Like c_strstr, here is a patch for c_strcasestr that avoids quadratic
asymptotic complexity.
2007-02-11 Bruno Haible <[EMAIL PROTECTED]>
Ensure O(n) worst-case complexity of c_strcasestr.
* lib/c-strcasestr.c: Include stdbool.h, string.h.
(knuth_morris_pratt): New function.
(c_strcasestr): Add some bookkeeping. Invoke knuth_morris_pratt when
the bookkeeping indicates that it's worth it.
* modules/c-strcasestr (Depends-on): Add stdbool, strnlen.
* modules/c-strcasestr-tests: New file.
* tests/test-c-strcasestr.c: New file.
*** lib/c-strcasestr.c 14 Sep 2006 14:18:36 -0000 1.2
--- lib/c-strcasestr.c 11 Feb 2007 15:00:17 -0000
***************
*** 21,30 ****
--- 21,121 ----
/* Specification. */
#include "c-strcasestr.h"
+ #include <stdbool.h>
#include <stddef.h>
+ #include <string.h>
#include "c-ctype.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 = c_tolower ((unsigned char) needle[i - 1]);
+
+ for (;;)
+ {
+ if (b == c_tolower ((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 (c_tolower ((unsigned char) needle[j])
+ == c_tolower ((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, using case-insensitive
comparison.
Note: This function may, in multibyte locales, return success even if
***************
*** 39,44 ****
--- 130,155 ----
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 = c_tolower ((unsigned char) *needle);
***************
*** 49,54 ****
--- 160,196 ----
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 (c_tolower ((unsigned char) *haystack) == b)
/* The first character matches. */
{
***************
*** 63,68 ****
--- 205,211 ----
if (*rhaystack == '\0')
/* No match. */
return NULL;
+ comparison_count++;
if (c_tolower ((unsigned char) *rhaystack)
!= c_tolower ((unsigned char) *rneedle))
/* Nothing in this round. */
*** modules/c-strcasestr 11 Oct 2005 18:50:36 -0000 1.1
--- modules/c-strcasestr 11 Feb 2007 15:00:17 -0000
***************
*** 7,12 ****
--- 7,14 ----
Depends-on:
c-ctype
+ stdbool
+ strnlen
configure.ac:
