#include <stddef.h>
#include <string.h>
+#include <stdbool.h>
+
+#include "malloca.h"
#ifndef _LIBC
# define __builtin_expect(expr, val) (expr)
#endif
-#undef memmem
+/* Knuth-Morris-Pratt algorithm.
+ See http://en.wikipedia.org/wiki/Knuth-Morris-Pratt_algorithm
+ Return a boolean indicating success. */
+
+static bool
+knuth_morris_pratt (const char *haystack, const char *last_haystack,
+ const char *needle, size_t m,
+ const char **resultp)
+{
+ /* Allocate the table. */
+ size_t *table = (size_t *) malloca (m * sizeof (size_t));
+ if (table == NULL)
+ return false;
+ /* Fill the table.
+ For 0 < i < m:
+ 0 < table[i] <= i is defined such that
+ rhaystack[0..i-1] == needle[0..i-1] and rhaystack[i] != needle[i]
+ implies
+ forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1],
+ and table[i] is as large as possible with this property.
+ table[0] remains uninitialized. */
+ {
+ size_t i, j;
+
+ table[1] = 1;
+ j = 0;
+ for (i = 2; i < m; i++)
+ {
+ unsigned char b = (unsigned char) needle[i - 1];
-/* Return the first occurrence of NEEDLE in HAYSTACK. */
+ for (;;)
+ {
+ if (b == (unsigned char) needle[j])
+ {
+ table[i] = i - ++j;
+ break;
+ }
+ if (j == 0)
+ {
+ table[i] = i;
+ break;
+ }
+ j = j - table[j];
+ }
+ }
+ }
+
+ /* Search, using the table to accelerate the processing. */
+ {
+ size_t j;
+ const char *rhaystack;
+ const char *phaystack;
+
+ *resultp = NULL;
+ j = 0;
+ rhaystack = haystack;
+ phaystack = haystack;
+ /* Invariant: phaystack = rhaystack + j. */
+ while (phaystack != last_haystack)
+ if ((unsigned char) needle[j] == (unsigned char) *phaystack)
+ {
+ j++;
+ phaystack++;
+ if (j == m)
+ {
+ /* The entire needle has been found. */
+ *resultp = rhaystack;
+ break;
+ }
+ }
+ else if (j > 0)
+ {
+ /* Found a match of needle[0..j-1], mismatch at needle[j]. */
+ rhaystack += table[j];
+ j -= table[j];
+ }
+ else
+ {
+ /* Found a mismatch at needle[0] already. */
+ rhaystack++;
+ phaystack++;
+ }
+ }
+
+ freea (table);
+ return true;
+}
+
+/* Return the first occurrence of NEEDLE in HAYSTACK. Return HAYSTACK
+ if NEEDLE_LEN is 0, otherwise NULL if NEEDLE is not found in
+ HAYSTACK. */
void *
-memmem (haystack, haystack_len, needle, needle_len)
- const void *haystack;
- size_t haystack_len;
- const void *needle;
- size_t needle_len;
+memmem (const void *haystack, size_t haystack_len,
+ const void *needle, size_t needle_len)
{
- const char *begin;
- const char *const last_possible
- = (const char *) haystack + haystack_len - needle_len;
+ /* Operating with void * is awkward. */
+ const char *Haystack = (const char *) haystack;
+ const char *Needle = (const char *) needle;
+ const char *last_haystack = Haystack + haystack_len;
+ const char *last_needle = Needle + needle_len;
if (needle_len == 0)
/* The first occurrence of the empty string is deemed to occur at
if (__builtin_expect (haystack_len < needle_len, 0))
return NULL;
- for (begin = (const char *) haystack; begin <= last_possible; ++begin)
- if (begin[0] == ((const char *) needle)[0] &&
- !memcmp ((const void *) &begin[1],
- (const void *) ((const char *) needle + 1),
- needle_len - 1))
- return (void *) begin;
+ /* Use optimizations in memchr when possible. */
+ if (__builtin_expect (needle_len == 1, 0))
+ return memchr (haystack, (unsigned char) *Needle, haystack_len);
+
+ /* Minimizing the worst-case complexity:
+ Let n = haystack_len, m = needle_len.
+ The naïve algorithm is O(n*m) worst-case.
+ The Knuth-Morris-Pratt algorithm is O(n) worst-case but it needs a
+ memory allocation.
+ To achieve linear complexity and yet amortize the cost of the
+ memory allocation, we activate the Knuth-Morris-Pratt algorithm
+ only once the naïve algorithm has already run for some time; more
+ precisely, when
+ - the outer loop count is >= 10,
+ - the average number of comparisons per outer loop is >= 5,
+ - the total number of comparisons is >= m.
+ But we try it only once. If the memory allocation attempt failed,
+ we don't retry it. */
+ {
+ bool try_kmp = true;
+ size_t outer_loop_count = 0;
+ size_t comparison_count = 0;
+
+ /* Speed up the following searches of needle by caching its first
+ character. */
+ char b = *Needle++;
+
+ for (;; Haystack++)
+ {
+ if (Haystack == last_haystack)
+ /* No match. */
+ return NULL;
+
+ /* See whether it's advisable to use an asymptotically faster
+ algorithm. */
+ if (try_kmp
+ && outer_loop_count >= 10
+ && comparison_count >= 5 * outer_loop_count)
+ {
+ /* See if needle + comparison_count now reaches the end of
+ needle. */
+ if (comparison_count >= needle_len)
+ {
+ /* Try the Knuth-Morris-Pratt algorithm. */
+ const char *result;
+ if (knuth_morris_pratt (Haystack, last_haystack,
+ Needle - 1, needle_len, &result))
+ return (void *) result;
+ try_kmp = false;
+ }
+ }
+
+ outer_loop_count++;
+ comparison_count++;
+ if (*Haystack == b)
+ /* The first character matches. */
+ {
+ const char *rhaystack = Haystack + 1;
+ const char *rneedle = Needle;
+
+ for (;; rhaystack++, rneedle++)
+ {
+ if (rneedle == last_needle)
+ /* Found a match. */
+ return (void *) Haystack;
+ if (rhaystack == last_haystack)
+ /* No match. */
+ return NULL;
+ comparison_count++;
+ if (*rhaystack != *rneedle)
+ /* Nothing in this round. */
+ break;
+ }
+ }
+ }
+ }
return NULL;
}
projects / gnulib.git / blobdiff