// Inspired by github.com/SaveTheRbtz/generic-sync-map-go but technically
// written from scratch with Go 1.23's sync.Map.
// Copyright 2024 Runxi Yu (porting it to generics)
// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package lindenii_common
import (
"sync"
"sync/atomic"
"unsafe"
)
// Map[K comparable, V comparable] is like a Go map[K]V but is safe for concurrent use
// by multiple goroutines without additional locking or coordination. Loads,
// stores, and deletes run in amortized constant time.
//
// The Map type is optimized for two common use cases: (1) when the entry for a given
// key is only ever written once but read many times, as in caches that only grow,
// or (2) when multiple goroutines read, write, and overwrite entries for disjoint
// sets of keys. In these two cases, use of a Map may significantly reduce lock
// contention compared to a Go map paired with a separate [Mutex] or [RWMutex].
//
// The zero Map is empty and ready for use. A Map must not be copied after first use.
//
// In the terminology of [the Go memory model], Map arranges that a write operation
// “synchronizes before” any read operation that observes the effect of the write, where
// read and write operations are defined as follows.
// [Map.Load], [Map.LoadAndDelete], [Map.LoadOrStore], [Map.Swap], [Map.CompareAndSwap],
// and [Map.CompareAndDelete] are read operations;
// [Map.Delete], [Map.LoadAndDelete], [Map.Store], and [Map.Swap] are write operations;
// [Map.LoadOrStore] is a write operation when it returns loaded set to false;
// [Map.CompareAndSwap] is a write operation when it returns swapped set to true;
// and [Map.CompareAndDelete] is a write operation when it returns deleted set to true.
//
// [the Go memory model]: https://go.dev/ref/mem
type Map[K comparable, V comparable] struct {
mu sync.Mutex
// read contains the portion of the map's contents that are safe for
// concurrent access (with or without mu held).
//
// The read field itself is always safe to load, but must only be stored with
// mu held.
//
// Entries stored in read may be updated concurrently without mu, but updating
// a previously-expunged entry requires that the entry be copied to the dirty
// map and unexpunged with mu held.
read atomic.Pointer[readOnly[K, V]]
// dirty contains the portion of the map's contents that require mu to be
// held. To ensure that the dirty map can be promoted to the read map quickly,
// it also includes all of the non-expunged entries in the read map.
//
// Expunged entries are not stored in the dirty map. An expunged entry in the
// clean map must be unexpunged and added to the dirty map before a new value
// can be stored to it.
//
// If the dirty map is nil, the next write to the map will initialize it by
// making a shallow copy of the clean map, omitting stale entries.
dirty map[K]*entry[V]
// misses counts the number of loads since the read map was last updated that
// needed to lock mu to determine whether the key was present.
//
// Once enough misses have occurred to cover the cost of copying the dirty
// map, the dirty map will be promoted to the read map (in the unamended
// state) and the next store to the map will make a new dirty copy.
misses int
}
// readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly[K comparable, V comparable] struct {
m map[K]*entry[V]
amended bool // true if the dirty map contains some key not in m.
}
// expunged is an arbitrary pointer that marks entries which have been deleted
// from the dirty map.
var expunged = unsafe.Pointer(new(any))
// An entry is a slot in the map corresponding to a particular key.
type entry[V comparable] struct {
// p points to the interface{} value stored for the entry.
//
// If p == nil, the entry has been deleted, and either m.dirty == nil or
// m.dirty[key] is e.
//
// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry
// is missing from m.dirty.
//
// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty
// != nil, in m.dirty[key].
//
// An entry can be deleted by atomic replacement with nil: when m.dirty is
// next created, it will atomically replace nil with expunged and leave
// m.dirty[key] unset.
//
// An entry's associated value can be updated by atomic replacement, provided
// p != expunged. If p == expunged, an entry's associated value can be updated
// only after first setting m.dirty[key] = e so that lookups using the dirty
// map find the entry.
p unsafe.Pointer
}
func newEntry[V comparable](i V) *entry[V] {
return &entry[V]{p: unsafe.Pointer(&i)}
}
func (m *Map[K, V]) loadReadOnly() readOnly[K, V] {
if p := m.read.Load(); p != nil {
return *p
}
return readOnly[K, V]{}
}
// Load returns the value stored in the map for a key, or nil if no
// value is present.
