8000 gh-119105: difflib: improve recursion for degenerate cases by pulkin · Pull Request #119131 · python/cpython · GitHub
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gh-119105: difflib: improve recursion for degenerate cases #119131

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Merged
merged 10 commits into from
May 19, 2024
Merged

gh-119105: difflib: improve recursion for degenerate cases #119131

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9bb454c
difflib.py: fix recursion branching for degenerate cases
pulkin May 17, 2024
a159954
difflib.py: grammar
pulkin May 17, 2024
81b7dae
difflib.py: weights-free implementation
pulkin May 18, 2024
2184911
difflib.py: rm left-over
pulkin May 18, 2024
f9e3480
difflib.py: even simpler impl
pulkin May 19, 2024
b0be97f
📜🤖 Added by blurb_it.
blurb-it[bot] May 19, 2024
bea603e
difflib.py: add a desc
pulkin May 19, 2024
df83f50
📜🤖 Added by blurb_it.
blurb-it[bot] May 19, 2024
0953ede
news: attempt fix build
pulkin May 19, 2024
7237b48
difflib.py: NEWS message
pulkin May 19, 2024
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25 changes: 20 additions & 5 deletions Lib/difflib.py
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Original file line number Diff line number Diff line change
Expand Up @@ -911,33 +911,48 @@ def _fancy_replace(self, a, alo, ahi, b, blo, bhi):

# don't synch up unless the lines have a similarity score of at
# least cutoff; best_ratio tracks the best score seen so far
best_ratio, cutoff = 0.74, 0.75
best_ratio, cutoff = (0.74, 0), 0.75
cruncher = SequenceMatcher(self.charjunk)
eqi, eqj = None, None # 1st indices of equal lines (if any)

# search for the pair that matches best without being identical
# (identical lines must be junk lines, & we don't want to synch up
# on junk -- unless we have to)
alen = alo + ahi - 1
blen = blo + bhi - 1
# weight is used to balance the recursion by prioritizing
# i and j in the middle of their ranges
weight = 0
for j in range(blo, bhi):
bj = b[j]
cruncher.set_seq2(bj)
if j < blen / 2:
weight += alen
elif j > blen / 2:
weight -= alen
for i in range(alo, ahi):
ai = a[i]
if ai == bj:
if eqi is None:
eqi, eqj = i, j
continue
cruncher.set_seq1(ai)
if i < alen / 2:
weight += blen
elif i > alen / 2:
weight -= blen
# computing similarity is expensive, so use the quick
# upper bounds first -- have seen this speed up messy
# compares by a factor of 3.
# note that ratio() is only expensive to compute the first
# time it's called on a sequence pair; the expensive part
# of the computation is cached by cruncher
if cruncher.real_quick_ratio() > best_ratio and \
cruncher.quick_ratio() > best_ratio and \
cruncher.ratio() > best_ratio:
best_ratio, best_i, best_j = cruncher.ratio(), i, j
if (cruncher.real_quick_ratio(), weight) > best_ratio and \
(cruncher.quick_ratio(), weight) > best_ratio and \
(cruncher.ratio(), weight) > best_ratio:
best_ratio, best_i, best_j = (cruncher.ratio(), weight), i, j
# assert weight == 0, weight
best_ratio, _ = best_ratio
if best_ratio < cutoff:
# no non-identical "pretty close" pair
if eqi is None:
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