Source code
Revision control
Copy as Markdown
Other Tools
//! [](https://travis-ci.org/fitzgen/id-arena)
//!
//! A simple, id-based arena.
//!
//! ## Id-based
//!
//! Allocate objects and get an identifier for that object back, *not* a
//! reference to the allocated object. Given an id, you can get a shared or
//! exclusive reference to the allocated object from the arena. This id-based
//! approach is useful for constructing mutable graph data structures.
//!
//! If you want allocation to return a reference, consider [the `typed-arena`
//!
//! ## No Deletion
//!
//! This arena does not support deletion, which makes its implementation simple
//! and allocation fast. If you want deletion, you need a way to solve the ABA
//! problem. Consider using [the `generational-arena`
//!
//! ## Homogeneous
//!
//! This crate's arenas can only contain objects of a single type `T`. If you
//! need an arena of objects with heterogeneous types, consider another crate.
//!
//! ## `#![no_std]` Support
//!
//! Requires the `alloc` nightly feature. Disable the on-by-default `"std"` feature:
//!
//! ```toml
//! [dependencies.id-arena]
//! version = "2"
//! default-features = false
//! ```
//!
//! ## `rayon` Support
//!
//! If the `rayon` feature of this crate is activated:
//!
//! ```toml
//! [dependencies]
//! id-arena = { version = "2", features = ["rayon"] }
//! ```
//!
//! parallel iteration. The `Arena` type will have a `par_iter` family of
//! methods where appropriate.
//!
//! ## Example
//!
//! ```rust
//! use id_arena::{Arena, Id};
//!
//! type AstNodeId = Id<AstNode>;
//!
//! #[derive(Debug, Eq, PartialEq)]
//! pub enum AstNode {
//! Const(i64),
//! Var(String),
//! Add {
//! lhs: AstNodeId,
//! rhs: AstNodeId,
//! },
//! Sub {
//! lhs: AstNodeId,
//! rhs: AstNodeId,
//! },
//! Mul {
//! lhs: AstNodeId,
//! rhs: AstNodeId,
//! },
//! Div {
//! lhs: AstNodeId,
//! rhs: AstNodeId,
//! },
//! }
//!
//! let mut ast_nodes = Arena::<AstNode>::new();
//!
//! // Create the AST for `a * (b + 3)`.
//! let three = ast_nodes.alloc(AstNode::Const(3));
//! let b = ast_nodes.alloc(AstNode::Var("b".into()));
//! let b_plus_three = ast_nodes.alloc(AstNode::Add {
//! lhs: b,
//! rhs: three,
//! });
//! let a = ast_nodes.alloc(AstNode::Var("a".into()));
//! let a_times_b_plus_three = ast_nodes.alloc(AstNode::Mul {
//! lhs: a,
//! rhs: b_plus_three,
//! });
//!
//! // Can use indexing to access allocated nodes.
//! assert_eq!(ast_nodes[three], AstNode::Const(3));
//! ```
#![forbid(unsafe_code)]
#![deny(missing_debug_implementations)]
#![deny(missing_docs)]
// In no-std mode, use the alloc crate to get `Vec`.
#![no_std]
#![cfg_attr(not(feature = "std"), feature(alloc))]
use core::cmp::Ordering;
use core::fmt;
use core::hash::{Hash, Hasher};
use core::iter;
use core::marker::PhantomData;
use core::ops;
use core::slice;
use core::sync::atomic::{self, AtomicUsize, ATOMIC_USIZE_INIT};
#[cfg(not(feature = "std"))]
extern crate alloc;
#[cfg(not(feature = "std"))]
use alloc::vec::{self, Vec};
#[cfg(feature = "std")]
extern crate std;
#[cfg(feature = "std")]
use std::vec::{self, Vec};
#[cfg(feature = "rayon")]
mod rayon;
#[cfg(feature = "rayon")]
pub use rayon::*;
/// A trait representing the implementation behavior of an arena and how
/// identifiers are represented.
///
/// ## When should I implement `ArenaBehavior` myself?
///
/// Usually, you should just use `DefaultArenaBehavior`, which is simple and
/// correct. However, there are some scenarios where you might want to implement
/// `ArenaBehavior` yourself:
///
/// * **Space optimizations:** The default identifier is two words in size,
/// which is larger than is usually necessary. For example, if you know that an
/// arena *cannot* contain more than 256 items, you could make your own
/// identifier type that stores the index as a `u8` and then you can save some
/// space.
///
/// * **Trait Coherence:** If you need to implement an upstream crate's traits
/// for identifiers, then defining your own identifier type allows you to work
/// with trait coherence rules.
