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//! Pre-allocated storage for a uniform data type. //! //! `Slab` provides pre-allocated storage for a single data type. If many values //! of a single type are being allocated, it can be more efficient to //! pre-allocate the necessary storage. Since the size of the type is uniform, //! memory fragmentation can be avoided. Storing, clearing, and lookup //! operations become very cheap. //! //! While `Slab` may look like other Rust collections, it is not intended to be //! used as a general purpose collection. The primary difference between `Slab` //! and `Vec` is that `Slab` returns the key when storing the value. //! //! It is important to note that keys may be reused. In other words, once a //! value associated with a given key is removed from a slab, that key may be //! returned from future calls to `insert`. //! //! # Examples //! //! Basic storing and retrieval. //! //! ``` //! # use slab::*; //! let mut slab = Slab::new(); //! //! let hello = slab.insert("hello"); //! let world = slab.insert("world"); //! //! assert_eq!(slab[hello], "hello"); //! assert_eq!(slab[world], "world"); //! //! slab[world] = "earth"; //! assert_eq!(slab[world], "earth"); //! ``` //! //! Sometimes it is useful to be able to associate the key with the value being //! inserted in the slab. This can be done with the `vacant_entry` API as such: //! //! ``` //! # use slab::*; //! let mut slab = Slab::new(); //! //! let hello = { //! let entry = slab.vacant_entry(); //! let key = entry.key(); //! //! entry.insert((key, "hello")); //! key //! }; //! //! assert_eq!(hello, slab[hello].0); //! assert_eq!("hello", slab[hello].1); //! ``` //! //! It is generally a good idea to specify the desired capacity of a slab at //! creation time. Note that `Slab` will grow the internal capacity when //! attempting to insert a new value once the existing capacity has been reached. //! To avoid this, add a check. //! //! ``` //! # use slab::*; //! let mut slab = Slab::with_capacity(1024); //! //! // ... use the slab //! //! if slab.len() == slab.capacity() { //! panic!("slab full"); //! } //! //! slab.insert("the slab is not at capacity yet"); //! ``` //! //! # Capacity and reallocation //! //! The capacity of a slab is the amount of space allocated for any future //! values that will be inserted in the slab. This is not to be confused with //! the *length* of the slab, which specifies the number of actual values //! currently being inserted. If a slab's length is equal to its capacity, the //! next value inserted into the slab will require growing the slab by //! reallocating. //! //! For example, a slab with capacity 10 and length 0 would be an empty slab //! with space for 10 more stored values. Storing 10 or fewer elements into the //! slab will not change its capacity or cause reallocation to occur. However, //! if the slab length is increased to 11 (due to another `insert`), it will //! have to reallocate, which can be slow. For this reason, it is recommended to //! use [`Slab::with_capacity`] whenever possible to specify how many values the //! slab is expected to store. //! //! # Implementation //! //! `Slab` is backed by a `Vec` of slots. Each slot is either occupied or //! vacant. `Slab` maintains a stack of vacant slots using a linked list. To //! find a vacant slot, the stack is popped. When a slot is released, it is //! pushed onto the stack. //! //! If there are no more available slots in the stack, then `Vec::reserve(1)` is //! called and a new slot is created. //! //! [`Slab::with_capacity`]: struct.Slab.html#with_capacity #![deny(warnings, missing_docs, missing_debug_implementations)] #![doc(html_root_url = "https://docs.rs/slab/0.4.1")] #![crate_name = "slab"] use std::{fmt, mem}; use std::iter::IntoIterator; use std::ops; /// Pre-allocated storage for a uniform data type /// /// See the [module documentation] for more details. /// /// [module documentation]: index.html #[derive(Clone)] pub struct Slab<T> { // Chunk of memory entries: Vec<Entry<T>>, // Number of Filled elements currently in the slab len: usize, // Offset of the next available slot in the slab. Set to the slab's // capacity when the slab is full. next: usize, } impl<T> Default for Slab<T> { fn default() -> Self { Slab::new() } } /// A handle to a vacant entry in a `Slab`. /// /// `VacantEntry` allows constructing values with the key that they will be /// assigned to. