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//! Concurrent hash maps. //! //! This crate implements concurrent hash maps, based on bucket-level multi-reader locks. It has //! excellent performance characteristics¹ and supports resizing, in-place mutation and more. //! //! The API derives directly from `std::collections::HashMap`, giving it a familiar feel. //! //! ¹Note that it heavily depends on the behavior of your program, but in most cases, it's really //! good. In some (rare) cases you might want atomic hash maps instead. //! //! # How it works //! //! `chashmap` is not lockless, but it distributes locks across the map such that lock contentions //! (which is what could make accesses expensive) are very rare. //! //! Hash maps consists of so called "buckets", which each defines a potential entry in the table. //! The bucket of some key-value pair is determined by the hash of the key. By holding a read-write //! lock for each bucket, we ensure that you will generally be able to insert, read, modify, etc. //! with only one or two locking subroutines. //! //! There is a special-case: reallocation. When the table is filled up such that very few buckets //! are free (note that this is "very few" and not "no", since the load factor shouldn't get too //! high as it hurts performance), a global lock is obtained while rehashing the table. This is //! pretty inefficient, but it rarely happens, and due to the adaptive nature of the capacity, it //! will only happen a few times when the map has just been initialized. //! //! ## Collision resolution //! //! When two hashes collide, they cannot share the same bucket, so there must be an algorithm which //! can resolve collisions. In our case, we use linear probing, which means that we take the bucket //! following it, and repeat until we find a free bucket. //! //! This method is far from ideal, but superior methods like Robin-Hood hashing works poorly (if at //! all) in a concurrent structure. //! //! # The API //! //! The API should feel very familiar, if you are used to the libstd hash map implementation. They //! share many of the methods, and I've carefully made sure that all the items, which have similarly //! named items in libstd, matches in semantics and behavior. extern crate parking_lot; extern crate owning_ref; #[cfg(test)] mod tests; use owning_ref::{OwningHandle, OwningRef}; use parking_lot::{RwLock, RwLockWriteGuard, RwLockReadGuard}; use std::collections::hash_map; use std::hash::{Hash, Hasher, BuildHasher}; use std::sync::atomic::{self, AtomicUsize}; use std::{mem, ops, cmp, fmt, iter}; /// The atomic ordering used throughout the code. const ORDERING: atomic::Ordering = atomic::Ordering::SeqCst; /// The length-to-capacity factor. const LENGTH_MULTIPLIER: usize = 4; /// The maximal load factor's numerator. const MAX_LOAD_FACTOR_NUM: usize = 100 - 15; /// The maximal load factor's denominator. const MAX_LOAD_FACTOR_DENOM: usize = 100; /// The default initial capacity. const DEFAULT_INITIAL_CAPACITY: usize = 64; /// The lowest capacity a table can have. const MINIMUM_CAPACITY: usize = 8; /// A bucket state. /// /// Buckets are the bricks of hash tables. They represent a single entry into the table. #[derive(Clone)] enum Bucket<K, V> { /// The bucket contains a key-value pair. Contains(K, V), /// The bucket is empty and has never been used. /// /// Since hash collisions are resolved by jumping to the next bucket, some buckets can cluster /// together, meaning that they are potential candidates for lookups. Empty buckets can be seen /// as the delimiter of such cluters. Empty, /// The bucket was removed. /// /// The technique of distincting between "empty" and "removed" was first described by Knuth. /// The idea is that when you search for a key, you will probe over these buckets, since the /// key could have been pushed behind the removed element: /// /// Contains(k1, v1) // hash = h /// Removed /// Contains(k2, v2) // hash = h /// /// If we stopped at `Removed`, we won't be able to