///
/// * **Share identifiers across arenas:** You can coordinate and share
/// identifiers across different arenas to enable a "struct of arrays" style
/// data representation.
pub trait ArenaBehavior {
/// The identifier type.
type Id: Copy;
/// Construct a new object identifier from the given index and arena
/// identifier.
///
/// ## Panics
///
/// Implementations are allowed to panic if the given index is larger than
/// the underlying storage (e.g. the implementation uses a `u8` for storing
/// indices and the given index value is larger than 255).
fn new_id(arena_id: u32, index: usize) -> Self::Id;
/// Get the given identifier's index.
fn index(Self::Id) -> usize;
/// Get the given identifier's arena id.
fn arena_id(Self::Id) -> u32;
/// Construct a new arena identifier.
///
/// This is used to disambiguate `Id`s across different arenas. To make
/// identifiers with the same index from different arenas compare false for
/// equality, return a unique `u32` on every invocation. This is the
/// default, provided implementation's behavior.
///
/// To make identifiers with the same index from different arenas compare
/// true for equality, return the same `u32` on every invocation.
fn new_arena_id() -> u32 {
static ARENA_COUNTER: AtomicUsize = ATOMIC_USIZE_INIT;
ARENA_COUNTER.fetch_add(1, atomic::Ordering::SeqCst) as u32
}
}
/// An identifier for an object allocated within an arena.
pub struct Id<T> {
idx: usize,
arena_id: u32,
_ty: PhantomData<fn() -> T>,
}
impl<T> fmt::Debug for Id<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("Id").field("idx", &self.idx).finish()
}
}
impl<T> Copy for Id<T> {}
impl<T> Clone for Id<T> {
#[inline]
fn clone(&self) -> Id<T> {
*self
}
}
impl<T> PartialEq for Id<T> {
#[inline]
fn eq(&self, rhs: &Self) -> bool {
self.arena_id == rhs.arena_id && self.idx == rhs.idx
}
}
impl<T> Eq for Id<T> {}
impl<T> Hash for Id<T> {
#[inline]
fn hash<H: Hasher>(&self, h: &mut H) {
self.arena_id.hash(h);
self.idx.hash(h);
}
}
impl<T> PartialOrd for Id<T> {
fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
Some(self.cmp(rhs))
}
}
impl<T> Ord for Id<T> {
fn cmp(&self, rhs: &Self) -> Ordering {
self.arena_id
.cmp(&rhs.arena_id)
.then(self.idx.cmp(&rhs.idx))
}
}
impl<T> Id<T> {
/// Get the index within the arena that this id refers to.
#[inline]
pub fn index(&self) -> usize {
self.idx
}
}
/// The default `ArenaBehavior` implementation.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct DefaultArenaBehavior<T> {
_phantom: PhantomData<fn() -> T>,
}
impl<T> ArenaBehavior for DefaultArenaBehavior<T> {
type Id = Id<T>;
#[inline]
fn new_id(arena_id: u32, idx: usize) -> Self::Id {
Id {
idx,
arena_id,
_ty: PhantomData,
}
}
#[inline]
fn index(id: Self::Id) -> usize {
id.idx
}
#[inline]
fn arena_id(id: Self::Id) -> u32 {
id.arena_id
}
}
/// An arena of objects of type `T`.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<&str>::new();
///
/// let a = arena.alloc("Albert");
/// assert_eq!(arena[a], "Albert");
///
/// arena[a] = "Alice";
/// assert_eq!(arena[a], "Alice");
/// ```
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Arena<T, A = DefaultArenaBehavior<T>> {
arena_id: u32,
items: Vec<T>,
_phantom: PhantomData<fn() -> A>,
}
impl<T, A> Default for Arena<T, A>
where
A: ArenaBehavior,
{
#[inline]
fn default() -> Arena<T, A> {
Arena {
arena_id: A::new_arena_id(),
items: Vec::new(),
_phantom: PhantomData,
}
}
}
impl<T, A> Arena<T, A>
where
A: ArenaBehavior,
{
/// Construct a new, empty `Arena`.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<usize>::new();
/// arena.alloc(42);
/// ```
#[inline]
pub fn new() -> Arena<T, A> {
Default::default()
}
/// Construct a new, empty `Arena` with capacity for the given number of
/// elements.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<usize>::with_capacity(100);
/// for x in 0..100 {
/// arena.alloc(x * x);
/// }
/// ```
#[inline]
pub fn with_capacity(capacity: usize) -> Arena<T, A> {
Arena {
arena_id: A::new_arena_id(),
items: Vec::with_capacity(capacity),
_phantom: PhantomData,
}
}
/// Allocate `item` within this arena and return its id.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<usize>::new();
/// let _id = arena.alloc(42);
/// ```
///
/// ## Panics
///
/// Panics if the number of elements in the arena overflows a `usize` or
/// `Id`'s index storage representation.
