/* Byte-wise substring search, using the Two-Way algorithm.
- Copyright (C) 2008 Free Software Foundation, Inc.
+ Copyright (C) 2008-2014 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Written by Eric Blake <ebb9@byu.net>, 2008.
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, write to the Free Software Foundation,
- Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
+ with this program; if not, see <http://www.gnu.org/licenses/>. */
/* Before including this file, you need to include <config.h> and
<string.h>, and define:
RESULT_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.
+ 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.
+ 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.
+ 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, 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.
+/* 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 http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260
- and http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm
+ See http://www-igm.univ-mlv.fr/~lecroq/string/node26.html#SECTION00260,
+ http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm,
+ http://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
# define LONG_NEEDLE_THRESHOLD SIZE_MAX
#endif
-#define MAX(a, b) ((a < b) ? (b) : (a))
+#ifndef MAX
+# define MAX(a, b) ((a < b) ? (b) : (a))
+#endif
#ifndef CANON_ELEMENT
# define CANON_ELEMENT(c) c
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
- larger of two ordered maximal suffixes. The ordered maximal
- suffixes are determined by lexicographic comparison of
+ 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)
+ size_t *period)
{
/* Index of last byte of left half, or SIZE_MAX. */
size_t max_suffix, max_suffix_rev;
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)
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;
- }
+ {
+ /* 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;
- }
- }
+ {
+ /* 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;
- }
+ {
+ /* Suffix is larger, start over from current location. */
+ max_suffix = j++;
+ k = p = 1;
+ }
}
*period = p;
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;
- }
+ {
+ /* 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;
- }
- }
+ {
+ /* 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;
- }
+ {
+ /* Suffix is larger, start over from current location. */
+ max_suffix_rev = j++;
+ k = p = 1;
+ }
}
- /* Choose the longer suffix. Return the first byte of the right
- half, rather than the last byte of the left half. */
+ /* 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;
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)
+ 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. */
first. */
if (CMP_FUNC (needle, needle + period, suffix) == 0)
{
- /* Entire needle is periodic; a mismatch can only advance by the
- period, so use memory to avoid rescanning known occurrences
- of the period. */
+ /* 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;
- }
- }
+ {
+ /* 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. */
+ 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;
- }
+ {
+ /* 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;
}
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)
+ 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. */
first. */
if (CMP_FUNC (needle, needle + period, suffix) == 0)
{
- /* Entire needle is periodic; a mismatch can only advance by the
- period, so use memory to avoid rescanning known occurrences
- of the period. */
+ /* 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;
- }
- }
+ {
+ /* 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. */
+ 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;
- }
+ {
+ /* 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
projects / gnulib.git / blobdiff