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common.go
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common.go
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package treap
import (
"math/rand"
"time"
)
const (
// staticDepth is the size of the static array to use for keeping track of the parent stack during treap iteration.
//
// Since a treap has a very high probability that the tree height is logarithmic, it is exceedingly unlikely that
// the parent stack will ever exceed this size even for extremely large numbers of items.
staticDepth = 128
// nodeFieldsSize is the size the fields of each node takes excluding the contents of the key and value. It assumes
// 64-bit pointers so technically it is smaller on 32-bit platforms, but overestimating the size in that case is
// acceptable since it avoids the need to import unsafe. It consists of 24-bytes for each key and value + 8 bytes
// for each of the priority, left, and right fields (24*2 + 8*3).
nodeFieldsSize = 72
)
var (
// emptySlice is used for keys that have no value associated with them so callers can distinguish between a key that
// does not exist and one that has no value associated with it.
emptySlice = make([]byte, 0)
)
// treapNode represents a node in the treap.
type treapNode struct {
key []byte
value []byte
priority int
left *treapNode
right *treapNode
}
// nodeSize returns the number of bytes the specified node occupies including the struct fields and the contents of the
// key and value.
func nodeSize(node *treapNode) uint64 {
return nodeFieldsSize + uint64(len(node.key)+len(node.value))
}
// newTreapNode returns a new node from the given key, value, and priority. The node is not initially linked to any
// others.
func newTreapNode(key, value []byte, priority int) *treapNode {
return &treapNode{key: key, value: value, priority: priority}
}
// parentStack represents a stack of parent treap nodes that are used during iteration. It consists of a static array
// for holding the parents and a dynamic overflow slice. It is extremely unlikely the overflow will ever be hit during
// normal operation, however, since a treap's height is probabilistic, the overflow case needs to be handled properly.
//
// This approach is used because it is much more efficient for the majority case than dynamically allocating heap space
// every time the treap is iterated.
type parentStack struct {
index int
items [staticDepth]*treapNode
overflow []*treapNode
}
// Len returns the current number of items in the stack.
func (s *parentStack) Len() int {
return s.index
}
// At returns the item n number of items from the top of the stack, where 0 is the topmost item, without removing it. It
// returns nil if n exceeds the number of items on the stack.
func (s *parentStack) At(n int) *treapNode {
index := s.index - n - 1
if index < 0 {
return nil
}
if index < staticDepth {
return s.items[index]
}
return s.overflow[index-staticDepth]
}
// Pop removes the top item from the stack. It returns nil if the stack is empty.
func (s *parentStack) Pop() *treapNode {
if s.index == 0 {
return nil
}
s.index--
if s.index < staticDepth {
node := s.items[s.index]
s.items[s.index] = nil
return node
}
node := s.overflow[s.index-staticDepth]
s.overflow[s.index-staticDepth] = nil
return node
}
// Push pushes the passed item onto the top of the stack.
func (s *parentStack) Push(node *treapNode) {
if s.index < staticDepth {
s.items[s.index] = node
s.index++
return
}
// This approach is used over append because reslicing the slice to pop the item causes the compiler to make
// unneeded allocations. Also, since the max number of items is related to the tree depth which requires
// exponentially more items to increase, only increase the cap one item at a time. This is more intelligent than
// the generic append expansion algorithm which often doubles the cap.
index := s.index - staticDepth
if index+1 > cap(s.overflow) {
overflow := make([]*treapNode, index+1)
copy(overflow, s.overflow)
s.overflow = overflow
}
s.overflow[index] = node
s.index++
}
func init() {
rand.Seed(time.Now().UnixNano())
}