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#![deny(warnings, missing_docs, missing_debug_implementations)] #![doc(html_root_url = "https://docs.rs/slotmap/0.4.0")] #![crate_name = "slotmap"] #![cfg_attr(feature = "unstable", feature(untagged_unions, try_reserve))] //! # slotmap //! //! This library provides a container with persistent unique keys to access //! stored values, [`SlotMap`]. Upon insertion a key is returned that can be //! used to later access or remove the values. Insertion, removal and access all //! take O(1) time with low overhead. Great for storing collections of objects //! that need stable, safe references but have no clear ownership otherwise, //! such as game entities or graph nodes. //! //! The difference between a [`BTreeMap`] or [`HashMap`] and a slot map is //! that the slot map generates and returns the key when inserting a value. A //! key is always unique and will only refer to the value that was inserted. //! A slot map's main purpose is to simply own things in a safe and efficient //! manner. //! //! You can also create (multiple) secondary maps that can map the keys returned //! by [`SlotMap`] to other values, to associate arbitrary data with objects //! stored in slot maps, without hashing required - it's direct indexing under //! the hood. //! //! # Examples //! //! ``` //! # use slotmap::*; //! let mut sm = SlotMap::new(); //! let foo = sm.insert("foo"); // Key generated on insert. //! let bar = sm.insert("bar"); //! assert_eq!(sm[foo], "foo"); //! assert_eq!(sm[bar], "bar"); //! //! sm.remove(bar); //! let reuse = sm.insert("reuse"); // Space from bar reused. //! assert_eq!(sm.contains_key(bar), false); // After deletion a key stays invalid. //! //! let mut sec = SecondaryMap::new(); //! sec.insert(foo, "noun"); // We provide the key for secondary maps. //! sec.insert(reuse, "verb"); //! //! for (key, val) in sm { //! println!("{} is a {}", val, sec[key]); //! } //! ``` //! //! # Serialization through [`serde`] //! //! Both keys and the slot maps have full (de)seralization support through //! the [`serde`] library. A key remains valid for a slot map even after one or //! both have been serialized and deserialized! This makes storing or //! transferring complicated referential structures and graphs a breeze. Care has //! been taken such that deserializing keys and slot maps from untrusted sources //! is safe. If you wish to use these features you must enable the `serde` //! feature flag for `slotmap` in your `Cargo.toml`. //! //! ```text //! slotmap = { version = "...", features = ["serde"] } //! ``` //! //! # Why not [`slab`]? //! //! Unlike [`slab`], the keys returned by [`SlotMap`] are versioned. This means //! that once a key is removed, it stays removed, even if the physical storage //! inside the slotmap is reused for new elements. The key is a //! permanently unique<sup>*</sup> reference to the inserted value. Despite //! supporting versioning, a [`SlotMap`] is not slower than [`slab`], by //! internally using carefully checked unsafe code. A [`HopSlotMap`] //! also provides faster iteration than [`slab`] does, and [`DenseSlotMap`] even //! faster still. Additionally, at the time of writing [`slab`] does not support //! serialization. //! //! # Performance characteristics and implementation details //! //! Insertion, access and deletion is all O(1) with low overhead by storing the //! elements inside a [`Vec`]. Unlike references or indices into a vector, //! unless you remove a key it is never invalidated. Behind the scenes each //! slot in the vector is a `(value, version)` tuple. After insertion the //! returned key also contains a version. Only when the stored version and //! version in a key match is a key valid. This allows us to reuse space in the //! vector after deletion without letting removed keys point to spurious new //! elements. <sup>*</sup>After 2<sup>31</sup> deletions and insertions to the //! same underlying slot the version wraps around and such a spurious reference //! could potentially occur. It is incredibly unlikely however, and in all //! circumstances is the behavior safe. A slot map can hold up to //! 