mdiff: speed up showfunc for large diffs This addresses the following issues with showfunc: - Silly usage of regular expressions. - Doing str.rstrip() needlessly in an inner loop. - Doing catastrophic backtracking when trying to find a function line. Finding function text is now at worst O(n lines in the old file), and at best close to O(n hunks). Given a diff like this[1]: src/main/antlr3/uk/ac/cam/ch/wwmm/pregenerated/ChemicalChunker.g | 4 +- src/main/java/uk/ac/cam/ch/wwmm/pregenerated/ChemicalChunkerLexer.java | 2 +- src/main/java/uk/ac/cam/ch/wwmm/pregenerated/ChemicalChunkerParser.java | 29189 +++++---- 3 files changed, 14741 insertions(+), 14454 deletions(-) [1]: https://bitbucket.org/wwmm/chemicaltagger/changeset/d2bfbaecd4fc/raw Without this change, hg log --stat --config diff.showfunc=1 takes an absurdly long time to complete: CallCount Recursive Total(ms) Inline(ms) module:lineno(function) 32813 0 80.3546 40.6086 mercurial.mdiff:160(yieldhunk) +65062746 0 25.7227 25.7227 +<method 'match' of '_sre.SRE_Pattern' objects> +65062746 0 14.0221 14.0221 +<method 'rstrip' of 'str' objects> +1809 0 0.0009 0.0009 +mercurial.mdiff:148(contextend) +1809 0 0.0003 0.0003 +<len> 65062746 0 25.7227 25.7227 <method 'match' of '_sre.SRE_Pattern' objects> 65062763 0 14.0221 14.0221 <method 'rstrip' of 'str' objects> 543 0 0.1631 0.1631 <zlib.decompress> 3 0 0.0505 0.0505 <mercurial.bdiff.blocks> 31007 0 80.4564 0.0477 mercurial.mdiff:147(_unidiff) +32813 0 80.3546 40.6086 +mercurial.mdiff:160(yieldhunk) +3 0 0.0505 0.0505 +<mercurial.bdiff.blocks> +3618 0 0.0022 0.0022 +mercurial.mdiff:154(contextstart) +5427 0 0.0013 0.0013 +<len> +3 0 0.0001 0.0000 +re:188(compile) 1 0 80.8381 0.0322 mercurial.patch:1777(diffstatdata) +107499 0 0.0235 0.0235 +<method 'startswith' of 'str' objects> +31014 0 80.7820 0.0071 +mercurial.util:1284(iterlines) +3 0 0.0000 0.0000 +<method 'search' of '_sre.SRE_Pattern' objects> +4 0 0.0000 0.0000 +mercurial.patch:1783(addresult) +3 0 0.0000 0.0000 +<method 'group' of '_sre.SRE_Match' objects> 6 0 0.0444 0.0283 mercurial.mdiff:12(splitnewlines) +6 0 0.0160 0.0160 +<method 'split' of 'str' objects> 32 0 0.0246 0.0246 <method 'update' of '_hashlib.HASH' objects> 11 0 0.0236 0.0236 <method 'read' of 'file' objects> Time: real 80.880 secs (user 80.200+0.000 sys 0.380+0.000) With this change, it's almost as fast as not using showfunc at all: CallCount Recursive Total(ms) Inline(ms) module:lineno(function) 543 0 0.1699 0.1699 <zlib.decompress> 3 0 0.0501 0.0501 <mercurial.bdiff.blocks> 32813 0 0.0415 0.0348 mercurial.mdiff:161(yieldhunk) +70837 0 0.0058 0.0058 +<method 'isalnum' of 'str' objects> +1809 0 0.0006 0.0006 +mercurial.mdiff:148(contextend) +1809 0 0.0002 0.0002 +<len> 1 0 0.4879 0.0310 mercurial.patch:1777(diffstatdata) +107499 0 0.0230 0.0230 +<method 'startswith' of 'str' objects> +31014 0 0.4335 0.0065 +mercurial.util:1284(iterlines) +3 0 0.0000 0.0000 +<method 'search' of '_sre.SRE_Pattern' objects> +4 0 0.0000 0.0000 +mercurial.patch:1783(addresult) +1 0 0.0004 0.0000 +re:188(compile) 32 0 0.0293 0.0293 <method 'update' of '_hashlib.HASH' objects> 6 0 0.0427 0.0279 mercurial.mdiff:12(splitnewlines) +6 0 0.0147 0.0147 +<method 'split' of 'str' objects> 31007 0 0.1169 0.0235 mercurial.mdiff:147(_unidiff) +3 0 0.0501 0.0501 +<mercurial.bdiff.blocks> +32813 0 0.0415 0.0348 +mercurial.mdiff:161(yieldhunk) +3618 0 0.0012 0.0012 +mercurial.mdiff:154(contextstart) +5427 0 0.0006 0.0006 +<len> 107597 0 0.0230 0.0230 <method 'startswith' of 'str' objects> 16 0 0.0213 0.0213 <mercurial.mpatch.patches> 194 0 0.0149 0.0149 <method 'split' of 'str' objects> Time: real 0.530 secs (user 0.450+0.000 sys 0.070+0.000)