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heapdict: a heap with decreased-key and increase-key operations

heapdict implements the MutableMapping ABC, meaning it works pretty much like a regular Python dict. It's designed to be used as a priority queue, where items are added and consumed as follows:

hd = heapdict()
hd[obj1] = priority1
hd[obj2] = priority2
# ...
obj = hd.pop()

Compared to an ordinary dict, a heapdict has the following differences:

popitem():
Remove and return the (key, priority) pair with the lowest priority, instead of a random object.
peekitem():
Return the (key, priority) pair with the lowest priority, without removing it.

Unlike the Python standard library's heapq module, the heapdict supports efficiently changing the priority of an existing object (often called "decrease-key" in textbooks). Altering the priority is important for many algorithms such as Dijkstra's Algorithm and A*.

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