Implement Rabin-Karp algorithm for substring search

README.md

@@ -2193,6 +2193,101 @@ public class PalindromCheckSnippet {
   }
 }
 ```
+### Rabin-Karp Substring Search Algorithm
+
+```java
+public class RabinKarpSubstringSearchSnippet {
+
+    private static final int PRIME = 1_000_000_007; // Large prime to reduce collisions
+
+    /**
+     * Implements the Rabin-Karp algorithm to find the index of the first occurrence 
+     * of a substring in a given text.
+     *
+     * @param text The text in which the substring is to be searched.
+     * @param pattern The substring pattern to search for.
+     * @return The index of the first occurrence of the pattern in the text, 
+     *         or -1 if the pattern is not found.
+     */
+    public static int rabinKarpSearch(String text, String pattern) {
+        if (pattern == null || pattern.length() == 0) {
+            return 0; // Trivial case: empty pattern
+        }
+        if (text == null || text.length() < pattern.length()) {
+            return -1; // Pattern cannot be found
+        }
+
+        int m = pattern.length();
+        int n = text.length();
+        long patternHash = createHash(pattern, m);
+        long textHash = createHash(text, m);
+
+        for (int i = 0; i <= n - m; i++) {
+            if (patternHash == textHash && checkEqual(text, i, i + m - 1, pattern, 0, m - 1)) {
+                return i; // Match found
+            }
+            if (i < n - m) {
+                textHash = recalculateHash(text, i, i + m, textHash, m);
+            }
+        }
+        return -1; // No match found
+    }
+
+    /**
+     * Creates a hash for the given substring.
+     *
+     * @param str The string to hash.
+     * @param end The length of substring considered.
+     * @return The hash value.
+     */
+    private static long createHash(String str, int end) {
+        long hash = 0;
+        for (int i = 0; i < end; i++) {
+            hash += (long) str.charAt(i) * pow(PRIME, i);
+        }
+        return hash;
+    }
+
+    /**
+     * Recalculates hash by sliding the window by one character.
+     */
+    private static long recalculateHash(String str, int oldIndex, int newIndex, long oldHash, int patternLen) {
+        long newHash = oldHash - str.charAt(oldIndex);
+        newHash /= PRIME;
+        newHash += (long) str.charAt(newIndex) * pow(PRIME, patternLen - 1);
+        return newHash;
+    }
+
+    /**
+     * Checks if two substrings are equal.
+     */
+    private static boolean checkEqual(String str1, int start1, int end1, String str2, int start2, int end2) {
+        if (end1 - start1 != end2 - start2) {
+            return false;
+        }
+        while (start1 <= end1 && start2 <= end2) {
+            if (str1.charAt(start1) != str2.charAt(start2)) {
+                return false;
+            }
+            start1++;
+            start2++;
+        }
+        return true;
+    }
+
+    /**
+     * Efficient power function for modular hashing.
+     */
+    private static long pow(int base, int exp) {
+        long result = 1;
+        for (int i = 0; i < exp; i++) {
+            result *= base;
+        }
+        return result;
+    }
+}
+```
+
 
 ### Reverse String
 

src/main/java/string/RabinKarpSubstringSearchSnippet.java

@@ -0,0 +1,132 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2017-2024 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package string;
+
+/**
+ * RabinKarpSubstringSearchSnippet.
+ */
+public class RabinKarpSubstringSearchSnippet {
+
+  private static final int PRIME = 1_000_000_007; // A prime number used as modulus for hashing
+
+  /**
+   * Implements the Rabin-Karp algorithm to find the index of a substring.
+   *
+   * @param text The text in which the substring is to be searched.
+   * @param pattern The substring pattern to search for.
+   * @return The index of the first occurrence, or -1 if the pattern is not found.
+   */
+  public static int rabinKarpSearch(String text, String pattern) {
+    if (pattern == null || pattern.length() == 0) {
+      return 0; // Trivial case: empty pattern
+    }
+    if (text == null || text.length() < pattern.length()) {
+      return -1; // Pattern cannot be found
+    }
+
+    int m = pattern.length();
+    int n = text.length();
+    long patternHash = createHash(pattern, m);
+    long textHash = createHash(text, m);
+
+    for (int i = 0; i <= n - m; i++) {
+      if (patternHash == textHash && checkEqual(text, i, i + m - 1, pattern, 0, m - 1)) {
+        return i; // Match found
+      }
+      if (i < n - m) {
+        textHash = recalculateHash(text, i, i + m, textHash, m);
+      }
+    }
+    return -1; // No match found
+  }
+
+  /**
+   * Creates a hash for the given substring.
+   *
+   * @param str The string to hash.
+   * @param end The length of substring considered.
+   * @return The hash value.
+   */
+  private static long createHash(String str, int end) {
+    long hash = 0;
+    for (int i = 0; i < end; i++) {
+      hash += str.charAt(i) * Math.pow(PRIME, i);
+    }
+    return hash;
+  }
+
+  /**
+   * Recalculates hash by sliding the window by one character.
+   *
+   * @param str The original string.
+   * @param oldIndex The index of the outgoing character.
+   * @param newIndex The index of the incoming character.
+   * @param oldHash The previous hash value.
+   * @param patternLen The length of the pattern.
+   * @return The recalculated hash.
+   */
+  private static long recalculateHash(
+        String str, 
+        int oldIndex, 
+        int newIndex, 
+        long oldHash, 
+        int patternLen) {
+    long newHash = oldHash - str.charAt(oldIndex);
+    newHash /= PRIME;
+    newHash += str.charAt(newIndex) * Math.pow(PRIME, patternLen - 1);
+    return newHash;
+  }
+
+  /**
+   * Checks if two substrings are equal.
+   *
+   * @param str1 The first string.
+   * @param start1 Start index in str1.
+   * @param end1 End index in str1.
+   * @param str2 The second string.
+   * @param start2 Start index in str2.
+   * @param end2 End index in str2.
+   * @return True if substrings are equal, otherwise false.
+   */
+  private static boolean checkEqual(
+        String str1, 
+        int start1, 
+        int end1, 
+        String str2, 
+        int start2, 
+        int end2) {
+    if (end1 - start1 != end2 - start2) {
+      return false;
+    }
+    while (start1 <= end1 && start2 <= end2) {
+      if (str1.charAt(start1) != str2.charAt(start2)) {
+        return false;
+      }
+      start1++;
+      start2++;
+    }
+    return true;
+  }
+}

src/test/java/string/RabinKarpSubstringSearchSnippetTest.java

@@ -0,0 +1,49 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2017-2024 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package string;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for 30 Seconds of Java code library.
+ * 
+ */
+class RabinKarpSubstringSearchSnippetTest {
+
+  /**
+   * Tests for {@link RabinKarpSubstringSearchSnippet#rabinKarpSearch(String, String)}.
+   */
+  @Test
+  void testRabinKarpSearch() {
+    // Test cases for Rabin-Karp substring search
+    assertEquals(6, RabinKarpSubstringSearchSnippet.rabinKarpSearch("abxabcabcaby", "abcaby"));
+    assertEquals(7, RabinKarpSubstringSearchSnippet.rabinKarpSearch("subash pandey", "pandey"));
+    assertEquals(-1, RabinKarpSubstringSearchSnippet.rabinKarpSearch("abcd", "e"));
+    assertEquals(0, RabinKarpSubstringSearchSnippet.rabinKarpSearch("aaaaa", "a"));
+    assertEquals(2, RabinKarpSubstringSearchSnippet.rabinKarpSearch("abcdabcd", "cdab"));
+  }
+}