| /* Byte-wise substring search, using the Two-Way algorithm. |
| Copyright (C) 2008-2021 Free Software Foundation, Inc. |
| This file is part of the GNU C Library. |
| Written by Eric Blake <ebb9@byu.net>, 2008. |
| |
| This program is free software; you can redistribute it and/or modify |
| it under the terms of the GNU General Public License as published by |
| the Free Software Foundation; either version 3, or (at your option) |
| any later version. |
| |
| This program 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 General Public License for more details. |
| |
| You should have received a copy of the GNU General Public License along |
| with this program; if not, see <https://www.gnu.org/licenses/>. */ |
| |
| /* Before including this file, you need to include <config.h> and |
| <string.h>, and define: |
| RETURN_TYPE A macro that expands to the return type. |
| AVAILABLE(h, h_l, j, n_l) |
| A macro that returns nonzero if there are |
| at least N_L bytes left starting at H[J]. |
| H is 'unsigned char *', H_L, J, and N_L |
| are 'size_t'; H_L is an lvalue. For |
| NUL-terminated searches, H_L can be |
| modified each iteration to avoid having |
| to compute the end of H up front. |
| |
| For case-insensitivity, you may optionally define: |
| CMP_FUNC(p1, p2, l) A macro that returns 0 iff the first L |
| characters of P1 and P2 are equal. |
| CANON_ELEMENT(c) A macro that canonicalizes an element right after |
| it has been fetched from one of the two strings. |
| The argument is an 'unsigned char'; the result |
| must be an 'unsigned char' as well. |
| |
| This file undefines the macros documented above, and defines |
| LONG_NEEDLE_THRESHOLD. |
| */ |
| |
| #include <limits.h> |
| #include <stdint.h> |
| |
| /* We use the Two-Way string matching algorithm (also known as |
| Chrochemore-Perrin), which guarantees linear complexity with |
| constant space. Additionally, for long needles, we also use a bad |
| character shift table similar to the Boyer-Moore algorithm to |
| achieve improved (potentially sub-linear) performance. |
| |
| See https://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260, |
| https://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm, |
| https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.34.6641&rep=rep1&type=pdf |
| */ |
| |
| /* Point at which computing a bad-byte shift table is likely to be |
| worthwhile. Small needles should not compute a table, since it |
| adds (1 << CHAR_BIT) + NEEDLE_LEN computations of preparation for a |
| speedup no greater than a factor of NEEDLE_LEN. The larger the |
| needle, the better the potential performance gain. On the other |
| hand, on non-POSIX systems with CHAR_BIT larger than eight, the |
| memory required for the table is prohibitive. */ |
| #if CHAR_BIT < 10 |
| # define LONG_NEEDLE_THRESHOLD 32U |
| #else |
| # define LONG_NEEDLE_THRESHOLD SIZE_MAX |
| #endif |
| |
| #ifndef MAX |
| # define MAX(a, b) ((a < b) ? (b) : (a)) |
| #endif |
| |
| #ifndef CANON_ELEMENT |
| # define CANON_ELEMENT(c) c |
| #endif |
| #ifndef CMP_FUNC |
| # define CMP_FUNC memcmp |
| #endif |
| |
| /* Perform a critical factorization of NEEDLE, of length NEEDLE_LEN. |
| Return the index of the first byte in the right half, and set |
| *PERIOD to the global period of the right half. |
| |
| The global period of a string is the smallest index (possibly its |
| length) at which all remaining bytes in the string are repetitions |
| of the prefix (the last repetition may be a subset of the prefix). |
| |
| When NEEDLE is factored into two halves, a local period is the |
| length of the smallest word that shares a suffix with the left half |
| and shares a prefix with the right half. All factorizations of a |
| non-empty NEEDLE have a local period of at least 1 and no greater |
| than NEEDLE_LEN. |
| |
| A critical factorization has the property that the local period |
