working skiplist

2025-12-20 03:59:49 +08:00 · 2021-10-02 14:02:56 -07:00
parent b6694279d7
commit 4c1741fdbb
6 changed files with 1165 additions and 0 deletions
--- a/weed/util/skiplist/skiplist.go
+++ b/weed/util/skiplist/skiplist.go
@@ -0,0 +1,480 @@
+package skiplist
+
+import (
+	"bytes"
+	"fmt"
+	"math/bits"
+	"math/rand"
+	"time"
+)
+
+const (
+	// maxLevel denotes the maximum height of the skiplist. This height will keep the skiplist
+	// efficient for up to 34m entries. If there is a need for much more, please adjust this constant accordingly.
+	maxLevel = 25
+)
+
+type SkipList struct {
+	startLevels  [maxLevel]*SkipListElementReference
+	endLevels    [maxLevel]*SkipListElementReference
+	maxNewLevel  int
+	maxLevel     int
+	elementCount int
+}
+
+// NewSeedEps returns a new empty, initialized Skiplist.
+// Given a seed, a deterministic height/list behaviour can be achieved.
+// Eps is used to compare keys given by the ExtractKey() function on equality.
+func NewSeed(seed int64) *SkipList {
+
+	// Initialize random number generator.
+	rand.Seed(seed)
+	//fmt.Printf("SkipList seed: %v\n", seed)
+
+	list := &SkipList{
+		maxNewLevel:  maxLevel,
+		maxLevel:     0,
+		elementCount: 0,
+	}
+
+	return list
+}
+
+// New returns a new empty, initialized Skiplist.
+func New() *SkipList {
+	return NewSeed(time.Now().UTC().UnixNano())
+}
+
+// IsEmpty checks, if the skiplist is empty.
+func (t *SkipList) IsEmpty() bool {
+	return t.startLevels[0] == nil
+}
+
+func (t *SkipList) generateLevel(maxLevel int) int {
+	level := maxLevel - 1
+	// First we apply some mask which makes sure that we don't get a level
+	// above our desired level. Then we find the first set bit.
+	var x = rand.Uint64() & ((1 << uint(maxLevel-1)) - 1)
+	zeroes := bits.TrailingZeros64(x)
+	if zeroes <= maxLevel {
+		level = zeroes
+	}
+
+	return level
+}
+
+func (t *SkipList) findEntryIndex(key []byte, level int) int {
+	// Find good entry point so we don't accidentally skip half the list.
+	for i := t.maxLevel; i >= 0; i-- {
+		if t.startLevels[i] != nil && bytes.Compare(t.startLevels[i].Key, key) < 0 || i <= level {
+			return i
+		}
+	}
+	return 0
+}
+
+func (t *SkipList) findExtended(key []byte, findGreaterOrEqual bool) (foundElem *SkipListElement, ok bool) {
+
+	foundElem = nil
+	ok = false
+
+	if t.IsEmpty() {
+		return
+	}
+
+	index := t.findEntryIndex(key, 0)
+	var currentNode *SkipListElement
+
+	currentNode = t.startLevels[index].Load()
+
+	// In case, that our first element is already greater-or-equal!
+	if findGreaterOrEqual && compareElement(currentNode, key) > 0 {
+		foundElem = currentNode
+		ok = true
+		return
+	}
+
+	for {
+		if compareElement(currentNode, key) == 0 {
+			foundElem = currentNode
+			ok = true
+			return
+		}
+
+		// Which direction are we continuing next time?
+		if currentNode.Next[index] != nil && bytes.Compare(currentNode.Next[index].Key, key) <= 0 {
+			// Go right
+			currentNode = currentNode.Next[index].Load()
+		} else {
+			if index > 0 {
+
+				// Early exit
+				if currentNode.Next[0] != nil && bytes.Compare(currentNode.Next[0].Key, key) == 0 {
+					currentNodeNext := currentNode.Next[0].Load()
+					foundElem = currentNodeNext
+					ok = true
+					return
+				}
+				// Go down
+				index--
+			} else {
+				// Element is not found and we reached the bottom.
+				if findGreaterOrEqual {
+					foundElem = currentNode.Next[index].Load()
+					ok = foundElem != nil
+				}
+
+				return
+			}
+		}
+	}
+}
+
+// Find tries to find an element in the skiplist based on the key from the given ListElement.
+// elem can be used, if ok is true.
+// Find runs in approx. O(log(n))
+func (t *SkipList) Find(key []byte) (elem *SkipListElement, ok bool) {
+
+	if t == nil || key == nil {
+		return
+	}
+
+	elem, ok = t.findExtended(key, false)
+	return
+}
+
+// FindGreaterOrEqual finds the first element, that is greater or equal to the given ListElement e.
+// The comparison is done on the keys (So on ExtractKey()).
