Added Sentinel Linear Search . (#6671)

Th-Shivam · web-flow · commit b50d1d017b57 · 2025-10-14T17:22:15.000Z
* Added Sentinel Linear Search .

* Fixed linter error

* Fixed linter errors

* Fixed linter errors in test fike

* Fixed linter errors in test file

* Fixed linter errors
diff --git a/src/main/java/com/thealgorithms/searches/SentinelLinearSearch.java b/src/main/java/com/thealgorithms/searches/SentinelLinearSearch.java
@@ -0,0 +1,99 @@
+package com.thealgorithms.searches;
+
+import com.thealgorithms.devutils.searches.SearchAlgorithm;
+
+/**
+ * Sentinel Linear Search is a variation of linear search that eliminates the
+ * need to check the array bounds in each iteration by placing the search key
+ * at the end of the array as a sentinel value.
+ *
+ * <p>
+ * The algorithm works by:
+ * 1. Storing the last element of the array
+ * 2. Placing the search key at the last position (sentinel)
+ * 3. Searching from the beginning without bound checking
+ * 4. If found before the last position, return the index
+ * 5. If found at the last position, check if it was originally there
+ *
+ * <p>
+ * Time Complexity:
+ * - Best case: O(1) - when the element is at the first position
+ * - Average case: O(n) - when the element is in the middle
+ * - Worst case: O(n) - when the element is not present
+ *
+ * <p>
+ * Space Complexity: O(1) - only uses constant extra space
+ *
+ * <p>
+ * Advantages over regular linear search:
+ * - Reduces the number of comparisons by eliminating bound checking
+ * - Slightly more efficient in practice due to fewer conditional checks
+ *
+ * @author TheAlgorithms Contributors
+ * @see LinearSearch
+ * @see SearchAlgorithm
+ */
+public class SentinelLinearSearch implements SearchAlgorithm {
+    /**
+     * Performs sentinel linear search on the given array.
+     *
+     * @param array the array to search in
+     * @param key the element to search for
+     * @param <T> the type of elements in the array, must be Comparable
+     * @return the index of the first occurrence of the key, or -1 if not found
+     * @throws IllegalArgumentException if the array is null
+     */
+    @Override
+    public <T extends Comparable<T>> int find(T[] array, T key) {
+        if (array == null) {
+            throw new IllegalArgumentException("Array cannot be null");
+        }
+
+        if (array.length == 0) {
+            return -1;
+        }
+
+        if (key == null) {
+            return findNull(array);
+        }
+
+        // Store the last element
+        T lastElement = array[array.length - 1];
+
+        // Place the sentinel (search key) at the end
+        array[array.length - 1] = key;
+
+        int i = 0;
+        // Search without bound checking since sentinel guarantees we'll find the key
+        while (array[i].compareTo(key) != 0) {
+            i++;
+        }
+
+        // Restore the original last element
+        array[array.length - 1] = lastElement;
+
+        // Check if we found the key before the sentinel position
+        // or if the original last element was the key we were looking for
+        if (i < array.length - 1 || (lastElement != null && lastElement.compareTo(key) == 0)) {
+            return i;
+        }
+
+        return -1; // Key not found
+    }
+
+    /**
+     * Helper method to find null values in the array.
+     *
+     * @param array the array to search in
+     * @param <T> the type of elements in the array
+     * @return the index of the first null element, or -1 if not found
+     */
+    private <T extends Comparable<T>> int findNull(T[] array) {
+        for (int i = 0; i < array.length; i++) {
+            if (array[i] == null) {
+                return i;
+            }
+        }
+        return -1;
+    }
+}
diff --git a/src/test/java/com/thealgorithms/searches/SentinelLinearSearchTest.java b/src/test/java/com/thealgorithms/searches/SentinelLinearSearchTest.java
@@ -0,0 +1,225 @@
+package com.thealgorithms.searches;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.Random;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for the SentinelLinearSearch class.
+ */
+class SentinelLinearSearchTest {
+
+    /**
+     * Test for finding an element present in the array.
+     */
+    @Test
+    void testSentinelLinearSearchFound() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+        Integer key = 5; // Element to find
+        assertEquals(5, sentinelLinearSearch.find(array, key), "The index of the found element should be 5.");
+    }
+
+    /**
+     * Test for finding the first element in the array.
+     */
+    @Test
+    void testSentinelLinearSearchFirstElement() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+        Integer key = 0; // First element
+        assertEquals(0, sentinelLinearSearch.find(array, key), "The index of the first element should be 0.");
+    }
+
+    /**
+     * Test for finding the last element in the array.
+     */
+    @Test
+    void testSentinelLinearSearchLastElement() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+        Integer key = 10; // Last element
+        assertEquals(10, sentinelLinearSearch.find(array, key), "The index of the last element should be 10.");
+    }
+
+    /**
+     * Test for finding an element not present in the array.
+     */
+    @Test
+    void testSentinelLinearSearchNotFound() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
+        Integer key = -1; // Element not in the array
+        assertEquals(-1, sentinelLinearSearch.find(array, key), "The element should not be found in the array.");
+    }
+
+    /**
+     * Test for finding an element in an empty array.
+     */
+    @Test
+    void testSentinelLinearSearchEmptyArray() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {}; // Empty array
+        Integer key = 1; // Key not present
+        assertEquals(-1, sentinelLinearSearch.find(array, key), "The element should not be found in an empty array.");
+    }
+
+    /**
+     * Test for finding an element in a single-element array when present.
+     */
