add: initial implementation of Rabin-Karp algorithm for string matching

Author: Lohit-pro

src/main/java/com/thealgorithms/strings/PatternSearchUsingRabinKarpAlgo.java

@@ -0,0 +1,79 @@
+package com.thealgorithms.strings;
+import java.util.*;
+
+public class PatternSearchUsingRabinKarpAlgo {
+
+    // I'm using Rabin-Karp algorithm that uses hashing to find pattern strings in a text.
+    public static List<String> search(String text, String pattern) {
+        List<String> result = new ArrayList<>();
+        int m = pattern.length();
+        int n = text.length();
+        int prime = 101;            // A prime number to mod hash values
+
+        int patternHash = 0;                // Hash value for pattern
+        int textHash = 0;                   // Hash value for text window
+        int h = 1;
+
+        // The value of h would be "pow(d, m-1) % prime"
+        for (int i = 0; i < m - 1; i++) {
+            h = (h * 256) % prime;
+        }
+
+        // Calculating the hash value of pattern and first window of text
+        for (int i = 0; i < m; i++) {
+            patternHash = (256 * patternHash + pattern.charAt(i)) % prime;
+            textHash = (256 * textHash + text.charAt(i)) % prime;
+        }
+
+        // Iterating on pattern by single char again and again..
+        for (int i = 0; i <= n - m; i++) {
+
+            if (patternHash == textHash) {
+                // If the hash values match, then only checking for next char's
+                int j;
+                for (j = 0; j < m; j++) {
+                    if (text.charAt(i + j) != pattern.charAt(j)) {
+                        break;
+                    }
+                }
+
+                // If patternHash == textHash and pattern[0...m-1] == text[i...i+m-1]
+                if (j == m) {
+                    result.add("Start: " + i + ", End: " + (i + m - 1) + ", Substring: " + text.substring(i, i + m));
+                }
+            }
+
+            // Calculating hash value for next window of text: Remove leading digit, add trailing digit
+            if (i < n - m) {
+                textHash = (256 * (textHash - text.charAt(i) * h) + text.charAt(i + m)) % prime;
+
+                // We might get negative value of textHash,so converting it to positive
+                if (textHash < 0) {
+                    textHash = (textHash + prime);
+                }
+            }
+        }
+
+        return result;
+    }
+
+    public static void main(String[] args) {
+
+        Scanner in = new Scanner(System.in);
+        System.out.print("Enter the string: ");
+        String text = in.next();
+//        String text = "ABCCDDAEFG";  test
+
+        System.out.print("Enter the searching string: ");
+        String pattern = in.next();
+//        String pattern = "CDD";    testt
+
+        List<String> result = search(text.toLowerCase(), pattern.toLowerCase());
+
+        if (result.isEmpty()) {
+            System.out.println("Pattern not found in the given text.");
+        } else {
+            System.out.println("Pattern found: " + result);
+        }
+    }
+}