// 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 sync

import (
	
	
)

// Map is like a Go map[interface{}]interface{} 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 specialized. Most code should use a plain Go map instead,
// with separate locking or coordination, for better type safety and to make it
// easier to maintain other invariants along with the map content.
//
// 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.
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).
	//
	// 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.Value // 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.
	//
	// 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[interface{}]*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.
	//
	// 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 struct {
	m       map[interface{}]*entry
	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(interface{}))

// 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.
	//
	// If p == nil, the entry has been deleted and m.dirty == nil.
	//
	// 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 // *interface{}
}

func newEntry( interface{}) *entry {
	return &entry{p: unsafe.Pointer(&)}
}

// 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 ( *Map) ( interface{}) ( interface{},  bool) {
	,  := .read.Load().(readOnly)
	,  := .m[]
	if ! && .amended {
		.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.Load().(readOnly)
		,  = .m[]
		if ! && .amended {
			,  = .dirty[]
			// 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.
			.missLocked()
		}
		.mu.Unlock()
	}
	if ! {
		return nil, false
	}
	return .load()
}

func ( *entry) () ( interface{},  bool) {
	 := atomic.LoadPointer(&.p)
	if  == nil ||  == expunged {
		return nil, false
	}
	return *(*interface{})(), true
}

// Store sets the value for a key.
func ( *Map) (,  interface{}) {
	,  := .read.Load().(readOnly)
	if ,  := .m[];  && .tryStore(&) {
		return
	}

	.mu.Lock()
	, _ = .read.Load().(readOnly)
	if ,  := .m[];  {
		if .unexpungeLocked() {
			// The entry was previously expunged, which implies that there is a
			// non-nil dirty map and this entry is not in it.
			.dirty[] = 
		}
		.storeLocked(&)
	} else if ,  := .dirty[];  {
		.storeLocked(&)
	} else {
		if !.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.
			.dirtyLocked()
			.read.Store(readOnly{m: .m, amended: true})
		}
		.dirty[] = newEntry()
	}
	.mu.Unlock()
}

// tryStore stores a value if the entry has not been expunged.
//
// If the entry is expunged, tryStore returns false and leaves the entry
// unchanged.
func ( *entry) ( *interface{}) bool {
	for {
		 := atomic.LoadPointer(&.p)
		if  == expunged {
			return false
		}
		if atomic.CompareAndSwapPointer(&.p, , unsafe.Pointer()) {
			return true
		}
	}
}

// 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 ( *entry) () ( bool) {
	return atomic.CompareAndSwapPointer(&.p, expunged, nil)
}

// storeLocked unconditionally stores a value to the entry.
//
// The entry must be known not to be expunged.
func ( *entry) ( *interface{}) {
	atomic.StorePointer(&.p, unsafe.Pointer())
}

// 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 ( *Map) (,  interface{}) ( interface{},  bool) {
	// Avoid locking if it's a clean hit.
	,  := .read.Load().(readOnly)
	if ,  := .m[];  {
		, ,  := .tryLoadOrStore()
		if  {
			return , 
		}
	}

	.mu.Lock()
	, _ = .read.Load().(readOnly)
	if ,  := .m[];  {
		if .unexpungeLocked() {
			.dirty[] = 
		}
		, , _ = .tryLoadOrStore()
	} else if ,  := .dirty[];  {
		, , _ = .tryLoadOrStore()
		.missLocked()
	} else {
		if !.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.
			.dirtyLocked()
			.read.Store(readOnly{m: .m, amended: true})
		}
		.dirty[] = newEntry()
		,  = , false
	}
	.mu.Unlock()

	return , 
}

// 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 ( *entry) ( interface{}) ( interface{}, ,  bool) {
	 := atomic.LoadPointer(&.p)
	if  == expunged {
		return nil, false, false
	}
	if  != nil {
		return *(*interface{})(), 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.
	 := 
	for {
		if atomic.CompareAndSwapPointer(&.p, nil, unsafe.Pointer(&)) {
			return , false, true
		}
		 = atomic.LoadPointer(&.p)
		if  == expunged {
			return nil, false, false
		}
		if  != nil {
			return *(*interface{})(), 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 ( *Map) ( interface{}) ( interface{},  bool) {
	,  := .read.Load().(readOnly)
	,  := .m[]
	if ! && .amended {
		.mu.Lock()
		, _ = .read.Load().(readOnly)
		,  = .m[]
		if ! && .amended {
			,  = .dirty[]
			delete(.dirty, )
			// 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.
			.missLocked()
		}
		.mu.Unlock()
	}
	if  {
		return .delete()
	}
	return nil, false
}

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

func ( *entry) () ( interface{},  bool) {
	for {
		 := atomic.LoadPointer(&.p)
		if  == nil ||  == expunged {
			return nil, false
		}
		if atomic.CompareAndSwapPointer(&.p, , nil) {
			return *(*interface{})(), true
		}
	}
}

// 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, Range may reflect any mapping for that key
// from any point during the Range call.
//
// 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 ( *Map) ( func(,  interface{}) 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.Load().(readOnly)
	if .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!
		.mu.Lock()
		, _ = .read.Load().(readOnly)
		if .amended {
			 = readOnly{m: .dirty}
			.read.Store()
			.dirty = nil
			.misses = 0
		}
		.mu.Unlock()
	}

	for ,  := range .m {
		,  := .load()
		if ! {
			continue
		}
		if !(, ) {
			break
		}
	}
}

func ( *Map) () {
	.misses++
	if .misses < len(.dirty) {
		return
	}
	.read.Store(readOnly{m: .dirty})
	.dirty = nil
	.misses = 0
}

func ( *Map) () {
	if .dirty != nil {
		return
	}

	,  := .read.Load().(readOnly)
	.dirty = make(map[interface{}]*entry, len(.m))
	for ,  := range .m {
		if !.tryExpungeLocked() {
			.dirty[] = 
		}
	}
}

func ( *entry) () ( bool) {
	 := atomic.LoadPointer(&.p)
	for  == nil {
		if atomic.CompareAndSwapPointer(&.p, nil, expunged) {
			return true
		}
		 = atomic.LoadPointer(&.p)
	}
	return  == expunged
}