Nicolas Cellier uploaded a new version of Collections to project The Trunk: http://source.squeak.org/trunk/Collections-nice.385.mcz
==================== Summary ====================
Name: Collections-nice.385 Author: nice Time: 30 September 2010, 11:46:29.277 pm UUID: 4d54da08-721a-4a4d-8387-914046189879 Ancestors: Collections-ul.384
Improve comment of Heap. Please feel free to improve me. As the class comment did start with a bit misleading comparison against SortedCollection, provides an optional #fullySort operation to fully sort the Heap and restore comparison ground - hope I didn't spoil announced O(n log n) efficiency. Of course, #fullySort has no interest for priority queues, but a Heap could be of more general use.
=============== Diff against Collections-ul.384 ===============
Item was changed: SequenceableCollection subclass: #Heap instanceVariableNames: 'array tally sortBlock indexUpdateBlock' classVariableNames: '' poolDictionaries: '' category: 'Collections-Sequenceable'!
+ !Heap commentStamp: 'nice 9/30/2010 23:22' prior: 0! + Class Heap implements a special data structure commonly referred to as 'heap' [ http://en.wikipedia.org/wiki/Heap_%28data_structure%29 ] + A Heap is a kind of binary tree stored in a linear array - see details after the instance variables description. - !Heap commentStamp: '<historical>' prior: 0! - Class Heap implements a special data structure commonly referred to as 'heap'. Heaps are more efficient than SortedCollections if: - a) Elements are only removed at the beginning - b) Elements are added with arbitrary sort order. - The sort time for a heap is O(n log n) in all cases.
+ Heaps are good at handling priority queues because: + 1) greatest priority element according to the sort block will be stored in first position and thus accessed in O(1) operations + 2) worse time for inserting or removing an element is in O(log n) operations, where n is the size of the Heap + Insertion/Removal times are more efficient than above upper bound, provided that: + a) Elements are only removed at the beginning + b) Elements are added with arbitrary sort order. + 3) there is no need to fully sort the Heap, which makes it more efficient than a SortedCollection + + The heap can be fully sorted by sending the message #fullySort. + Worse time for fully sorting the Heap is in O(n log n) operations, but this is rarely used a feature. + Remind that the Heap does not fully sort the collection if you don't ask. + Thus don't expect #do: and other iterators to enumerate elements according to the sortBlock order. + Instance variables: array <Array> The data repository tally <Integer> The number of elements in the heap sortBlock <Block|nil> A two-argument block defining the sort order, or nil in which case the default sort order is [:element1 :element2| element1 <= element2] indexUpdateBlock <Block|nil> A two-argument block of the form [:data :index | ... ] which allows an application object to keep track of its index within the heap. Useful for quick heap update when object's sort value changes (for example, when an object in a priority queue has its priority increased by an external event, you don't want to have to search through the whole heap to find the index before fixing + the heap). No update occurs if nil. + + The Heap can be viewed as a binary tree (every node in the tree has at most two children). + The root is stored in first slot of internal array. + The children are stored in next two slots. + The children of children in next four slots. + etc... + For a node A of index i (1 based), the two children B1 and B2 are thus stored in indices (2*i) and (2*i+1). + Of course, the children indices must be less than the tally otherwise they are considered inexistent. + + The Heap does arrange to preserve the following invariant: + For any children B of a node A, A is sorted before B, in other words, (self sort: A before: B) = true + This implies that the root is always the first element according to sort order. + + ! - the heap). No update occurs if nil.!
Item was added: + ----- Method: Heap>>fullySort (in category 'accessing') ----- + fullySort + "Fully sort the heap. + This method preserves the heap invariants and can thus be sent safely" + self privateReverseSort. + 1 to: tally // 2 do: [:i | array swap: i with: 1 + tally - i]!
Item was added: + ----- Method: Heap>>privateReverseSort (in category 'private') ----- + privateReverseSort + "Arrange to have the array sorted in reverse order. + WARNING: this method breaks the heap invariants. It's up to the sender to restore them afterwards." + | oldTally | + oldTally := tally. + [tally > 1] whileTrue: + [array swap: 1 with: tally. + tally := tally - 1. + self downHeapSingle: 1]. + tally := oldTally!