Merge a (possibly infinite) number of already sorted inputs (each of possibly infinite length) into a single sorted output.
Similar to heapq.merge and sorted(itertools.chain(*iterables)).
Like heapq.merge, returns a generator, does not pull the data into memory all at once, and assumes that each of the input iterables is already sorted (smallest to largest).
Unlike heapq.merge, accepts an infinite number of input iterables, but requires all of them to come in ascending order (that is, their starting point must come in ascending order).
In addition, accepts a key function (like sorted, min, max, etc.)
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Copy to clipboard1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 | # mergeinf.py
# (C) 2010 Gabriel Genellina
from heapq import heappop, heapreplace, heapify
from operator import attrgetter
__all__ = ['imerge']
# 3.x compatibility
try:
iter(()).next
except AttributeError:
next_function_getter = attrgetter('__next__')
class IterRecord(list):
def __eq__(self, other): return self[0]==other[0]
def __lt__(self, other): return self[0]<other[0]
def __le__(self, other): return self[0]<=other[0]
def __ne__(self, other): return self[0]!=other[0]
def __gt__(self, other): return self[0]>other[0]
def __ge__(self, other): return self[0]>=other[0]
else:
next_function_getter = attrgetter('next')
IterRecord = list
def imerge(iterables, key=None):
'''Merge a (possibly infinite) number of already sorted inputs
(each of possibly infinite length) into a single sorted output.
Similar to heapq.merge and sorted(itertools.chain(*iterables)).
Like heapq.merge, returns a generator, does not pull the data
into memory all at once, and assumes that each of the input
iterables is already sorted (smallest to largest).
Unlike heapq.merge, accepts an infinite number of input iterables,
but requires all of them to come in ascending order (that is,
their starting point must come in ascending order).
In addition, accepts a *key* function (like `sorted`, `min`,
`max`, etc.)
>>> list(imerge([[1,3,5,7], [2,4,8], [5,10,15,20], [], [25]]))
[1, 2, 3, 4, 5, 5, 7, 8, 10, 15, 20, 25]
'''
_heappop, _heapreplace, _heapify, _StopIteration = heappop, heapreplace, heapify, StopIteration
_iter, _next, _len, _next_function_getter = iter, next, len, next_function_getter
h = []
h_append = h.append
iterables = _iter(iterables)
more_iterables = True
while _len(h)<2:
try:
# raises StopIteration when no more iterables
next_item = _next_function_getter(_iter(_next(iterables)))
except _StopIteration:
more_iterables = False
break
try:
v = next_item()
except _StopIteration:
# ignore empty iterables
continue
if key is not None:
highest = key(v)
else:
highest = v
h_append(IterRecord([highest, v, next_item]))
if _len(h) >= 2:
# the heap invariant should hold, if input iterables come already sorted
if h[1][0] < h[0][0]:
raise ValueError('items out of order: %r and %r' % (h[0][0], h[1][0]))
elif _len(h) == 1:
# a single iterable, just send it
assert not more_iterables
_, v, next_item = h[0]
yield v
try:
while True:
yield next_item()
except _StopIteration:
return
else:
# empty
return
cur = highest
while h:
_, v, next_item = s = h[0]
yield v
try:
v = s[1] = next_item() # raises StopIteration when no more items
except _StopIteration:
_heappop(h) # remove empty iterator
else:
if key is not None:
cur = s[0] = key(v)
else:
cur = s[0] = v
_heapreplace(h, s) # restore heap condition
# 'highest' is the highest known item in the heap.
# Any time we advance an iterable and get an item ('cur')
# greater than 'highest', we must bring more enough iterables
# into play to ensure no items are missed.
if more_iterables and (cur >= highest or _len(h) < 2):
while cur >= highest or _len(h)<2:
try:
# raises StopIteration when no more iterables
next_item = _next_function_getter(_iter(_next(iterables)))
except _StopIteration:
more_iterables = False
break
try:
v = next_item()
except _StopIteration:
# ignore empty iterables
continue
if key is not None:
highest = key(v)
else:
highest = v
h_append(IterRecord([highest, v, next_item]))
_heapify(h)
|
This recipe is based on heapq.merge in the standard library, modified to be able to accept an infinite number of iterables.
heapq.merge requires all iterables to be already ordered; in addition, this recipe requires all iterables to come in ascending order (that is, their starting point must be in ascending order). This is important in order to be able to handle an infinite number of input streams. Input iterables start to be consumed only when needed, e.g.:
imerge([[0,5,10,15,20],
xrange(10,40,2),
xrange(30,40,3),
[],
[35,38,42]])
starts to look at the last iterable [35 ...] only after yielding the number 28 (taken from the second one).
Example: Enumerate a regular language (that is, generate all possible strings matching a given regular expression), so that shorter strings come before the longer ones.
If the regular expression contains unbounded repeaters (like *, + or {n,})
there will be infinite matches; we want the shorter ones first,
or else we might never see them (think of 'a*|b';
we want 'b' to appear early, or it could be infinitely delayed after all those
'aaaaaaaaaaaaaaaa...').
Given '(a|bc)*', we can think of * as
meaning '(a|bc){0}' (repeat zero times) merged with '(a|bc){1}' (repeat once) merged
with '(a|bc){2}' merged with ...
'(a|bc){0}' = ''
'(a|bc){1}' = 'a', 'bc'
'(a|bc){2}' = 'aa', 'abc', 'bca', 'bcbc'
'(a|bc){3}' = 'aaa', 'aabc', 'abca', 'bcaa', 'abcbc', 'bcabc', 'bcbca', 'bcbcbc'
...
Merging all those (infinite) sequences gives:
'', 'a', 'aa', 'bc',
'aaa', 'abc', 'bca',
'aaaa', 'aabc', 'abca', 'bcaa', 'bcbc',
'aaaaa', 'aaabc', 'aabca', 'abcaa', 'abcbc', 'bcaaa', 'bcabc', 'bcbca',
...
The code to generate those strings (just a demo, real usage in another recipe):
from itertools import product, islice
def by_length(s):
return (len(s), s)
def products(xs):
current = ['']
yield current
while True:
current = sorted((x+y for x,y in product(current, xs)), key=by_length)
yield current
def closure(xs):
return imerge(products(xs), key=by_length)
for x in closure(['a', 'bc']):
print(x)
We can look at any part of the output sequence with:
for x in islice(closure(['a', 'bc']), 2000, 2010):
print(x)
abcaaaabcabcabc
abcaaaabcabcbca
abcaaaabcbcaaaa
abcaaaabcbcaabc
...
