416 lines
13 KiB
C
416 lines
13 KiB
C
/* Byte-wise substring search, using the Two-Way algorithm.
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* Copyright (C) 2008 Eric Blake
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* Permission to use, copy, modify, and distribute this software
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* is freely granted, provided that this notice is preserved.
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*/
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/* Before including this file, you need to include <string.h>, and define:
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RESULT_TYPE A macro that expands to the return type.
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AVAILABLE(h, h_l, j, n_l) A macro that returns nonzero if there are
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at least N_L bytes left starting at
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H[J]. H is 'unsigned char *', H_L, J,
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and N_L are 'size_t'; H_L is an
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lvalue. For NUL-terminated searches,
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H_L can be modified each iteration to
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avoid having to compute the end of H
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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
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right after it has been fetched from
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one of the two strings. The argument
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is an 'unsigned char'; the result must
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be an 'unsigned char' as well.
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This file undefines the macros documented above, and defines
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LONG_NEEDLE_THRESHOLD.
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*/
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#include <limits.h>
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#include <stdint.h>
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/* We use the Two-Way string matching algorithm, which guarantees
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linear complexity with constant space. Additionally, for long
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needles, we also use a bad character shift table similar to the
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Boyer-Moore algorithm to achieve improved (potentially sub-linear)
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performance.
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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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/* Point at which computing a bad-byte shift table is likely to be
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worthwhile. Small needles should not compute a table, since it
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adds (1 << CHAR_BIT) + NEEDLE_LEN computations of preparation for a
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speedup no greater than a factor of NEEDLE_LEN. The larger the
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needle, the better the potential performance gain. On the other
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hand, on non-POSIX systems with CHAR_BIT larger than eight, the
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memory required for the table is prohibitive. */
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#if CHAR_BIT < 10
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# define LONG_NEEDLE_THRESHOLD 32U
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#else
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# define LONG_NEEDLE_THRESHOLD SIZE_MAX
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#endif
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#define MAX(a, b) ((a < b) ? (b) : (a))
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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 memcmp
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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 byte 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 unsigned char *needle, size_t needle_len,
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size_t *period)
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{
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/* Index of last byte 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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unsigned char a, b; /* Current comparison bytes. */
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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[(size_t)(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 longer suffix. Return the first byte of the right
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half, rather than the last byte 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. This
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method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD.
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Performance is guaranteed to be linear, with an initialization cost
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of 2 * NEEDLE_LEN comparisons.
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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 RETURN_TYPE
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two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
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const unsigned char *needle, size_t needle_len)
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{
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size_t i; /* Index into current byte 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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/* Scan for matches in right half. */
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i = MAX (suffix, memory);
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while (i < needle_len && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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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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while (memory < i + 1 && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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--i;
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if (i + 1 < memory + 1)
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return (RETURN_TYPE) (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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/* 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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/* Scan for matches in right half. */
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i = suffix;
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while (i < needle_len && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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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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while (i != SIZE_MAX && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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--i;
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if (i == SIZE_MAX)
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return (RETURN_TYPE) (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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return NULL;
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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. This
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method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN.
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Performance is guaranteed to be linear, with an initialization cost
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of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations.
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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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and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible.
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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, and
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sublinear performance is not possible. */
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static RETURN_TYPE
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two_way_long_needle (const unsigned char *haystack, size_t haystack_len,
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const unsigned char *needle, size_t needle_len)
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{
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size_t i; /* Index into current byte 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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size_t shift_table[1U << CHAR_BIT]; /* See below. */
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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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/* Populate shift_table. For each possible byte value c,
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shift_table[c] is the distance from the last occurrence of c to
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the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE.
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shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0. */
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for (i = 0; i < 1U << CHAR_BIT; i++)
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shift_table[i] = needle_len;
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for (i = 0; i < needle_len; i++)
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shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1;
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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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size_t shift;
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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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/* Check the last byte first; if it does not match, then
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shift to the next possible match location. */
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shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])];
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if (0 < shift)
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{
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if (memory && shift < period)
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{
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/* Since needle is periodic, but the last period has
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a byte out of place, there can be no match until
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after the mismatch. */
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shift = needle_len - period;
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memory = 0;
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}
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j += shift;
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continue;
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}
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/* Scan for matches in right half. The last byte has
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already been matched, by virtue of the shift table. */
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i = MAX (suffix, memory);
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while (i < needle_len - 1 && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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++i;
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if (needle_len - 1 <= 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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while (memory < i + 1 && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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--i;
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if (i + 1 < memory + 1)
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return (RETURN_TYPE) (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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/* 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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size_t 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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/* Check the last byte first; if it does not match, then
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shift to the next possible match location. */
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shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])];
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if (0 < shift)
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{
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j += shift;
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continue;
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}
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/* Scan for matches in right half. The last byte has
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already been matched, by virtue of the shift table. */
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i = suffix;
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while (i < needle_len - 1 && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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++i;
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if (needle_len - 1 <= 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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while (i != SIZE_MAX && (CANON_ELEMENT (needle[i])
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== CANON_ELEMENT (haystack[i + j])))
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--i;
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if (i == SIZE_MAX)
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return (RETURN_TYPE) (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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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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#undef MAX
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#undef RETURN_TYPE
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