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cf11e74b0d
It uses the same two-way algorithm used on strstr, strcasestr, and memmem. Different than strstr, neither the "shift table" optimization nor the self-adapting filtering check is used because it would result in a too-large shift table (and it also simplifies the implementation bit). Checked on x86_64-linux-gnu and aarch64-linux-gnu. Reviewed-by: DJ Delorie <dj@redhat.com>
313 lines
9.5 KiB
C
313 lines
9.5 KiB
C
/* Wide character substring search, using the Two-Way algorithm.
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Copyright (C) 2008-2024 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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/* Before including this file, you need to include <string.h> (and
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<config.h> before that, if not part of libc), and define:
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AVAILABLE(h, h_l, j, n_l)
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A macro that returns nonzero if there are
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at least N_L characters left starting at H[J].
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H is 'wchar_t *', H_L, J, and N_L are 'size_t';
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H_L is an lvalue. For NUL-terminated searches,
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H_L can be modified each iteration to avoid
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having to compute the end of H up front.
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For case-insensitivity, you may optionally define:
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CMP_FUNC(p1, p2, l) A macro that returns 0 iff the first L
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characters of P1 and P2 are equal.
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CANON_ELEMENT(c) A macro that canonicalizes an element right after
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it has been fetched from one of the two strings.
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The argument is an 'wchar_t'; the result must
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be an 'wchar_t' as well.
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*/
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#include <limits.h>
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#include <stdint.h>
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#include <sys/param.h> /* Defines MAX. */
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/* We use the Two-Way string matching algorithm, which guarantees
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linear complexity with constant space.
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See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260
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and http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm
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*/
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#ifndef CANON_ELEMENT
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# define CANON_ELEMENT(c) c
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#endif
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#ifndef CMP_FUNC
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# define CMP_FUNC __wmemcmp
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#endif
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/* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN.
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Return the index of the first character in the right half, and set
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*PERIOD to the global period of the right half.
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The global period of a string is the smallest index (possibly its
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length) at which all remaining bytes in the string are repetitions
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of the prefix (the last repetition may be a subset of the prefix).
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When NEEDLE is factored into two halves, a local period is the
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length of the smallest word that shares a suffix with the left half
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and shares a prefix with the right half. All factorizations of a
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non-empty NEEDLE have a local period of at least 1 and no greater
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than NEEDLE_LEN.
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A critical factorization has the property that the local period
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equals the global period. All strings have at least one critical
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factorization with the left half smaller than the global period.
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Given an ordered alphabet, a critical factorization can be computed
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in linear time, with 2 * NEEDLE_LEN comparisons, by computing the
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larger of two ordered maximal suffixes. The ordered maximal
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suffixes are determined by lexicographic comparison of
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periodicity. */
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static size_t
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critical_factorization (const wchar_t *needle, size_t needle_len,
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size_t *period)
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{
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/* Index of last character of left half, or SIZE_MAX. */
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size_t max_suffix, max_suffix_rev;
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size_t j; /* Index into NEEDLE for current candidate suffix. */
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size_t k; /* Offset into current period. */
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size_t p; /* Intermediate period. */
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wchar_t a, b; /* Current comparison bytes. */
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/* Special case NEEDLE_LEN of 1 or 2 (all callers already filtered
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out 0-length needles. */
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if (needle_len < 3)
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{
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*period = 1;
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return needle_len - 1;
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}
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/* Invariants:
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0 <= j < NEEDLE_LEN - 1
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-1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed)
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min(max_suffix, max_suffix_rev) < global period of NEEDLE
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1 <= p <= global period of NEEDLE
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p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j]
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1 <= k <= p
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*/
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/* Perform lexicographic search. */
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max_suffix = SIZE_MAX;
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j = 0;
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k = p = 1;
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while (j + k < needle_len)
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{
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a = CANON_ELEMENT (needle[j + k]);
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b = CANON_ELEMENT (needle[max_suffix + k]);
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if (a < b)
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{
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/* Suffix is smaller, period is entire prefix so far. */
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j += k;
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k = 1;
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p = j - max_suffix;
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}
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else if (a == b)
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{
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/* Advance through repetition of the current period. */
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if (k != p)
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++k;
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else
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{
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j += p;
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k = 1;
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}
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}
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else /* b < a */
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{
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/* Suffix is larger, start over from current location. */
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max_suffix = j++;
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k = p = 1;
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}
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}
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*period = p;
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/* Perform reverse lexicographic search. */
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max_suffix_rev = SIZE_MAX;
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j = 0;
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k = p = 1;
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while (j + k < needle_len)
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{
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a = CANON_ELEMENT (needle[j + k]);
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b = CANON_ELEMENT (needle[max_suffix_rev + k]);
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if (b < a)
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{
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/* Suffix is smaller, period is entire prefix so far. */
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j += k;
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k = 1;
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p = j - max_suffix_rev;
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}
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else if (a == b)
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{
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/* Advance through repetition of the current period. */
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if (k != p)
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++k;
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else
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{
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j += p;
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k = 1;
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}
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}
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else /* a < b */
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{
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/* Suffix is larger, start over from current location. */
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max_suffix_rev = j++;
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k = p = 1;
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}
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}
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/* Choose the shorter suffix. Return the first character of the right
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half, rather than the last character of the left half. */
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if (max_suffix_rev + 1 < max_suffix + 1)
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return max_suffix + 1;
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*period = p;
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return max_suffix_rev + 1;
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}
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/* Return the first location of non-empty NEEDLE within HAYSTACK, or
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NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK.