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let hello = { /// let entry = slab.vacant_entry(); /// let key = entry.key(); /// /// entry.insert((key, "hello")); /// key /// }; /// /// assert_eq!(hello, slab[hello].0); /// assert_eq!("hello", slab[hello].1); /// ``` #[derive(Debug)] pub struct VacantEntry<'a, T: 'a> { slab: &'a mut Slab<T>, key: usize, } /// An iterator over the values stored in the `Slab` pub struct Iter<'a, T: 'a> { entries: std::slice::Iter<'a, Entry<T>>, curr: usize, } /// A mutable iterator over the values stored in the `Slab` pub struct IterMut<'a, T: 'a> { entries: std::slice::IterMut<'a, Entry<T>>, curr: usize, } #[derive(Clone)] enum Entry<T> { Vacant(usize), Occupied(T), } impl<T> Slab<T> { /// Construct a new, empty `Slab`. /// /// The function does not allocate and the returned slab will have no /// capacity until `insert` is called or capacity is explicitly reserved. /// /// # Examples /// /// ``` /// # use slab::*; /// let slab: Slab<i32> = Slab::new(); /// ``` pub fn new() -> Slab<T> { Slab::with_capacity(0) } /// Construct a new, empty `Slab` with the specified capacity. /// /// The returned slab will be able to store exactly `capacity` without /// reallocating. If `capacity` is 0, the slab will not allocate. /// /// It is important to note that this function does not specify the *length* /// of the returned slab, but only the capacity. For an explanation of the /// difference between length and capacity, see [Capacity and /// reallocation](index.html#capacity-and-reallocation). /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::with_capacity(10); /// /// // The slab contains no values, even though it has capacity for more /// assert_eq!(slab.len(), 0); /// /// // These are all done without reallocating... /// for i in 0..10 { /// slab.insert(i); /// } /// /// // ...but this may make the slab reallocate /// slab.insert(11); /// ``` pub fn with_capacity(capacity: usize) -> Slab<T> { Slab { entries: Vec::with_capacity(capacity), next: 0, len: 0, } } /// Return the number of values the slab can store without reallocating. /// /// # Examples /// /// ``` /// # use slab::*; /// let slab: Slab<i32> = Slab::with_capacity(10); /// assert_eq!(slab.capacity(), 10); /// ``` pub fn capacity(&self) -> usize { self.entries.capacity() } /// Reserve capacity for at least `additional` more values to be stored /// without allocating. /// /// `reserve` does nothing if the slab already has sufficient capacity for /// `additional` more values. If more capacity is required, a new segment of /// memory will be allocated and all existing values will be copied into it. /// As such, if the slab is already very large, a call to `reserve` can end /// up being expensive. /// /// The slab may reserve more than `additional` extra space in order to /// avoid frequent reallocations. Use `reserve_exact` instead to guarantee /// that only the requested space is allocated. /// /// # Panics /// /// Panics if the new capacity overflows `usize`. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// slab.insert("hello"); /// slab.reserve(10); /// assert!(slab.capacity() >= 11); /// ``` pub fn reserve(&mut self, additional: usize) { if self.capacity() - self.len >= additional { return; } let need_add = self.len + additional - self.entries.len(); self.entries.reserve(need_add); } /// Reserve the minimum capacity required to store exactly `additional` /// more values. /// /// `reserve_exact` does nothing if the slab already has sufficient capacity /// for `additional` more valus. If more capacity is required, a new segment /// of memory will be allocated and all existing values will be copied into /// it. As such, if the slab is already very large, a call to `reserve` can /// end up being expensive. /// /// Note that the allocator may give the slab more space than it requests. /// Therefore capacity can not be relied upon to be precisely minimal. /// Prefer `reserve` if future insertions are expected. /// /// # Panics /// /// Panics if the new capacity overflows `usize`. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// slab.insert("hello"); /// slab.reserve_exact(10); /// assert!(slab.capacity() >= 11); /// ``` pub fn reserve_exact(&mut self, additional: usize) { if self.capacity() - self.len >= additional { return; } let need_add = self.len + additional - self.entries.len(); self.entries.reserve_exact(need_add); } /// Shrink the capacity of the slab as much as possible. /// /// It will drop down as close as possible to the length but the allocator /// may still inform the vector that there is space for a few more elements. /// Also, since values are not moved, the slab cannot shrink past any stored /// values. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::with_capacity(10); /// /// for i in 0..3 { /// slab.insert(i); /// } /// /// assert_eq!(slab.capacity(), 10); /// slab.shrink_to_fit(); /// assert!(slab.capacity() >= 3); /// ``` /// /// In this case, even though two values are removed, the slab cannot shrink /// past the last value. /// /// ``` /// # use slab::*; /// let mut slab = Slab::with_capacity(10); /// /// for i in 0..3 { /// slab.insert(i); /// } /// /// slab.remove(0); /// slab.remove(1); /// /// assert_eq!(slab.capacity(), 10); /// slab.shrink_to_fit(); /// assert!(slab.capacity() >= 3); /// ``` pub fn shrink_to_fit(&mut self) { self.entries.shrink_to_fit(); } /// Clear the slab of all values. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// for i in 0..3 { /// slab.insert(i); /// } /// /// slab.clear(); /// assert!(slab.is_empty()); /// ``` pub fn clear(&mut self) { self.entries.clear(); self.len = 0; self.next = 0; } /// Return the number of stored values. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// for i in 0..3 { /// slab.insert(i); /// } /// /// assert_eq!(3, slab.len()); /// ``` pub fn len(&self) -> usize { self.len } /// Return `true` if there are no values stored in the slab. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// assert!(slab.is_empty()); /// /// slab.insert(1); /// assert!(!slab.is_empty()); /// ``` pub fn is_empty(&self) -> bool { self.len == 0 } /// Return an iterator over the slab. /// /// This function should generally be **avoided** as it is not efficient. /// Iterators must iterate over every slot in the slab even if it is /// vacant. As such, a slab with a capacity of 1 million but only one /// stored value must still iterate the million slots. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// for i in 0..3 { /// slab.insert(i); /// } /// /// let mut iterator = slab.iter(); /// /// assert_eq!(iterator.next(), Some((0, &0))); /// assert_eq!(iterator.next(), Some((1, &1))); /// assert_eq!(iterator.next(), Some((2, &2))); /// assert_eq!(iterator.next(), None); /// ``` pub fn iter(&self) -> Iter<T> { Iter { entries: self.entries.iter(), curr: 0, } } /// Return an iterator that allows modifying each value. /// /// This function should generally be **avoided** as it is not efficient. /// Iterators must iterate over every slot in the slab even if it is /// vacant. As such, a slab with a capacity of 1 million but only one /// stored value must still iterate the million slots. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let key1 = slab.insert(0); /// let key2 = slab.insert(1); /// /// for (key, val) in slab.iter_mut() { /// if key == key1 { /// *val += 2; /// } /// } /// /// assert_eq!(slab[key1], 2); /// assert_eq!(slab[key2], 1); /// ``` pub fn iter_mut(&mut self) -> IterMut<T> { IterMut { entries: self.entries.iter_mut(), curr: 0, } } /// Return a reference to the value associated with the given key. /// /// If the given key is not associated with a value, then `None` is /// returned. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// let key = slab.insert("hello"); /// /// assert_eq!(slab.get(key), Some(&"hello")); /// assert_eq!