2<sup>32</sup> - 2 elements at a time. //! //! The memory usage for each slot in [`SlotMap`] is `4 + max(sizeof(T), 4)` //! rounded up to the alignment of `T`. Similarly it is `4 + max(sizeof(T), 12)` //! for [`HopSlotMap`]. [`DenseSlotMap`] has an overhead of 8 bytes per element //! and 8 bytes per slot. //! //! # Choosing `SlotMap`, `HopSlotMap` or `DenseSlotMap` //! //! A [`SlotMap`] can never shrink the size of its underlying storage, because //! for each storage slot it must remember what the latest stored version was, //! even if the slot is empty now. This means that iteration can be slow as it //! must iterate over potentially a lot of empty slots. //! //! [`HopSlotMap`] solves this by maintaining more information on //! insertion/removal, allowing it to iterate only over filled slots by 'hopping //! over' contiguous blocks of vacant slots. This can give it significantly //! better iteration speed. If you expect to iterate over all elements in a //! [`SlotMap`] a lot, choose [`HopSlotMap`]. The downside is that insertion and //! removal is roughly twice as slow. Random access is the same speed for both. //! //! [`DenseSlotMap`] goes even further and stores all elements on a contiguous //! block of memory. It uses two indirects per random access; the slots contain //! indices used to access the contiguous memory. This means random access is //! slower than both [`SlotMap`] and [`HopSlotMap`], but iteration is //! significantly faster. Finally, there is no trait requirement on the value //! type of a [`DenseSlotMap`], see [`Slottable`] for more details. //! //! # Choosing `SecondaryMap` or `SparseSecondaryMap` //! //! You want to associate extra data with objects stored in a slot map, so you //! use (multiple) secondary maps to map keys to that data. //! //! A [`SecondaryMap`] is simply a [`Vec`] of slots like slot map is, and //! essentially provides all the same guarantees as [`SlotMap`] does for its //! operations (with the exception that you provide the keys as produced by the //! primary slot map). This does mean that even if you associate data to only //! a single element from the primary slot map, you could need and have to //! initialize as much memory as the original. //! //! A [`SparseSecondaryMap`] is like a [`HashMap`] from keys to objects, however //! it automatically removes outdated keys for slots that had their space //! reused. You should use this variant if you expect to store some associated //! data for only a small portion of the primary slot map. //! //! # Custom key types //! //! If you have multiple slot maps it's an error to use the key of one slot map //! on another slot map. The result is safe, but unspecified, and can not be //! detected at runtime, so it can lead to a hard to find bug. //! //! To prevent this, slot maps allow you to specify what the type is of the key //! they return, as long as that type implements the [`Key`] trait. To aid with //! this, the [`new_key_type!`] macro is provided that builds such a type for //! you. The resulting type is exactly like [`DefaultKey`]. So instead of simply //! using `SlotMap<DefaultKey, Player>` you would use: //! //! ``` //! # use slotmap::*; //! # #[derive(Copy, Clone)] //! # struct Player; //! new_key_type! { struct PlayerKey; } //! let sm: SlotMap<PlayerKey, Player> = SlotMap::with_key(); //! ``` //! //! [`Vec`]: https://doc.rust-lang.org/std/vec/struct.Vec.html //! [`BTreeMap`]: https://doc.rust-lang.org/std/collections/struct.BTreeMap.html //! [`HashMap`]: https://doc.rust-lang.org/std/collections/struct.HashMap.html //! [`SlotMap`]: struct.SlotMap.html //! [`HopSlotMap`]: hop/struct.HopSlotMap.html //! [`DenseSlotMap`]: dense/struct.DenseSlotMap.html //! [`SecondaryMap`]: secondary/struct.SecondaryMap.html //! [`SparseSecondaryMap`]: sparse_secondary/struct.SparseSecondaryMap.html //! [`Slottable`]: trait.Slottable.html //! [`Key`]: trait.Key.html //! [`new_key_type!`]: macro.new_key_type.html //! [`serde`]: https://github.com/serde-rs/serde //! [`slab`]: https://github.com/carllerche/slab //! [`DefaultKey`]: struct.DefaultKey.html #[cfg(feature = "serde")] extern crate serde; // So our macros can refer to these. #[cfg(feature = "serde")] #[doc(hidden)] pub mod __impl { pub use serde::{Deserialize, Deserializer, Serialize, Serializer}; } #[cfg(test)] #[macro_use] extern crate quickcheck; #[cfg(test)] extern crate serde_json; pub(crate) mod normal; pub use crate::normal::*; pub mod dense; pub use crate::dense::DenseSlotMap; pub mod hop; pub use crate::hop::HopSlotMap; pub mod secondary; pub use crate::secondary::SecondaryMap; pub mod sparse_secondary; pub use crate::sparse_secondary::SparseSecondaryMap; use std::fmt::{self, Debug, Formatter}; use std::num::NonZeroU32; /// A trait for items that can go in a [`SlotMap`] or [`HopSlotMap`]. Due to /// current stable Rust restrictions a type must be [`Copy`] to be placed in one /// of those slot maps. This restriction does not apply to [`DenseSlotMap`], /// [`SecondaryMap`] or [`SparseSecondaryMap`]. It also does not apply if you /// use nightly Rust and enable the `unstable` feature for `slotmap` by editing /// your `Cargo.toml`: /// /// ```text /// slotmap = { version = "...", features = ["unstable"] } /// ``` /// /// This trait should already be automatically implemented for any type that is /// slottable. /// /// [`Copy`]: https://doc.rust-lang.org/std/marker/trait.Copy.html /// [`SecondaryMap`]: secondary/struct.SecondaryMap.html /// [`SparseSecondaryMap`]: sparse_secondary/struct.SparseSecondaryMap.html /// [`SlotMap`]: struct.SlotMap.html /// [`HopSlotMap`]: hop/struct.HopSlotMap.html /// [`DenseSlotMap`]: dense/struct.DenseSlotMap.html #[cfg(not(feature = "unstable"))] pub trait Slottable: Copy {} /// A trait for items that can go in a [`SlotMap`] or [`HopSlotMap`]. Due to /// current stable Rust restrictions a type must be [`Copy`] to be placed in one /// of those slot maps. This restriction does not apply to [`DenseSlotMap`], /// [`SecondaryMap`] or [`SparseSecondaryMap`]. It also does not apply if you /// use nightly Rust and enable the `unstable` feature for `slotmap` by editing /// your `Cargo.toml`: /// /// ```text /// slotmap = { version = "...", features = ["unstable"] } /// ``` /// /// This trait should already be automatically implemented for any type that is /// slottable. /// /// [`Copy`]: https://doc.rust-lang.org/std/marker/trait.Copy.html /// [`SecondaryMap`]: secondary/struct.SecondaryMap.html /// [`SparseSecondaryMap`]: sparse_secondary/struct.SparseSecondaryMap.html /// [`SlotMap`]: struct.SlotMap.html /// [`HopSlotMap`]: hop/struct.HopSlotMap.html /// [`DenseSlotMap`]: dense/struct.DenseSlotMap.html #[cfg(feature = "unstable")] pub trait Slottable {} #[cfg(not(feature = "unstable"))] impl<T: Copy> Slottable for T {} #[cfg(feature = "unstable")] impl<T> Slottable for T {} /// The actual data stored in a [`Key`]. /// /// This implements `Ord` so keys can be stored in e.g. [`BTreeMap`], but the /// order of keys is unspecified. /// /// [`Key`]: trait.Key.html /// [`BTreeMap`]: https://doc.rust-lang.org/std/collections/struct.BTreeMap.html #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)] pub struct KeyData { idx: u32, version: NonZeroU32, } impl KeyData { fn new(idx: u32, version: u32) -> Self { Self { idx, version: NonZeroU32::new(version).expect("KeyData constructed with zero version"), } } fn null() -> Self { Self::new(std::u32::MAX, 1) } fn is_null(self) -> bool { self.idx == std::u32::MAX } /// Returns the key data as a 64-bit integer. No guarantees about its value /// are made other than that passing it to `from_ffi` will return a key /// equal to the original. /// /// With this you can easily pass slot map keys as opaque handles to foreign /// code. After you get them back you can confidently use them in your slot /// map without worrying about unsafe behavior as you would with passing and /// receiving back references or pointers. /// /// This is not a substitute for proper serialization, use [`serde`] for /// that. If you are not doing FFI, you almost surely do not need this /// function. /// /// [`serde`]: index.html#serialization-through-serde pub fn as_ffi(self) -> u64 { (u64::from(self.version.get()) << 32) | u64::from(self.idx) } /// Iff `value` is a value received from `k.as_ffi()`, returns a key equal /// to `k`. Otherwise the behavior is safe but unspecified. pub fn from_ffi(value: u64) -> Self { let idx = value & 0xffff_ffff; let version = (value >> 32) | 1; // Ensure version is odd. Self::new(idx as u32, version as u32) } } impl Debug for KeyData { fn fmt(&self, f: &mut Formatter) -> fmt::Result { write!(f, "{}v{}", self.idx, self.version.get()) } } impl Default for KeyData { fn default() -> Self { Self::null() } } /// Key used to access stored values in a slot map. /// /// Do not use a key from one slot map in another. The behavior is safe but /// non-sensical (and might panic in case of out-of-bounds). /// /// To prevent this, it is suggested to have a unique key type for each slot /// map. The easiest way to do this is through [`new_key_type!`], which /// makes a new type identical to [`DefaultKey`], just with a different name. /// /// [`new_key_type!`]: macro.new_key_type.html /// [`DefaultKey`]: struct.DefaultKey.html pub trait Key: From<KeyData> + Into<KeyData> + Clone { /// Creates a new key that is always invalid and distinct from any non-null /// key. A null key can only be created through this method (or default /// initialization of keys made with [`new_key_type!`], which calls this /// method). /// /// A null key is always invalid, but an invalid key (that is, a key that /// has been removed from the slot map) does not become a null key. A null /// is safe to use with any safe method of any slot map instance. /// /// # Examples /// /// ``` /// # use slotmap::*; /// let mut sm = SlotMap::new(); /// let k = sm.insert(42); /// let nk = DefaultKey::null(); /// assert!