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130 lines
4.3 KiB
C
130 lines
4.3 KiB
C
/* Copyright (C) 1991-2020 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#ifndef _LIBC
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# include <config.h>
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#endif
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#include <string.h>
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#ifndef _LIBC
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# define __memmem memmem
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#endif
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#define RETURN_TYPE void *
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#define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
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#define FASTSEARCH(S,C,N) (void*) memchr ((void *)(S), (C), (N))
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#include "str-two-way.h"
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#undef memmem
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/* Hash character pairs so a small shift table can be used. All bits of
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p[0] are included, but not all bits from p[-1]. So if two equal hashes
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match on p[-1], p[0] matches too. Hash collisions are harmless and result
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in smaller shifts. */
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#define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift))
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/* Fast memmem algorithm with guaranteed linear-time performance.
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Small needles up to size 2 use a dedicated linear search. Longer needles
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up to size 256 use a novel modified Horspool algorithm. It hashes pairs
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of characters to quickly skip past mismatches. The main search loop only
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exits if the last 2 characters match, avoiding unnecessary calls to memcmp
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and allowing for a larger skip if there is no match. A self-adapting
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filtering check is used to quickly detect mismatches in long needles.
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By limiting the needle length to 256, the shift table can be reduced to 8
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bits per entry, lowering preprocessing overhead and minimizing cache effects.
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The limit also implies worst-case performance is linear.
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Needles larger than 256 characters use the linear-time Two-Way algorithm. */
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void *
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__memmem (const void *haystack, size_t hs_len,
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const void *needle, size_t ne_len)
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{
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const unsigned char *hs = (const unsigned char *) haystack;
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const unsigned char *ne = (const unsigned char *) needle;
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if (ne_len == 0)
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return (void *) hs;
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if (ne_len == 1)
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return (void *) memchr (hs, ne[0], hs_len);
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/* Ensure haystack length is >= needle length. */
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if (hs_len < ne_len)
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return NULL;
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const unsigned char *end = hs + hs_len - ne_len;
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if (ne_len == 2)
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{
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uint32_t nw = ne[0] << 16 | ne[1], hw = hs[0] << 16 | hs[1];
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for (hs++; hs <= end && hw != nw; )
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hw = hw << 16 | *++hs;
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return hw == nw ? (void *)hs - 1 : NULL;
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}
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/* Use Two-Way algorithm for very long needles. */
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if (__builtin_expect (ne_len > 256, 0))
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return two_way_long_needle (hs, hs_len, ne, ne_len);
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uint8_t shift[256];
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size_t tmp, shift1;
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size_t m1 = ne_len - 1;
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size_t offset = 0;
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memset (shift, 0, sizeof (shift));
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for (int i = 1; i < m1; i++)
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shift[hash2 (ne + i)] = i;
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/* Shift1 is the amount we can skip after matching the hash of the
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needle end but not the full needle. */
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shift1 = m1 - shift[hash2 (ne + m1)];
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shift[hash2 (ne + m1)] = m1;
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for ( ; hs <= end; )
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{
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/* Skip past character pairs not in the needle. */
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do
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{
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hs += m1;
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tmp = shift[hash2 (hs)];
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}
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while (tmp == 0 && hs <= end);
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/* If the match is not at the end of the needle, shift to the end
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and continue until we match the hash of the needle end. */
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hs -= tmp;
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if (tmp < m1)
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continue;
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/* Hash of the last 2 characters matches. If the needle is long,
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try to quickly filter out mismatches. */
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if (m1 < 15 || memcmp (hs + offset, ne + offset, 8) == 0)
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{
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if (memcmp (hs, ne, m1) == 0)
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return (void *) hs;
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/* Adjust filter offset when it doesn't find the mismatch. */
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offset = (offset >= 8 ? offset : m1) - 8;
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}
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/* Skip based on matching the hash of the needle end. */
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hs += shift1;
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}
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return NULL;
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}
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libc_hidden_def (__memmem)
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weak_alias (__memmem, memmem)
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libc_hidden_weak (memmem)
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