(slab.get(123), None); /// ``` pub fn get(&self, key: usize) -> Option<&T> { match self.entries.get(key) { Some(&Entry::Occupied(ref val)) => Some(val), _ => None, } } /// Return a mutable reference to the value associated with the given key. /// /// If the given key is not associated with a value, then `None` is /// returned. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// let key = slab.insert("hello"); /// /// *slab.get_mut(key).unwrap() = "world"; /// /// assert_eq!(slab[key], "world"); /// assert_eq!(slab.get_mut(123), None); /// ``` pub fn get_mut(&mut self, key: usize) -> Option<&mut T> { match self.entries.get_mut(key) { Some(&mut Entry::Occupied(ref mut val)) => Some(val), _ => None, } } /// Return a reference to the value associated with the given key without /// performing bounds checking. /// /// This function should be used with care. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// let key = slab.insert(2); /// /// unsafe { /// assert_eq!(slab.get_unchecked(key), &2); /// } /// ``` pub unsafe fn get_unchecked(&self, key: usize) -> &T { match *self.entries.get_unchecked(key) { Entry::Occupied(ref val) => val, _ => unreachable!(), } } /// Return a mutable reference to the value associated with the given key /// without performing bounds checking. /// /// This function should be used with care. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// let key = slab.insert(2); /// /// unsafe { /// let val = slab.get_unchecked_mut(key); /// *val = 13; /// } /// /// assert_eq!(slab[key], 13); /// ``` pub unsafe fn get_unchecked_mut(&mut self, key: usize) -> &mut T { match *self.entries.get_unchecked_mut(key) { Entry::Occupied(ref mut val) => val, _ => unreachable!(), } } /// Insert a value in the slab, returning key assigned to the value. /// /// The returned key can later be used to retrieve or remove the value using indexed /// lookup and `remove`. Additional capacity is allocated if needed. See /// [Capacity and reallocation](index.html#capacity-and-reallocation). /// /// # Panics /// /// Panics if the number of elements in the vector overflows a `usize`. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// let key = slab.insert("hello"); /// assert_eq!(slab[key], "hello"); /// ``` pub fn insert(&mut self, val: T) -> usize { let key = self.next; self.insert_at(key, val); key } /// Return a handle to a vacant entry allowing for further manipulation. /// /// This function is useful when creating values that must contain their /// slab key. The returned `VacantEntry` reserves a slot in the slab and is /// able to query the associated key. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let hello = { /// let entry = slab.vacant_entry(); /// let key = entry.key(); /// /// entry.insert((key, "hello")); /// key /// }; /// /// assert_eq!(hello, slab[hello].0); /// assert_eq!("hello", slab[hello].1); /// ``` pub fn vacant_entry(&mut self) -> VacantEntry<T> { VacantEntry { key: self.next, slab: self, } } fn insert_at(&mut self, key: usize, val: T) { self.len += 1; if key == self.entries.len() { self.entries.push(Entry::Occupied(val)); self.next = key + 1; } else { let prev = mem::replace( &mut self.entries[key], Entry::Occupied(val)); match prev { Entry::Vacant(next) => { self.next = next; } _ => unreachable!(), } } } /// Remove and return the value associated with the given key. /// /// The key is then released and may be associated with future stored /// values. /// /// # Panics /// /// Panics if `key` is not associated with a value. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let hello = slab.insert("hello"); /// /// assert_eq!(slab.remove(hello), "hello"); /// assert!(!slab.contains(hello)); /// ``` pub fn remove(&mut self, key: usize) -> T { // Swap the entry at the provided value let prev = mem::replace( &mut self.entries[key], Entry::Vacant(self.next)); match prev { Entry::Occupied(val) => { self.len -= 1; self.next = key; val } _ => { // Woops, the entry is actually vacant, restore the state self.entries[key] = prev; panic!("invalid key"); } } } /// Return `true` if a value is associated with the given key. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let hello = slab.insert("hello"); /// assert!