From 9e66c13fe293aefa49d93dc70c017f31eca4e078 Mon Sep 17 00:00:00 2001 From: Bruno Haible Date: Sun, 11 Feb 2007 15:54:58 +0000 Subject: [PATCH] Ensure O(n) worst-case complexity of strcasestr substitute. --- ChangeLog | 12 +++++ lib/strcasestr.c | 142 +++++++++++++++++++++++++++++++++++++++++++++++++++++ modules/strcasestr | 2 + 3 files changed, 156 insertions(+) diff --git a/ChangeLog b/ChangeLog index 4250e1913..b752d049b 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,5 +1,17 @@ 2007-02-11 Bruno Haible + Ensure O(n) worst-case complexity of strcasestr substitute. + * lib/strcasestr.c: Include stdbool.h. + (knuth_morris_pratt): New function. + (strcasestr): Add some bookkeeping. Invoke knuth_morris_pratt when the + bookkeeping indicates that it's worth it. + * modules/strcasestr (Depends-on): Add stdbool, strnlen. + + * modules/strcasestr-tests: New file. + * tests/test-strcasestr.c: New file. + +2007-02-11 Bruno Haible + Ensure O(n) worst-case complexity of c_strcasestr. * lib/c-strcasestr.c: Include stdbool.h, string.h. (knuth_morris_pratt): New function. diff --git a/lib/strcasestr.c b/lib/strcasestr.c index e1f5952c7..24610f7f3 100644 --- a/lib/strcasestr.c +++ b/lib/strcasestr.c @@ -22,10 +22,100 @@ #include #include +#include #include /* for NULL, in case a nonstandard string.h lacks it */ #define TOLOWER(Ch) (isupper (Ch) ? tolower (Ch) : (Ch)) +/* 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 *needle, + const char **resultp) +{ + size_t m = strlen (needle); + + /* Allocate the table. */ + size_t *table = (size_t *) malloc (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 = TOLOWER ((unsigned char) needle[i - 1]); + + for (;;) + { + if (b == TOLOWER ((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 != '\0') + if (TOLOWER ((unsigned char) needle[j]) + == TOLOWER ((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++; + } + } + + free (table); + return true; +} + /* Find the first occurrence of NEEDLE in HAYSTACK, using case-insensitive comparison. Note: This function may, in multibyte locales, return success even if @@ -35,6 +125,26 @@ strcasestr (const char *haystack, const char *needle) { if (*needle != '\0') { + /* Minimizing the worst-case complexity: + Let n = strlen(haystack), m = strlen(needle). + 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; + size_t last_ccount = 0; /* last comparison count */ + const char *needle_last_ccount = needle; /* = needle + last_ccount */ + /* Speed up the following searches of needle by caching its first character. */ unsigned char b = TOLOWER ((unsigned char) *needle); @@ -45,6 +155,37 @@ strcasestr (const char *haystack, const char *needle) if (*haystack == '\0') /* 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 (needle_last_ccount != NULL) + { + needle_last_ccount += + strnlen (needle_last_ccount, comparison_count - last_ccount); + if (*needle_last_ccount == '\0') + needle_last_ccount = NULL; + last_ccount = comparison_count; + } + if (needle_last_ccount == NULL) + { + /* Try the Knuth-Morris-Pratt algorithm. */ + const char *result; + bool success = + knuth_morris_pratt (haystack, needle - 1, &result); + if (success) + return (char *) result; + try_kmp = false; + } + } + + outer_loop_count++; + comparison_count++; if (TOLOWER ((unsigned char) *haystack) == b) /* The first character matches. */ { @@ -59,6 +200,7 @@ strcasestr (const char *haystack, const char *needle) if (*rhaystack == '\0') /* No match. */ return NULL; + comparison_count++; if (TOLOWER ((unsigned char) *rhaystack) != TOLOWER ((unsigned char) *rneedle)) /* Nothing in this round. */ diff --git a/modules/strcasestr b/modules/strcasestr index 35632bc55..9f9553820 100644 --- a/modules/strcasestr +++ b/modules/strcasestr @@ -7,6 +7,8 @@ m4/strcasestr.m4 Depends-on: string +stdbool +strnlen configure.ac: gl_FUNC_STRCASESTR -- 2.11.0