Suggested improvement to radixsort()

[Markus B Krüger]

(I hope this is the correct forum for this post; if not, please let me
know where it should go.)

I've found a possible improvement to the implementation of radixsort()
located in /usr/src/lib/libc/stdlib/radixsort.c, which will make the
implementation more efficient when sorting data sets containing many
strings that share a common prefix.

The improvement consists in checking for bins where all strings have
the same character at the current position of the radix sort, and
moving on to the next character instead of needlessly shuffling the
strings in the bin around.  When I used the modified radixsort() to
sort a web log file, which typically contains many strings with common
prefixes (IP addresses), I registered a speedup of approximately 15%.
On a constructed dataset where all strings had a long common prefix,
the speedup was 50%.

My modifications are listed below in patch format.  Corrections,
comments and questions are welcome.  Is this patch something that I
should send with send-pr, or should it be submitted elsewhere?

--- begin radixsort.c.patch ---
*** radixsort.c	Tue Mar 11 12:39:57 1997
--- radixsort.c.patch	Fri Oct 31 12:55:10 2003
***************
*** 175,180 ****
--- 175,191 ----
  		}

  		/*
+ 		 * Special case: if all strings have the same
+ 		 * character at position i, move on to the next
+ 		 * character.
+ 		 */
+ 		if (nc == 1 && count[bmin] == n) {
+ 			push(a, n, i+1);
+ 			nc = count[bmin] = 0;
+ 			continue;
+ 		}
+
+ 		/*
  		 * Set top[]; push incompletely sorted bins onto stack.
  		 * top[] = pointers to last out-of-place element in bins.
  		 * count[] = counts of elements in bins.
--- end radixsort.c.patch ---

-- 
 ,-------------------  Markus Bjartveit Krüger  ---------------------.
'                                                                     `
` E-mail: markusk at pvv.org           WWW: http://www.pvv.org/~markusk/ '
 )-------------------------------------------------------------------(