+// FindGreaterOrEqual runs in approx. O(log(n))
+func (t *SkipList) FindGreaterOrEqual(key []byte) (elem *SkipListElement, ok bool) {
+
+	if t == nil || key == nil {
+		return
+	}
+
+	elem, ok = t.findExtended(key, true)
+	return
+}
+
+// Delete removes an element equal to e from the skiplist, if there is one.
+// If there are multiple entries with the same value, Delete will remove one of them
+// (Which one will change based on the actual skiplist layout)
+// Delete runs in approx. O(log(n))
+func (t *SkipList) Delete(key []byte) {
+
+	if t == nil || t.IsEmpty() || key == nil {
+		return
+	}
+
+	index := t.findEntryIndex(key, t.maxLevel)
+
+	var currentNode *SkipListElement
+	var nextNode *SkipListElement
+
+	for {
+
+		if currentNode == nil {
+			nextNode = t.startLevels[index].Load()
+		} else {
+			nextNode = currentNode.Next[index].Load()
+		}
+
+		// Found and remove!
+		if nextNode != nil && compareElement(nextNode, key) == 0 {
+
+			if currentNode != nil {
+				currentNode.Next[index] = nextNode.Next[index]
+				currentNode.Save()
+			}
+
+			if index == 0 {
+				if nextNode.Next[index] != nil {
+					nextNextNode := nextNode.Next[index].Load()
+					nextNextNode.Prev = currentNode.Reference()
+					nextNextNode.Save()
+				}
+				t.elementCount--
+				nextNode.DeleteSelf()
+			}
+
+			// Link from start needs readjustments.
+			startNextKey := t.startLevels[index].Key
+			if compareElement(nextNode, startNextKey) == 0 {
+				t.startLevels[index] = nextNode.Next[index]
+				// This was our currently highest node!
+				if t.startLevels[index] == nil {
+					t.maxLevel = index - 1
+				}
+			}
+
+			// Link from end needs readjustments.
+			if nextNode.Next[index] == nil {
+				t.endLevels[index] = currentNode.Reference()
+			}
+			nextNode.Next[index] = nil
+		}
+
+		if nextNode != nil && compareElement(nextNode, key) < 0 {
+			// Go right
+			currentNode = nextNode
+		} else {
+			// Go down
+			index--
+			if index < 0 {
+				break
+			}
+		}
+	}
+
+}
+
+// Insert inserts the given ListElement into the skiplist.
+// Insert runs in approx. O(log(n))
+func (t *SkipList) Insert(key []byte) {
+
+	if t == nil || key == nil {
+		return
+	}
+
+	level := t.generateLevel(t.maxNewLevel)
+
+	// Only grow the height of the skiplist by one at a time!
+	if level > t.maxLevel {
+		level = t.maxLevel + 1
+		t.maxLevel = level
+	}
+
+	elem := &SkipListElement{
+		Id:     rand.Int63(),
+		Next:   make([]*SkipListElementReference, t.maxNewLevel, t.maxNewLevel),
+		Level:  int32(level),
+		Values: [][]byte{key},
+	}
+
+	t.elementCount++
+
+	newFirst := true
+	newLast := true
+	if !t.IsEmpty() {
+		newFirst = compareElement(elem, t.startLevels[0].Key) < 0
+		newLast = compareElement(elem, t.endLevels[0].Key) > 0
+	}
+
+	normallyInserted := false
+	if !newFirst && !newLast {
+
+		normallyInserted = true
+
+		index := t.findEntryIndex(key, level)
+
+		var currentNode *SkipListElement
+		var nextNodeRef *SkipListElementReference
+
+		for {
+
+			if currentNode == nil {
+				nextNodeRef = t.startLevels[index]
+			} else {
+				nextNodeRef = currentNode.Next[index]
+			}
+
+			var nextNode *SkipListElement
+
+			// Connect node to next
+			if index <= level && (nextNodeRef == nil || bytes.Compare(nextNodeRef.Key, key) > 0) {
+				elem.Next[index] = nextNodeRef
+				if currentNode != nil {
+					currentNode.Next[index] = elem.Reference()
+					currentNode.Save()
+				}
+				if index == 0 {
+					elem.Prev = currentNode.Reference()
+					if nextNodeRef != nil {
+						nextNode = nextNodeRef.Load()
+						nextNode.Prev = elem.Reference()
+						nextNode.Save()
+					}
+				}
+			}
+
+			if nextNodeRef != nil && bytes.Compare(nextNodeRef.Key, key) <= 0 {
+				// Go right
+				if nextNode == nil {
+					// reuse nextNode when index == 0
+					nextNode = nextNodeRef.Load()
+				}
+				currentNode = nextNode
+			} else {
+				// Go down
+				index--
+				if index < 0 {
+					break
+				}
+			}
+		}
+	}
+
+	// Where we have a left-most position that needs to be referenced!