find the second KV pair. So `Removed` is /// semantically different from `Empty`, as the search won't stop. /// /// However, we are still able to insert new pairs at the removed buckets. Removed, } impl<K, V> Bucket<K, V> { /// Is this bucket 'empty'? fn is_empty(&self) -> bool { if let Bucket::Empty = *self { true } else { false } } /// Is this bucket 'removed'? fn is_removed(&self) -> bool { if let Bucket::Removed = *self { true } else { false } } /// Is this bucket free? /// /// "Free" means that it can safely be replace by another bucket — namely that the bucket is /// not occupied. fn is_free(&self) -> bool { match *self { // The two replacable bucket types are removed buckets and empty buckets. Bucket::Removed | Bucket::Empty => true, // KV pairs can't be replaced as they contain data. Bucket::Contains(..) => false, } } /// Get the value (if any) of this bucket. /// /// This gets the value of the KV pair, if any. If the bucket is not a KV pair, `None` is /// returned. fn value(self) -> Option<V> { if let Bucket::Contains(_, val) = self { Some(val) } else { None } } /// Get a reference to the value of the bucket (if any). /// /// This returns a reference to the value of the bucket, if it is a KV pair. If not, it will /// return `None`. /// /// Rather than `Option`, it returns a `Result`, in order to make it easier to work with the /// `owning_ref` crate (`try_new` and `try_map` of `OwningHandle` and `OwningRef` /// respectively). fn value_ref(&self) -> Result<&V, ()> { if let Bucket::Contains(_, ref val) = *self { Ok(val) } else { Err(()) } } /// Does the bucket match a given key? /// /// This returns `true` if the bucket is a KV pair with key `key`. If not, `false` is returned. fn key_matches(&self, key: &K) -> bool where K: PartialEq { if let Bucket::Contains(ref candidate_key, _) = *self { // Check if the keys matches. candidate_key == key } else { // The bucket isn't a KV pair, so we'll return false, since there is no key to test // against. false } } } /// The low-level representation of the hash table. /// /// This is different from `CHashMap` in two ways: /// /// 1. It is not wrapped in a lock, meaning that resizing and reallocation is not possible. /// 2. It does not track the number of occupied buckets, making it expensive to obtain the load /// factor. struct Table<K, V> { /// The hash function builder. /// /// This randomly picks a hash function from some family of functions in libstd. This /// effectively eliminates the issue of hash flooding. hash_builder: hash_map::RandomState, /// The bucket array. /// /// This vector stores the buckets. The order in which they're stored is far from arbitrary: A /// KV pair `(key, val)`'s first priority location is at `self.hash(&key) % len`. If not /// possible, the next bucket is used, and this process repeats until the bucket is free (or /// the end is reached, in which we simply wrap around). buckets: Vec<RwLock<Bucket<K, V>>>, } impl<K, V> Table<K, V> { /// Create a table with a certain number of buckets. fn new(buckets: usize) -> Table<K, V> { // TODO: For some obscure reason `RwLock` doesn't implement `Clone`. // Fill a vector with `buckets` of `Empty` buckets. let mut vec = Vec::with_capacity(buckets); for _ in 0..buckets { vec.push(RwLock::new(Bucket::Empty)); } Table { // Generate a hash function. hash_builder: hash_map::RandomState::new(), buckets: vec, } } /// Create a table with at least some capacity. fn with_capacity(cap: usize) -> Table<K, V> { Table::new(cmp::max(MINIMUM_CAPACITY, cap * LENGTH_MULTIPLIER)) } } impl<K: PartialEq + Hash, V> Table<K, V> { /// Hash some key through the internal hash function. fn hash(&self, key: &K) -> usize { // Build the initial hash function state. let mut hasher = self.hash_builder.build_hasher(); // Hash the key. key.hash(&mut hasher); // Cast to `usize`. Since the hash function returns `u64`, this cast won't ever cause // entropy less than the ouput space. hasher.finish() as usize } /// Scan from the first priority of a key until a match is