(nk.is_null()); /// assert!(k != nk); /// assert_eq!(sm.get(nk), None); /// ``` /// /// [`new_key_type!`]: macro.new_key_type.html fn null() -> Self { KeyData::null().into() } /// Checks if a key is null. There is only a single null key, that is /// `a.is_null() && b.is_null()` implies `a == b`. /// /// # Examples /// /// ``` /// # use slotmap::*; /// new_key_type! { struct MyKey; } /// let a = MyKey::null(); /// let b = MyKey::default(); /// assert_eq!(a, b); /// assert!(a.is_null()); /// ``` fn is_null(self) -> bool { self.into().is_null() } } /// A helper macro to conveniently create new key types. If you use a new key /// type for each slot map you create you can entirely prevent using the wrong /// key on the wrong slot map. /// /// The type constructed by this macro is identical to [`DefaultKey`], just with /// a different name. /// /// [`DefaultKey`]: struct.DefaultKey.html /// /// # Examples /// /// ``` /// # extern crate slotmap; /// # use slotmap::*; /// new_key_type! { /// struct EntityKey; /// /// /// Key for the Player slot map. /// pub struct PlayerKey; /// } /// /// fn main() { /// let mut players = SlotMap::with_key(); /// let mut entities: SlotMap<EntityKey, (f64, f64)> = SlotMap::with_key(); /// let bob: PlayerKey = players.insert("bobby"); /// // Now this is a type error because entities.get expects an EntityKey: /// // entities.get(bob); /// } /// ``` #[macro_export(local_inner_macros)] macro_rules! new_key_type { ( $(#[$outer:meta])* $vis:vis struct $name:ident; $($rest:tt)* ) => { $(#[$outer])* #[derive(Copy, Clone, Default, Eq, PartialEq, Ord, PartialOrd, Hash, Debug)] #[repr(transparent)] $vis struct $name($crate::KeyData); impl From<$crate::KeyData> for $name { fn from(k: $crate::KeyData) -> Self { $name(k) } } impl From<$name> for $crate::KeyData { fn from(k: $name) -> Self { k.0 } } impl $crate::Key for $name { } $crate::__serialize_key!($name); $crate::new_key_type!($($rest)*); }; () => {} } #[cfg(feature = "serde")] #[doc(hidden)] #[macro_export] macro_rules! __serialize_key { ( $name:ty ) => { impl $crate::__impl::Serialize for $name { fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: $crate::__impl::Serializer, { $crate::KeyData::from(*self).serialize(serializer) } } impl<'de> $crate::__impl::Deserialize<'de> for $name { fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> where D: $crate::__impl::Deserializer<'de>, { let key_data: $crate::KeyData = $crate::__impl::Deserialize::deserialize(deserializer)?; Ok(key_data.into()) } } }; } #[cfg(not(feature = "serde"))] #[doc(hidden)] #[macro_export] macro_rules! __serialize_key { ( $name:ty ) => {}; } new_key_type! { /// The default slot map key type. pub struct DefaultKey; } // Returns if a is an older version than b, taking into account wrapping of // versions. fn is_older_version(a: u32, b: u32) -> bool { let diff = a.wrapping_sub(b); diff >= (1 << 31) } // Serialization with serde. #[cfg(feature = "serde")] mod serialize { use super::*; use serde::{Deserialize, Deserializer, Serialize, Serializer}; #[derive(Serialize, Deserialize)] pub struct SerKey { idx: u32, version: u32, } impl Serialize for KeyData { fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer, { let ser_key = SerKey { idx: self.idx, version: self.version.get(), }; ser_key.serialize(serializer) } } impl<'de> Deserialize<'de> for KeyData { fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> where D: Deserializer<'de>, { let mut ser_key: SerKey = Deserialize::deserialize(deserializer)?; // Ensure a.is_null() && b.is_null() implies a == b. if ser_key.idx == std::u32::MAX { ser_key.version = 1; } ser_key.version |= 1; // Ensure version is odd. Ok(Self::new(ser_key.idx, ser_key.version)) } } } #[cfg(test)] mod tests { use super::*; #[test] fn check_is_older_version() { let is_older = |a, b| is_older_version(a, b); assert!(!is_older(42, 42)); assert!(is_older(0, 1)); assert!(is_older(0, 1 << 31)); assert!(!is_older(0, (1 << 31) + 1)); assert!(is_older((-1i32) as u32, 0)); } #[cfg(feature = "serde")] #[test] fn key_serde() { // Check round-trip through serde. let mut sm = SlotMap::new(); let k = sm.insert(42); let ser = serde_json::to_string(&k).unwrap(); let de: DefaultKey = serde_json::from_str(&ser).unwrap(); assert_eq!(k, de); // Even if a malicious entity sends up even (unoccupied) versions in the // key, we make the version point to the occupied version. let malicious: KeyData = serde_json::from_str(&r#"{"idx":0,"version":4}"#).unwrap(); assert_eq!(malicious.version.get(), 5); } }