#[inline]
pub fn alloc(&mut self, item: T) -> A::Id {
let id = self.next_id();
self.items.push(item);
id
}
/// Allocate an item with the id that it will be assigned.
///
/// This is useful for structures that want to store their id as their own
/// member.
///
/// ```
/// use id_arena::{Arena, Id};
///
/// struct Cat {
/// id: Id<Cat>,
/// }
///
/// let mut arena = Arena::<Cat>::new();
///
/// let kitty = arena.alloc_with_id(|id| Cat { id });
/// assert_eq!(arena[kitty].id, kitty);
/// ```
#[inline]
pub fn alloc_with_id(&mut self, f: impl FnOnce(A::Id) -> T) -> A::Id {
let id = self.next_id();
let val = f(id);
self.alloc(val)
}
/// Get the id that will be used for the next item allocated into this
/// arena.
///
/// If you are allocating a `struct` that wants to have its id as a member
/// of itself, prefer the less error-prone `Arena::alloc_with_id` method.
#[inline]
pub fn next_id(&self) -> A::Id {
let arena_id = self.arena_id;
let idx = self.items.len();
A::new_id(arena_id, idx)
}
/// Get a shared reference to the object associated with the given `id` if
/// it exists.
///
/// If there is no object associated with `id` (for example, it might
/// reference an object allocated within a different arena) then return
/// `None`.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<usize>::new();
/// let id = arena.alloc(42);
/// assert!(arena.get(id).is_some());
///
/// let other_arena = Arena::<usize>::new();
/// assert!(other_arena.get(id).is_none());
/// ```
#[inline]
pub fn get(&self, id: A::Id) -> Option<&T> {
if A::arena_id(id) != self.arena_id {
None
} else {
self.items.get(A::index(id))
}
}
/// Get an exclusive reference to the object associated with the given `id`
/// if it exists.
///
/// If there is no object associated with `id` (for example, it might
/// reference an object allocated within a different arena) then return
/// `None`.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<usize>::new();
/// let id = arena.alloc(42);
/// assert!(arena.get_mut(id).is_some());
///
/// let mut other_arena = Arena::<usize>::new();
/// assert!(other_arena.get_mut(id).is_none());
/// ```
#[inline]
pub fn get_mut(&mut self, id: A::Id) -> Option<&mut T> {
if A::arena_id(id) != self.arena_id {
None
} else {
self.items.get_mut(A::index(id))
}
}
/// Iterate over this arena's items and their ids.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<&str>::new();
///
/// arena.alloc("hello");
/// arena.alloc("hi");
/// arena.alloc("yo");
///
/// for (id, s) in arena.iter() {
/// assert_eq!(arena.get(id).unwrap(), s);
/// println!("{:?} -> {}", id, s);
/// }
/// ```
#[inline]
pub fn iter(&self) -> Iter<T, A> {
IntoIterator::into_iter(self)
}
/// Iterate over this arena's items and their ids, allowing mutation of each
/// item.
#[inline]
pub fn iter_mut(&mut self) -> IterMut<T, A> {
IntoIterator::into_iter(self)
}
/// Get the number of objects allocated in this arena.
///
/// ```
/// use id_arena::Arena;
///
/// let mut arena = Arena::<&str>::new();
///
/// arena.alloc("hello");
/// arena.alloc("hi");
///
/// assert_eq!(arena.len(), 2);
/// ```
#[inline]
pub fn len(&self) -> usize {
self.items.len()
}
}
impl<T, A> ops::Index<A::Id> for Arena<T, A>
where
A: ArenaBehavior,
{
type Output = T;
#[inline]
fn index(&self, id: A::Id) -> &T {
assert_eq!(self.arena_id, A::arena_id(id));
&self.items[A::index(id)]
}
}
impl<T, A> ops::IndexMut<A::Id> for Arena<T, A>
where
A: ArenaBehavior,
{
#[inline]
fn index_mut(&mut self, id: A::Id) -> &mut T {
assert_eq!(self.arena_id, A::arena_id(id));
&mut self.items[A::index(id)]
}
}
fn add_id<A, T>(item: Option<(usize, T)>, arena_id: u32) -> Option<(A::Id, T)>
where
A: ArenaBehavior,
{
item.map(|(idx, item)| (A::new_id(arena_id, idx), item))
}
/// An iterator over `(Id, &T)` pairs in an arena.
///
/// See [the `Arena::iter()` method](./struct.Arena.html#method.iter) for details.
