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/main/java/com/thealgorithms/bitmanipulation/CountSetBits.java

@@ -1,79 +1,79 @@
 package com.thealgorithms.bitmanipulation;
 
-public class CountSetBits {
+/**
+ * Utility class to count total set bits from 1 to N
+ * A set bit is a bit in binary representation that is 1
+ *
+ * @author navadeep
+ */
+public final class CountSetBits {
 
-    /**
-     * The below algorithm is called as Brian Kernighan's algorithm
-     * We can use Brian Kernighan’s algorithm to improve the above naive algorithm’s performance.
-     The idea is to only consider the set bits of an integer by turning off its rightmost set bit
-     (after counting it), so the next iteration of the loop considers the next rightmost bit.
-
-        The expression n & (n-1) can be used to turn off the rightmost set bit of a number n. This
-     works as the expression n-1 flips all the bits after the rightmost set bit of n, including the
-     rightmost set bit itself. Therefore, n & (n-1) results in the last bit flipped of n.
-
-        For example, consider number 52, which is 00110100 in binary, and has a total 3 bits set.
-
-        1st iteration of the loop: n = 52
-
-        00110100    &               (n)
-        00110011                    (n-1)
-        ~~~~~~~~
-        00110000
-
-
-        2nd iteration of the loop: n = 48
-
-        00110000    &               (n)
-        00101111                    (n-1)
-        ~~~~~~~~
-        00100000
-
-
-        3rd iteration of the loop: n = 32
-
-        00100000    &               (n)
-        00011111                    (n-1)
-        ~~~~~~~~
-        00000000                    (n = 0)
-
-     * @param num takes Long number whose number of set bit is to be found
-     * @return the count of set bits in the binary equivalent
-    */
-    public long countSetBits(long num) {
-        long cnt = 0;
-        while (num > 0) {
-            cnt++;
-            num &= (num - 1);
-        }
-        return cnt;
+    private CountSetBits() {
+        // Utility class, prevent instantiation
     }
 
     /**
-     * This approach takes O(1) running time to count the set bits, but requires a pre-processing.
+     * Counts total number of set bits in all numbers from 1 to n
+     * Time Complexity: O(log n)
      *
-     * So, we divide our 32-bit input into 8-bit chunks, with four chunks. We have 8 bits in each chunk.
-     *
-     * Then the range is from 0-255 (0 to 2^7).
-     * So, we may need to count set bits from 0 to 255 in individual chunks.
-     *
-     * @param num takes a long number
-     * @return the count of set bits in the binary equivalent
+     * @param n the upper limit (inclusive)
+     * @return total count of set bits from 1 to n
+     * @throws IllegalArgumentException if n is negative
      */
-    public int lookupApproach(int num) {
-        int[] table = new int[256];
-        table[0] = 0;
-
-        for (int i = 1; i < 256; i++) {
-            table[i] = (i & 1) + table[i >> 1]; // i >> 1 equals to i/2
+    public static int countSetBits(int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("Input must be non-negative");
         }
 
-        int res = 0;
-        for (int i = 0; i < 4; i++) {
-            res += table[num & 0xff];
-            num >>= 8;
+        if (n == 0) {
+            return 0;
         }
 
-        return res;
+        // Find the largest power of 2 <= n
+        int x = largestPowerOf2InNumber(n);
+
+        // Total bits at position x: x * 2^(x-1)
+        int bitsAtPositionX = x * (1 << (x - 1));
+
+        // Remaining numbers after 2^x
+        int remainingNumbers = n - (1 << x) + 1;
+
+        // Recursively count for the rest
+        int rest = countSetBits(n - (1 << x));
+
+        return bitsAtPositionX + remainingNumbers + rest;
+    }
+
+    /**
+     * Finds the position of the most significant bit in n
+     *
+     * @param n the number
+     * @return position of MSB (0-indexed from right)
+     */
+    private static int largestPowerOf2InNumber(int n) {
+        int position = 0;
+        while ((1 << position) <= n) {
+            position++;
+        }
+        return position - 1;
+    }
+
+    /**
+     * Alternative naive approach - counts set bits by iterating through all numbers
+     * Time Complexity: O(n log n)
+     *
+     * @param n the upper limit (inclusive)
+     * @return total count of set bits from 1 to n
+     */
+    public static int countSetBitsNaive(int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("Input must be non-negative");
+        }
+
+        int count = 0;
+        for (int i = 1; i <= n; i++) {
+            count += Integer.bitCount(i);
+        }
+        return count;
     }
 }

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);
+        }
     }
 }