// The ok result indicates whether value was found in the map.
func (m *Map[K, V]) Load(key K) (value V, ok bool) {
read := m.loadReadOnly()
e, ok := read.m[key]
if !ok && read.amended {
m.mu.Lock()
// Avoid reporting a spurious miss if m.dirty got promoted while we were
// blocked on m.mu. (If further loads of the same key will not miss, it's
// not worth copying the dirty map for this key.)
read = m.loadReadOnly()
e, ok = read.m[key]
if !ok && read.amended {
e, ok = m.dirty[key]
// Regardless of whether the entry was present, record a miss: this key
// will take the slow path until the dirty map is promoted to the read
// map.
m.missLocked()
}
m.mu.Unlock()
}
if !ok {
return *new(V), false
}
return e.load()
}
func (e *entry[V]) load() (value V, ok bool) {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged {
return value, false
}
return *(*V)(p), true
}
// Store sets the value for a key.
func (m *Map[K, V]) Store(key K, value V) {
_, _ = m.Swap(key, value)
}
// Clear deletes all the entries, resulting in an empty Map.
func (m *Map[K, V]) Clear() {
read := m.loadReadOnly()
if len(read.m) == 0 && !read.amended {
// Avoid allocating a new readOnly when the map is already clear.
return
}
m.mu.Lock()
defer m.mu.Unlock()
read = m.loadReadOnly()
if len(read.m) > 0 || read.amended {
m.read.Store(&readOnly[K, V]{})
}
clear(m.dirty)
// Don't immediately promote the newly-cleared dirty map on the next operation.
m.misses = 0
}
// tryCompareAndSwap compare the entry with the given old value and swaps
// it with a new value if the entry is equal to the old value, and the entry
// has not been expunged.
//
// If the entry is expunged, tryCompareAndSwap returns false and leaves
// the entry unchanged.
func (e *entry[V]) tryCompareAndSwap(old V, new V) bool {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged || *(*V)(p) != old { // XXX
return false
}
// Copy the interface after the first load to make this method more amenable
// to escape analysis: if the comparison fails from the start, we shouldn't
// bother heap-allocating an interface value to store.
nc := new
for {
if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(&nc)) {
return true
}
p = atomic.LoadPointer(&e.p)
if p == nil || p == expunged || *(*V)(p) != old {
return false
}
}
}
// unexpungeLocked ensures that the entry is not marked as expunged.
//
// If the entry was previously expunged, it must be added to the dirty map
// before m.mu is unlocked.
func (e *entry[V]) unexpungeLocked() (wasExpunged bool) {
return atomic.CompareAndSwapPointer(&e.p, expunged, nil)
}
// swapLocked unconditionally swaps a value into the entry.
//
// The entry must be known not to be expunged.
func (e *entry[V]) swapLocked(i *V) *V {
return (*V)(atomic.SwapPointer(&e.p, unsafe.Pointer(i)))
}
// LoadOrStore returns the existing value for the key if present.
// Otherwise, it stores and returns the given value.
// The loaded result is true if the value was loaded, false if stored.
func (m *Map[K, V]) LoadOrStore(key K, value V) (actual V, loaded bool) {
// Avoid locking if it's a clean hit.
read := m.loadReadOnly()
if e, ok := read.m[key]; ok {
actual, loaded, ok := e.tryLoadOrStore(value)
if ok {
return actual, loaded
}
}
m.mu.Lock()
read = m.loadReadOnly()
if e, ok := read.m[key]; ok {
if e.unexpungeLocked() {
m.dirty[key] = e
}
actual, loaded, _ = e.tryLoadOrStore(value)
} else if e, ok := m.dirty[key]; ok {
actual, loaded, _ = e.tryLoadOrStore(value)
m.missLocked()
} else {
if !read.amended {
// We're adding the first new key to the dirty map.
