Sunday Algorithm Research-string matching beyond KMP

The first time I heard about the Sunday algorithm, I said it was a big pie. In his illustrated explanations, I found that this algorithm is an easy-to-understand algorithm that is more efficient than KMP and BM.

So I tried to write one. If it was really good, share it.

First of all:

The Sunday algorithm is a faster algorithm proposed by Daniel M. Sunday in 1990 than the BM algorithm. The core idea is: During the matching process, the pattern string is not required to be compared from left to right or from right to left. When a mismatch is found, the algorithm can skip as many characters as possible to perform the next matching, thus improving the matching efficiency.

 

Assume that s [I] ≈ T [J], 1 ≤ I ≤ n, 1 ≤ j ≤ m in case of mismatch. At this time, the matched part is U, and the length of the string U is assumed to be L. 1. Obviously, s [L + I + 1] must participate in the next round of matching, and t [m] should at least move to this position (that is, the mode string T should move at least one character to the right ).

Figure 1 Sunday Algorithm Mismatch There are two cases: (1) s [L + I + 1] does not appear in the mode string T. At this time, the mode string T [0] is moved to the character position after s [T + I + 1. 2.

Figure 2 Sunday The number 1 Situation(2) S [L + I + 1] appears in the mode string. Here s [L + I + 1] from the right side of the pattern string T, that is, by T [M-1], t [M-2],… T [0. If it is found that s [L + I + 1] is the same as a character in T, write down this position as K, 1 ≤ k ≤ m, T [k] = s [L + I + 1]. In this case, the pattern string T should be moved to the right M-K character position, that is, to the T [k] And s [L + I + 1] Alignment position. 3.

Figure 3 Sunday The number 2 SituationAnd so on. If the match is complete, the match is successful. Otherwise, move the next round until the rightmost end of the Main string s ends. The worst case of this algorithm is O (n * m ). This algorithm is faster to match short mode strings. Code: Int Sunday (const char * SRC, const char * des) <br/>{< br/> int I, j, Pos = 0; <br/> int len_s, len_d; <br/> int next [26] = {0}; // next array, preprocessing initialization <br/> len_s = strlen (SRC ); <br/> len_d = strlen (DES); <br/> for (j = 0; j <26; ++ J) // initialize the next array <br/> next [J] = len_d; <br/> for (j = 0; j <len_d; ++ J) // set the next array <br/> next [des [J]-'a'] = len_d-j; <br/> while (Pos <(len_s-len_d + 1 )) // traverse the original string <br/> {<br/> I = Pos; <br/> for (j = 0; j <len_d; ++ J, ++ I) // compare <br />{< Br/> If (SRC [I]! = Des [J]) // if it does not match, the original string will jump to the next link <br/>{< br/> POS + = next [SRC [POS + len_d]-'a']; <br/> break; <br/>}< br/> If (j = len_d) <br/> return Pos; <br/>}< br/> return-1; // if no substring exists,-1 is returned. <br/>}</P> <p> int main () <br/>{< br/> char SRC [] = "abcdacdaahfacabcdabcdeaa"; <br/> char des [] = "ABCDE "; <br/> cout <Sunday (SRC, des) <Endl; <br/> return 0; <br/>}