feat: add Count Set Bits algorithm (#7072)

* feat: add Count Set Bits algorithm (issue #6931)

* fix: correct CountSetBits algorithm logic

* style: apply clang-format to CountSetBits files

* fix: correct test expectations for CountSetBits

* fix: correct test expectations for CountSetBits

src/test/java/com/thealgorithms/bitmanipulation/CountSetBitsTest.java

@@ -1,26 +1,56 @@
 package com.thealgorithms.bitmanipulation;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
 
-public class CountSetBitsTest {
+class CountSetBitsTest {
 
     @Test
-    void testSetBits() {
-        CountSetBits csb = new CountSetBits();
-        assertEquals(1L, csb.countSetBits(16));
-        assertEquals(4, csb.countSetBits(15));
-        assertEquals(5, csb.countSetBits(10000));
-        assertEquals(5, csb.countSetBits(31));
+    void testCountSetBitsZero() {
+        assertEquals(0, CountSetBits.countSetBits(0));
     }
 
     @Test
-    void testSetBitsLookupApproach() {
-        CountSetBits csb = new CountSetBits();
-        assertEquals(1L, csb.lookupApproach(16));
-        assertEquals(4, csb.lookupApproach(15));
-        assertEquals(5, csb.lookupApproach(10000));
-        assertEquals(5, csb.lookupApproach(31));
+    void testCountSetBitsOne() {
+        assertEquals(1, CountSetBits.countSetBits(1));
+    }
+
+    @Test
+    void testCountSetBitsSmallNumber() {
+        assertEquals(4, CountSetBits.countSetBits(3)); // 1(1) + 10(1) + 11(2) = 4
+    }
+
+    @Test
+    void testCountSetBitsFive() {
+        assertEquals(7, CountSetBits.countSetBits(5)); // 1 + 1 + 2 + 1 + 2 = 7
+    }
+
+    @Test
+    void testCountSetBitsTen() {
+        assertEquals(17, CountSetBits.countSetBits(10));
+    }
+
+    @Test
+    void testCountSetBitsLargeNumber() {
+        assertEquals(42, CountSetBits.countSetBits(20)); // Changed from 93 to 42
+    }
+
+    @Test
+    void testCountSetBitsPowerOfTwo() {
+        assertEquals(13, CountSetBits.countSetBits(8)); // Changed from 9 to 13
+    }
+
+    @Test
+    void testCountSetBitsNegativeInput() {
+        assertThrows(IllegalArgumentException.class, () -> CountSetBits.countSetBits(-1));
+    }
+
+    @Test
+    void testNaiveApproachMatchesOptimized() {
+        for (int i = 0; i <= 100; i++) {
+            assertEquals(CountSetBits.countSetBitsNaive(i), CountSetBits.countSetBits(i), "Mismatch at n = " + i);
+        }
     }
 }