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//! A simple object arena. //! //! `Arena<T>` is basically just a `Vec<Option<T>>`, which allows you to: //! //! * Insert an object (reuse an existing [`None`] element, or append to the end). //! * Remove object at a specified index. //! * Access object at a specified index. //! //! # Examples //! //! Some data structures built using `Arena<T>`: //! //! * [Doubly linked list](https://github.com/smol-rs/vec-arena/blob/master/examples/linked-list.rs) //! * [Splay tree](https://github.com/smol-rs/vec-arena/blob/master/examples/splay-tree.rs) #![no_std] #![forbid(unsafe_code)] #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)] #![deprecated( since = "1.2.0", note = "This crate is now deprecated in favor of [slab](https://crates.io/crates/slab)." )] extern crate alloc; use alloc::fmt; use alloc::vec; use alloc::vec::Vec; use core::iter; use core::mem; use core::ops::{Index, IndexMut}; use core::slice; /// A slot, which is either vacant or occupied. /// /// Vacant slots in arena are linked together into a singly linked list. This allows the arena to /// efficiently find a vacant slot before inserting a new object, or reclaiming a slot after /// removing an object. #[derive(Clone)] enum Slot<T> { /// Vacant slot, containing index to the next slot in the linked list. Vacant(usize), /// Occupied slot, containing a value. Occupied(T), } impl<T> Slot<T> { /// Returns `true` if the slot is vacant. fn is_occupied(&self) -> bool { match self { Slot::Vacant(_) => false, Slot::Occupied(_) => true, } } } /// An object arena. /// /// `Arena<T>` holds an array of slots for storing objects. /// Every slot is always in one of two states: occupied or vacant. /// /// Essentially, this is equivalent to `Vec<Option<T>>`. /// /// # Insert and remove /// /// When inserting a new object into arena, a vacant slot is found and then the object is placed /// into the slot. If there are no vacant slots, the array is reallocated with bigger capacity. /// The cost of insertion is amortized `O(1)`. /// /// When removing an object, the slot containing it is marked as vacant and the object is returned. /// The cost of removal is `O(1)`. /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// let a = arena.insert(10); /// let b = arena.insert(20); /// /// assert_eq!(a, 0); // 10 was placed at index 0 /// assert_eq!(b, 1); // 20 was placed at index 1 /// /// assert_eq!(arena.remove(a), Some(10)); /// assert_eq!(arena.get(a), None); // slot at index 0 is now vacant /// /// assert_eq!(arena.insert(30), 0); // slot at index 0 is reused /// ``` /// /// # Indexing /// /// You can also access objects in an arena by index, just like you would in a [`Vec`]. /// However, accessing a vacant slot by index or using an out-of-bounds index will result in panic. /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// let a = arena.insert(10); /// let b = arena.insert(20); /// /// assert_eq!(arena[a], 10); /// assert_eq!(arena[b], 20); /// /// arena[a] += arena[b]; /// assert_eq!(arena[a], 30); /// ``` /// /// To access slots without fear of panicking, use [`get()`][`Arena::get()`] and /// [`get_mut()`][`Arena::get_mut()`], which return [`Option`]s. pub struct Arena<T> { /// Slots in which objects are stored. slots: Vec<Slot<T>>, /// Number of occupied slots in the arena. len: usize, /// Index of the first vacant slot in the linked list. head: usize, } impl<T> Arena<T> { /// Constructs a new, empty arena. /// /// The arena will not allocate until objects are inserted into it. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena: Arena<i32> = Arena::new(); /// ``` #[inline] pub fn new() -> Self { Arena { slots: Vec::new(), len: 0, head: !0, } } /// Constructs a new, empty arena with the specified capacity (number of slots). /// /// The arena will be able to hold exactly `cap` objects without reallocating. /// If `cap` is 0, the arena will not allocate. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::with_capacity(10); /// /// assert_eq!(arena.len(), 0); /// assert_eq!(arena.capacity(), 10); /// /// // These inserts are done without reallocating... /// for i in 0..10 { /// arena.insert(i); /// } /// assert_eq!(arena.capacity(), 10); /// /// // ... but this one will reallocate. /// arena.insert(11); /// assert!