| equals the global period. All strings have at least one critical |
| factorization with the left half smaller than the global period. |
| And while some strings have more than one critical factorization, |
| it is provable that with an ordered alphabet, at least one of the |
| critical factorizations corresponds to a maximal suffix. |
| |
| Given an ordered alphabet, a critical factorization can be computed |
| in linear time, with 2 * NEEDLE_LEN comparisons, by computing the |
| shorter of two ordered maximal suffixes. The ordered maximal |
| suffixes are determined by lexicographic comparison while tracking |
| periodicity. */ |
| static size_t |
| critical_factorization (const unsigned char *needle, size_t needle_len, |
| size_t *period) |
| { |
| /* Index of last byte of left half, or SIZE_MAX. */ |
| size_t max_suffix, max_suffix_rev; |
| size_t j; /* Index into NEEDLE for current candidate suffix. */ |
| size_t k; /* Offset into current period. */ |
| size_t p; /* Intermediate period. */ |
| unsigned char a, b; /* Current comparison bytes. */ |
| |
| /* Special case NEEDLE_LEN of 1 or 2 (all callers already filtered |
| out 0-length needles. */ |
| if (needle_len < 3) |
| { |
| *period = 1; |
| return needle_len - 1; |
| } |
| |
| /* Invariants: |
| 0 <= j < NEEDLE_LEN - 1 |
| -1 <= max_suffix{,_rev} < j (treating SIZE_MAX as if it were signed) |
| min(max_suffix, max_suffix_rev) < global period of NEEDLE |
| 1 <= p <= global period of NEEDLE |
| p == global period of the substring NEEDLE[max_suffix{,_rev}+1...j] |
| 1 <= k <= p |
| */ |
| |
| /* Perform lexicographic search. */ |
| max_suffix = SIZE_MAX; |
| j = 0; |
| k = p = 1; |
| while (j + k < needle_len) |
| { |
| a = CANON_ELEMENT (needle[j + k]); |
| b = CANON_ELEMENT (needle[max_suffix + k]); |
| if (a < b) |
| { |
| /* Suffix is smaller, period is entire prefix so far. */ |
| j += k; |
| k = 1; |
| p = j - max_suffix; |
| } |
| else if (a == b) |
| { |
| /* Advance through repetition of the current period. */ |
| if (k != p) |
| ++k; |
| else |
| { |
| j += p; |
| k = 1; |
| } |
| } |
| else /* b < a */ |
| { |
| /* Suffix is larger, start over from current location. */ |
| max_suffix = j++; |
| k = p = 1; |
| } |
| } |
| *period = p; |
| |
| /* Perform reverse lexicographic search. */ |
| max_suffix_rev = SIZE_MAX; |
| j = 0; |
| k = p = 1; |
| while (j + k < needle_len) |
| { |
| a = CANON_ELEMENT (needle[j + k]); |
| b = CANON_ELEMENT (needle[max_suffix_rev + k]); |
| if (b < a) |
| { |
| /* Suffix is smaller, period is entire prefix so far. */ |
| j += k; |
| k = 1; |
| p = j - max_suffix_rev; |
| } |
| else if (a == b) |
| { |
| /* Advance through repetition of the current period. */ |
| if (k != p) |
| ++k; |
| else |
| { |
| j += p; |
| k = 1; |
| } |
| } |
| else /* a < b */ |
| { |
| /* Suffix is larger, start over from current location. */ |
| max_suffix_rev = j++; |
| k = p = 1; |
| } |
| } |
| |
| /* Choose the shorter suffix. Return the index of the first byte of |
| the right half, rather than the last byte of the left half. |
| |
| For some examples, 'banana' has two critical factorizations, both |
| exposed by the two lexicographic extreme suffixes of 'anana' and |
| 'nana', where both suffixes have a period of 2. On the other |
| hand, with 'aab' and 'bba', both strings have a single critical |
| factorization of the last byte, with the suffix having a period |
| of 1. While the maximal lexicographic suffix of 'aab' is 'b', |
| the maximal lexicographic suffix of 'bba' is 'ba', which is not a |
| critical factorization. Conversely, the maximal reverse |
| lexicographic suffix of 'a' works for 'bba', but not 'ab' for |
| 'aab'. The shorter suffix of the two will always be a critical |
| factorization. */ |
| if (max_suffix_rev + 1 < max_suffix + 1) |
| return max_suffix + 1; |
| *period = p; |
| return max_suffix_rev + 1; |
