SCALA-728: Find Twos Complement of Number (#1242)

* SCALA-728: Find Twos Complement of Number

* SCALA-728: Corrected variable name

* SCALA-728: Corrected variable name

* Update TwosComplementTest.scala

* Update TwosComplementTest.scala

---------

Co-authored-by: Callum Gibbons <[email protected]>
Co-authored-by: Grzegorz Piwowarek <[email protected]>

Author: 3 people

scala-core-modules/scala-core-9/src/main/scala/com/baeldung/scala/twoscomplement/TwosComplement.scala

@@ -0,0 +1,53 @@
+package com.baeldung.scala.twoscomplement
+
+import scala.annotation.tailrec
+
+object TwosComplement {
+  def convertWithFold(bin: String): String = {
+    addOneWithFold(onesComplement(bin))
+  }
+
+  def convertUsingRecursion(bin: String): String = {
+    addOneWithRec(onesComplement(bin))
+  }
+
+  private def onesComplement(bin: String): String = {
+    bin.map {
+      case '1' => '0'
+      case '0' => '1'
+    }
+  }
+
+  private def addOneWithFold(bin: String): String = {
+    bin.reverse
+      .foldLeft((true, ""))((added, bit) => {
+        val (needsAdding, acc) = added
+        if (needsAdding) {
+          if (bit == '0') (false, acc + '1') else (true, acc + '0')
+        } else {
+          (false, acc + bit)
+        }
+      })
+      ._2
+      .reverse
+  }
+
+  private def addOneWithRec(bin: String): String = {
+    @tailrec
+    def addOne(needsAdding: Boolean, acc: String)(binString: String): String = {
+      if (needsAdding) {
+        binString match
+          case "" => acc
+          case _ => {
+            val (bit, tail) = binString.splitAt(1)
+            if (bit == "0") addOne(false, acc + '1')(tail.mkString)
+            else addOne(true, acc + '0')(tail.mkString)
+          }
+      } else {
+        acc + binString
+      }
+    }
+
+    addOne(true, "")(bin.reverse).reverse
+  }
+}

scala-core-modules/scala-core-9/src/test/scala/com/baeldung/scala/twoscomplement/TwosComplementTest.scala

@@ -0,0 +1,31 @@
+package com.baeldung.scala.twoscomplement
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class TwosComplementTest extends AnyFlatSpec with Matchers {
+
+  val testExamples = List(
+    ("00110100", "11001100"),
+    ("00110101", "11001011"),
+    ("01111101", "10000011"),
+    ("00000000", "00000000"),
+    ("01111111", "10000001")
+  )
+
+  "convertWithFold" should "convert binary to twos complement" in {
+    val comparer = compare(TwosComplement.convertWithFold)
+    comparer(testExamples)
+  }
+
+  "convertUsingRecursion" should "convert binary to twos complement" in {
+    val comparer = compare(TwosComplement.convertUsingRecursion)
+    comparer(testExamples)
+  }
+
+  private def compare(
+    fn: String => String
+  )(tests: List[(String, String)]): Unit = {
+    tests.foreach((input, expected) => fn(input) shouldBe expected)
+  }
+}