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/* Copyright (C) 1991-2021 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)
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