mirror of
https://github.com/jlengrand/project_euler.git
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1c880bda8c
Searches for some data about dijkstra on the web. Creates the utilities folder in the repo.
68 lines
2.6 KiB
Python
68 lines
2.6 KiB
Python
# Priority dictionary using binary heaps
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# David Eppstein, UC Irvine, 8 Mar 2002
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from __future__ import generators
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class priorityDictionary(dict):
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def __init__(self):
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'''Initialize priorityDictionary by creating binary heap
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of pairs (value,key). Note that changing or removing a dict entry will
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not remove the old pair from the heap until it is found by smallest() or
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until the heap is rebuilt.'''
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self.__heap = []
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dict.__init__(self)
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def smallest(self):
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'''Find smallest item after removing deleted items from heap.'''
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if len(self) == 0:
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raise IndexError, "smallest of empty priorityDictionary"
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heap = self.__heap
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while heap[0][1] not in self or self[heap[0][1]] != heap[0][0]:
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lastItem = heap.pop()
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insertionPoint = 0
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while 1:
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smallChild = 2*insertionPoint+1
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if smallChild+1 < len(heap) and \
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heap[smallChild] > heap[smallChild+1]:
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smallChild += 1
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if smallChild >= len(heap) or lastItem <= heap[smallChild]:
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heap[insertionPoint] = lastItem
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break
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heap[insertionPoint] = heap[smallChild]
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insertionPoint = smallChild
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return heap[0][1]
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def __iter__(self):
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'''Create destructive sorted iterator of priorityDictionary.'''
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def iterfn():
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while len(self) > 0:
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x = self.smallest()
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yield x
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del self[x]
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return iterfn()
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def __setitem__(self,key,val):
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'''Change value stored in dictionary and add corresponding
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pair to heap. Rebuilds the heap if the number of deleted items grows
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too large, to avoid memory leakage.'''
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dict.__setitem__(self,key,val)
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heap = self.__heap
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if len(heap) > 2 * len(self):
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self.__heap = [(v,k) for k,v in self.iteritems()]
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self.__heap.sort() # builtin sort likely faster than O(n) heapify
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else:
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newPair = (val,key)
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insertionPoint = len(heap)
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heap.append(None)
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while insertionPoint > 0 and \
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newPair < heap[(insertionPoint-1)//2]:
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heap[insertionPoint] = heap[(insertionPoint-1)//2]
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insertionPoint = (insertionPoint-1)//2
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heap[insertionPoint] = newPair
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def setdefault(self,key,val):
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'''Reimplement setdefault to call our customized __setitem__.'''
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if key not in self:
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self[key] = val
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return self[key]
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