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POFastest way to do a case-insensitive substring search in C/C++?
 

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    <h2>Note</h2> <p>The question below was asked in 2008 about some code from 2003. As the OP's <strong>update</strong> shows, this entire post has been obsoleted by vintage 2008 algorithms and persists here only as a historical curiosity.</p> <hr> <p>I need to do a fast case-insensitive substring search in C/C++. My requirements are as follows:</p> <ul> <li>Should behave like strstr() (i.e. return a pointer to the match point).</li> <li>Must be case-insensitive (doh).</li> <li>Must support the current locale.</li> <li>Must be available on Windows (MSVC++ 8.0) or easily portable to Windows (i.e. from an open source library).</li> </ul> <p>Here is the current implementation I am using (taken from the GNU C Library):</p> <pre><code>/* Return the offset of one string within another. Copyright (C) 1994,1996,1997,1998,1999,2000 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with the GNU C Library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA. */ /* * My personal strstr() implementation that beats most other algorithms. * Until someone tells me otherwise, I assume that this is the * fastest implementation of strstr() in C. * I deliberately chose not to comment it. You should have at least * as much fun trying to understand it, as I had to write it :-). * * Stephen R. van den Berg, berg@pool.informatik.rwth-aachen.de */ /* * Modified to use table lookup instead of tolower(), since tolower() isn't * worth s*** on Windows. * * -- Anders Sandvig (anders@wincue.org) */ #if HAVE_CONFIG_H # include &lt;config.h&gt; #endif #include &lt;ctype.h&gt; #include &lt;string.h&gt; typedef unsigned chartype; char char_table[256]; void init_stristr(void) { int i; char string[2]; string[1] = '\0'; for (i = 0; i &lt; 256; i++) { string[0] = i; _strlwr(string); char_table[i] = string[0]; } } #define my_tolower(a) ((chartype) char_table[a]) char * my_stristr (phaystack, pneedle) const char *phaystack; const char *pneedle; { register const unsigned char *haystack, *needle; register chartype b, c; haystack = (const unsigned char *) phaystack; needle = (const unsigned char *) pneedle; b = my_tolower (*needle); if (b != '\0') { haystack--; /* possible ANSI violation */ do { c = *++haystack; if (c == '\0') goto ret0; } while (my_tolower (c) != (int) b); c = my_tolower (*++needle); if (c == '\0') goto foundneedle; ++needle; goto jin; for (;;) { register chartype a; register const unsigned char *rhaystack, *rneedle; do { a = *++haystack; if (a == '\0') goto ret0; if (my_tolower (a) == (int) b) break; a = *++haystack; if (a == '\0') goto ret0; shloop: ; } while (my_tolower (a) != (int) b); jin: a = *++haystack; if (a == '\0') goto ret0; if (my_tolower (a) != (int) c) goto shloop; rhaystack = haystack-- + 1; rneedle = needle; a = my_tolower (*rneedle); if (my_tolower (*rhaystack) == (int) a) do { if (a == '\0') goto foundneedle; ++rhaystack; a = my_tolower (*++needle); if (my_tolower (*rhaystack) != (int) a) break; if (a == '\0') goto foundneedle; ++rhaystack; a = my_tolower (*++needle); } while (my_tolower (*rhaystack) == (int) a); needle = rneedle; /* took the register-poor approach */ if (a == '\0') break; } } foundneedle: return (char*) haystack; ret0: return 0; }</code></pre> <p>Can you make this code faster, or do you know of a better implementation?</p> <p><strong>Note:</strong> I noticed that the GNU C Library now has <a href="http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/string/strstr.c?rev=1.2&amp;content-type=text/x-cvsweb-markup&amp;cvsroot=glibc" rel="nofollow noreferrer">a new implementation of <code>strstr()</code></a>, but I am not sure how easily it can be modified to be case-insensitive, or if it is in fact faster than the old one (in my case). I also noticed that <a href="http://sources.redhat.com/cgi-bin/cvsweb.cgi/libc/wcsmbs/wcsstr.c?rev=1.4&amp;content-type=text/x-cvsweb-markup&amp;cvsroot=glibc" rel="nofollow noreferrer">the old implementation is still used for wide character strings</a>, so if anyone knows why, please share.</p> <p><strong>Update</strong></p> <p>Just to make things clear&mdash;in case it wasn't already&mdash;I didn't write this function, it's a part of the GNU C Library. I only modified it to be case-insensitive.</p> <p>Also, thanks for the tip about <code>strcasestr()</code> and checking out other implementations from other sources (like OpenBSD, FreeBSD, etc.). It seems to be the way to go. The code above is from 2003, which is why I posted it here in hope for a better version being available, which apparently it is. :)</p>
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    1. COThis tip won't help but you should at least clean out all unnecessary code, like the code you skip with the 'goto jin' statement.
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      1. USLasse Vågsæther Karlsen
    2. CO... and I am not very impressed by your lack of reading skills. I did not write this code, as is stated both in the source code comments and in my note below.
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    3. COI wanted to see if people knew of faster ways to do a case-insensitive substring search--because I need it to provide fast searching in my program--and as it turned out, a faster strcasestr() is in fact now available.
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