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"{ Package: 'stx:libbasic2' }"
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"{ NameSpace: Smalltalk }"
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Collection subclass:#SkipList
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instanceVariableNames:'sortBlock pointers numElements level splice'
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classVariableNames:'Rand'
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poolDictionaries:''
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category:'Collections-Ordered-Trees'
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!
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SkipList comment:'From "Skip Lists: A Probabilistic Alternative to Balanced Trees" by William Pugh ( http://epaperpress.com/sortsearch/download/skiplist.pdf ):
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"Skip lists are a data structure that can be used in place of balanced trees. Skip lists use probabilistic balancing rather than strictly enforcing balancing and as a result the algorithms for insertion and deletion in skip lists are much simpler and significantly faster than equivalent algorithms for balanced trees."
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Notes:
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The elements of the skip list must implement #< or you must provide a sort block.
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'
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!
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!SkipList class methodsFor:'documentation'!
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documentation
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"
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From 'Skip Lists: A Probabilistic Alternative to Balanced Trees' by William Pugh
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( http://epaperpress.com/sortsearch/download/skiplist.pdf ):
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Skip lists are a data structure that can be used in place of balanced trees.
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Skip lists use probabilistic balancing rather than strictly enforcing balancing
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and as a result the algorithms for insertion and deletion in skip lists are much simpler
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and significantly faster than equivalent algorithms for balanced trees.
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Notes:
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The elements of the skip list must implement #< or you must provide a sort block.
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"
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! !
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!SkipList class methodsFor:'instance creation'!
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maxLevel: maxLevel
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"
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SkipList maxLevel: 5
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"
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^ super new initialize: maxLevel
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!
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maxLevel: anInteger sortBlock: aBlock
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^ (self maxLevel: anInteger) sortBlock: aBlock
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!
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new
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"
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SkipList new
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"
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^ super new initialize: 10
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!
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new: anInteger
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^ self maxLevel: (anInteger log: 2) ceiling
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!
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new: anInteger sortBlock: aBlock
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^ (self new: anInteger) sortBlock: aBlock
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!
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sortBlock: aBlock
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^ self new sortBlock: aBlock
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! !
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!SkipList methodsFor:'accessing'!
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level
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^ level
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!
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maxLevel
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^ pointers size
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!
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maxLevel: n
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| newLevel oldPointers |
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newLevel := n max:level.
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oldPointers := pointers.
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pointers := Array new:newLevel.
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splice := Array new:newLevel.
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1 to: level do: [:i | pointers at: i put: (oldPointers at: i)]
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"Modified (format): / 18-06-2017 / 17:42:51 / cg"
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!
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size
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^ numElements
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!
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sortBlock
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^ sortBlock
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!
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sortBlock: aBlock
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sortBlock := aBlock
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"Modified (format): / 18-06-2017 / 17:44:44 / cg"
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! !
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!SkipList methodsFor:'adding'!
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add: element
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self add: element ifPresent: nil.
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^ element
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!
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add: element ifPresent: aBlock
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| node lvl s |
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node := self search:element updating:splice.
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node notNil ifTrue: [aBlock notNil ifTrue: [^ aBlock value: node]].
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lvl := self randomLevel.
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node := SkipListNode on:element level:lvl.
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level + 1 to: lvl do: [:i | splice at: i put: self].
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1 to: lvl do: [:i |
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s := splice at:i.
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node atForward: i put: (s forward: i).
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s atForward: i put: node].
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numElements := numElements + 1.
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splice atAllPut: nil.
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^ element
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"Modified: / 18-06-2017 / 17:32:23 / cg"
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! !
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!SkipList methodsFor:'element comparison'!
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is: element1 equalTo: element2
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^ element1 = element2
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! !
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!SkipList methodsFor:'enumerating'!
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do: aBlock
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self nodesDo: [:node | aBlock value: node object]
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! !
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!SkipList methodsFor:'initialization'!
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initialize: maxLevel
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pointers := Array new:maxLevel.
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splice := Array new:maxLevel.
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numElements := 0.
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level := 0.
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Rand ifNil: [Rand := RandomGenerator new]
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"Modified: / 18-06-2017 / 17:40:56 / cg"
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! !
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!SkipList methodsFor:'node enumeration'!
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nodesDo: aBlock
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| node |
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node := pointers first.
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[node notNil]
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whileTrue:
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[aBlock value: node.
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node := node next]
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"Modified (format): / 18-06-2017 / 17:31:41 / cg"
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! !
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!SkipList methodsFor:'private'!
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atForward: i put: node
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level := node
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ifNil: [pointers findLast: [:n | n notNil]]
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ifNotNil: [level max: i].
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^ pointers at: i put: node
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"Modified (format): / 18-06-2017 / 17:32:30 / cg"
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!
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forward: i
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^ pointers at: i
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!
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is: node before: element
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| object |
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node isNil ifTrue: [^ false].
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object := node object.
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^ sortBlock isNil
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ifTrue: [object < element]
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ifFalse: [
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(self is: object equalTo: element)
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ifTrue: [ false]
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ifFalse:[ sortBlock value: object value: element ]
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]
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"Modified: / 18-06-2017 / 17:42:31 / cg"
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!
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is: node theNodeFor: element
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node isNil ifTrue: [^ false].
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node == self ifTrue: [^ false].
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^ self is: node object equalTo: element
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"Modified: / 18-06-2017 / 17:42:42 / cg"
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!
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next
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^ pointers first
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!
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randomLevel
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| p answer max |
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p := 0.5.
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answer := 1.
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max := self maxLevel.
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[Rand next < p and: [answer < max]]
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whileTrue: [answer := answer + 1].
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^ answer
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"Modified (format): / 18-06-2017 / 17:42:59 / cg"
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!
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search: element updating: array
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| node forward |
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node := self.
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level to: 1 by: -1 do: [:i |
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[forward := node forward: i.
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self is: forward before: element] whileTrue: [node := forward].
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"At this point: node < element <= forward"
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array ifNotNil: [array at: i put: node]].
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node := node next.
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^ (self is: node theNodeFor: element)
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ifTrue: [node]
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ifFalse:[nil]
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"Modified: / 18-06-2017 / 17:44:37 / cg"
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! !
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!SkipList methodsFor:'removing'!
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remove: element ifAbsent: aBlock
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| node i s |
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node := self search:element updating:splice.
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node isNil ifTrue:[
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^ aBlock value
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].
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i := 1.
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[s := splice at:i.
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i <= level and: [(s forward: i) == node]] whileTrue: [
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s atForward: i put: (node forward: i).
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i := i + 1
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].
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numElements := numElements - 1.
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splice atAllPut: nil.
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^ node object
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"Modified (format): / 18-06-2017 / 17:43:30 / cg"
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!
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removeAll
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pointers atAllPut: nil.
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splice atAllPut: nil.
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numElements := 0.
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level := 0.
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"Modified (format): / 18-06-2017 / 17:43:39 / cg"
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! !
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!SkipList methodsFor:'testing'!
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includes: element
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^ (self search: element updating: nil) notNil
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!
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isEmpty
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^ numElements = 0
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! !
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!SkipList class methodsFor:'documentation'!
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version
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^ '$Header$'
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!
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version_CVS
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^ '$Header$'
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! !
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