found. /// /// This scans from the first priority of `key` (as defined by its hash), until a match is /// found (will wrap on end), i.e. `matches` returns `true` with the bucket as argument. /// /// The read guard from the RW-lock of the bucket is returned. fn scan<F>(&self, key: &K, matches: F) -> RwLockReadGuard<Bucket<K, V>> where F: Fn(&Bucket<K, V>) -> bool { // Hash the key. let hash = self.hash(key); // Start at the first priority bucket, and then move upwards, searching for the matching // bucket. for i in 0.. { // Get the lock of the `i`'th bucket after the first priority bucket (wrap on end). let lock = self.buckets[(hash + i) % self.buckets.len()].read(); // Check if it is a match. if matches(&lock) { // Yup. Return. return lock; } } // TODO unreachable!(); } /// Scan from the first priority of a key until a match is found (mutable guard). /// /// This is similar to `scan`, but instead of an immutable lock guard, a mutable lock guard is /// returned. fn scan_mut<F>(&self, key: &K, matches: F) -> RwLockWriteGuard<Bucket<K, V>> where F: Fn(&Bucket<K, V>) -> bool { // Hash the key. let hash = self.hash(key); // Start at the first priority bucket, and then move upwards, searching for the matching // bucket. for i in 0.. { // Get the lock of the `i`'th bucket after the first priority bucket (wrap on end). let lock = self.buckets[(hash + i) % self.buckets.len()].write(); // Check if it is a match. if matches(&lock) { // Yup. Return. return lock; } } // TODO unreachable!(); } /// Scan from the first priority of a key until a match is found (bypass locks). /// /// This is similar to `scan_mut`, but it safely bypasses the locks by making use of the /// aliasing invariants of `&mut`. fn scan_mut_no_lock<F>(&mut self, key: &K, matches: F) -> &mut Bucket<K, V> where F: Fn(&Bucket<K, V>) -> bool { // Hash the key. let hash = self.hash(key); // TODO: To tame the borrowchecker, we fetch this in advance. let len = self.buckets.len(); // Start at the first priority bucket, and then move upwards, searching for the matching // bucket. for i in 0.. { // TODO: hacky hacky let idx = (hash + i) % len; // Get the lock of the `i`'th bucket after the first priority bucket (wrap on end). // Check if it is a match. if { let bucket = self.buckets[idx].get_mut(); matches(&bucket) } { // Yup. Return. return self.buckets[idx].get_mut(); } } // TODO unreachable!(); } /// Find a bucket with some key, or a free bucket in same cluster. /// /// This scans for buckets with key `key`. If one is found, it will be returned. If none are /// found, it will return a free bucket in the same cluster. fn lookup_or_free(&self, key: &K) -> RwLockWriteGuard<Bucket<K, V>> { // Hash the key. let hash = self.hash(key); // The encountered free bucket. let mut free = None; // Start at the first priority bucket, and then move upwards, searching for the matching // bucket. for i in 0.. { // Get the lock of the `i`'th bucket after the first priority bucket (wrap on end). let lock = self.buckets[(hash + i) % self.buckets.len()].write(); if lock.key_matches(key) { // We found a match. return lock; } else if lock.is_empty() { // The cluster is over. Use the encountered free bucket, if any. return free.unwrap_or(lock); } else if lock.is_removed() && free.is_none() { // We found a free bucket, so we can store it to later (if we don't already have // one). free = Some(lock) } } // TODO unreachable!(); } /// Lookup some key. /// /// This searches some key `key`, and returns a immutable lock guard to its bucket. If the key /// couldn't be found, the returned value will be an `Empty` cluster. fn lookup(&self, key: &K) -> RwLockReadGuard<Bucket<K, V>> { self.scan(key, |x| match *x { // We'll check that the keys does indeed match, as the chance of hash collisions // happening is inevitable Bucket::Contains(ref candidate_key, _) if key == candidate_key => true, // We reached an empty bucket, meaning that there are no more buckets, not even removed // ones, to search. Bucket::Empty => true, _ => false, }) } /// Lookup