+    @Test
+    void testSentinelLinearSearchSingleElementFound() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {42}; // Single element array
+        Integer key = 42; // Element present
+        assertEquals(0, sentinelLinearSearch.find(array, key), "The element should be found at index 0.");
+    }
+
+    /**
+     * Test for finding an element in a single-element array when not present.
+     */
+    @Test
+    void testSentinelLinearSearchSingleElementNotFound() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {42}; // Single element array
+        Integer key = 24; // Element not present
+        assertEquals(-1, sentinelLinearSearch.find(array, key), "The element should not be found in the array.");
+    }
+
+    /**
+     * Test for finding multiple occurrences of the same element in the array.
+     */
+    @Test
+    void testSentinelLinearSearchMultipleOccurrences() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {1, 2, 3, 4, 5, 3, 6, 7, 3}; // 3 occurs multiple times
+        Integer key = 3; // Key to find
+        assertEquals(2, sentinelLinearSearch.find(array, key), "The index of the first occurrence of the element should be 2.");
+    }
+
+    /**
+     * Test for finding an element in a large array.
+     */
+    @Test
+    void testSentinelLinearSearchLargeArray() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = new Integer[1000];
+        for (int i = 0; i < array.length; i++) {
+            array[i] = i; // Fill the array with integers 0 to 999
+        }
+        Integer key = 256; // Present in the array
+        assertEquals(256, sentinelLinearSearch.find(array, key), "The index of the found element should be 256.");
+    }
+
+    /**
+     * Test for finding an element in a large array when it is not present.
+     */
+    @Test
+    void testSentinelLinearSearchLargeArrayNotFound() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = new Integer[1000];
+        for (int i = 0; i < array.length; i++) {
+            array[i] = i; // Fill the array with integers 0 to 999
+        }
+        Integer key = 1001; // Key not present
+        assertEquals(-1, sentinelLinearSearch.find(array, key), "The element should not be found in the array.");
+    }
+
+    /**
+     * Test for performance with random large array.
+     */
+    @Test
+    void testSentinelLinearSearchRandomArray() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Random random = new Random();
+        Integer[] array = random.ints(0, 1000).distinct().limit(1000).boxed().toArray(Integer[] ::new);
+        Integer key = array[random.nextInt(array.length)]; // Key should be in the array
+        assertEquals(java.util.Arrays.asList(array).indexOf(key), sentinelLinearSearch.find(array, key), "The index of the found element should match.");
+    }
+
+    /**
+     * Test for handling null array.
+     */
+    @Test
+    void testSentinelLinearSearchNullArray() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = null; // Null array
+        Integer key = 1; // Any key
+        assertThrows(IllegalArgumentException.class, () -> sentinelLinearSearch.find(array, key), "Should throw IllegalArgumentException for null array.");
+    }
+
+    /**
+     * Test for handling null key in array with null elements.
+     */
+    @Test
+    void testSentinelLinearSearchNullKey() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {1, null, 3, 4, null}; // Array with null elements
+        Integer key = null; // Null key
+        assertEquals(1, sentinelLinearSearch.find(array, key), "The index of the first null element should be 1.");
+    }
+
+    /**
+     * Test for handling null key when not present in array.
+     */
+    @Test
+    void testSentinelLinearSearchNullKeyNotFound() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {1, 2, 3, 4, 5}; // Array without null elements
+        Integer key = null; // Null key
+        assertEquals(-1, sentinelLinearSearch.find(array, key), "Null key should not be found in array without null elements.");
+    }
+
+    /**
+     * Test with String array to verify generic functionality.
+     */
+    @Test
+    void testSentinelLinearSearchStringArray() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        String[] array = {"apple", "banana", "cherry", "date", "elderberry"};
+        String key = "cherry"; // Element to find
+        assertEquals(2, sentinelLinearSearch.find(array, key), "The index of 'cherry' should be 2.");
+    }
+
+    /**
+     * Test with String array when element not found.
+     */
+    @Test
+    void testSentinelLinearSearchStringArrayNotFound() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        String[] array = {"apple", "banana", "cherry", "date", "elderberry"};
+        String key = "grape"; // Element not in array
+        assertEquals(-1, sentinelLinearSearch.find(array, key), "The element 'grape' should not be found in the array.");
+    }
+
+    /**
+     * Test that the original array is not modified after search.
+     */
+    @Test
+    void testSentinelLinearSearchArrayIntegrity() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {1, 2, 3, 4, 5};
+        Integer[] originalArray = array.clone(); // Keep a copy of the original
+        Integer key = 3; // Element to find
+
+        sentinelLinearSearch.find(array, key);
+
+        // Verify array is unchanged
+        for (int i = 0; i < array.length; i++) {
+            assertEquals(originalArray[i], array[i], "Array should remain unchanged after search.");
+        }
+    }
+
+    /**
+     * Test edge case where the key is the same as the last element.
+     */
+    @Test
+    void testSentinelLinearSearchKeyEqualsLastElement() {
+        SentinelLinearSearch sentinelLinearSearch = new SentinelLinearSearch();
+        Integer[] array = {1, 2, 3, 4, 5, 3}; // Last element is 3, and 3 also appears earlier
+        Integer key = 3; // Key equals last element
+        assertEquals(2, sentinelLinearSearch.find(array, key), "Should find the first occurrence at index 2, not the last.");
+    }
+}