// Make sure it is allocated and mark the read-only map as incomplete.
m.dirtyLocked()
m.read.Store(&readOnly[K, V]{m: read.m, amended: true})
}
m.dirty[key] = newEntry(value)
actual, loaded = value, false
}
m.mu.Unlock()
return actual, loaded
}
// tryLoadOrStore atomically loads or stores a value if the entry is not
// expunged.
//
// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and
// returns with ok==false.
func (e *entry[V]) tryLoadOrStore(i V) (actual V, loaded, ok bool) {
p := atomic.LoadPointer(&e.p)
if p == expunged {
return actual, false, false
}
if p != nil {
return *(*V)(p), true, true
}
// Copy the interface after the first load to make this method more amenable
// to escape analysis: if we hit the "load" path or the entry is expunged, we
// shouldn't bother heap-allocating.
ic := i
for {
if atomic.CompareAndSwapPointer(&e.p, nil, unsafe.Pointer(&ic)) {
return i, false, true
}
p = atomic.LoadPointer(&e.p)
if p == expunged {
return actual, false, false
}
if p != nil {
return *(*V)(p), true, true
}
}
}
// LoadAndDelete deletes the value for a key, returning the previous value if any.
// The loaded result reports whether the key was present.
func (m *Map[K, V]) LoadAndDelete(key K) (value V, loaded bool) {
read := m.loadReadOnly()
e, ok := read.m[key]
if !ok && read.amended {
m.mu.Lock()
read = m.loadReadOnly()
e, ok = read.m[key]
if !ok && read.amended {
e, ok = m.dirty[key]
delete(m.dirty, key)
// Regardless of whether the entry was present, record a miss: this key
// will take the slow path until the dirty map is promoted to the read
// map.
m.missLocked()
}
m.mu.Unlock()
}
if ok {
return e.delete()
}
return value, false
}
// Delete deletes the value for a key.
func (m *Map[K, V]) Delete(key K) {
m.LoadAndDelete(key)
}
func (e *entry[V]) delete() (value V, ok bool) {
for {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged {
return value, false
}
if atomic.CompareAndSwapPointer(&e.p, p, nil) {
return *(*V)(p), true
}
}
}
// trySwap swaps a value if the entry has not been expunged.
//
// If the entry is expunged, trySwap returns false and leaves the entry
// unchanged.
func (e *entry[V]) trySwap(i *V) (*V, bool) {
for {
p := atomic.LoadPointer(&e.p)
if p == expunged {
return nil, false
}
if atomic.CompareAndSwapPointer(&e.p, p, unsafe.Pointer(i)) {
return (*V)(p), true
}
}
}
// Swap swaps the value for a key and returns the previous value if any.
// The loaded result reports whether the key was present.
func (m *Map[K, V]) Swap(key K, value V) (previous V, loaded bool) {
read := m.loadReadOnly()
if e, ok := read.m[key]; ok {
if v, ok := e.trySwap(&value); ok {
if v == nil {
return previous, false
}
return *v, true
}
}
m.mu.Lock()
read = m.loadReadOnly()
if e, ok := read.m[key]; ok {
if e.unexpungeLocked() {
// The entry was previously expunged, which implies that there is a
// non-nil dirty map and this entry is not in it.
m.dirty[key] = e
}
if v := e.swapLocked(&value); v != nil {
loaded = true
previous = *v
}
} else if e, ok := m.dirty[key]; ok {
if v := e.swapLocked(&value); v != nil {
loaded = true
previous = *v
}
} else {
if !read.amended {
// We're adding the first new key to the dirty map.
// Make sure it is allocated and mark the read-only map as incomplete.
m.dirtyLocked()
m.read.Store(&readOnly[K, V]{m: read.m, amended: true})
}
m.dirty[key] = newEntry(value)
}
m.mu.Unlock()
return previous, loaded
}
// CompareAndSwap swaps the old and new values for key
// if the value stored in the map is equal to old.