some key, mutably. /// /// This is similar to `lookup`, but it returns a mutable guard. /// /// Replacing at this bucket is safe as the bucket will be in the same cluster of buckets as /// the first priority cluster. fn lookup_mut(&self, key: &K) -> RwLockWriteGuard<Bucket<K, V>> { self.scan_mut(key, |x| match *x { // We'll check that the keys does indeed match, as the chance of hash collisions // happening is inevitable Bucket::Contains(ref candidate_key, _) if key == candidate_key => true, // We reached an empty bucket, meaning that there are no more buckets, not even removed // ones, to search. Bucket::Empty => true, _ => false, }) } /// Find a free bucket in the same cluster as some key. /// /// This means that the returned lock guard defines a valid, free bucket, where `key` can be /// inserted. fn find_free(&self, key: &K) -> RwLockWriteGuard<Bucket<K, V>> { self.scan_mut(key, |x| x.is_free()) } /// Find a free bucket in the same cluster as some key (bypassing locks). /// /// This is similar to `find_free`, except that it safely bypasses locks through the aliasing /// guarantees of `&mut`. fn find_free_no_lock(&mut self, key: &K) -> &mut Bucket<K, V> { self.scan_mut_no_lock(key, |x| x.is_free()) } /// Fill the table with data from another table. /// /// This is used to efficiently copy the data of `table` into `self`. /// /// # Important /// /// The table should be empty for this to work correctly/logically. fn fill(&mut self, table: Table<K, V>) { // Run over all the buckets. for i in table.buckets { // We'll only transfer the bucket if it is a KV pair. if let Bucket::Contains(key, val) = i.into_inner() { // Find a bucket where the KV pair can be inserted. let mut bucket = self.scan_mut_no_lock(&key, |x| match *x { // Halt on an empty bucket. Bucket::Empty => true, // We'll assume that the rest of the buckets either contains other KV pairs (in // particular, no buckets have been removed in the newly construct table). _ => false, }); // Set the bucket to the KV pair. *bucket = Bucket::Contains(key, val); } } } } impl<K: Clone, V: Clone> Clone for Table<K, V> { fn clone(&self) -> Table<K, V> { Table { // Since we copy plainly without rehashing etc., it is important that we keep the same // hash function. hash_builder: self.hash_builder.clone(), // Lock and clone every bucket individually. buckets: self.buckets.iter().map(|x| RwLock::new(x.read().clone())).collect(), } } } impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Table<K, V> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { // We'll just run over all buckets and output one after one. for i in &self.buckets { // Acquire the lock. let lock = i.read(); // Check if the bucket actually contains anything. if let Bucket::Contains(ref key, ref val) = *lock { // Write it to the output stream in a nice format. write!(f, "{:?} => {:?}", key, val)?; } } Ok(()) } } /// An iterator over the entries of some table. pub struct IntoIter<K, V> { /// The inner table. table: Table<K, V>, } impl<K, V> Iterator for IntoIter<K, V> { type Item = (K, V); fn next(&mut self) -> Option<(K, V)> { // We own the table, and can thus do what we want with it. We'll simply pop from the // buckets until we find a bucket containing data. while let Some(bucket) = self.table.buckets.pop() { // We can bypass dem ebil locks. if let Bucket::Contains(key, val) = bucket.into_inner() { // The bucket contained data, so we'll return the pair. return Some((key, val)); } } // We've exhausted all the buckets, and no more data could be found. None } } impl<K, V> IntoIterator for Table<K, V> { type Item = (K, V); type IntoIter = IntoIter<K, V>; fn into_iter(self) -> IntoIter<K, V> { IntoIter { table: self, } } } /// A RAII guard for reading an entry of a hash map. /// /// This is an access type dereferencing to the inner value of the entry. It will handle unlocking /// on drop. pub struct ReadGuard<'a, K: 'a, V: 'a> { /// The inner hecking long type. inner: OwningRef<OwningHandle<RwLockReadGuard<'a, Table<K, V>>, RwLockReadGuard<'a, Bucket<K, V>>>, V>, } impl<'a, K, V> ops::Deref for ReadGuard<'a, K, V> { type Target = V; fn deref(&self) -> &V { &self.inner } } impl<'a, K, V: PartialEq> cmp::PartialEq for ReadGuard<'a, K, V> { fn eq(&self, other: &ReadGuard<'a, K, V>) -> bool { self == other } } impl<'a, K, V: Eq> cmp::Eq for ReadGuard<'a, K, V> {} impl<'a, K: fmt::Debug, V: fmt::Debug> fmt::Debug for ReadGuard<'a, K, V> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "ReadGuard({:?