bug-gnulib
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: c-strstr: add testcase


From: Bruno Haible
Subject: Re: c-strstr: add testcase
Date: Sun, 11 Feb 2007 15:13:54 +0100
User-agent: KMail/1.5.4

Now here's the fix that brings the worst-case complexity down from O(n*m)
to O(n). One could use either the Knuth-Morris-Pratt or the Boyer-Moore
algorithm. I'm using the former one, because Boyer-Moore works "backwards",
which is harder to generalize to the mbsstr case.

2007-02-11  Bruno Haible  <address@hidden>

        Ensure O(n) worst-case complexity of c_strstr.
        * lib/c-strstr.c: Include stdbool.h, string.h.
        (knuth_morris_pratt): New function.
        (c_strstr): Add some bookkeeping. Invoke knuth_morris_pratt when the
        bookkeeping indicates that it's worth it.
        * modules/c-strstr (Depends-on): Add stdbool, strnlen.

*** lib/c-strstr.c      11 Feb 2007 13:58:43 -0000      1.4
--- lib/c-strstr.c      11 Feb 2007 14:01:35 -0000
***************
*** 21,27 ****
  /* Specification.  */
  #include "c-strstr.h"
  
! #include <stddef.h>
  
  /* Find the first occurrence of NEEDLE in HAYSTACK.  */
  char *
--- 21,117 ----
  /* Specification.  */
  #include "c-strstr.h"
  
! #include <stdbool.h>
! #include <stdlib.h>
! #include <string.h>
! 
! /* Knuth-Morris-Pratt algorithm.
!    See http://en.wikipedia.org/wiki/Knuth-Morris-Pratt_algorithm
!    Return a boolean indicating success.  */
! static bool
! knuth_morris_pratt (const char *haystack, const char *needle,
!                   const char **resultp)
! {
!   size_t m = strlen (needle);
! 
!   /* Allocate the table.  */
!   size_t *table = (size_t *) malloc (m * sizeof (size_t));
!   if (table == NULL)
!     return false;
!   /* Fill the table.
!      For 0 < i < m:
!        0 < table[i] <= i is defined such that
!        rhaystack[0..i-1] == needle[0..i-1] and rhaystack[i] != needle[i]
!        implies
!        forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1],
!        and table[i] is as large as possible with this property.
!      table[0] remains uninitialized.  */
!   {
!     size_t i, j;
! 
!     table[1] = 1;
!     j = 0;
!     for (i = 2; i < m; i++)
!       {
!       unsigned char b = (unsigned char) needle[i - 1];
! 
!       for (;;)
!         {
!           if (b == (unsigned char) needle[j])
!             {
!               table[i] = i - ++j;
!               break;
!             }
!           if (j == 0)
!             {
!               table[i] = i;
!               break;
!             }
!           j = j - table[j];
!         }
!       }
!   }
! 
!   /* Search, using the table to accelerate the processing.  */
!   {
!     size_t j;
!     const char *rhaystack;
!     const char *phaystack;
! 
!     *resultp = NULL;
!     j = 0;
!     rhaystack = haystack;
!     phaystack = haystack;
!     /* Invariant: phaystack = rhaystack + j.  */
!     while (*phaystack != '\0')
!       if ((unsigned char) needle[j] == (unsigned char) *phaystack)
!       {
!         j++;
!         phaystack++;
!         if (j == m)
!           {
!             /* The entire needle has been found.  */
!             *resultp = rhaystack;
!             break;
!           }
!       }
!       else if (j > 0)
!       {
!         /* Found a match of needle[0..j-1], mismatch at needle[j].  */
!         rhaystack += table[j];
!         j -= table[j];
!       }
!       else
!       {
!         /* Found a mismatch at needle[0] already.  */
!         rhaystack++;
!         phaystack++;
!       }
!   }
! 
!   free (table);
!   return true;
! }
  
  /* Find the first occurrence of NEEDLE in HAYSTACK.  */
  char *
***************
*** 34,39 ****
--- 124,149 ----
           needle may be found in haystack.  */
    if (*needle != '\0')
      {
+       /* Minimizing the worst-case complexity:
+        Let n = strlen(haystack), m = strlen(needle).
+        The naïve algorithm is O(n*m) worst-case.
+        The Knuth-Morris-Pratt algorithm is O(n) worst-case but it needs a
+        memory allocation.
+        To achieve linear complexity and yet amortize the cost of the memory
+        allocation, we activate the Knuth-Morris-Pratt algorithm only once
+        the naïve algorithm has already run for some time; more precisely,
+        when
+          - the outer loop count is >= 10,
+          - the average number of comparisons per outer loop is >= 5,
+          - the total number of comparisons is >= m.
+        But we try it only once.  If the memory allocation attempt failed,
+        we don't retry it.  */
+       bool try_kmp = true;
+       size_t outer_loop_count = 0;
+       size_t comparison_count = 0;
+       size_t last_ccount = 0;                 /* last comparison count */
+       const char *needle_last_ccount = needle;        /* = needle + 
last_ccount */
+ 
        /* Speed up the following searches of needle by caching its first
         character.  */
        unsigned char b = (unsigned char) *needle;
***************
*** 44,49 ****
--- 154,190 ----
          if (*haystack == '\0')
            /* No match.  */
            return NULL;
+ 
+         /* See whether it's advisable to use an asymptotically faster
+            algorithm.  */
+         if (try_kmp
+             && outer_loop_count >= 10
+             && comparison_count >= 5 * outer_loop_count)
+           {
+             /* See if needle + comparison_count now reaches the end of
+                needle.  */
+             if (needle_last_ccount != NULL)
+               {
+                 needle_last_ccount +=
+                   strnlen (needle_last_ccount, comparison_count - 
last_ccount);
+                 if (*needle_last_ccount == '\0')
+                   needle_last_ccount = NULL;
+                 last_ccount = comparison_count;
+               }
+             if (needle_last_ccount == NULL)
+               {
+                 /* Try the Knuth-Morris-Pratt algorithm.  */
+                 const char *result;
+                 bool success =
+                   knuth_morris_pratt (haystack, needle - 1, &result);
+                 if (success)
+                   return (char *) result;
+                 try_kmp = false;
+               }
+           }
+ 
+         outer_loop_count++;
+         comparison_count++;
          if ((unsigned char) *haystack == b)
            /* The first character matches.  */
            {
***************
*** 58,63 ****
--- 199,205 ----
                  if (*rhaystack == '\0')
                    /* No match.  */
                    return NULL;
+                 comparison_count++;
                  if ((unsigned char) *rhaystack != (unsigned char) *rneedle)
                    /* Nothing in this round.  */
                    break;
*** modules/c-strstr    28 Aug 2006 13:04:33 -0000      1.1
--- modules/c-strstr    11 Feb 2007 14:01:35 -0000
***************
*** 6,11 ****
--- 6,13 ----
  lib/c-strstr.c
  
  Depends-on:
+ stdbool
+ strnlen
  
  configure.ac:
  





reply via email to

[Prev in Thread] Current Thread [Next in Thread]