots.reserve_exact(additional - vacant);
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}
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}
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/// Returns an iterator over occupied slots.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// arena.insert(1);
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/// arena.insert(2);
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/// arena.insert(4);
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///
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/// let mut iterator = arena.iter();
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/// assert_eq!(iterator.next(), Some((0, &1)));
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/// assert_eq!(iterator.next(), Some((1, &2)));
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/// assert_eq!(iterator.next(), Some((2, &4)));
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/// ```
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#[inline]
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pub fn iter(&self) -> Iter<'_, T> {
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Iter {
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slots: self.slots.iter().enumerate(),
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}
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}
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/// Returns an iterator that returns mutable references to objects.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::new();
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/// arena.insert("zero".to_string());
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/// arena.insert("one".to_string());
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/// arena.insert("two".to_string());
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///
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/// for (index, object) in arena.iter_mut() {
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/// *object = index.to_string() + " " + object;
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/// }
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///
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/// let mut iterator = arena.iter();
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/// assert_eq!(iterator.next(), Some((0, &"0 zero".to_string())));
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/// assert_eq!(iterator.next(), Some((1, &"1 one".to_string())));
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/// assert_eq!(iterator.next(), Some((2, &"2 two".to_string())));
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/// ```
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#[inline]
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pub fn iter_mut(&mut self) -> IterMut<'_, T> {
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IterMut {
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slots: self.slots.iter_mut().enumerate(),
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}
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}
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/// Shrinks the capacity of the arena as much as possible.
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///
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/// It will drop down as close as possible to the length but the allocator may still inform
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/// the arena that there is space for a few more elements.
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///
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/// # Examples
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///
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/// ```
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/// use vec_arena::Arena;
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///
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/// let mut arena = Arena::with_capacity(10);
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/// arena.insert("first".to_string());
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/// arena.insert("second".to_string());
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/// arena.insert("third".to_string());
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/// assert_eq!(arena.capacity(), 10);
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/// arena.shrink_to_fit();
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/// assert!(arena.capacity() >= 3);
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/// ```
|
|
pub fn shrink_to_fit(&mut self) {
|
|
self.slots.shrink_to_fit();
|
|
}
|
|
}
|
|
|
|
impl<T> fmt::Debug for Arena<T> {
|
|
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
|
|
write!(f, "Arena {{ ... }}")
|
|
}
|
|
}
|
|
|
|
impl<T> Index<usize> for Arena<T> {
|
|
type Output = T;
|
|
|
|
#[inline]
|
|
fn index(&self, index: usize) -> &T {
|
|
self.get(index).expect("vacant slot at `index`")
|
|
}
|
|
}
|
|
|
|
impl<T> IndexMut<usize> for Arena<T> {
|
|
#[inline]
|
|
fn index_mut(&mut self, index: usize) -> &mut T {
|
|
self.get_mut(index).expect("vacant slot at `index`")
|
|
}
|
|
}
|
|
|
|
impl<T> Default for Arena<T> {
|
|
fn default() -> Self {
|
|
Arena::new()
|
|
}
|
|
}
|
|
|
|
impl<T: Clone> Clone for Arena<T> {
|
|
fn clone(&self) -> Self {
|
|
Arena {
|
|
slots: self.slots.clone(),
|
|
len: self.len,
|
|
head: self.head,
|
|
}
|
|
}
|
|
}
|
|
|
|
/// An iterator over the occupied slots in an [`Arena`].
|
|
pub struct IntoIter<T> {
|
|
slots: iter::Enumerate<vec::IntoIter<Slot<T>>>,
|
|
}
|
|
|
|
impl<T> Iterator for IntoIter<T> {
|
|
type Item = (usize, T);
|
|
|
|
#[inline]
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
while let Some((index, slot)) = self.slots.next() {
|
|
if let Slot::Occupied(object) = slot {
|
|
return Some((index, object));
|
|
}
|
|
}
|
|
None
|
|
}
|
|
}
|
|
|
|
impl<T> IntoIterator for Arena<T> {
|
|
type Item = (usize, T);
|
|
type IntoIter = IntoIter<T>;
|
|
|
|
#[inline]
|
|
fn into_iter(self) -> Self::IntoIter {
|
|
IntoIter {
|
|
slots: self.slots.into_iter().enumerate(),
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<T> iter::FromIterator<T> for Arena<T> {
|
|
fn from_iter<U: IntoIterator<Item = T>>(iter: U) -> Arena<T> {
|
|
let iter = iter.into_iter();
|
|
let mut arena = Arena::with_capacity(iter.size_hint().0);
|
|
for i in iter {
|
|
arena.insert(i);
|
|
}
|
|
arena
|
|
}
|
|
}
|
|
|
|
impl<T> fmt::Debug for IntoIter<T> {
|
|
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
|
|
write!(f, "IntoIter {{ ... }}")
|
|
}
|
|
}
|
|
|
|
/// An iterator over references to the occupied slots in an [`Arena`].
|
|
pub struct Iter<'a, T> {
|
|
slots: iter::Enumerate<slice::Iter<'a, Slot<T>>>,
|
|
}
|
|
|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
type Item = (usize, &'a T);
|
|
|
|
#[inline]
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
while let Some((index, slot)) = self.slots.next() {
|
|
if let Slot::Occupied(ref object) = *slot {
|
|
return Some((index, object));
|
|
}
|
|
}
|
|
None
|
|
}
|
|
}
|
|
|
|
impl<'a, T> IntoIterator for &'a Arena<T> {
|
|
type Item = (usize, &'a T);
|
|
type IntoIter = Iter<'a, T>;
|
|
|
|
#[inline]
|
|
fn into_iter(self) -> Self::IntoIter {
|
|
self.iter()
|
|
}
|
|
}
|
|
|
|
impl<'a, T> fmt::Debug for Iter<'a, T> {
|
|
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
|
|
write!(f, "Iter {{ ... }}")
|
|
}
|
|
}
|
|
|
|
/// An iterator over mutable references to the occupied slots in a `Arena`.
|
|
pub struct IterMut<'a, T> {
|
|
slots: iter::Enumerate<slice::IterMut<'a, Slot<T>>>,
|
|
}
|
|
|
|
impl<'a, T> Iterator for IterMut<'a, T> {
|
|
type Item = (usize, &'a mut T);
|
|
|
|
#[inline]
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
while let Some((index, slot)) = self.slots.next() {
|
|
if let Slot::Occupied(ref mut object) = *slot {
|
|
return Some((index, object));
|
|
}
|
|
}
|
|
None
|
|
}
|
|
}
|
|
|
|
impl<'a, T> IntoIterator for &'a mut Arena<T> {
|
|
type Item = (usize, &'a mut T);
|
|
type IntoIter = IterMut<'a, T>;
|
|
|
|
#[inline]
|
|
fn into_iter(self) -> Self::IntoIter {
|
|
self.iter_mut()
|
|
}
|
|
}
|
|
|
|
impl<'a, T> fmt::Debug for IterMut<'a, T> {
|
|
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
|
|
write!(f, "IterMut {{ ... }}")
|
|
}
|
|
}
|