go src - sync.Map

var m sync.Map

type Map struct {
	mu Mutex

	// read contains the portion of the map's contents that are safe for
	// concurrent access (with or without mu held).
	read atomic.Pointer[readOnly]

	// 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.
	dirty map[any]*entry

	// 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.
	misses int
}

// readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly struct {
	m       map[any]*entry
	amended bool // true if the dirty map contains some key not in m.
}

// An entry is a slot in the map corresponding to a particular key.
type entry struct {
	// p points to the interface{} value stored for the entry.
	p atomic.Pointer[any]
}

// expunged is an arbitrary pointer that marks entries which have been deleted
// from the dirty map.
var expunged = new(any)

// 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) Load(key any) (value any, 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 nil, false
	}
	return e.load()
}

func (m *Map) loadReadOnly() readOnly {
	if p := m.read.Load(); p != nil {
		return *p
	}
	return readOnly{}
}

// 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) LoadOrStore(key, value any) (actual any, 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{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) tryLoadOrStore(i any) (actual any, loaded, ok bool) {
	p := e.p.Load()
	if p == expunged {
		return nil, false, false
	}
	if p != nil {
		return *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 e.p.CompareAndSwap(nil, &ic) {
			return i, false, true
		}
		p = e.p.Load()
		if p == expunged {
			return nil, false, false
		}
		if p != nil {
			return *p, true, true
		}
	}
}

// Delete deletes the value for a key.
func (m *Map) Delete(key any) {
	m.LoadAndDelete(key)
}

// 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) LoadAndDelete(key any) (value any, 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 nil, false
}

// Store sets the value for a key.
func (m *Map) Store(key, value any) {
	_, _ = m.Swap(key, value)
}

// 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) Swap(key, value any) (previous any, loaded bool) {
	read := m.loadReadOnly()
	if e, ok := read.m[key]; ok {
		if v, ok := e.trySwap(&value); ok {
			if v == nil {
				return nil, 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{m: read.m, amended: true})
		}
		m.dirty[key] = newEntry(value)
	}
	m.mu.Unlock()
	return previous, loaded
}

// 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) trySwap(i *any) (*any, bool) {
	for {
		p := e.p.Load()
		if p == expunged {
			return nil, false
		}
		if e.p.CompareAndSwap(p, i) {
			return p, true
		}
	}
}

// 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) CompareAndSwap(key, old, new any) 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
}

// 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) tryCompareAndSwap(old, new any) bool {
	p := e.p.Load()
	if p == nil || p == expunged || *p != old {
		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 e.p.CompareAndSwap(p, &nc) {
			return true
		}
		p = e.p.Load()
		if p == nil || p == expunged || *p != old {
			return false
		}
	}
}

// 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) CompareAndDelete(key, old any) (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 := e.p.Load()
		if p == nil || p == expunged || *p != old {
			return false
		}
		if e.p.CompareAndSwap(p, nil) {
			return true
		}
	}
	return false
}