map.go

     1  // Copyright 2024 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package sync
     6  
     7  import (
     8  	isync "internal/sync"
     9  )
    10  
    11  // Map is like a Go map[any]any but is safe for concurrent use
    12  // by multiple goroutines without additional locking or coordination.
    13  // Loads, stores, and deletes run in amortized constant time.
    14  //
    15  // The Map type is specialized. Most code should use a plain Go map instead,
    16  // with separate locking or coordination, for better type safety and to make it
    17  // easier to maintain other invariants along with the map content.
    18  //
    19  // The Map type is optimized for two common use cases: (1) when the entry for a given
    20  // key is only ever written once but read many times, as in caches that only grow,
    21  // or (2) when multiple goroutines read, write, and overwrite entries for disjoint
    22  // sets of keys. In these two cases, use of a Map may significantly reduce lock
    23  // contention compared to a Go map paired with a separate [Mutex] or [RWMutex].
    24  //
    25  // The zero Map is empty and ready for use. A Map must not be copied after first use.
    26  //
    27  // In the terminology of [the Go memory model], Map arranges that a write operation
    28  // “synchronizes before” any read operation that observes the effect of the write, where
    29  // read and write operations are defined as follows.
    30  // [Map.Load], [Map.LoadAndDelete], [Map.LoadOrStore], [Map.Swap], [Map.CompareAndSwap],
    31  // and [Map.CompareAndDelete] are read operations;
    32  // [Map.Delete], [Map.LoadAndDelete], [Map.Store], and [Map.Swap] are write operations;
    33  // [Map.LoadOrStore] is a write operation when it returns loaded set to false;
    34  // [Map.CompareAndSwap] is a write operation when it returns swapped set to true;
    35  // and [Map.CompareAndDelete] is a write operation when it returns deleted set to true.
    36  //
    37  // [the Go memory model]: https://go.dev/ref/mem
    38  type Map struct {
    39  	_ noCopy
    40  
    41  	m isync.HashTrieMap[any, any]
    42  }
    43  
    44  // Load returns the value stored in the map for a key, or nil if no
    45  // value is present.
    46  // The ok result indicates whether value was found in the map.
    47  func (m *Map) Load(key any) (value any, ok bool) {
    48  	return m.m.Load(key)
    49  }
    50  
    51  // Store sets the value for a key.
    52  func (m *Map) Store(key, value any) {
    53  	m.m.Store(key, value)
    54  }
    55  
    56  // Clear deletes all the entries, resulting in an empty Map.
    57  func (m *Map) Clear() {
    58  	m.m.Clear()
    59  }
    60  
    61  // LoadOrStore returns the existing value for the key if present.
    62  // Otherwise, it stores and returns the given value.
    63  // The loaded result is true if the value was loaded, false if stored.
    64  func (m *Map) LoadOrStore(key, value any) (actual any, loaded bool) {
    65  	return m.m.LoadOrStore(key, value)
    66  }
    67  
    68  // LoadAndDelete deletes the value for a key, returning the previous value if any.
    69  // The loaded result reports whether the key was present.
    70  func (m *Map) LoadAndDelete(key any) (value any, loaded bool) {
    71  	return m.m.LoadAndDelete(key)
    72  }
    73  
    74  // Delete deletes the value for a key.
    75  // If the key is not in the map, Delete does nothing.
    76  func (m *Map) Delete(key any) {
    77  	m.m.Delete(key)
    78  }
    79  
    80  // Swap swaps the value for a key and returns the previous value if any.
    81  // The loaded result reports whether the key was present.
    82  func (m *Map) Swap(key, value any) (previous any, loaded bool) {
    83  	return m.m.Swap(key, value)
    84  }
    85  
    86  // CompareAndSwap swaps the old and new values for key
    87  // if the value stored in the map is equal to old.
    88  // The old value must be of a comparable type.
    89  func (m *Map) CompareAndSwap(key, old, new any) (swapped bool) {
    90  	return m.m.CompareAndSwap(key, old, new)
    91  }
    92  
    93  // CompareAndDelete deletes the entry for key if its value is equal to old.
    94  // The old value must be of a comparable type.
    95  //
    96  // If there is no current value for key in the map, CompareAndDelete
    97  // returns false (even if the old value is the nil interface value).
    98  func (m *Map) CompareAndDelete(key, old any) (deleted bool) {
    99  	return m.m.CompareAndDelete(key, old)
   100  }
   101  
   102  // Range calls f sequentially for each key and value present in the map.
   103  // If f returns false, range stops the iteration.
   104  //
   105  // Range does not necessarily correspond to any consistent snapshot of the Map's
   106  // contents: no key will be visited more than once, but if the value for any key
   107  // is stored or deleted concurrently (including by f), Range may reflect any
   108  // mapping for that key from any point during the Range call. Range does not
   109  // block other methods on the receiver; even f itself may call any method on m.
   110  //
   111  // Range may be O(N) with the number of elements in the map even if f returns
   112  // false after a constant number of calls.
   113  func (m *Map) Range(f func(key, value any) bool) {
   114  	m.m.Range(f)
   115  }
   116
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