})", self) } } /// A mutable RAII guard for reading an entry of a hash map. /// /// This is an access type dereferencing to the inner value of the entry. It will handle unlocking /// on drop. pub struct WriteGuard<'a, K: 'a, V: 'a> { /// The inner hecking long type. inner: OwningHandle<OwningHandle<RwLockReadGuard<'a, Table<K, V>>, RwLockWriteGuard<'a, Bucket<K, V>>>, &'a mut V>, } impl<'a, K, V> ops::Deref for WriteGuard<'a, K, V> { type Target = V; fn deref(&self) -> &V { &self.inner } } impl<'a, K, V> ops::DerefMut for WriteGuard<'a, K, V> { fn deref_mut(&mut self) -> &mut V { &mut self.inner } } impl<'a, K, V: PartialEq> cmp::PartialEq for WriteGuard<'a, K, V> { fn eq(&self, other: &WriteGuard<'a, K, V>) -> bool { self == other } } impl<'a, K, V: Eq> cmp::Eq for WriteGuard<'a, K, V> {} impl<'a, K: fmt::Debug, V: fmt::Debug> fmt::Debug for WriteGuard<'a, K, V> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "WriteGuard({:?})", self) } } /// A concurrent hash map. /// /// This type defines a concurrent associative array, based on hash tables with linear probing and /// dynamic resizing. /// /// The idea is to let each entry hold a multi-reader lock, effectively limiting lock contentions /// to writing simultaneously on the same entry, and resizing the table. /// /// It is not an atomic or lockless hash table, since such construction is only useful in very few /// cases, due to limitations on in-place operations on values. pub struct CHashMap<K, V> { /// The inner table. table: RwLock<Table<K, V>>, /// The total number of KV pairs in the table. /// /// This is used to calculate the load factor. len: AtomicUsize, } impl<K, V> CHashMap<K, V> { /// Create a new hash map with a certain capacity. /// /// "Capacity" means the amount of entries the hash map can hold before reallocating. This /// function allocates a hash map with at least the capacity of `cap`. pub fn with_capacity(cap: usize) -> CHashMap<K, V> { CHashMap { // Start at 0 KV pairs. len: AtomicUsize::new(0), // Make a new empty table. We will make sure that it is at least one. table: RwLock::new(Table::with_capacity(cap)), } } /// Create a new hash map. /// /// This creates a new hash map with some fixed initial capacity. pub fn new() -> CHashMap<K, V> { CHashMap::with_capacity(DEFAULT_INITIAL_CAPACITY) } /// Get the number of entries in the hash table. /// /// This is entirely atomic, and will not acquire any locks. /// /// This is guaranteed to reflect the number of entries _at this particular moment. pub fn len(&self) -> usize { self.len.load(ORDERING) } /// Get the capacity of the hash table. /// /// The capacity is equal to the number of entries the table can hold before reallocating. pub fn capacity(&self) -> usize { self.buckets() * MAX_LOAD_FACTOR_NUM / MAX_LOAD_FACTOR_DENOM } /// Get the number of buckets of the hash table. /// /// "Buckets" refers to the amount of potential entries in the inner table. It is different /// from capacity, in the sense that the map cannot hold this number of entries, since it needs /// to keep the load factor low. pub fn buckets(&self) -> usize { self.table.read().buckets.len() } /// Is the hash table empty? pub fn is_empty(&self) -> bool { self.len() == 0 } /// Clear the map. /// /// This clears the hash map and returns the previous version of the map. /// /// It is relatively efficient, although it needs to write lock a RW lock. pub fn clear(&self) -> CHashMap<K, V> { // Acquire a writable lock. let mut lock = self.table.write(); CHashMap { // Replace the old table with an