| } |
| |
| /* Return the first location of non-empty NEEDLE within HAYSTACK, or |
| NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This |
| method is optimized for NEEDLE_LEN < LONG_NEEDLE_THRESHOLD. |
| Performance is guaranteed to be linear, with an initialization cost |
| of 2 * NEEDLE_LEN comparisons. |
| |
| If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at |
| most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. |
| If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * |
| HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */ |
| static RETURN_TYPE |
| two_way_short_needle (const unsigned char *haystack, size_t haystack_len, |
| const unsigned char *needle, size_t needle_len) |
| { |
| size_t i; /* Index into current byte of NEEDLE. */ |
| size_t j; /* Index into current window of HAYSTACK. */ |
| size_t period; /* The period of the right half of needle. */ |
| size_t suffix; /* The index of the right half of needle. */ |
| |
| /* Factor the needle into two halves, such that the left half is |
| smaller than the global period, and the right half is |
| periodic (with a period as large as NEEDLE_LEN - suffix). */ |
| suffix = critical_factorization (needle, needle_len, &period); |
| |
| /* Perform the search. Each iteration compares the right half |
| first. */ |
| if (CMP_FUNC (needle, needle + period, suffix) == 0) |
| { |
| /* Entire needle is periodic; a mismatch in the left half can |
| only advance by the period, so use memory to avoid rescanning |
| known occurrences of the period in the right half. */ |
| size_t memory = 0; |
| j = 0; |
| while (AVAILABLE (haystack, haystack_len, j, needle_len)) |
| { |
| /* Scan for matches in right half. */ |
| i = MAX (suffix, memory); |
| while (i < needle_len && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| ++i; |
| if (needle_len <= i) |
| { |
| /* Scan for matches in left half. */ |
| i = suffix - 1; |
| while (memory < i + 1 && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| --i; |
| if (i + 1 < memory + 1) |
| return (RETURN_TYPE) (haystack + j); |
| /* No match, so remember how many repetitions of period |
| on the right half were scanned. */ |
| j += period; |
| memory = needle_len - period; |
| } |
| else |
| { |
| j += i - suffix + 1; |
| memory = 0; |
| } |
| } |
| } |
| else |
| { |
| /* The two halves of needle are distinct; no extra memory is |
| required, and any mismatch results in a maximal shift. */ |
| period = MAX (suffix, needle_len - suffix) + 1; |
| j = 0; |
| while (AVAILABLE (haystack, haystack_len, j, needle_len)) |
| { |
| /* Scan for matches in right half. */ |
| i = suffix; |
| while (i < needle_len && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| ++i; |
| if (needle_len <= i) |
| { |
| /* Scan for matches in left half. */ |
| i = suffix - 1; |
| while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| --i; |
| if (i == SIZE_MAX) |
| return (RETURN_TYPE) (haystack + j); |
| j += period; |
| } |
| else |
| j += i - suffix + 1; |
| } |
| } |
| return NULL; |
| } |
| |
| /* Return the first location of non-empty NEEDLE within HAYSTACK, or |
| NULL. HAYSTACK_LEN is the minimum known length of HAYSTACK. This |
| method is optimized for LONG_NEEDLE_THRESHOLD <= NEEDLE_LEN. |
| Performance is guaranteed to be linear, with an initialization cost |
| of 3 * NEEDLE_LEN + (1 << CHAR_BIT) operations. |
| |
| If AVAILABLE does not modify HAYSTACK_LEN (as in memmem), then at |
| most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, |
| and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible. |
| If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 * |
| HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and |
| sublinear performance is not possible. */ |
| static RETURN_TYPE |
| two_way_long_needle (const unsigned char *haystack, size_t haystack_len, |
