Rabin_Karp_Problem hactoberfest2022

Rabin_Karp_Problem.java

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+module Rabin_Karp_Problem {
+	// Following program is a Java implementation
+	// of Rabin Karp Algorithm given in the CLRS book
+
+	public class Main {
+		// d is the number of characters in the input alphabet
+		public final static int d = 256;
+
+		/* pat -> pattern
+			txt -> text
+			q -> A prime number
+		*/
+		static void search(String pat, String txt, int q)
+		{
+			int M = pat.length();
+			int N = txt.length();
+			int i, j;
+			int p = 0; // hash value for pattern
+			int t = 0; // hash value for txt
+			int h = 1;
+
+			// The value of h would be "pow(d, M-1)%q"
+			for (i = 0; i < M - 1; i++)
+				h = (h * d) % q;
+
+			// Calculate the hash value of pattern and first
+			// window of text
+			for (i = 0; i < M; i++) {
+				p = (d * p + pat.charAt(i)) % q;
+				t = (d * t + txt.charAt(i)) % q;
+			}
+
+			// Slide the pattern over text one by one
+			for (i = 0; i <= N - M; i++) {
+
+				// Check the hash values of current window of
+				// text and pattern. If the hash values match
+				// then only check for characters one by one
+				if (p == t) {
+					/* Check for characters one by one */
+					for (j = 0; j < M; j++) {
+						if (txt.charAt(i + j) != pat.charAt(j))
+							break;
+					}
+
+					// if p == t and pat[0...M-1] = txt[i, i+1,
+					// ...i+M-1]
+					if (j == M)
+						System.out.println(
+							"Pattern found at index " + i);
+				}
+
+				// Calculate hash value for next window of text:
+				// Remove leading digit, add trailing digit
+				if (i < N - M) {
+					t = (d * (t - txt.charAt(i) * h)
+						+ txt.charAt(i + M))
+						% q;
+
+					// We might get negative value of t,
+					// converting it to positive
+					if (t < 0)
+						t = (t + q);
+				}
+			}
+		}
+
+		/* Driver Code */
+		public static void main(String[] args)
+		{
+			String txt = "GEEKS FOR GEEKS";
+			String pat = "GEEK";
+
+			// A prime number
+			int q = 101;
+
+			// Function Call
+			search(pat, txt, q);
+		}
+	}
+
+}