The bad character heuristic method is one of the approaches of Boyer Moore Algorithm. Another approach is Good Suffix Heuristic. In this method we will try to find a bad character, that means a character of the main string, which is not matching with the pattern. When the mismatch has occurred, we will shift the entire pattern until the mismatch becomes a match, otherwise, pattern moves past the bad character.
Here the time complexity is O(m/n) for best case and O(mn)for the worst case, where n is the length of the text and m is the length of the pattern.
Input and Output
Input: Main String: “ABAAABCDBBABCDDEBCABC”, Pattern “ABC” Output: Pattern found at position: 4 Pattern found at position: 10 Pattern found at position: 18
Algorithm
badCharacterHeuristic(pattern, badCharacterArray)
Input − pattern, which will be searched, the bad character array to store location
Output: Fill the bad character array for future use
Begin n := pattern length for all entries of badCharacterArray, do set all entries to -1 done for all characters of the pattern, do set last position of each character in badCharacterArray. done End
searchPattern(pattern, text)
Input − pattern, which will be searched and the main text
Output − the locations where the pattern is found
Begin patLen := length of pattern strLen := length of text. call badCharacterHeuristic(pattern, badCharacterArray) shift := 0 while shift <= (strLen - patLen), do j := patLen -1 while j >= 0 and pattern[j] = text[shift + j], do decrease j by 1 done if j < 0, then print the shift as, there is a match if shift + patLen < strLen, then shift:= shift + patLen – badCharacterArray[text[shift + patLen]] else increment shift by 1 else shift := shift + max(1, j-badCharacterArray[text[shift+j]]) done End
Example
#include<iostream> #define MAXCHAR 256 using namespace std; int maximum(int data1, int data2) { if(data1 > data2) return data1; return data2; } void badCharacterHeuristic(string pattern, int badCharacter[MAXCHAR]) { int n = pattern.size(); //find length of pattern for(int i = 0; i<MAXCHAR; i++) badCharacter[i] = -1; //set all character distance as -1 for(int i = 0; i < n; i++) { badCharacter[(int)pattern[i]] = i; //set position of character in the array. } } void searchPattern(string mainString, string pattern, int *array, int *index) { int patLen = pattern.size(); int strLen = mainString.size(); int badCharacter[MAXCHAR]; //make array for bad character position badCharacterHeuristic(pattern, badCharacter); //fill bad character array int shift = 0; while(shift <= (strLen - patLen)) { int j = patLen - 1; while(j >= 0 && pattern[j] == mainString[shift+j]) { j--; //reduce j when pattern and main string character is matching } if(j < 0) { (*index)++; array[(*index)] = shift; if((shift + patLen) < strLen) { shift += patLen - badCharacter[mainString[shift + patLen]]; }else { shift += 1; } }else { shift += maximum(1, j - badCharacter[mainString[shift+j]]); } } } int main() { string mainString = "ABAAABCDBBABCDDEBCABC"; string pattern = "ABC"; int locArray[mainString.size()]; int index = -1; searchPattern(mainString, pattern, locArray, &index); for(int i = 0; i <= index; i++) { cout << "Pattern found at position: " << locArray[i]<<endl; } }
Output
Pattern found at position: 4 Pattern found at position: 10 Pattern found at position: 18