From 6d61ab37611fe39c8f753d32c14ee54605983541 Mon Sep 17 00:00:00 2001
From: Yadukrishnan <yadavan88@gmail.com>
Date: Thu, 25 Apr 2024 09:44:27 +0530
Subject: [PATCH] Added another approach for the calculation

---
 .../twoscomplement/TwosComplement.java        | 36 +++++++++++++++++++
 .../TwosComplementUnitTest.java               | 15 ++++++++
 2 files changed, 51 insertions(+)

diff --git a/core-java-modules/core-java-numbers-7/src/main/java/com/baeldung/twoscomplement/TwosComplement.java b/core-java-modules/core-java-numbers-7/src/main/java/com/baeldung/twoscomplement/TwosComplement.java
index 0bff5abc11..094114422f 100644
--- a/core-java-modules/core-java-numbers-7/src/main/java/com/baeldung/twoscomplement/TwosComplement.java
+++ b/core-java-modules/core-java-numbers-7/src/main/java/com/baeldung/twoscomplement/TwosComplement.java
@@ -1,6 +1,7 @@
 package com.baeldung.twoscomplement;
 
 import java.math.BigInteger;
+import java.util.stream.Collectors;
 
 public class TwosComplement {
 
@@ -74,6 +75,41 @@ public class TwosComplement {
         return number.compareTo(minValue) >= 0 && number.compareTo(maxValue) <= 0;
     }
 
+    public static String decimalToTwosComplementBinaryUsingShortCut(BigInteger num, int numBits) {
+        if (!canRepresentInNBits(num, numBits)) {
+            throw new IllegalArgumentException(numBits + " bits is not enough to represent the number " + num);
+        }
+        var isNegative = num.signum() == -1;
+        var absNum = num.abs();
+
+        // Convert the abs value of the number to its binary representation
+        String binary = absNum.toString(2);
+
+        // Pad the binary representation with zeros to make it numBits long
+        while (binary.length() < numBits) {
+            binary = "0" + binary;
+        }
+
+        // If the input number is negative, calculate two's complement
+        if (isNegative) {
+            binary = performTwosComplementUsingShortCut(binary);
+        }
+
+        return formatInNibbles(binary);
+    }
+
+    private static String performTwosComplementUsingShortCut(String binary) {
+        int firstOneIndexFromRight = binary.lastIndexOf('1');
+        if (firstOneIndexFromRight == -1) {
+            return binary;
+        }
+        String rightPart = binary.substring(firstOneIndexFromRight);
+        String leftPart = binary.substring(0, firstOneIndexFromRight);
+        String leftWithOnes = leftPart.chars().mapToObj(c -> c == '0' ? '1' : '0')
+                .map(String::valueOf).collect(Collectors.joining(""));
+        return leftWithOnes + rightPart;
+    }
+
 }
 
 
diff --git a/core-java-modules/core-java-numbers-7/src/test/java/com/baeldung/twoscomplement/TwosComplementUnitTest.java b/core-java-modules/core-java-numbers-7/src/test/java/com/baeldung/twoscomplement/TwosComplementUnitTest.java
index 248727f2ab..d48a010ffe 100644
--- a/core-java-modules/core-java-numbers-7/src/test/java/com/baeldung/twoscomplement/TwosComplementUnitTest.java
+++ b/core-java-modules/core-java-numbers-7/src/test/java/com/baeldung/twoscomplement/TwosComplementUnitTest.java
@@ -24,6 +24,21 @@ public class TwosComplementUnitTest {
         Assertions.assertEquals(expected, twosComplement);
     }
 
+    @ParameterizedTest(name = "Twos Complement of {0} with number of bits {1}")
+    @CsvSource({
+            "0, 4, 0000",
+            "1, 4, 0001",
+            "-1, 4, 1111",
+            "7, 4, 0111",
+            "-7, 4, 1001",
+            "12345, 16, 0011 0000 0011 1001",
+            "-12345, 16, 1100 1111 1100 0111"
+    })
+    public void givenNumberAndBits_getTwosComplementUsingShortcut(String number, int noOfBits, String expected) {
+        String twosComplement = TwosComplement.decimalToTwosComplementBinary(new BigInteger(number), noOfBits);
+        Assertions.assertEquals(expected, twosComplement);
+    }
+
     @Test
     public void givenNumberOutsideBitsRange_throwException() {
         Exception ex = Assertions.assertThrows(IllegalArgumentException.class, () -> {