|
| 1 | +package tree |
| 2 | + |
| 3 | +import ( |
| 4 | + "sync" |
| 5 | + "sync/atomic" |
| 6 | +) |
| 7 | + |
| 8 | +// SmallBTree is a small-optimized ordered set/map. |
| 9 | +// |
| 10 | +// It stores 0/1 item inline (lock-free reads) and promotes to a lock-based |
| 11 | +// *BTree when a second distinct item is inserted. |
| 12 | +type SmallBTree[T any] struct { |
| 13 | + mu sync.Mutex |
| 14 | + less func(a, b T) bool |
| 15 | + degree int |
| 16 | + |
| 17 | + slotA T |
| 18 | + slotB T |
| 19 | + |
| 20 | + // single points to either slotA or slotB. |
| 21 | + single atomic.Pointer[T] |
| 22 | + bt atomic.Pointer[BTree[T]] |
| 23 | +} |
| 24 | + |
| 25 | +func NewSmallBTree[T any](less func(a, b T) bool, degree int) *SmallBTree[T] { |
| 26 | + return &SmallBTree[T]{ |
| 27 | + less: less, |
| 28 | + degree: degree, |
| 29 | + } |
| 30 | +} |
| 31 | + |
| 32 | +func (t *SmallBTree[T]) equal(a, b T) bool { |
| 33 | + return !t.less(a, b) && !t.less(b, a) |
| 34 | +} |
| 35 | + |
| 36 | +func (t *SmallBTree[T]) Get(item T) (result T, ok bool) { |
| 37 | + if bt := t.bt.Load(); bt != nil { |
| 38 | + return bt.Get(item) |
| 39 | + } |
| 40 | + if p := t.single.Load(); p != nil { |
| 41 | + if t.equal(*p, item) { |
| 42 | + return *p, true |
| 43 | + } |
| 44 | + } |
| 45 | + var zero T |
| 46 | + return zero, false |
| 47 | +} |
| 48 | + |
| 49 | +func (t *SmallBTree[T]) Set(item T) (prev T, ok bool) { |
| 50 | + if bt := t.bt.Load(); bt != nil { |
| 51 | + return bt.Set(item) |
| 52 | + } |
| 53 | + |
| 54 | + t.mu.Lock() |
| 55 | + defer t.mu.Unlock() |
| 56 | + |
| 57 | + if bt := t.bt.Load(); bt != nil { |
| 58 | + return bt.Set(item) |
| 59 | + } |
| 60 | + |
| 61 | + if p := t.single.Load(); p != nil { |
| 62 | + cur := *p |
| 63 | + if t.equal(cur, item) { |
| 64 | + dst := &t.slotA |
| 65 | + if p == dst { |
| 66 | + dst = &t.slotB |
| 67 | + } |
| 68 | + *dst = item |
| 69 | + t.single.Store(dst) |
| 70 | + return cur, true |
| 71 | + } |
| 72 | + |
| 73 | + // Promote to a full B-tree. |
| 74 | + btNew := NewBTree(t.less, t.degree) |
| 75 | + btNew.Set(cur) |
| 76 | + btNew.Set(item) |
| 77 | + t.bt.Store(btNew) |
| 78 | + // Keep reads correct during promotion by storing bt first, then clearing single. |
| 79 | + t.single.Store(nil) |
| 80 | + var zero T |
| 81 | + return zero, false |
| 82 | + } |
| 83 | + |
| 84 | + dst := &t.slotA |
| 85 | + *dst = item |
| 86 | + t.single.Store(dst) |
| 87 | + var zero T |
| 88 | + return zero, false |
| 89 | +} |
| 90 | + |
| 91 | +func (t *SmallBTree[T]) Delete(item T) (prev T, ok bool) { |
| 92 | + if bt := t.bt.Load(); bt != nil { |
| 93 | + return bt.Delete(item) |
| 94 | + } |
| 95 | + |
| 96 | + t.mu.Lock() |
| 97 | + defer t.mu.Unlock() |
| 98 | + |
| 99 | + if bt := t.bt.Load(); bt != nil { |
| 100 | + return bt.Delete(item) |
| 101 | + } |
| 102 | + |
| 103 | + if p := t.single.Load(); p != nil { |
| 104 | + cur := *p |
| 105 | + if t.equal(cur, item) { |
| 106 | + t.single.Store(nil) |
| 107 | + return cur, true |
| 108 | + } |
| 109 | + } |
| 110 | + var zero T |
| 111 | + return zero, false |
| 112 | +} |
| 113 | + |
| 114 | +// ReverseSeek returns the first item that is less than or equal to the pivot. |
| 115 | +func (t *SmallBTree[T]) ReverseSeek(pivot T) (result T, ok bool) { |
| 116 | + if bt := t.bt.Load(); bt != nil { |
| 117 | + return bt.ReverseSeek(pivot) |
| 118 | + } |
| 119 | + if p := t.single.Load(); p != nil { |
| 120 | + // If pivot < item, there is no <= match. |
| 121 | + if t.less(pivot, *p) { |
| 122 | + var zero T |
| 123 | + return zero, false |
| 124 | + } |
| 125 | + return *p, true |
| 126 | + } |
| 127 | + var zero T |
| 128 | + return zero, false |
| 129 | +} |
| 130 | + |
| 131 | +func (t *SmallBTree[T]) Max() (result T, ok bool) { |
| 132 | + if bt := t.bt.Load(); bt != nil { |
| 133 | + return bt.Max() |
| 134 | + } |
| 135 | + if p := t.single.Load(); p != nil { |
| 136 | + return *p, true |
| 137 | + } |
| 138 | + var zero T |
| 139 | + return zero, false |
| 140 | +} |
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