empty initial table. table: RwLock::new(mem::replace(&mut *lock, Table::new(DEFAULT_INITIAL_CAPACITY))), // Replace the length with 0 and use the old length. len: AtomicUsize::new(self.len.swap(0, ORDERING)), } } /// Filter the map based on some predicate /// /// This tests every entry in the hash map by closure `predicate`. If it returns `true`, the /// map will retain the entry. If not, the entry will be removed. /// /// This won't lock the table. This can be a major performance trade-off, as it means that it /// must lock on every table entry. However, it won't block other operations of the table, /// while filtering. pub fn filter<F>(&self, predicate: F) where F: Fn(&K, &V) -> bool { // Acquire the read lock to the table. let table = self.table.read(); // Run over every bucket and apply the filter. for bucket in &table.buckets { // Acquire the read lock, which we will upgrade if necessary. // TODO: Use read lock and upgrade later. let mut lock = bucket.write(); // Skip the free buckets. // TODO: Fold the `if` into the `match` when the borrowck gets smarter. if match *lock { Bucket::Contains(ref key, ref val) => !predicate(key, val), _ => false, } { // Predicate didn't match. Set the bucket to removed. *lock = Bucket::Removed; // Decrement the length to account for the removed bucket. // TODO: Can we somehow bundle these up to reduce the overhead of atomic // operations? Storing in a local variable and then subtracting causes // issues with consistency. self.len.fetch_sub(1, ORDERING); } } } } impl<K: PartialEq + Hash, V> CHashMap<K, V> { /// Get the value of some key. /// /// This will lookup the entry of some key `key`, and acquire the read-only lock. This means /// that all other parties are blocked from _writing_ (not reading) this value while the guard /// is held. pub fn get(&self, key: &K) -> Option<ReadGuard<K, V>> { // Acquire the read lock and lookup in the table. if let Ok(inner) = OwningRef::new(OwningHandle::new(self.table.read(), |x| unsafe { &*x }.lookup(key))) .try_map(|x| x.value_ref()) { // The bucket contains data. Some(ReadGuard { inner: inner, }) } else { // The bucket is empty/removed. None } } /// Get the (mutable) value of some key. /// /// This will lookup the entry of some key `key`, and acquire the writable lock. This means /// that all other parties are blocked from both reading and writing this value while the guard /// is held. pub fn get_mut(&self, key: &K) -> Option<WriteGuard<K, V>> { // Acquire the write lock and lookup in the table. if let Ok(inner) = OwningHandle::try_new(OwningHandle::new(self.table.read(), |x| unsafe { &*x }.lookup_mut(key)), |x| if let &mut Bucket::Contains(_, ref mut val) = unsafe { &mut *(x as *mut Bucket<K, V>) } { // The bucket contains data. Ok(val) } else { // The bucket is empty/removed. Err(()) }) { Some(WriteGuard { inner: inner, }) } else { None } } /// Does the hash map contain this key? pub fn contains_key(&self, key: &K) -> bool { // Acquire the lock. let lock = self.table.read(); // Look the key up in the table let bucket = lock.lookup(key); // Test if it is free or not. !bucket.is_free() // fuck im sleepy rn } /// Insert a **new** entry. /// /// This inserts an entry, which the map does not already contain, into the table. If the entry /// exists, the old entry won't be replaced, nor will an error be returned. It will possibly /// introduce silent bugs. /// /// To be more specific, it assumes that the entry does not already exist, and will simply skip /// to the end of the cluster, even if it does exist. /// /// This is faster than e.g. `insert`, but should only be used, if you know that the entry /// doesn't already exist. /// /// # Warning /// /// Only use this, if you know what you're doing. This can easily introduce very complex logic /// errors. /// /// For most other purposes, use `insert`. /// /// # Panics /// /// This might perform checks in debug mode testing if the key exists already. pub fn insert_new(&self, key: K, val: V) { debug_assert!