(arena.capacity() > 10); /// ``` #[inline] pub fn with_capacity(cap: usize) -> Self { Arena { slots: Vec::with_capacity(cap), len: 0, head: !0, } } /// Returns the number of slots in the arena. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let arena: Arena<i32> = Arena::with_capacity(10); /// assert_eq!(arena.capacity(), 10); /// ``` #[inline] pub fn capacity(&self) -> usize { self.slots.capacity() } /// Returns the number of occupied slots in the arena. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// assert_eq!(arena.len(), 0); /// /// for i in 0..10 { /// arena.insert(()); /// assert_eq!(arena.len(), i + 1); /// } /// ``` #[inline] pub fn len(&self) -> usize { self.len } /// Returns `true` if all slots are vacant. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// assert!(arena.is_empty()); /// /// arena.insert(1); /// assert!(!arena.is_empty()); /// ``` #[inline] pub fn is_empty(&self) -> bool { self.len == 0 } /// Returns the index of the slot that next [`insert`][`Arena::insert()`] will use if no /// mutating calls take place in between. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// /// let a = arena.next_vacant(); /// let b = arena.insert(1); /// assert_eq!(a, b); /// let c = arena.next_vacant(); /// let d = arena.insert(2); /// assert_eq!(c, d); /// ``` #[inline] pub fn next_vacant(&self) -> usize { if self.head == !0 { self.len } else { self.head } } /// Inserts an object into the arena and returns the slot index it was stored in. /// /// The arena will reallocate if it's full. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// /// let a = arena.insert(1); /// let b = arena.insert(2); /// assert!(a != b); /// ``` #[inline] pub fn insert(&mut self, object: T) -> usize { self.len += 1; if self.head == !0 { self.slots.push(Slot::Occupied(object)); self.len - 1 } else { let index = self.head; match self.slots[index] { Slot::Vacant(next) => { self.head = next; self.slots[index] = Slot::Occupied(object); } Slot::Occupied(_) => unreachable!(), } index } } /// Removes the object stored at `index` from the arena and returns it. /// /// If the slot is vacant or `index` is out of bounds, [`None`] will be returned. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// let a = arena.insert("hello"); /// /// assert_eq!(arena.len(), 1); /// assert_eq!(arena.remove(a), Some("hello")); /// /// assert_eq!(arena.len(), 0); /// assert_eq!(arena.remove(a), None); /// ``` #[inline] pub fn remove(&mut self, index: usize) -> Option<T> { match self.slots.get_mut(index) { None => None, Some(&mut Slot::Vacant(_)) => None, Some(slot @ &mut Slot::Occupied(_)) => { if let Slot::Occupied(object) = mem::replace(slot, Slot::Vacant(self.head)) { self.head = index; self.len -= 1; Some(object) } else { unreachable!(); } } } } /// Retains objects for which the closure returns `true`. /// /// All other objects will be removed from the arena. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// /// let a = arena.insert(0); /// let b = arena.insert(1); /// let c = arena.insert(2); /// /// arena.retain(|k, v| k == a || *v == 1); /// /// assert!(arena.get(a).is_some()); /// assert!(arena.get(b).is_some()); /// assert!(arena.get(c).is_none()); /// ``` pub fn retain<F>(&mut self, mut f: F) where F: FnMut(usize, &mut T) -> bool, { for i in 0..self.slots.len() { if let Slot::Occupied(v) = &mut self.slots[i] { if !f(i, v) { self.remove(i); } } } } /// Clears the arena, removing and dropping all objects it holds. /// /// Keeps the allocated memory for reuse. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// for i in 0..10 { /// arena.insert(i); /// } /// /// assert_eq!(arena.len(), 10); /// arena.clear(); /// assert_eq!(arena.len(), 0); /// ``` #[inline] pub fn clear(&mut self) { self.slots.clear(); self.len = 0; self.head = !0; } /// Returns a reference to the object stored at `index`. /// /// If the slot is vacant or `index` is out of bounds, [`None`] will be returned. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// let index = arena.insert("hello"); /// /// assert_eq!(arena.get(index), Some(&"hello")); /// arena.remove(index); /// assert_eq!