| const unsigned char *needle, size_t needle_len) |
| { |
| size_t i; /* Index into current byte of NEEDLE. */ |
| size_t j; /* Index into current window of HAYSTACK. */ |
| size_t period; /* The period of the right half of needle. */ |
| size_t suffix; /* The index of the right half of needle. */ |
| size_t shift_table[1U << CHAR_BIT]; /* See below. */ |
| |
| /* Factor the needle into two halves, such that the left half is |
| smaller than the global period, and the right half is |
| periodic (with a period as large as NEEDLE_LEN - suffix). */ |
| suffix = critical_factorization (needle, needle_len, &period); |
| |
| /* Populate shift_table. For each possible byte value c, |
| shift_table[c] is the distance from the last occurrence of c to |
| the end of NEEDLE, or NEEDLE_LEN if c is absent from the NEEDLE. |
| shift_table[NEEDLE[NEEDLE_LEN - 1]] contains the only 0. */ |
| for (i = 0; i < 1U << CHAR_BIT; i++) |
| shift_table[i] = needle_len; |
| for (i = 0; i < needle_len; i++) |
| shift_table[CANON_ELEMENT (needle[i])] = needle_len - i - 1; |
| |
| /* Perform the search. Each iteration compares the right half |
| first. */ |
| if (CMP_FUNC (needle, needle + period, suffix) == 0) |
| { |
| /* Entire needle is periodic; a mismatch in the left half can |
| only advance by the period, so use memory to avoid rescanning |
| known occurrences of the period in the right half. */ |
| size_t memory = 0; |
| size_t shift; |
| j = 0; |
| while (AVAILABLE (haystack, haystack_len, j, needle_len)) |
| { |
| /* Check the last byte first; if it does not match, then |
| shift to the next possible match location. */ |
| shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; |
| if (0 < shift) |
| { |
| if (memory && shift < period) |
| { |
| /* Since needle is periodic, but the last period has |
| a byte out of place, there can be no match until |
| after the mismatch. */ |
| shift = needle_len - period; |
| } |
| memory = 0; |
| j += shift; |
| continue; |
| } |
| /* Scan for matches in right half. The last byte has |
| already been matched, by virtue of the shift table. */ |
| i = MAX (suffix, memory); |
| while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| ++i; |
| if (needle_len - 1 <= i) |
| { |
| /* Scan for matches in left half. */ |
| i = suffix - 1; |
| while (memory < i + 1 && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| --i; |
| if (i + 1 < memory + 1) |
| return (RETURN_TYPE) (haystack + j); |
| /* No match, so remember how many repetitions of period |
| on the right half were scanned. */ |
| j += period; |
| memory = needle_len - period; |
| } |
| else |
| { |
| j += i - suffix + 1; |
| memory = 0; |
| } |
| } |
| } |
| else |
| { |
| /* The two halves of needle are distinct; no extra memory is |
| required, and any mismatch results in a maximal shift. */ |
| size_t shift; |
| period = MAX (suffix, needle_len - suffix) + 1; |
| j = 0; |
| while (AVAILABLE (haystack, haystack_len, j, needle_len)) |
| { |
| /* Check the last byte first; if it does not match, then |
| shift to the next possible match location. */ |
| shift = shift_table[CANON_ELEMENT (haystack[j + needle_len - 1])]; |
| if (0 < shift) |
| { |
| j += shift; |
| continue; |
| } |
| /* Scan for matches in right half. The last byte has |
| already been matched, by virtue of the shift table. */ |
| i = suffix; |
| while (i < needle_len - 1 && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| ++i; |
| if (needle_len - 1 <= i) |
| { |
| /* Scan for matches in left half. */ |
| i = suffix - 1; |
| while (i != SIZE_MAX && (CANON_ELEMENT (needle[i]) |
| == CANON_ELEMENT (haystack[i + j]))) |
| --i; |
| if (i == SIZE_MAX) |
| return (RETURN_TYPE) (haystack + j); |
| j += period; |
| } |
| else |
| j += i - suffix + 1; |
| } |
| } |
| return NULL; |
| } |
| |
| #undef AVAILABLE |
| #undef CANON_ELEMENT |
| #undef CMP_FUNC |
| #undef MAX |
| #undef RETURN_TYPE |
