Re: why is this code so slow?

From:
"Somebody" <somebody@cox.net>
Newsgroups:
comp.lang.c++
Date:
Tue, 1 May 2007 19:01:04 -0700
Message-ID:
<eiSZh.24232$OT4.21493@newsfe19.lga>
It doesn't work actually :). It seems like you only look at one digit at a
time rather then the number as a whole...

It fails on:

String [1/10]
String [01/10]
String[10/10]
String [2/10]

But it was faster though... LOL...

Anyways, I got my working algorithm optimized down so that its as fast as
yours (but works :) )...

strcmp() = 36ms
my logical string compare = 175ms

This is about the timing I expected, not a MINUTE+ with my original
implementation.

I got it down this fast by:

1) writing a very small streamlined macro for "IsDigit()". That chopped off
a SIGNIFICANT amount of time since its not an intrinsic function and the CRT
function (for VS2005) does a bunch of locale crap.

2) writing a very small streamlined INLINE block of code for atoi()... Yes,
it was copy & pasted twice since it would be easier to read :). We are only
talking about 4 lines here. This allowed me to combine the atoi() code and
the "find end of number" loops that followed. Another significant chunk of
time.

3) finally, I wrote a streamlined macro for tolower()... that had a big
effect on the case insensitive compares.

Calling conventions had almost zero effect on performance.

"Obnoxious User" <OU@127.0.0.1> wrote in message
news:463718a3$0$13594$88260bb3@alt.teranews.com...

Somebody skrev:

<cpunerd@gmail.com> wrote in message
news:1177990346.022115.312180@n76g2000hsh.googlegroups.com...

On Apr 30, 8:24 pm, "Somebody" <someb...@cox.net> wrote:

<snip>

the string library is probably intrinsic with your compiler, and
strcmp works off the knowledge that one can compare multiple bytes at
a time before the end of the string. it may be faster for you to write
an inlined function that finds the first number in a string, then
strcmp the bytes previous to the numbers, and then do your inlined
compare of the numbers


Hmm...

I did think of doing something like that, but even though the example
string I gave above was "nice"... it isn't always going to be that way.
Imagine if you have something like "DVD-ISO9817-FULLIMAGE-1-07-2005" as
an example. I'd have to compare every string segment of the string
seperately and keep track of where I'm at. I thought it would be more
overhead.


What about:
(Written without enough debugging, but I expect it to work. Not
benchmarked)

//---------------------------------------------------------------------------
#include <set>
#include <ostream>
#include <algorithm>
#include <iterator>

inline bool isd(char c) {
return (c >= '0') && (c <= '9');
}

int cmplogic(char const * a, char const * b) {
  int result = 0;
  bool ad,bd,ab,ob,pb = false;
  while(*a && *b) {
    ad = isd(*a);
    bd = isd(*b);
    ab = ad && bd;
    ob = !ab && (ad || bd);
    if(*a < *b) {
      if(ab) { if(!result) result = -1; }
      else if(pb && ob) return 1;
      else if(pb && !ab) return result?result:-1;
      else return -1;
    } else
    if(*a > *b) {
      if(ab) { if(!result) result = 1; }
      else if(pb && ob) return -1;
      else if(pb && !ab) return result?result:1;
      else return 1;
    } else
    if(result) return result;
    pb = ab;
    ++a;
    ++b;
  }
  if(!(*a || *b)) {
    return result;
  }
  if(*a) {
    if(isd(*a)) return 1;
    return result?result:1;
  }
  if(isd(*b)) return -1;
  return result?result:-1;
}

struct diff {
  bool operator()(char const * a, char const * b) {
    return cmplogic(a,b) == -1;
  }
};

int main(int argc, char* argv[])
{
  std::set<char const *,diff> s;
  s.insert("string11");
  s.insert("string1");
  s.insert("2u");
  s.insert("963y");
  s.insert("963h");
  s.insert("string481");
  s.insert("DVD-ISO9817-FULLIMAGE-1-08-2006");
  s.insert("string23u4");
  s.insert("string25");
  s.insert("string10");
  s.insert("41");
  s.insert("string110");
  s.insert("string23a");
  s.insert("string2");
  s.insert("string23u21");
  s.insert("963");
  s.insert("957");
  s.insert("DVD-ISO9817-FULLIMAGE-1-07-2006");
  s.insert("DVD-ISO9819-FULLIMAGE-2-07-2005");
  std::copy(s.begin(),s.end(),
            std::ostream_iterator<char const*>(std::cout,"\n"));
  return 0;
}

//---------------------------------------------------------------------------

Outputs:

2u
41
957
963
963h
963y
DVD-ISO9817-FULLIMAGE-1-07-2006
DVD-ISO9817-FULLIMAGE-1-08-2006
DVD-ISO9819-FULLIMAGE-2-07-2005
string1
string10
string11
string23
string23a
string23u4
string23u21
string25
string110
string481

--
OU

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