#[derive(Debug)]
pub struct Iter<'a, T: 'a, A: 'a> {
arena_id: u32,
iter: iter::Enumerate<slice::Iter<'a, T>>,
_phantom: PhantomData<fn() -> A>,
}
impl<'a, T: 'a, A: 'a> Iterator for Iter<'a, T, A>
where
A: ArenaBehavior,
{
type Item = (A::Id, &'a T);
#[inline]
fn next(&mut self) -> Option<Self::Item> {
add_id::<A, _>(self.iter.next(), self.arena_id)
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.iter.size_hint()
}
}
impl<'a, T: 'a, A: 'a> DoubleEndedIterator for Iter<'a, T, A>
where
A: ArenaBehavior,
{
fn next_back(&mut self) -> Option<Self::Item> {
add_id::<A, _>(self.iter.next_back(), self.arena_id)
}
}
impl<'a, T: 'a, A: 'a> ExactSizeIterator for Iter<'a, T, A>
where
A: ArenaBehavior,
{
fn len(&self) -> usize {
self.iter.len()
}
}
impl<'a, T, A> IntoIterator for &'a Arena<T, A>
where
A: ArenaBehavior,
{
type Item = (A::Id, &'a T);
type IntoIter = Iter<'a, T, A>;
#[inline]
fn into_iter(self) -> Iter<'a, T, A> {
Iter {
arena_id: self.arena_id,
iter: self.items.iter().enumerate(),
_phantom: PhantomData,
}
}
}
/// An iterator over `(Id, &mut T)` pairs in an arena.
///
/// See [the `Arena::iter_mut()` method](./struct.Arena.html#method.iter_mut)
/// for details.
#[derive(Debug)]
pub struct IterMut<'a, T: 'a, A: 'a> {
arena_id: u32,
iter: iter::Enumerate<slice::IterMut<'a, T>>,
_phantom: PhantomData<fn() -> A>,
}
impl<'a, T: 'a, A: 'a> Iterator for IterMut<'a, T, A>
where
A: ArenaBehavior,
{
type Item = (A::Id, &'a mut T);
#[inline]
fn next(&mut self) -> Option<Self::Item> {
add_id::<A, _>(self.iter.next(), self.arena_id)
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.iter.size_hint()
}
}
impl<'a, T: 'a, A: 'a> DoubleEndedIterator for IterMut<'a, T, A>
where
A: ArenaBehavior,
{
fn next_back(&mut self) -> Option<Self::Item> {
add_id::<A, _>(self.iter.next_back(), self.arena_id)
}
}
impl<'a, T: 'a, A: 'a> ExactSizeIterator for IterMut<'a, T, A>
where
A: ArenaBehavior,
{
fn len(&self) -> usize {
self.iter.len()
}
}
impl<'a, T, A> IntoIterator for &'a mut Arena<T, A>
where
A: ArenaBehavior,
{
type Item = (A::Id, &'a mut T);
type IntoIter = IterMut<'a, T, A>;
#[inline]
fn into_iter(self) -> IterMut<'a, T, A> {
IterMut {
arena_id: self.arena_id,
iter: self.items.iter_mut().enumerate(),
_phantom: PhantomData,
}
}
}
/// An iterator over `(Id, T)` pairs in an arena.
#[derive(Debug)]
pub struct IntoIter<T, A> {
arena_id: u32,
iter: iter::Enumerate<vec::IntoIter<T>>,
_phantom: PhantomData<fn() -> A>,
}
impl<T, A> Iterator for IntoIter<T, A>
where
A: ArenaBehavior,
{
type Item = (A::Id, T);
#[inline]
fn next(&mut self) -> Option<Self::Item> {
add_id::<A, _>(self.iter.next(), self.arena_id)
}
fn size_hint(&self) -> (usize, Option<usize>) {
self.iter.size_hint()
}
}
impl<T, A> DoubleEndedIterator for IntoIter<T, A>
where
A: ArenaBehavior,
{
fn next_back(&mut self) -> Option<Self::Item> {
add_id::<A, _>(self.iter.next_back(), self.arena_id)
}
}
impl<T, A> ExactSizeIterator for IntoIter<T, A>
where
A: ArenaBehavior,
{
fn len(&self) -> usize {
self.iter.len()
}
}
impl<T, A> IntoIterator for Arena<T, A>
where
A: ArenaBehavior,
{
type Item = (A::Id, T);
type IntoIter = IntoIter<T, A>;
#[inline]
fn into_iter(self) -> IntoIter<T, A> {
IntoIter {
arena_id: self.arena_id,
iter: self.items.into_iter().enumerate(),
_phantom: PhantomData,
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn ids_are_send_sync() {
fn assert_send_sync<T: Send + Sync>() {}
struct Foo;
assert_send_sync::<Id<Foo>>();
}
}