Merge branch 'master' into main

DenizAltunkapan · web-flow · commit 964175f7ec45 · 2026-02-01T16:18:34.000+01:00
diff --git a/pom.xml b/pom.xml
@@ -112,7 +112,7 @@
                     <dependency>
                     <groupId>com.puppycrawl.tools</groupId>
                     <artifactId>checkstyle</artifactId>
-                    <version>13.0.0</version>
+                    <version>13.1.0</version>
                     </dependency>
                 </dependencies>
             </plugin>
diff --git a/src/main/java/com/thealgorithms/searches/BinarySearch.java b/src/main/java/com/thealgorithms/searches/BinarySearch.java
@@ -3,14 +3,32 @@
 import com.thealgorithms.devutils.searches.SearchAlgorithm;
 
 /**
- * Binary search is one of the most popular algorithms The algorithm finds the
- * position of a target value within a sorted array
- * IMPORTANT
- * This algorithm works correctly only if the input array is sorted
- * in ascending order.
- * <p>
- * Worst-case performance O(log n) Best-case performance O(1) Average
- * performance O(log n) Worst-case space complexity O(1)
+ * Binary Search Algorithm Implementation
+ *
+ * <p>Binary search is one of the most efficient searching algorithms for finding a target element
+ * in a SORTED array. It works by repeatedly dividing the search space in half, eliminating half of
+ * the remaining elements in each step.
+ *
+ * <p>IMPORTANT: This algorithm ONLY works correctly if the input array is sorted in ascending
+ * order.
+ *
+ * <p>Algorithm Overview: 1. Start with the entire array (left = 0, right = array.length - 1) 2.
+ * Calculate the middle index 3. Compare the middle element with the target: - If middle element
+ * equals target: Found! Return the index - If middle element is less than target: Search the right
+ * half - If middle element is greater than target: Search the left half 4. Repeat until element is
+ * found or search space is exhausted
+ *
+ * <p>Performance Analysis: - Best-case time complexity: O(1) - Element found at middle on first
+ * try - Average-case time complexity: O(log n) - Most common scenario - Worst-case time
+ * complexity: O(log n) - Element not found or at extreme end - Space complexity: O(1) - Only uses
+ * a constant amount of extra space
+ *
+ * <p>Example Walkthrough: Array: [1, 3, 5, 7, 9, 11, 13, 15, 17, 19] Target: 7
+ *
+ * <p>Step 1: left=0, right=9, mid=4, array[4]=9 (9 &gt; 7, search left half) Step 2: left=0,
+ * right=3, mid=1, array[1]=3 (3 &lt; 7, search right half) Step 3: left=2, right=3, mid=2,
+ * array[2]=5 (5 &lt; 7, search right half) Step 4: left=3, right=3, mid=3, array[3]=7 (Found!
+ * Return index 3)
  *
  * @author Varun Upadhyay (https://github.com/varunu28)
  * @author Podshivalov Nikita (https://github.com/nikitap492)
@@ -20,41 +38,89 @@
 class BinarySearch implements SearchAlgorithm {
 
     /**
-     * @param array is an array where the element should be found
-     * @param key is an element which should be found
-     * @param <T> is any comparable type
-     * @return index of the element
+     * Generic method to perform binary search on any comparable type. This is the main entry point
+     * for binary search operations.
+     *
+     * <p>Example Usage:
+     * <pre>
+     * Integer[] numbers = {1, 3, 5, 7, 9, 11};
+     * int result = new BinarySearch().find(numbers, 7);
+     * // result will be 3 (index of element 7)
+     *
+     * int notFound = new BinarySearch().find(numbers, 4);
+     * // notFound will be -1 (element 4 does not exist)
+     * </pre>
+     *
+     * @param <T> The type of elements in the array (must be Comparable)
+     * @param array The sorted array to search in (MUST be sorted in ascending order)
+     * @param key The element to search for
+     * @return The index of the key if found, -1 if not found or if array is null/empty
      */
     @Override
     public <T extends Comparable<T>> int find(T[] array, T key) {
+        // Handle edge case: empty array
         if (array == null || array.length == 0) {
             return -1;
         }
+
+        // Delegate to the core search implementation
         return search(array, key, 0, array.length - 1);
     }
 
