Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
162 lines (138 sloc) 5.39 KB
/* Return the offset of one string within another.
Copyright (C) 1994-2019 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifndef _LIBC
# include <config.h>
#endif
#include <string.h>
#define RETURN_TYPE char *
#define AVAILABLE(h, h_l, j, n_l) \
(((j) + (n_l) <= (h_l)) \
|| ((h_l) += __strnlen ((void*)((h) + (h_l)), (n_l) + 512), \
(j) + (n_l) <= (h_l)))
#include "str-two-way.h"
#undef strstr
#ifndef STRSTR
#define STRSTR strstr
#endif
static inline char *
strstr2 (const unsigned char *hs, const unsigned char *ne)
{
uint32_t h1 = (ne[0] << 16) | ne[1];
uint32_t h2 = 0;
for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
h2 = (h2 << 16) | c;
return h1 == h2 ? (char *)hs - 2 : NULL;
}
static inline char *
strstr3 (const unsigned char *hs, const unsigned char *ne)
{
uint32_t h1 = ((uint32_t)ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
uint32_t h2 = 0;
for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
h2 = (h2 | c) << 8;
return h1 == h2 ? (char *)hs - 3 : NULL;
}
/* Hash character pairs so a small shift table can be used. All bits of
p[0] are included, but not all bits from p[-1]. So if two equal hashes
match on p[-1], p[0] matches too. Hash collisions are harmless and result
in smaller shifts. */
#define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift))
/* Fast strstr algorithm with guaranteed linear-time performance.
Small needles up to size 3 use a dedicated linear search. Longer needles
up to size 256 use a novel modified Horspool algorithm. It hashes pairs
of characters to quickly skip past mismatches. The main search loop only
exits if the last 2 characters match, avoiding unnecessary calls to memcmp
and allowing for a larger skip if there is no match. A self-adapting
filtering check is used to quickly detect mismatches in long needles.
By limiting the needle length to 256, the shift table can be reduced to 8
bits per entry, lowering preprocessing overhead and minimizing cache effects.
The limit also implies worst-case performance is linear.
Needles larger than 256 characters use the linear-time Two-Way algorithm. */
char *
STRSTR (const char *haystack, const char *needle)
{
const unsigned char *hs = (const unsigned char *) haystack;
const unsigned char *ne = (const unsigned char *) needle;
/* Handle short needle special cases first. */
if (ne[0] == '\0')
return (char *)hs;
hs = (const unsigned char *)strchr ((const char*)hs, ne[0]);
if (hs == NULL || ne[1] == '\0')
return (char*)hs;
if (ne[2] == '\0')
return strstr2 (hs, ne);
if (ne[3] == '\0')
return strstr3 (hs, ne);
/* Ensure haystack length is at least as long as needle length.
Since a match may occur early on in a huge haystack, use strnlen
and read ahead a few cachelines for improved performance. */
size_t ne_len = strlen ((const char*)ne);
size_t hs_len = __strnlen ((const char*)hs, ne_len | 512);
if (hs_len < ne_len)
return NULL;
/* Check whether we have a match. This improves performance since we
avoid initialization overheads. */
if (memcmp (hs, ne, ne_len) == 0)
return (char *) hs;
/* Use Two-Way algorithm for very long needles. */
if (__glibc_unlikely (ne_len > 256))
return two_way_long_needle (hs, hs_len, ne, ne_len);
const unsigned char *end = hs + hs_len - ne_len;
uint8_t shift[256];
size_t tmp, shift1;
size_t m1 = ne_len - 1;
size_t offset = 0;
/* Initialize bad character shift hash table. */
memset (shift, 0, sizeof (shift));
for (int i = 1; i < m1; i++)
shift[hash2 (ne + i)] = i;
/* Shift1 is the amount we can skip after matching the hash of the
needle end but not the full needle. */
shift1 = m1 - shift[hash2 (ne + m1)];
shift[hash2 (ne + m1)] = m1;
while (1)
{
if (__glibc_unlikely (hs > end))
{
end += __strnlen ((const char*)end + m1 + 1, 2048);
if (hs > end)
return NULL;
}
/* Skip past character pairs not in the needle. */
do
{
hs += m1;
tmp = shift[hash2 (hs)];
}
while (tmp == 0 && hs <= end);
/* If the match is not at the end of the needle, shift to the end
and continue until we match the hash of the needle end. */
hs -= tmp;
if (tmp < m1)
continue;
/* Hash of the last 2 characters matches. If the needle is long,
try to quickly filter out mismatches. */
if (m1 < 15 || memcmp (hs + offset, ne + offset, 8) == 0)
{
if (memcmp (hs, ne, m1) == 0)
return (void *) hs;
/* Adjust filter offset when it doesn't find the mismatch. */
offset = (offset >= 8 ? offset : m1) - 8;
}
/* Skip based on matching the hash of the needle end. */
hs += shift1;
}
}
libc_hidden_builtin_def (strstr)
You can’t perform that action at this time.