Merge pull request #267 from Abhishek-kumar82078/master

Added Boyer_Moore Algorithm

Author: master-fury

Pattern_Matching/src/Boyer_Moore.cpp

@@ -0,0 +1,85 @@
+// Boyer Moore String Matching Algorithm
+// Best Case time complexity O(n/m)
+// Average Case time complexity O(n)
+// Worst Case time complexity O(nm)
+
+#include <bits/stdc++.h>
+using namespace std;
+# define NO_OF_CHARS 256
+
+// The preprocessing function for Boyer Moore's
+// bad character Function
+void badCharacter( string str, int size, int badChar[NO_OF_CHARS])
+{
+	// Fill the last occurrence of a character
+	for (int 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]={-1};
+
+	/* Fill the bad character array by function badCharHeuristic() for given pattern */
+	badCharacter(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 otherwise not less than 0*/
+		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]]);
+	}
+}
+
+
+int main()
+{
+	string txt= "CCACACCABCABCD";
+	string pat = "ABCD";
+	search(txt, pat);
+	return 0;
+}
+
+// Description
+// Boyer Moore algorithm starts matching from the last character of the pattern
+// It preprocesses the pattern and creates different arrays for last occurrence of each of the characters in string .
+// At every step, it slides the pattern by the max of the slides suggested .