     /**
-     * This method implements the Generic Binary Search
+     * Core recursive implementation of binary search algorithm. This method divides the problem
+     * into smaller subproblems recursively.
+     *
+     * <p>How it works:
+     * <ol>
+     * <li>Calculate the middle index to avoid integer overflow</li>
+     * <li>Check if middle element matches the target</li>
+     * <li>If not, recursively search either left or right half</li>
+     * <li>Base case: left &gt; right means element not found</li>
+     * </ol>
+     *
+     * <p>Time Complexity: O(log n) because we halve the search space each time.
+     * Space Complexity: O(log n) due to recursive call stack.
      *
-     * @param array The array to make the binary search
-     * @param key The number you are looking for
-     * @param left The lower bound
-     * @param right The upper bound
-     * @return the location of the key
+     * @param <T> The type of elements (must be Comparable)
+     * @param array The sorted array to search in
+     * @param key The element we're looking for
+     * @param left The leftmost index of current search range (inclusive)
+     * @param right The rightmost index of current search range (inclusive)
+     * @return The index where key is located, or -1 if not found
      */
     private <T extends Comparable<T>> int search(T[] array, T key, int left, int right) {
+        // Base case: Search space is exhausted
+        // This happens when left pointer crosses right pointer
         if (right < left) {
-            return -1; // this means that the key not found
+            return -1; // Key not found in the array
         }
-        // find median
-        int median = (left + right) >>> 1;
+
+        // Calculate middle index
+        // Using (left + right) / 2 could cause integer overflow for large arrays
+        // So we use: left + (right - left) / 2 which is mathematically equivalent
+        // but prevents overflow
+        int median = (left + right) >>> 1; // Unsigned right shift is faster division by 2
+
+        // Get the value at middle position for comparison
         int comp = key.compareTo(array[median]);
 
+        // Case 1: Found the target element at middle position
         if (comp == 0) {
-            return median;
-        } else if (comp < 0) {
+            return median; // Return the index where element was found
+        }
+        // Case 2: Target is smaller than middle element
+        // This means if target exists, it must be in the LEFT half
+        else if (comp < 0) {
+            // Recursively search the left half
+            // New search range: [left, median - 1]
             return search(array, key, left, median - 1);
-        } else {
+        }
+        // Case 3: Target is greater than middle element
+        // This means if target exists, it must be in the RIGHT half
+        else {
+            // Recursively search the right half
+            // New search range: [median + 1, right]
             return search(array, key, median + 1, right);
         }
     }
diff --git a/src/main/java/com/thealgorithms/strings/KMP.java b/src/main/java/com/thealgorithms/strings/KMP.java
@@ -1,5 +1,8 @@
 package com.thealgorithms.strings;
 
+import java.util.ArrayList;
+import java.util.List;
+
 /**
  * Implementation of Knuth–Morris–Pratt algorithm Usage: see the main function
  * for an example
@@ -8,16 +11,19 @@ public final class KMP {
     private KMP() {
     }
 
-    // a working example
-
-    public static void main(String[] args) {
-        final String haystack = "AAAAABAAABA"; // This is the full string
-        final String needle = "AAAA"; // This is the substring that we want to find
-        kmpMatcher(haystack, needle);
-    }
+    /**
+     * find the starting index in string haystack[] that matches the search word P[]
+     *
+     * @param haystack The text to be searched
+     * @param needle   The pattern to be searched for
+     * @return A list of starting indices where the pattern is found
+     */
+    public static List<Integer> kmpMatcher(final String haystack, final String needle) {
+        List<Integer> occurrences = new ArrayList<>();
+        if (haystack == null || needle == null || needle.isEmpty()) {
+            return occurrences;
+        }
 
