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/* c-strstr.c -- substring search in C locale
Copyright (C) 2005-2007 Free Software Foundation, Inc.
Written by Bruno Haible <bruno@clisp.org>, 2005, 2007.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program 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 General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#include <config.h>
/* Specification. */
#include "c-strstr.h"
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include "malloca.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 *) malloca (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++;
}
}
freea (table);
return true;
}
/* Find the first occurrence of NEEDLE in HAYSTACK. */
char *
c_strstr (const char *haystack, const char *needle)
{
/* Be careful not to look at the entire extent of haystack or needle
until needed. This is useful because of these two cases:
- haystack may be very long, and a match of needle found early,
- needle may be very long, and not even a short initial segment of
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;
needle++;
for (;; haystack++)
{
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. */
{
const char *rhaystack = haystack + 1;
const char *rneedle = needle;
for (;; rhaystack++, rneedle++)
{
if (*rneedle == '\0')
/* Found a match. */
return (char *) haystack;
if (*rhaystack == '\0')
/* No match. */
return NULL;
comparison_count++;
if ((unsigned char) *rhaystack != (unsigned char) *rneedle)
/* Nothing in this round. */
break;
}
}
}
}
else
return (char *) haystack;
}
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