X-Git-Url: http://erislabs.net/gitweb/?a=blobdiff_plain;f=lib%2Fmemmem.c;h=57c6560323c042b8060c012c4711b3328e5dccee;hb=fec00c917a873fe54b61a911ea22e07f7c48386b;hp=e8822221c7051ecd9fd4466fdb409089f45e36ef;hpb=ed081024933a7c415cb2872f847357102cf6595b;p=gnulib.git diff --git a/lib/memmem.c b/lib/memmem.c index e8822221c..57c656032 100644 --- a/lib/memmem.c +++ b/lib/memmem.c @@ -21,24 +21,147 @@ #include #include +#include + +#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 *) nmalloca (m, sizeof (size_t)); + if (table == NULL) + return false; + /* Fill the table. + For 0 < i < m: + 0 < table[i] <= i is defined such that + forall 0 < x < table[i]: needle[x..i-1] != needle[0..i-1-x], + and table[i] is as large as possible with this property. + This implies: + 1) For 0 < i < m: + If table[i] < i, + needle[table[i]..i-1] = needle[0..i-1-table[i]]. + 2) For 0 < i < m: + rhaystack[0..i-1] == needle[0..i-1] + and exists h, i <= h < m: rhaystack[h] != needle[h] + implies + forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1]. + table[0] remains uninitialized. */ + { + size_t i, j; + + /* i = 1: Nothing to verify for x = 0. */ + table[1] = 1; + j = 0; + + for (i = 2; i < m; i++) + { + /* Here: j = i-1 - table[i-1]. + The inequality needle[x..i-1] != needle[0..i-1-x] is known to hold + for x < table[i-1], by induction. + Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1]. */ + unsigned char b = (unsigned char) needle[i - 1]; + + for (;;) + { + /* Invariants: The inequality needle[x..i-1] != needle[0..i-1-x] + is known to hold for x < i-1-j. + Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1]. */ + if (b == (unsigned char) needle[j]) + { + /* Set table[i] := i-1-j. */ + table[i] = i - ++j; + break; + } + /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds + for x = i-1-j, because + needle[i-1] != needle[j] = needle[i-1-x]. */ + if (j == 0) + { + /* The inequality holds for all possible x. */ + table[i] = i; + break; + } + /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds + for i-1-j < x < i-1-j+table[j], because for these x: + needle[x..i-2] + = needle[x-(i-1-j)..j-1] + != needle[0..j-1-(x-(i-1-j))] (by definition of table[j]) + = needle[0..i-2-x], + hence needle[x..i-1] != needle[0..i-1-x]. + Furthermore + needle[i-1-j+table[j]..i-2] + = needle[table[j]..j-1] + = needle[0..j-1-table[j]] (by definition of table[j]). */ + j = j - table[j]; + } + /* Here: j = i - table[i]. */ + } + } + + /* 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++; + } + } -/* Return the first occurrence of NEEDLE in HAYSTACK. */ + 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_start, size_t haystack_len, + const void *needle_start, 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_start; + const char *needle = (const char *) needle_start; + 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 @@ -50,12 +173,82 @@ memmem (haystack, haystack_len, needle, needle_len) 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 + byte. */ + 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 byte 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; }