+	for i := level; i >= 0; i-- {
+
+		didSomething := false
+
+		if newFirst || normallyInserted {
+
+			if t.startLevels[i] == nil || bytes.Compare(t.startLevels[i].Key, key) > 0 {
+				if i == 0 && t.startLevels[i] != nil {
+					startLevelElement := t.startLevels[i].Load()
+					startLevelElement.Prev = elem.Reference()
+					startLevelElement.Save()
+				}
+				elem.Next[i] = t.startLevels[i]
+				t.startLevels[i] = elem.Reference()
+			}
+
+			// link the endLevels to this element!
+			if elem.Next[i] == nil {
+				t.endLevels[i] = elem.Reference()
+			}
+
+			didSomething = true
+		}
+
+		if newLast {
+			// Places the element after the very last element on this level!
+			// This is very important, so we are not linking the very first element (newFirst AND newLast) to itself!
+			if !newFirst {
+				if t.endLevels[i] != nil {
+					endLevelElement := t.endLevels[i].Load()
+					endLevelElement.Next[i] = elem.Reference()
+					endLevelElement.Save()
+				}
+				if i == 0 {
+					elem.Prev = t.endLevels[i]
+				}
+				t.endLevels[i] = elem.Reference()
+			}
+
+			// Link the startLevels to this element!
+			if t.startLevels[i] == nil || bytes.Compare(t.startLevels[i].Key, key) > 0 {
+				t.startLevels[i] = elem.Reference()
+			}
+
+			didSomething = true
+		}
+
+		if !didSomething {
+			break
+		}
+	}
+
+	elem.Save()
+
+}
+
+// GetValue extracts the ListElement value from a skiplist node.
+func (e *SkipListElement) GetValue() []byte {
+	return e.Values[0]
+}
+
+// GetSmallestNode returns the very first/smallest node in the skiplist.
+// GetSmallestNode runs in O(1)
+func (t *SkipList) GetSmallestNode() *SkipListElement {
+	return t.startLevels[0].Load()
+}
+
+// GetLargestNode returns the very last/largest node in the skiplist.
+// GetLargestNode runs in O(1)
+func (t *SkipList) GetLargestNode() *SkipListElement {
+	return t.endLevels[0].Load()
+}
+
+// Next returns the next element based on the given node.
+// Next will loop around to the first node, if you call it on the last!
+func (t *SkipList) Next(e *SkipListElement) *SkipListElement {
+	if e.Next[0] == nil {
+		return t.startLevels[0].Load()
+	}
+	return e.Next[0].Load()
+}
+
+// Prev returns the previous element based on the given node.
+// Prev will loop around to the last node, if you call it on the first!
+func (t *SkipList) Prev(e *SkipListElement) *SkipListElement {
+	if e.Prev == nil {
+		return t.endLevels[0].Load()
+	}
+	return e.Prev.Load()
+}
+
+// GetNodeCount returns the number of nodes currently in the skiplist.
+func (t *SkipList) GetNodeCount() int {
+	return t.elementCount
+}
+
+// String returns a string format of the skiplist. Useful to get a graphical overview and/or debugging.
+func (t *SkipList) println() {
+
+	print("start --> ")
+	for i, l := range t.startLevels {
+		if l == nil {
+			break
+		}
+		if i > 0 {
+			print(" -> ")
+		}
+		next := "---"
+		if l != nil {
+			next = string(l.Key)
+		}
+		print(fmt.Sprintf("[%v]", next))
+	}
+	println()
+
+	nodeRef := t.startLevels[0]
+	for nodeRef != nil {
+		print(fmt.Sprintf("%v: ", string(nodeRef.Key)))
+		node := nodeRef.Load()
+		for i := 0; i <= int(node.Level); i++ {
+
+			l := node.Next[i]
+
+			next := "---"
+			if l != nil {
+				next = string(l.Key)
+			}
+
+			if i == 0 {
+				prev := "---"
+
+				if node.Prev != nil {
+					prev = string(node.Prev.Key)
+				}
+				print(fmt.Sprintf("[%v|%v]", prev, next))
+			} else {
+				print(fmt.Sprintf("[%v]", next))
+			}
+			if i < int(node.Level) {
+				print(" -> ")
+			}
+
+		}
+		println()
+		nodeRef = node.Next[0]
+	}
+
+	print("end --> ")
+	for i, l := range t.endLevels {
+		if l == nil {
+			break
+		}
+		if i > 0 {
+			print(" -> ")
+		}
+		next := "---"
+		if l != nil {
+			next = string(l.Key)
+		}
+		print(fmt.Sprintf("[%v]", next))
+	}
+	println()
+}