(!self.contains_key(&key), "Hash table contains already key, contrary to \ the assumptions about `insert_new`'s arguments."); // Expand and lock the table. We need to expand to ensure the bounds on the load factor. let lock = self.table.read(); { // Find the free bucket. let mut bucket = lock.find_free(&key); // Set the bucket to the new KV pair. *bucket = Bucket::Contains(key, val); } // Expand the table (we know beforehand that the entry didn't already exist). self.expand(lock); } /// Replace an existing entry, or insert a new one. /// /// This will replace an existing entry and return the old entry, if any. If no entry exists, /// it will simply insert the new entry and return `None`. pub fn insert(&self, key: K, val: V) -> Option<V> { let ret; // Expand and lock the table. We need to expand to ensure the bounds on the load factor. let lock = self.table.read(); { // Lookup the key or a free bucket in the inner table. let mut bucket = lock.lookup_or_free(&key); // Replace the bucket. ret = mem::replace(&mut *bucket, Bucket::Contains(key, val)).value(); } // Expand the table if no bucket was overwritten (i.e. the entry is fresh). if ret.is_none() { self.expand(lock); } ret } /// Insert or update. /// /// This looks up `key`. If it exists, the reference to its value is passed through closure /// `update`. If it doesn't exist, the result of closure `insert` is inserted. pub fn upsert<F, G>(&self, key: K, insert: F, update: G) where F: FnOnce() -> V, G: FnOnce(&mut V) { // Expand and lock the table. We need to expand to ensure the bounds on the load factor. let lock = self.table.read(); { // Lookup the key or a free bucket in the inner table. let mut bucket = lock.lookup_or_free(&key); match *bucket { // The bucket had KV pair! Bucket::Contains(_, ref mut val) => { // Run it through the closure. update(val); // TODO: We return to stop the borrowck to yell at us. This prevents the control flow // from reaching the expansion after the match if it has been right here. return; }, // The bucket was empty, simply insert. ref mut x => *x = Bucket::Contains(key, insert()), } } // Expand the table (this will only happen if the function haven't returned yet). self.expand(lock); } /// Map or insert an entry. /// /// This sets the value associated with key `key` to `f(Some(old_val))` (if it returns `None`, /// the entry is removed) if it exists. If it does not exist, it inserts it with value /// `f(None)`, unless the closure returns `None`. /// /// Note that if `f` returns `None`, the entry of key `key` is removed unconditionally. pub fn alter<F>(&self, key: K, f: F) where F: FnOnce(Option<V>) -> Option<V> { // Expand and lock the table. We need to expand to ensure the bounds on the load factor. let lock = self.table.read(); { // Lookup the key or a free bucket in the inner table. let mut bucket = lock.lookup_or_free(&key); match mem::replace(&mut *bucket, Bucket::Removed) { Bucket::Contains(_, val) => if let Some(new_val) = f(Some(val)) { // Set the bucket to a KV pair with the new value. *bucket = Bucket::Contains(key, new_val); // No extension required, as the bucket already had a KV pair previously. } else { // The old entry was removed, so we decrement the length of the map. self.len.fetch_sub(1, ORDERING); // TODO: We return as a hack to avoid the borrowchecker from thinking we moved a // referenced object. Namely, under this match arm the expansion after the match // statement won't ever be reached. return; }, _ => if let Some(new_val) = f(None) { // The previously free cluster will get a KV pair with the new value. *bucket = Bucket::Contains(key, new_val); } else { return; }, } } // A new entry was inserted, so naturally, we expand the table. self.expand(lock); } /// Remove an entry. /// /// This removes and returns the entry with key `key`. If no entry with said key exists, it /// will simply return `None`. pub fn remove(&self, key: &K) -> Option<V> { // Acquire the read lock of the table. let lock = self.table.read(); // Lookup the table, mutably. let mut bucket = lock.lookup_mut(&key); // Remove the bucket. match &mut *bucket { // There was nothing to remove. &mut Bucket::Removed | &mut Bucket::Empty => None, // TODO: We know that this is a `Bucket::Contains` variant, but to bypass borrowck // madness, we do weird weird stuff. bucket => { // Decrement the length of the map. self.len.fetch_sub(1, ORDERING); // Set the bucket to "removed" and return its value. mem::replace(bucket, Bucket::Removed).value() }, } } /// Reserve additional space. /// /// This reserves additional `additional` buckets to the table. Note that it might reserve more /// in order make reallocation less common. pub fn reserve(&self, additional: usize) { // Get the new length. let len = self.len() + additional; // Acquire the write lock (needed because we'll mess with the table). let mut lock = self.table.write(); // Handle the case where another thread has resized the table while we were acquiring the // lock. if lock.buckets.len() < len * LENGTH_MULTIPLIER { // Swap the table out with a new table of desired size (multiplied by some factor). let table = mem::replace(&mut *lock, Table::with_capacity(len)); // Fill the new table with the data from the old table. lock.fill(table); } } /// Shrink the capacity of the map to reduce space usage. /// /// This will shrink the capacity of the map to the needed amount (plus some additional space /// to avoid reallocations), effectively reducing memory usage in cases where there is /// excessive space. /// /// It is healthy to run this once in a while, if the size of your hash map changes a lot (e.g. /// has a high maximum case). pub fn shrink_to_fit(&self) { // Acquire the write lock (needed because we'll mess with the table). let mut lock = self.table.write(); // Swap the table out with a new table of desired size (multiplied by some factor). let table = mem::replace(&mut *lock, Table::with_capacity(self.len())); // Fill the new table with the data from the old table. lock.fill(table); } /// Increment the size of the hash map and expand it so one more entry can fit in. /// /// This returns the read lock, such that the caller won't have to acquire it twice. fn expand(&self, lock: RwLockReadGuard<Table<K, V>>) { // Increment the length to take the new element into account. let len = self.len.fetch_add(1, ORDERING) + 1; // Extend if necessary. We multiply by some constant to adjust our load factor. if len * MAX_LOAD_FACTOR_DENOM > lock.buckets.len() * MAX_LOAD_FACTOR_NUM { // Drop the read lock to avoid deadlocks when acquiring the write lock. drop(lock); // Reserve 1 entry in space (the function will handle the excessive space logic). self.reserve(1); } } } impl<K, V> Default for CHashMap<K, V> { fn default() -> CHashMap<K, V> { // Forward the call to `new`. CHashMap::new() } } impl<K: Clone, V: Clone> Clone for CHashMap<K, V> { fn clone(&self) -> CHashMap<K, V> { CHashMap { table: RwLock::new(self.table.read().clone()), len: AtomicUsize::new(self.len.load(ORDERING)), } } } impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for CHashMap<K, V> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { write!(f, "{:?}", *self.table.read()) } } impl<K, V> IntoIterator for CHashMap<K, V> { type Item = (K, V); type IntoIter = IntoIter<K, V>; fn into_iter(self) -> IntoIter<K, V> { self.table.into_inner().into_iter() } } impl<K: PartialEq + Hash, V> iter::FromIterator<(K, V)> for CHashMap<K, V> { fn from_iter<I: IntoIterator<Item = (K, V)>>(iter: I) -> CHashMap<K, V> { // TODO: This step is required to obtain the length of the iterator. Eliminate it. let vec: Vec<_> = iter.into_iter().collect(); let len = vec.len(); // Start with an empty table. let mut table = Table::with_capacity(len); // Fill the table with the pairs from the iterator. for (key, val) in vec { // Insert the KV pair. This is fine, as we are ensured that there are no duplicates in // the iterator. let bucket = table.find_free_no_lock(&key); *bucket = Bucket::Contains(key, val); } CHashMap { table: RwLock::new(table), len: AtomicUsize::new(len), } } }