(slab.contains(hello)); /// /// slab.remove(hello); /// /// assert!(!slab.contains(hello)); /// ``` pub fn contains(&self, key: usize) -> bool { self.entries.get(key) .map(|e| { match *e { Entry::Occupied(_) => true, _ => false, } }) .unwrap_or(false) } /// Retain only the elements specified by the predicate. /// /// In other words, remove all elements `e` such that `f(usize, &mut e)` /// returns false. This method operates in place and preserves the key /// associated with the retained values. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let k1 = slab.insert(0); /// let k2 = slab.insert(1); /// let k3 = slab.insert(2); /// /// slab.retain(|key, val| key == k1 || *val == 1); /// /// assert!(slab.contains(k1)); /// assert!(slab.contains(k2)); /// assert!(!slab.contains(k3)); /// /// assert_eq!(2, slab.len()); /// ``` pub fn retain<F>(&mut self, mut f: F) where F: FnMut(usize, &mut T) -> bool { for i in 0..self.entries.len() { let keep = match self.entries[i] { Entry::Occupied(ref mut v) => f(i, v), _ => true, }; if !keep { self.remove(i); } } } } impl<T> ops::Index<usize> for Slab<T> { type Output = T; fn index(&self, key: usize) -> &T { match self.entries[key] { Entry::Occupied(ref v) => v, _ => panic!("invalid key"), } } } impl<T> ops::IndexMut<usize> for Slab<T> { fn index_mut(&mut self, key: usize) -> &mut T { match self.entries[key] { Entry::Occupied(ref mut v) => v, _ => panic!("invalid key"), } } } impl<'a, T> IntoIterator for &'a Slab<T> { type Item = (usize, &'a T); type IntoIter = Iter<'a, T>; fn into_iter(self) -> Iter<'a, T> { self.iter() } } impl<'a, T> IntoIterator for &'a mut Slab<T> { type Item = (usize, &'a mut T); type IntoIter = IterMut<'a, T>; fn into_iter(self) -> IterMut<'a, T> { self.iter_mut() } } impl<T> fmt::Debug for Slab<T> where T: fmt::Debug { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { write!(fmt, "Slab {{ len: {}, cap: {} }}", self.len, self.capacity()) } } impl<'a, T: 'a> fmt::Debug for Iter<'a, T> where T: fmt::Debug { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { fmt.debug_struct("Iter") .field("curr", &self.curr) .field("remaining", &self.entries.len()) .finish() } } impl<'a, T: 'a> fmt::Debug for IterMut<'a, T> where T: fmt::Debug { fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result { fmt.debug_struct("IterMut") .field("curr", &self.curr) .field("remaining", &self.entries.len()) .finish() } } // ===== VacantEntry ===== impl<'a, T> VacantEntry<'a, T> { /// Insert a value in the entry, returning a mutable reference to the value. /// /// To get the key associated with the value, use `key` prior to calling /// `insert`. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let hello = { /// let entry = slab.vacant_entry(); /// let key = entry.key(); /// /// entry.insert((key, "hello")); /// key /// }; /// /// assert_eq!(hello, slab[hello].0); /// assert_eq!("hello", slab[hello].1); /// ``` pub fn insert(self, val: T) -> &'a mut T { self.slab.insert_at(self.key, val); match self.slab.entries[self.key] { Entry::Occupied(ref mut v) => v, _ => unreachable!(), } } /// Return the key associated with this entry. /// /// A value stored in this entry will be associated with this key. /// /// # Examples /// /// ``` /// # use slab::*; /// let mut slab = Slab::new(); /// /// let hello = { /// let entry = slab.vacant_entry(); /// let key = entry.key(); /// /// entry.insert((key, "hello")); /// key /// }; /// /// assert_eq!(hello, slab[hello].0); /// assert_eq!("hello", slab[hello].1); /// ``` pub fn key(&self) -> usize { self.key } } // ===== Iter ===== impl<'a, T> Iterator for Iter<'a, T> { type Item = (usize, &'a T); fn next(&mut self) -> Option<(usize, &'a T)> { while let Some(entry) = self.entries.next() { let curr = self.curr; self.curr += 1; if let Entry::Occupied(ref v) = *entry { return Some((curr, v)); } } None } } // ===== IterMut ===== impl<'a, T> Iterator for IterMut<'a, T> { type Item = (usize, &'a mut T); fn next(&mut self) -> Option<(usize, &'a mut T)> { while let Some(entry) = self.entries.next() { let curr = self.curr; self.curr += 1; if let Entry::Occupied(ref mut v) = *entry { return Some((curr, v)); } } None } }