-    // find the starting index in string haystack[] that matches the search word P[]
-    public static void kmpMatcher(final String haystack, final String needle) {
         final int m = haystack.length();
         final int n = needle.length();
         final int[] pi = computePrefixFunction(needle);
@@ -32,10 +38,11 @@ public static void kmpMatcher(final String haystack, final String needle) {
             }
 
             if (q == n) {
-                System.out.println("Pattern starts: " + (i + 1 - n));
+                occurrences.add(i + 1 - n);
                 q = pi[q - 1];
             }
         }
+        return occurrences;
     }
 
     // return the prefix function
diff --git a/src/main/java/com/thealgorithms/strings/RabinKarp.java b/src/main/java/com/thealgorithms/strings/RabinKarp.java
@@ -1,32 +1,30 @@
 package com.thealgorithms.strings;
 
-import java.util.Scanner;
+import java.util.ArrayList;
+import java.util.List;
 
 /**
  * @author Prateek Kumar Oraon (https://github.com/prateekKrOraon)
  *
- An implementation of Rabin-Karp string matching algorithm
- Program will simply end if there is no match
+ *         An implementation of Rabin-Karp string matching algorithm
+ *         Program will simply end if there is no match
  */
 public final class RabinKarp {
     private RabinKarp() {
     }
 
-    public static Scanner scanner = null;
-    public static final int ALPHABET_SIZE = 256;
+    private static final int ALPHABET_SIZE = 256;
 
-    public static void main(String[] args) {
-        scanner = new Scanner(System.in);
-        System.out.println("Enter String");
-        String text = scanner.nextLine();
-        System.out.println("Enter pattern");
-        String pattern = scanner.nextLine();
-
-        int q = 101;
-        searchPat(text, pattern, q);
+    public static List<Integer> search(String text, String pattern) {
+        return search(text, pattern, 101);
     }
 
-    private static void searchPat(String text, String pattern, int q) {
+    public static List<Integer> search(String text, String pattern, int q) {
+        List<Integer> occurrences = new ArrayList<>();
+        if (text == null || pattern == null || pattern.isEmpty()) {
+            return occurrences;
+        }
+
         int m = pattern.length();
         int n = text.length();
         int t = 0;
@@ -35,48 +33,42 @@ private static void searchPat(String text, String pattern, int q) {
         int j = 0;
         int i = 0;
 
-        h = (int) Math.pow(ALPHABET_SIZE, m - 1) % q;
+        if (m > n) {
+            return new ArrayList<>();
+        }
+
+        // h = pow(ALPHABET_SIZE, m-1) % q
+        for (i = 0; i < m - 1; i++) {
+            h = h * ALPHABET_SIZE % q;
+        }
 
         for (i = 0; i < m; i++) {
-            // hash value is calculated for each character and then added with the hash value of the
-            // next character for pattern as well as the text for length equal to the length of
-            // pattern
             p = (ALPHABET_SIZE * p + pattern.charAt(i)) % q;
             t = (ALPHABET_SIZE * t + text.charAt(i)) % q;
         }
 
         for (i = 0; i <= n - m; i++) {
-            // if the calculated hash value of the pattern and text matches then
-            // all the characters of the pattern is matched with the text of length equal to length
-            // of the pattern if all matches then pattern exist in string if not then the hash value
-            // of the first character of the text is subtracted and hash value of the next character
-            // after the end of the evaluated characters is added
             if (p == t) {
-                // if hash value matches then the individual characters are matched
                 for (j = 0; j < m; j++) {
-                    // if not matched then break out of the loop
                     if (text.charAt(i + j) != pattern.charAt(j)) {
                         break;
                     }
                 }
 
-                // if all characters are matched then pattern exist in the string
                 if (j == m) {
-                    System.out.println("Pattern found at index " + i);
+                    occurrences.add(i);
                 }
             }
 
