Enhance docs, add more tests in TwosComplement (#5862)

Author: Hardvan

src/main/java/com/thealgorithms/bitmanipulation/TwosComplement.java

@@ -1,41 +1,62 @@
 package com.thealgorithms.bitmanipulation;
 
 /**
- * @wikipedia - https://en.wikipedia.org/wiki/Two%27s_complement
- *            This Algorithm was first suggested by Jon Von Neumann
- * @author - https://github.com/Monk-AbhinayVerma
- * @return the two's complement of any binary number
+ * This class provides a method to compute the Two's Complement of a given binary number.
+ *
+ * <p>In two's complement representation, a binary number's negative value is obtained
+ * by taking the one's complement (inverting all bits) and then adding 1 to the result.
+ * This method handles both small and large binary strings and ensures the output is
+ * correct for all binary inputs, including edge cases like all zeroes and all ones.
+ *
+ * <p>For more information on Two's Complement:
+ * @see <a href="https://en.wikipedia.org/wiki/Two%27s_complement">Wikipedia - Two's Complement</a>
+ *
+ * <p>Algorithm originally suggested by Jon von Neumann.
+ *
+ * @author Abhinay Verma (https://github.com/Monk-AbhinayVerma)
  */
 public final class TwosComplement {
     private TwosComplement() {
     }
 
-    // Function to get the 2's complement of a binary number
+    /**
+     * Computes the Two's Complement of the given binary string.
+     * Steps:
+     * 1. Compute the One's Complement (invert all bits).
+     * 2. Add 1 to the One's Complement to get the Two's Complement.
+     * 3. Iterate from the rightmost bit to the left, adding 1 and carrying over as needed.
+     * 4. If a carry is still present after the leftmost bit, prepend '1' to handle overflow.
+     *
+     * @param binary The binary number as a string (only '0' and '1' characters allowed).
+     * @return The two's complement of the input binary string as a new binary string.
+     * @throws IllegalArgumentException If the input contains non-binary characters.
+     */
     public static String twosComplement(String binary) {
+        if (!binary.matches("[01]+")) {
+            throw new IllegalArgumentException("Input must contain only '0' and '1'.");
+        }
+
         StringBuilder onesComplement = new StringBuilder();
-        // Step 1: Find the 1's complement (invert the bits)
-        for (int i = 0; i < binary.length(); i++) {
-            if (binary.charAt(i) == '0') {
-                onesComplement.append('1');
-            } else {
-                onesComplement.append('0');
-            }
+        for (char bit : binary.toCharArray()) {
+            onesComplement.append(bit == '0' ? '1' : '0');
         }
-        // Step 2: Add 1 to the 1's complement
+
         StringBuilder twosComplement = new StringBuilder(onesComplement);
         boolean carry = true;
-        for (int i = onesComplement.length() - 1; i >= 0; i--) {
-            if (onesComplement.charAt(i) == '1' && carry) {
+
+        for (int i = onesComplement.length() - 1; i >= 0 && carry; i--) {
+            if (onesComplement.charAt(i) == '1') {
                 twosComplement.setCharAt(i, '0');
-            } else if (onesComplement.charAt(i) == '0' && carry) {
+            } else {
                 twosComplement.setCharAt(i, '1');
                 carry = false;
             }
         }
-        // If there is still a carry, append '1' at the beginning
+
         if (carry) {
             twosComplement.insert(0, '1');
         }
+
         return twosComplement.toString();
     }
 }

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

@@ -1,6 +1,7 @@
 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;
 
@@ -12,7 +13,6 @@ public class TwosComplementTest {
 
     @Test
     public void testTwosComplementAllZeroes() {
-        // Test with a binary number consisting entirely of zeroes
         assertEquals("10000", TwosComplement.twosComplement("0000"));
         assertEquals("1000", TwosComplement.twosComplement("000"));
         assertEquals("100", TwosComplement.twosComplement("00"));
@@ -21,7 +21,6 @@ public void testTwosComplementAllZeroes() {
 
     @Test
     public void testTwosComplementAllOnes() {
-        // Test with a binary number consisting entirely of ones
         assertEquals("00001", TwosComplement.twosComplement("11111"));
         assertEquals("0001", TwosComplement.twosComplement("1111"));
         assertEquals("001", TwosComplement.twosComplement("111"));
@@ -30,25 +29,36 @@ public void testTwosComplementAllOnes() {
 
     @Test
     public void testTwosComplementMixedBits() {
-        // Test with binary numbers with mixed bits
-        assertEquals("1111", TwosComplement.twosComplement("0001")); // 1's complement: 1110, then add 1: 1111
-        assertEquals("1001", TwosComplement.twosComplement("0111")); // 1's complement: 1000
-        assertEquals("11001", TwosComplement.twosComplement("00111")); // 1's complement: 11000, add 1: 11001
-        assertEquals("011", TwosComplement.twosComplement("101")); // 1's complement: 010, add 1: 011
+        assertEquals("1111", TwosComplement.twosComplement("0001")); // 1 -> 1111
+        assertEquals("1001", TwosComplement.twosComplement("0111")); // 0111 -> 1001
+        assertEquals("11001", TwosComplement.twosComplement("00111")); // 00111 -> 11001
+        assertEquals("011", TwosComplement.twosComplement("101")); // 101 -> 011
     }
 
     @Test
     public void testTwosComplementSingleBit() {
-        // Test with single bit
-        assertEquals("10", TwosComplement.twosComplement("0"));
-        assertEquals("1", TwosComplement.twosComplement("1"));
+        assertEquals("10", TwosComplement.twosComplement("0")); // 0 -> 10
+        assertEquals("1", TwosComplement.twosComplement("1")); // 1 -> 1
     }
 
     @Test
     public void testTwosComplementWithLeadingZeroes() {
-        // Test with leading zeroes in the input
-        assertEquals("1111", TwosComplement.twosComplement("0001"));
-        assertEquals("101", TwosComplement.twosComplement("011"));
-        assertEquals("110", TwosComplement.twosComplement("010"));
+        assertEquals("1111", TwosComplement.twosComplement("0001")); // 0001 -> 1111
+        assertEquals("101", TwosComplement.twosComplement("011")); // 011 -> 101
+        assertEquals("110", TwosComplement.twosComplement("010")); // 010 -> 110
+    }
+
+    @Test
+    public void testInvalidBinaryInput() {
+        // Test for invalid input that contains non-binary characters.
+        assertThrows(IllegalArgumentException.class, () -> TwosComplement.twosComplement("102"));
+        assertThrows(IllegalArgumentException.class, () -> TwosComplement.twosComplement("abc"));
+        assertThrows(IllegalArgumentException.class, () -> TwosComplement.twosComplement("10a01"));
+    }
+
+    @Test
+    public void testEmptyInput() {
+        // Edge case: Empty input should result in an IllegalArgumentException.
+        assertThrows(IllegalArgumentException.class, () -> TwosComplement.twosComplement(""));
     }
 }