boyer-moore-algorithm-for-pattern-searching.cpp

/* C++ Program for Bad Character Heuristic of Boyer 
Moore String Matching Algorithm */
#include <bits/stdc++.h> 
using namespace std; 
# define NO_OF_CHARS 256 

// The preprocessing function for Boyer Moore's 
// bad character heuristic 
void badCharHeuristic( string str, int size, 
						int badchar[NO_OF_CHARS]) 
{ 
	int i; 

	// Initialize all occurrences as -1 
	for (i = 0; i < NO_OF_CHARS; i++) 
		badchar[i] = -1; 

	// Fill the actual value of last occurrence 
	// of a character 
	for (i = 0; i < size; i++) 
		badchar[(int) str[i]] = i; 
} 

/* A pattern searching function that uses Bad 
Character Heuristic of Boyer Moore Algorithm */
void search( string txt, string pat) 
{ 
	int m = pat.size(); 
	int n = txt.size(); 

	int badchar[NO_OF_CHARS]; 

	/* Fill the bad character array by calling 
	the preprocessing function badCharHeuristic() 
	for given pattern */
	badCharHeuristic(pat, m, badchar); 

	int s = 0; // s is shift of the pattern with 
				// respect to text 
	while(s <= (n - m)) 
	{ 
		int j = m - 1; 

		/* Keep reducing index j of pattern while 
		characters of pattern and text are 
		matching at this shift s */
		while(j >= 0 && pat[j] == txt[s + j]) 
			j--; 

		/* If the pattern is present at current 
		shift, then index j will become -1 after 
		the above loop */
		if (j < 0) 
		{ 
			cout << "pattern occurs at shift = " << s << endl; 

			/* Shift the pattern so that the next 
			character in text aligns with the last 
			occurrence of it in pattern. 
			The condition s+m < n is necessary for 
			the case when pattern occurs at the end 
			of text */
			s += (s + m < n)? m-badchar[txt[s + m]] : 1; 

		} 

		else
			/* Shift the pattern so that the bad character 
			in text aligns with the last occurrence of 
			it in pattern. The max function is used to 
			make sure that we get a positive shift. 
			We may get a negative shift if the last 
			occurrence of bad character in pattern 
			is on the right side of the current 
			character. */
			s += max(1, j - badchar[txt[s + j]]); 
	} 
} 

/* Driver code */
int main() 
{ 
	string txt= "ABAAABCD"; 
	string pat = "ABC"; 
	search(txt, pat); 
	return 0; 
} 

// This code is contributed by rathbhupendra