mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-21 20:40:05 +00:00
130 lines
4.3 KiB
C
130 lines
4.3 KiB
C
/* Copyright (C) 1991-2023 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>
|
|
|
|
#ifndef _LIBC
|
|
# define __memmem memmem
|
|
#endif
|
|
|
|
#define RETURN_TYPE void *
|
|
#define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
|
|
#define FASTSEARCH(S,C,N) (void*) memchr ((void *)(S), (C), (N))
|
|
#include "str-two-way.h"
|
|
|
|
#undef memmem
|
|
|
|
/* 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 memmem algorithm with guaranteed linear-time performance.
|
|
Small needles up to size 2 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. */
|
|
void *
|
|
__memmem (const void *haystack, size_t hs_len,
|
|
const void *needle, size_t ne_len)
|
|
{
|
|
const unsigned char *hs = (const unsigned char *) haystack;
|
|
const unsigned char *ne = (const unsigned char *) needle;
|
|
|
|
if (ne_len == 0)
|
|
return (void *) hs;
|
|
if (ne_len == 1)
|
|
return (void *) memchr (hs, ne[0], hs_len);
|
|
|
|
/* Ensure haystack length is >= needle length. */
|
|
if (hs_len < ne_len)
|
|
return NULL;
|
|
|
|
const unsigned char *end = hs + hs_len - ne_len;
|
|
|
|
if (ne_len == 2)
|
|
{
|
|
uint32_t nw = ne[0] << 16 | ne[1], hw = hs[0] << 16 | hs[1];
|
|
for (hs++; hs <= end && hw != nw; )
|
|
hw = hw << 16 | *++hs;
|
|
return hw == nw ? (void *)hs - 1 : NULL;
|
|
}
|
|
|
|
/* Use Two-Way algorithm for very long needles. */
|
|
if (__builtin_expect (ne_len > 256, 0))
|
|
return two_way_long_needle (hs, hs_len, ne, ne_len);
|
|
|
|
uint8_t shift[256];
|
|
size_t tmp, shift1;
|
|
size_t m1 = ne_len - 1;
|
|
size_t offset = 0;
|
|
|
|
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;
|
|
|
|
for ( ; hs <= end; )
|
|
{
|
|
/* 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;
|
|
}
|
|
return NULL;
|
|
}
|
|
libc_hidden_def (__memmem)
|
|
weak_alias (__memmem, memmem)
|
|
libc_hidden_weak (memmem)
|