bdiff: further restrain potential quadratic performance This causes the longest_match search to limit itself to a window of 30000 lines during search (roughly 1MB), thus avoiding a full O(N*M) search that might occur in repetitive structured inputs. For a particular class of many MB pathological test cases, this generated the following timings: size before after 10x 1.25s 1.24s 100x 57s 33s 1000x >8400s 400s The times on the right quickly become much faster and appear more linear. While windowing means deltas are no longer "optimal", the resulting deltas were within a couple percent of expected size. While we've yet to have a report of a file with the level of repetition necessary to hit this case, some JSON/XML database dump scenario is fairly likely to hit it. This may also slightly improve the average-case performance for deltas of large binaries.