gh-119105: Differ.compare is too slow [for degenerate cases] (#119492)

``_fancy_replace()`` is no longer recursive. and a single call does a worst-case linear number of ratio() computations instead of quadratic. This renders toothless a universe of pathological cases. Some inputs may produce different output, but that's rare, and I didn't find a case where the final diff appeared to be of materially worse quality. To the contrary, by refusing to even consider synching on lines "far apart", there was more easy-to-digest locality in the output.

Lib/difflib.py

@@ -908,79 +908,52 @@ def _fancy_replace(self, a, alo, ahi, b, blo, bhi):
         + abcdefGhijkl
         ?    ^  ^  ^
         """
-        from operator import ge, gt
-        # Don't synch up unless the lines have a similarity score of at
-        # least cutoff; best_ratio tracks the best score seen so far.
-        # Keep track of all index pairs achieving the best ratio and
-        # deal with them here. Previously only the smallest pair was
-        # handled here, and if there are many pairs with the best ratio,
-        # recursion could grow very deep, and runtime cubic. See:
+        # Don't synch up unless the lines have a similarity score above
+        # cutoff. Previously only the smallest pair was handled here,
+        # and if there are many pairs with the best ratio, recursion
+        # could grow very deep, and runtime cubic. See:
         # https://github.com/python/cpython/issues/119105
-        best_ratio, cutoff = 0.74, 0.75
+        #
+        # Later, more pathological cases prompted removing recursion
+        # entirely.
+        cutoff = 0.74999
         cruncher = SequenceMatcher(self.charjunk)
-        eqi, eqj = None, None   # 1st indices of equal lines (if any)
-        # List of index pairs achieving best_ratio. Strictly increasing
-        # in both index positions.
-        max_pairs = []
-        maxi = -1 # `i` index of last pair in max_pairs
-
-        # 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)
         crqr = cruncher.real_quick_ratio
         cqr = cruncher.quick_ratio
         cr = cruncher.ratio
+
+        WINDOW = 10
+        best_i = best_j = None
+        dump_i, dump_j = alo, blo # smallest indices not yet resolved
         for j in range(blo, bhi):
-            bj = b[j]
-            cruncher.set_seq2(bj)
-            # Find new best, if possible. Else search for the smallest i
-            # (if any) > maxi that equals the best ratio
-            search_equal = True
-            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)
-                # computing similarity is expensive, so use the quick
-                # upper bounds first -- have seen this speed up messy
-                # compares by a factor of 3.
-                cmp = ge if search_equal and i > maxi else gt
-                if (cmp(crqr(), best_ratio)
-                      and cmp(cqr(), best_ratio)
-                      and cmp((ratio := cr()), best_ratio)):
-                    if ratio > best_ratio:
-                        best_ratio = ratio
-                        max_pairs.clear()
-                    else:
-                        assert best_ratio == ratio and search_equal
-                        assert i > maxi
-                    max_pairs.append((i, j))
-                    maxi = i
-                    search_equal = False
-        if best_ratio < cutoff:
-            assert not max_pairs
-            # no non-identical "pretty close" pair
-            if eqi is None:
-                # no identical pair either -- treat it as a straight replace
-                yield from self._plain_replace(a, alo, ahi, b, blo, bhi)
-                return
-            # no close pair, but an identical pair -- synch up on that
-            max_pairs = [(eqi, eqj)]
-        else:
-            # there's a close pair, so forget the identical pair (if any)
-            assert max_pairs
-            eqi = None
-
-        last_i, last_j = alo, blo
-        for this_i, this_j in max_pairs:
-            # pump out diffs from before the synch point
-            yield from self._fancy_helper(a, last_i, this_i,
-                                          b, last_j, this_j)
+            cruncher.set_seq2(b[j])
+            # Search the corresponding i's within WINDOW for rhe highest
+            # ratio greater than `cutoff`.
+            aequiv = alo + (j - blo)
+            arange = range(max(aequiv - WINDOW, dump_i),
+                           min(aequiv + WINDOW + 1, ahi))
+            if not arange: # likely exit if `a` is shorter than `b`
+                break
+            best_ratio = cutoff
+            for i in arange:
+                cruncher.set_seq1(a[i])
+                # Ordering by cheapest to most expensive ratio is very
+                # valuable, most often getting out early.
+                if (crqr() > best_ratio
+                      and cqr() > best_ratio
+                      and cr() > best_ratio):
+                    best_i, best_j, best_ratio = i, j, cr()
+
+            if best_i is None:
+                # found nothing to synch on yet - move to next j
+                continue
+
+            # pump out straight replace from before this synch pair
+            yield from self._fancy_helper(a, dump_i, best_i,
+                                          b, dump_j, best_j)
             # do intraline marking on the synch pair
-            aelt, belt = a[this_i], b[this_j]
-            if eqi is None:
+            aelt, belt = a[best_i], b[best_j]
+            if aelt != belt:
                 # pump out a '-', '?', '+', '?' quad for the synched lines
                 atags = btags = ""
                 cruncher.set_seqs(aelt, belt)
@@ -1002,17 +975,18 @@ def _fancy_replace(self, a, alo, ahi, b, blo, bhi):
             else:
                 # the synch pair is identical
                 yield '  ' + aelt
-            last_i, last_j = this_i + 1, this_j + 1
+            dump_i, dump_j = best_i + 1, best_j + 1
+            best_i = best_j = None
 
-        # pump out diffs from after the last synch point
-        yield from self._fancy_helper(a, last_i, ahi,
-                                      b, last_j, bhi)
+        # pump out straight replace from after the last synch pair
+        yield from self._fancy_helper(a, dump_i, ahi,
+                                      b, dump_j, bhi)
 
     def _fancy_helper(self, a, alo, ahi, b, blo, bhi):
         g = []
         if alo < ahi:
             if blo < bhi:
-                g = self._fancy_replace(a, alo, ahi, b, blo, bhi)
+                g = self._plain_replace(a, alo, ahi, b, blo, bhi)
             else:
                 g = self._dump('-', a, alo, ahi)
         elif blo < bhi:

Misc/NEWS.d/next/Library/2024-05-24-04-05-37.gh-issue-119105.aDSRFn.rst

@@ -0,0 +1 @@
+``difflib``'s ``DIffer.compare()`` (and so also ``ndiff``) can no longer be provoked into cubic-time behavior, or into unbounded recursion, and should generally be faster in ordinary cases too. Results may change in some cases, although that should be rare. Correctness of diffs is not affected. Some similar lines far apart may be reported as deleting one and adding the other, where before they were displayed on adjacent output lines with markup showing the intraline differences.