// The old value must be of a comparable type.
func (m *Map[K, V]) CompareAndSwap(key K, old, new V) (swapped bool) {
read := m.loadReadOnly()
if e, ok := read.m[key]; ok {
return e.tryCompareAndSwap(old, new)
} else if !read.amended {
return false // No existing value for key.
}
m.mu.Lock()
defer m.mu.Unlock()
read = m.loadReadOnly()
swapped = false
if e, ok := read.m[key]; ok {
swapped = e.tryCompareAndSwap(old, new)
} else if e, ok := m.dirty[key]; ok {
swapped = e.tryCompareAndSwap(old, new)
// We needed to lock mu in order to load the entry for key,
// and the operation didn't change the set of keys in the map
// (so it would be made more efficient by promoting the dirty
// map to read-only).
// Count it as a miss so that we will eventually switch to the
// more efficient steady state.
m.missLocked()
}
return swapped
}
// CompareAndDelete deletes the entry for key if its value is equal to old.
// The old value must be of a comparable type.
//
// If there is no current value for key in the map, CompareAndDelete
// returns false (even if the old value is the nil interface value).
func (m *Map[K, V]) CompareAndDelete(key K, old V) (deleted bool) {
read := m.loadReadOnly()
e, ok := read.m[key]
if !ok && read.amended {
m.mu.Lock()
read = m.loadReadOnly()
e, ok = read.m[key]
if !ok && read.amended {
e, ok = m.dirty[key]
// Don't delete key from m.dirty: we still need to do the “compare” part
// of the operation. The entry will eventually be expunged when the
// dirty map is promoted to the read map.
//
// Regardless of whether the entry was present, record a miss: this key
// will take the slow path until the dirty map is promoted to the read
// map.
m.missLocked()
}
m.mu.Unlock()
}
for ok {
p := atomic.LoadPointer(&e.p)
if p == nil || p == expunged || *(*V)(p) != old {
return false
}
if atomic.CompareAndSwapPointer(&e.p, p, nil) {
return true
}
}
return false
}
// Range calls f sequentially for each key and value present in the map.
// If f returns false, range stops the iteration.
//
// Range does not necessarily correspond to any consistent snapshot of the Map's
// contents: no key will be visited more than once, but if the value for any key
// is stored or deleted concurrently (including by f), Range may reflect any
// mapping for that key from any point during the Range call. Range does not
// block other methods on the receiver; even f itself may call any method on m.
//
// Range may be O(N) with the number of elements in the map even if f returns
// false after a constant number of calls.
func (m *Map[K, V]) Range(f func(key K, value V) bool) {
// We need to be able to iterate over all of the keys that were already
// present at the start of the call to Range.
// If read.amended is false, then read.m satisfies that property without
// requiring us to hold m.mu for a long time.
read := m.loadReadOnly()
if read.amended {
// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)
// (assuming the caller does not break out early), so a call to Range
// amortizes an entire copy of the map: we can promote the dirty copy
// immediately!
m.mu.Lock()
read = m.loadReadOnly()
if read.amended {
read = readOnly[K, V]{m: m.dirty}
copyRead := read
m.read.Store(©Read)
m.dirty = nil
m.misses = 0
}
m.mu.Unlock()
}
for k, e := range read.m {
v, ok := e.load()
if !ok {
continue
}
if !f(k, v) {
break
}
}
}
func (m *Map[K, V]) missLocked() {
m.misses++
if m.misses < len(m.dirty) {
return
}
m.read.Store(&readOnly[K, V]{m: m.dirty})
m.dirty = nil
m.misses = 0
}
func (m *Map[K, V]) dirtyLocked() {
if m.dirty != nil {
return
}
read := m.loadReadOnly()
m.dirty = make(map[K]*entry[V], len(read.m))
for k, e := range read.m {
if !e.tryExpungeLocked() {
m.dirty[k] = e
}
}
}
func (e *entry[V]) tryExpungeLocked() (isExpunged bool) {
p := atomic.LoadPointer(&e.p)
for p == nil {
if atomic.CompareAndSwapPointer(&e.p, nil, expunged) {
return true
}
p = atomic.LoadPointer(&e.p)
}
return p == expunged
}