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The scan over `candidates` made LCS quadratic for inputs with many equal elements, such as text with many blank lines (bhousel#84). The `buffer2index` values from `r` on are strictly increasing, so the same candidate is found by binary search. The results are unchanged.
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Fixes the slowdown in #84 for inputs with many equal lines, such as text with many blank lines.
In
LCS, the candidate thatjextends was found by a linear scan fromr. Thebuffer2indexvalues fromron are strictly increasing, so the same candidate can be found by binary search. The output is unchanged.Measured on a 4,000-line text that is 45% blank lines, with five changes on each side: 11.2 s before, 0.15 s after. I compared the output with 3.2.1 on 50,000 small inputs and three large ones, and it was identical in every case.
The worst case left is a large input of only one repeated line (20,000 blank lines still takes about 17 s).