depq

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================================== depq - Double-ended priority queue

• Python implementation of a thread-safe and efficient double-ended priority queue (DEPQ) in which items and their priority values are stored in a deque object as tuples.
• This of course can also be used as a regular priority queue, or simply a FIFO/LIFO queue.
• Priority queues have many uses such as scheduling, event driven simulation, heuristic analysis, spam filtering, graph searching, etc.

Features & advantages of this implementation:

• Completely thread-safe
• Serializable via pickling or JSON
• Priority values can be ints/floats, numpy types, strings, or any other comparable type you choose!
• popfirst() and poplast() have O(1) performance instead of running in logarithmic time like in a standard DEPQ or other heap-derived structure
• Naturally fast also because deque object is implemented in C
• Items with equal priorities are sorted in the order they were originally added
• Specific items can be deleted or their priorities changed
• Membership testing with 'in' operator occurs in O(1) as does getting an item's frequency in DEPQ via count(item)

Implementation:

• Priorities are always in proper order, thus, a binary search is performed to find the right index with which to insert new items when specifying priority. Normally, this would result in O(n log n) performance when adding items via insert(item, priority) where self.high() > priority > self.low() because deque (as a doubly linked list) random access is O(n).

  Though, ACTUALLY that is not the case here as I've been able to reduce that to O(n) by modifying the binary search to operate while the internal deque is concurrently rotating.

Examples:

from textwrap import fill # For nice wrapped printing from depq import DEPQ

Defaults. If iterable is not None, extend(iterable) will be

called (example below). If maxlen is not None, abs(int(maxlen))

will become the length limit. If a maxlen is set and an item

is added with a priority > lowest prioritized item, it will be

added and the last item will be popped. After instantiation, the

maxlen can be retrieved with maxlen() and set with set_maxlen(length).

depq = DEPQ(iterable=None, maxlen=None)

Add some characters with their ordinal

values as priority and keep count

for c in 'AN_ERRONEOUS_STRING': ... count = list( # This is hacky and not important, skip next 4 lines :) ... x + 1 if '{} #{}'.format(c, x + 1) in depq ... else next(iter(())) if x != 0 else 0 ... for x in range(len(depq) + 1) ... )[-1] ... ... depq.insert('{} #{}'.format(c, count + 1), ord(c)) # item, priority ... print(fill(str(depq), 77)) DEPQ([('_ #1', 95), ('_ #2', 95), ('U #1', 85), ('T #1', 84), ('S #1', 83), ('S #2', 83), ('R #1', 82), ('R #2', 82), ('R #3', 82), ('O #1', 79), ('O #2', 79), ('N #1', 78), ('N #2', 78), ('N #3', 78), ('I #1', 73), ('G #1', 71), ('E #1', 69), ('E #2', 69), ('A #1', 65)])

As you can see items with equal priorities are sorted in the order

they were originally added. Also note DEPQ root (depq[0]) is highest

priority like a max heap.

depq.first() '_ #1' depq.last() 'A #1' depq.high() 95 depq.low() 65 depq[7] # Returns tuple(item, priority) ('R #2', 82)

depq.poplast() ('A #1', 65) depq.last() 'E #2'

depq.size() # Alias for len(DEPQ) 18 depq.is_empty() False depq.clear() depq.is_empty() True

Extend any length iterable of iterables of length >= 2

depq.extend([('bar', 1, 'arbitrary'), (None, 5), ('foo', 2, 'blah')]) depq DEPQ([(None, 5), ('foo', 2), ('bar', 1)])

depq.clear()

depq.addfirst('starter') # For an empty DEPQ, addfirst & addlast are # functionally identical; they add item to DEPQ depq # with given priority, or default 0 DEPQ([('starter', 0)])

depq.addfirst('high', depq.high() + 1) depq.addlast('low', depq.low() - 1) depq DEPQ([('high', 1), ('starter', 0), ('low', -1)])

depq.addfirst('higher') # Default priority DEPQ.high() depq.addlast('lower') # Default priority DEPQ.low() depq DEPQ([('higher', 1), ('high', 1), ('starter', 0), ('low', -1), ('lower', -1)])

depq.addfirst('highest', 0) # Invalid priority raises exception Traceback (most recent call last): File "", line 1, in File "C:\Python34\lib\depq.py", line 340, in addfirst raise ValueError('Priority must be >= ' ValueError: Priority must be >= highest priority.