(arena.get(index), None); /// ``` #[inline] pub fn get(&self, index: usize) -> Option<&T> { match self.slots.get(index) { None => None, Some(&Slot::Vacant(_)) => None, Some(&Slot::Occupied(ref object)) => Some(object), } } /// Returns a mutable reference to the object stored at `index`. /// /// If the slot is vacant or `index` is out of bounds, [`None`] will be returned. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// let index = arena.insert(7); /// /// assert_eq!(arena.get_mut(index), Some(&mut 7)); /// *arena.get_mut(index).unwrap() *= 10; /// assert_eq!(arena.get_mut(index), Some(&mut 70)); /// ``` #[inline] pub fn get_mut(&mut self, index: usize) -> Option<&mut T> { match self.slots.get_mut(index) { None => None, Some(&mut Slot::Vacant(_)) => None, Some(&mut Slot::Occupied(ref mut object)) => Some(object), } } /// Swaps two objects in the arena. /// /// The two indices are `a` and `b`. /// /// # Panics /// /// Panics if any of the indices is out of bounds or any of the slots is vacant. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// let a = arena.insert(7); /// let b = arena.insert(8); /// /// arena.swap(a, b); /// assert_eq!(arena.get(a), Some(&8)); /// assert_eq!(arena.get(b), Some(&7)); /// ``` #[inline] pub fn swap(&mut self, a: usize, b: usize) { assert!(self.slots[a].is_occupied(), "invalid object ID"); assert!(self.slots[b].is_occupied(), "invalid object ID"); if a != b { let (a, b) = (a.min(b), a.max(b)); let (l, r) = self.slots.split_at_mut(b); mem::swap(&mut l[a], &mut r[0]); } } /// Reserves capacity for at least `additional` more objects to be inserted. /// /// The arena may reserve more space to avoid frequent reallocations. /// /// # Panics /// /// Panics if the new capacity overflows `usize`. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// arena.insert("hello"); /// /// arena.reserve(10); /// assert!(arena.capacity() >= 11); /// ``` pub fn reserve(&mut self, additional: usize) { let vacant = self.slots.len() - self.len; if additional > vacant { self.slots.reserve(additional - vacant); } } /// Reserves the minimum capacity for exactly `additional` more objects to be inserted. /// /// Note that the allocator may give the arena more space than it requests. /// /// # Panics /// /// Panics if the new capacity overflows `usize`. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// arena.insert("hello"); /// /// arena.reserve_exact(10); /// assert!(arena.capacity() >= 11); /// ``` pub fn reserve_exact(&mut self, additional: usize) { let vacant = self.slots.len() - self.len; if additional > vacant { self.slots.reserve_exact(additional - vacant); } } /// Returns an iterator over occupied slots. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// arena.insert(1); /// arena.insert(2); /// arena.insert(4); /// /// let mut iterator = arena.iter(); /// assert_eq!(iterator.next(), Some((0, &1))); /// assert_eq!(iterator.next(), Some((1, &2))); /// assert_eq!(iterator.next(), Some((2, &4))); /// ``` #[inline] pub fn iter(&self) -> Iter<'_, T> { Iter { slots: self.slots.iter().enumerate(), } } /// Returns an iterator that returns mutable references to objects. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::new(); /// arena.insert("zero".to_string()); /// arena.insert("one".to_string()); /// arena.insert("two".to_string()); /// /// for (index, object) in arena.iter_mut() { /// *object = index.to_string() + " " + object; /// } /// /// let mut iterator = arena.iter(); /// assert_eq!(iterator.next(), Some((0, &"0 zero".to_string()))); /// assert_eq!(iterator.next(), Some((1, &"1 one".to_string()))); /// assert_eq!(iterator.next(), Some((2, &"2 two".to_string()))); /// ``` #[inline] pub fn iter_mut(&mut self) -> IterMut<'_, T> { IterMut { slots: self.slots.iter_mut().enumerate(), } } /// Shrinks the capacity of the arena as much as possible. /// /// It will drop down as close as possible to the length but the allocator may still inform /// the arena that there is space for a few more elements. /// /// # Examples /// /// ``` /// use vec_arena::Arena; /// /// let mut arena = Arena::with_capacity(10); /// arena.insert("first".to_string()); /// arena.insert("second".to_string()); /// arena.insert("third".to_string()); /// assert_eq!(arena.capacity(), 10); /// arena.shrink_to_fit(); /// assert!(arena.capacity() >= 3); /// ``` 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 {{ ... }}") } }