On Thu, 03 Jan 2013 11:40:31 +0100, Gustavo Lopes <glo...@nebm.ist.utl.pt>
wrote:
The algorithm behaves very poorly in this case because at each position
of the text, all the substrings starting there and with size between m
and n (where m is the size of the smallest pattern and n is the largest)
are checked, even if there are only two patterns with size m and n. We
could fix this easily by building a set of the pattern sizes found and
try only with those. The hashing of the substrings could also be
improved; we don't have to recalculate everything when we advance in the
text.
Both optimizations (the hash rolling and limiting the substrings hashed on
each iteration) worked quite well.
But I got much better results with another algorithm [1], so I'm going to
merge the branch with it [2] instead. I get these results with a 1.7 MB
string and 13 replacement strings, the smallest with 6 characters and 30
iterations (x86-64, gcc -O3):
strtr: 0.1387
str_replace: 0.4471
The algorithm doesn't perform as well when the replacement strings are
small. Adding a replacement for the pattern '_' (1 character) yields:
strtr: 0.6157
str_replace: 0.6230
But even in this case, it works better than my optimized version of the
current algorithm.
I plan on merging to 5.4 and 5.5; you may want to review it as introducing
completely new code carries some risk.
[1] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.13.2927
[2] https://github.com/cataphract/php-src/compare/strtr_wu94
--
Gustavo Lopes
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