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If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at
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most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.
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If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
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HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */
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static inline wchar_t *
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two_way_short_needle (const wchar_t *haystack, size_t haystack_len,
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const wchar_t *needle, size_t needle_len)
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{
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size_t i; /* Index into current character of NEEDLE. */
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size_t j; /* Index into current window of HAYSTACK. */
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size_t period; /* The period of the right half of needle. */
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size_t suffix; /* The index of the right half of needle. */
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/* Factor the needle into two halves, such that the left half is
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smaller than the global period, and the right half is
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periodic (with a period as large as NEEDLE_LEN - suffix). */
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suffix = critical_factorization (needle, needle_len, &period);
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/* Perform the search. Each iteration compares the right half
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first. */
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if (CMP_FUNC (needle, needle + period, suffix) == 0)
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{
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/* Entire needle is periodic; a mismatch can only advance by the
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period, so use memory to avoid rescanning known occurrences
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of the period. */
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size_t memory = 0;
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j = 0;
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while (AVAILABLE (haystack, haystack_len, j, needle_len))
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{
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const wchar_t *pneedle;
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const wchar_t *phaystack;
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/* Scan for matches in right half. */
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i = MAX (suffix, memory);
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pneedle = &needle[i];
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phaystack = &haystack[i + j];
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while (i < needle_len && (CANON_ELEMENT (*pneedle++)
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== CANON_ELEMENT (*phaystack++)))
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++i;
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if (needle_len <= i)
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{
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/* Scan for matches in left half. */
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i = suffix - 1;
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pneedle = &needle[i];
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phaystack = &haystack[i + j];
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while (memory < i + 1 && (CANON_ELEMENT (*pneedle--)
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== CANON_ELEMENT (*phaystack--)))
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--i;
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if (i + 1 < memory + 1)
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return (wchar_t *) (haystack + j);
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/* No match, so remember how many repetitions of period
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on the right half were scanned. */
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j += period;
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memory = needle_len - period;
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}
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else
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{
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j += i - suffix + 1;
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memory = 0;
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}
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}
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}
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else
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{
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const wchar_t *phaystack;
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/* The comparison always starts from needle[suffix], so cache it
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and use an optimized first-character loop. */
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wchar_t needle_suffix = CANON_ELEMENT (needle[suffix]);
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/* The two halves of needle are distinct; no extra memory is
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required, and any mismatch results in a maximal shift. */
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period = MAX (suffix, needle_len - suffix) + 1;
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j = 0;
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while (AVAILABLE (haystack, haystack_len, j, needle_len))
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{
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wchar_t haystack_char;
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const wchar_t *pneedle;
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phaystack = &haystack[suffix + j];
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while (needle_suffix
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!= (haystack_char = CANON_ELEMENT (*phaystack++)))
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{
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++j;
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if (!AVAILABLE (haystack, haystack_len, j, needle_len))
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goto ret0;
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}
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/* Scan for matches in right half. */
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i = suffix + 1;
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pneedle = &needle[i];
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while (i < needle_len)
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{
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if (CANON_ELEMENT (*pneedle++)
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!= (haystack_char = CANON_ELEMENT (*phaystack++)))
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break;
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++i;
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}
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if (needle_len <= i)
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{
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/* Scan for matches in left half. */
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i = suffix - 1;
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pneedle = &needle[i];
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phaystack = &haystack[i + j];
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while (i != SIZE_MAX)
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{
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if (CANON_ELEMENT (*pneedle--)
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!= (haystack_char = CANON_ELEMENT (*phaystack--)))
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break;
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--i;
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}
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if (i == SIZE_MAX)
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return (wchar_t *) (haystack + j);
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j += period;
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}
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else
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j += i - suffix + 1;
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}
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}
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ret0: __attribute__ ((unused))
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return NULL;
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}
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#undef AVAILABLE
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#undef CANON_ELEMENT
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#undef CMP_FUNC
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