KMP string pattern matching algorithm

Overview

?? KMP algorithm I think there are two key points: 1. Calculate a partial match table for a pattern string (at this point, compare yourself to yourself) 2. When the main string is matched, the main string is only traversed once, and the matching table calculates where the pattern string should move, depending on the part of the pattern string. The time complexity of the KMP algorithm is O (m+n); The algorithm code (PHP) I implemented below

theory

About the KMP theory section, this article is well written: http://kb.cnblogs.com/page/176818/. I will not dwell on it.

calculate partial Match table

 function kmp_next($string){    $length= Strlen ($string);//Get string length    $next[0] =0;$j=0;$i=1; while($i<$length){if($string{$j} ==$string{$i}){$j++;$next[$i]=$j;$i++; }Else if($j==0){$next[$i] =$j;$i++; }Else{$j=$next[$j]; }    }return $next;}

KMP Algorithm

 function kmp($text,$mode){    $t _length= Strlen ($text);$m _length= Strlen ($mode);if($t _length<$m _length){return-1; }$arr= Kmp_next ($mode);$j=0;$i=0; while($i<$t _length){if($text{$i}==$mode{$j}){if($j<$m _length-1){$j++;$i++; }Else{return $i-$m _length+1; }        }Else if($j==0){$i++; }Else{$j=$arr[$j-1]; }    }return-1;}

called

$string‘BBC ABCDAB ABCDABCDABDE‘;$mode‘ABCDABD‘;$key = kmp($string,$mode);var_dump(kmp_next($mode));var_dump($key);var_dump(substr($string,$key,strlen($mode)));

Results

KMP string pattern matching algorithm