The Trunk: Collections-nice.385.mcz

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!