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gh-106865: Improve difflib.find_longest_match performance with cache #106877

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gh-106865: Improve difflib.find_longest_match performance with cache #106877

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5f35b0d
Improve difflib performance with a cache
gaogaotiantian Jul 18, 2023
0396f81
📜🤖 Added by blurb_it.
blurb-it[bot] Jul 18, 2023
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32 changes: 23 additions & 9 deletions Lib/difflib.py
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Original file line number Diff line number Diff line change
Expand Up @@ -371,20 +371,34 @@ def find_longest_match(self, alo=0, ahi=None, blo=0, bhi=None):
# junk-free match ending with a[i-1] and b[j]
j2len = {}
nothing = []
b2jcache = {}
for i in range(alo, ahi):
# look at all instances of a[i] in b; note that because
# b2j has no junk keys, the loop is skipped if a[i] is junk
j2lenget = j2len.get
newj2len = {}
for j in b2j.get(a[i], nothing):
# a[i] matches b[j]
if j < blo:
continue
if j >= bhi:
break
k = newj2len[j] = j2lenget(j-1, 0) + 1
if k > bestsize:
besti, bestj, bestsize = i-k+1, j-k+1, k
if a[i] not in b2jcache:
cache = []
for j in b2j.get(a[i], nothing):
# a[i] matches b[j]
if j < blo:
continue
if j >= bhi:
break
cache.append(j)
k = newj2len[j] = j2lenget(j-1, 0) + 1
if k > bestsize:
besti, bestj, bestsize = i-k+1, j-k+1, k
b2jcache[a[i]] = cache
else:
for j in b2jcache[a[i]]:
# a[i] matches b[j]
if j-1 in j2len:
k = newj2len[j] = j2len[j-1] + 1
if k > bestsize:
besti, bestj, bestsize = i-k+1, j-k+1, k
else:
k = 1
j2len = newj2len

# Extend the best by non-junk elements on each end. In particular,
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2023-07-18-23-45-19.gh-issue-106865.dxO3Qr.rst
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@@ -0,0 +1 @@
Improve performance of :func:`difflib.SequenceMatcher.find_longest_match` with a cache on hot path