del depq[0] # As does del Traceback (most recent call last): File "", line 1, in File "C:\Python34\lib\depq.py", line 639, in delitem raise NotImplementedError('Items cannot be deleted by ' NotImplementedError: Items cannot be deleted by referencing arbitrary indices.

depq.clear() depq.count(None) 0 for i in range(10): ... depq.insert(None, i) ... print(fill(str(depq), 77)) DEPQ([(None, 9), (None, 8), (None, 7), (None, 6), (None, 5), (None, 4), (None, 3), (None, 2), (None, 1), (None, 0)])

None in depq True depq.count(None) 10 depq.remove(None) # Removes item from DEPQ, default # of removals is 1 [(None, 0)]

print(fill(str(depq), 77)) DEPQ([(None, 9), (None, 8), (None, 7), (None, 6), (None, 5), (None, 4), (None, 3), (None, 2), (None, 1)])

depq.remove(None, 4) # As you see, returns list of tuple(item, priority) [(None, 1), (None, 2), (None, 3), (None, 4)] print(fill(str(depq), 77)) DEPQ([(None, 9), (None, 8), (None, 7), (None, 6), (None, 5)])

depq[None] = 7 # Alias for DEPQ.insert(item, priority) print(fill(str(depq), 77)) DEPQ([(None, 9), (None, 8), (None, 7), (None, 7), (None, 6), (None, 5)])

depq.elim(None) # This simply calls DEPQ.remove(item, -1) [(None, 5), (None, 6), (None, 7), (None, 7), (None, 8), (None, 9)] print(fill(str(depq), 77)) DEPQ([])

import pickle # Pickling won't work if items aren't picklable import json # JSON won't work if items aren't JSON serializable

for i in range(5): ... depq.insert([i], i) # Unhashable types allowed but don't mutate them! ... depq DEPQ([([4], 4), ([3], 3), ([2], 2), ([1], 1), ([0], 0)])

binary_depq = pickle.dumps(depq) print(fill(str(binary_depq), 77)) b'\x80\x03cdepq\nDEPQ\nq\x00)\x81q\x01}q\x02(X\x05\x00\x00\x00itemsq\x03}q\x0 4(X\x03\x00\x00\x00[1]q\x05K\x01X\x03\x00\x00\x00[3]q\x06K\x01X\x03\x00\x00\x 00[2]q\x07K\x01X\x03\x00\x00\x00[4]q\x08K\x01X\x03\x00\x00\x00[0]q\tK\x01uX\x 04\x00\x00\x00dataq\nccollections\ndeque\nq\x0b]q\x0c(]q\rK\x04aK\x04\x86q\x0 e]q\x0fK\x03aK\x03\x86q\x10]q\x11K\x02aK\x02\x86q\x12]q\x13K\x01aK\x01\x86q\x 14]q\x15K\x00aK\x00\x86q\x16e\x85q\x17Rq\x18X\x05\x00\x00\x00startq\x19K\x00u b.'

json_depq = json.dumps(depq.to_json()) print(fill(json_depq, 77)) {"items": {"[1]": 1, "[3]": 1, "[2]": 1, "[4]": 1, "[0]": 1}, "data": [[[4], 4], [[3], 3], [[2], 2], [[1], 1], [[0], 0]], "start": 0}

depq_from_pickle = pickle.loads(binary_depq) depq_from_json = DEPQ.from_json(json_depq) # Classmethod returns new DEPQ

depq DEPQ([([4], 4), ([3], 3), ([2], 2), ([1], 1), ([0], 0)]) depq_from_pickle DEPQ([([4], 4), ([3], 3), ([2], 2), ([1], 1), ([0], 0)]) depq_from_json DEPQ([([4], 4), ([3], 3), ([2], 2), ([1], 1), ([0], 0)])

Notes:

• The items in DEPQ are also stored along with their frequency in a separate dict for O(1) lookup. If item is un-hashable, the repr() of that item is stored instead. So 'item in DEPQ' would check the dict for item and if TypeError is raised it would try repr(item).
• This implementation inserts in the middle in linear time whereas a textbook DEPQ is O(log n). In actual use cases though, this infinitesimal increase in run time is irrelevant, especially when one considers the extra functionality gained coupled with the fact that the other 2 main operations popfirst() and poplast() now occur in constant time.