-            // if i<n-m then hash value of the first character of the text is subtracted and hash
-            // value of the next character after the end of the evaluated characters is added to get
-            // the hash value of the next window of characters in the text
             if (i < n - m) {
-                t = (ALPHABET_SIZE * (t - text.charAt(i) * h) + text.charAt(i + m)) % q;
-
-                // if hash value becomes less than zero than q is added to make it positive
+                t = (t - text.charAt(i) * h % q);
                 if (t < 0) {
-                    t = (t + q);
+                    t += q;
                 }
+                t = t * ALPHABET_SIZE % q;
+                t = (t + text.charAt(i + m)) % q;
             }
         }
+        return occurrences;
     }
 }
diff --git a/src/test/java/com/thealgorithms/strings/KMPTest.java b/src/test/java/com/thealgorithms/strings/KMPTest.java
@@ -0,0 +1,29 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class KMPTest {
+
+    @Test
+    public void testNullInputs() {
+        assertEquals(List.of(), KMP.kmpMatcher(null, "A"));
+        assertEquals(List.of(), KMP.kmpMatcher("A", null));
+        assertEquals(List.of(), KMP.kmpMatcher(null, null));
+    }
+
+    @Test
+    public void testKMPMatcher() {
+        assertEquals(List.of(0, 1), KMP.kmpMatcher("AAAAABAAABA", "AAAA"));
+        assertEquals(List.of(0, 3), KMP.kmpMatcher("ABCABC", "ABC"));
+        assertEquals(List.of(10), KMP.kmpMatcher("ABABDABACDABABCABAB", "ABABCABAB"));
+        assertEquals(List.of(), KMP.kmpMatcher("ABCDE", "FGH"));
+        assertEquals(List.of(), KMP.kmpMatcher("A", "AA"));
+        assertEquals(List.of(0, 1, 2), KMP.kmpMatcher("AAA", "A"));
+        assertEquals(List.of(0), KMP.kmpMatcher("A", "A"));
+        assertEquals(List.of(), KMP.kmpMatcher("", "A"));
+        assertEquals(List.of(), KMP.kmpMatcher("A", ""));
+    }
+}
diff --git a/src/test/java/com/thealgorithms/strings/RabinKarpTest.java b/src/test/java/com/thealgorithms/strings/RabinKarpTest.java
@@ -0,0 +1,46 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class RabinKarpTest {
+
+    @Test
+    public void testNullInputs() {
+        assertEquals(List.of(), RabinKarp.search(null, "A"));
+        assertEquals(List.of(), RabinKarp.search("A", null));
+        assertEquals(List.of(), RabinKarp.search(null, null));
+    }
+
+    @Test
+    public void testHashCollision() {
+        // 'a' = 97. (char)198 % 101 = 97.
+        // For length 1, h = 1. p = 97. t = 198 % 101 = 97.
+        // Collision occurs, loop checks characters: 198 != 97, breaks.
+        char collisionChar = (char) 198;
+        String text = String.valueOf(collisionChar);
+        String pattern = "a";
+        assertEquals(List.of(), RabinKarp.search(text, pattern));
+    }
+
+    @Test
+    public void testSearchWithCustomQ() {
+        // Using a different prime
+        assertEquals(List.of(0, 1), RabinKarp.search("AAAA", "AAA", 13));
+    }
+
+    @Test
+    public void testRabinKarpSearch() {
+        assertEquals(List.of(0, 1), RabinKarp.search("AAAAABAAABA", "AAAA"));
+        assertEquals(List.of(0, 3), RabinKarp.search("ABCABC", "ABC"));
+        assertEquals(List.of(10), RabinKarp.search("ABABDABACDABABCABAB", "ABABCABAB"));
+        assertEquals(List.of(), RabinKarp.search("ABCDE", "FGH"));
+        assertEquals(List.of(), RabinKarp.search("A", "AA"));
+        assertEquals(List.of(0, 1, 2), RabinKarp.search("AAA", "A"));
+        assertEquals(List.of(0), RabinKarp.search("A", "A"));
+        assertEquals(List.of(), RabinKarp.search("", "A"));
+        assertEquals(List.of(), RabinKarp.search("A", ""));
+    }
+}
