[KTLN-872] Convert hex string to byte array in Kotlin (#968)

* added unit tests

* made code fixes

Author: albertache1998

core-kotlin-modules/core-kotlin-strings-5/src/test/kotlin/com/baeldung/hexToByte/HexToByteUnitTest.kt

@@ -0,0 +1,60 @@
+package com.baeldung.hexToByte
+
+import org.junit.jupiter.api.Assertions.assertArrayEquals
+import org.junit.jupiter.api.Test
+import java.math.BigInteger
+
+class HexToByteUnitTest {
+
+    @Test
+    fun `convert hex string to byte array using for loop`() {
+        val hexString = "48656C6C6F20576F726C64"
+        val expectedByteArray = byteArrayOf(72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100)
+        assertArrayEquals(expectedByteArray, hexStringToByteArrayUsingForLoop(hexString))
+    }
+
+    @Test
+    fun `convert hex string to byte array using BigInteger`() {
+        val hexString = "48656C6C6F20576F726C64"
+        val expectedByteArray = byteArrayOf(72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100)
+
+        assertArrayEquals(expectedByteArray, hexStringToByteArrayUsingBigInteger(hexString))
+    }
+
+    @Test
+    fun `convert hex string to byte array using BigInteger using standard libraries methods`() {
+        val hexString = "48656C6C6F20576F726C64"
+        val expectedByteArray = byteArrayOf(72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100)
+
+        assertArrayEquals(expectedByteArray, hexStringToByteArrayUsingStandardLibraryMethods(hexString))
+    }
+
+    @OptIn(ExperimentalStdlibApi::class)
+    @Test
+    fun `convert hex string to byte array using hexToByteArray`() {
+        val hexString = "48656C6C6F20576F726C64"
+        val expectedByteArray = byteArrayOf(72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100)
+
+        assertArrayEquals(expectedByteArray, hexString.hexToByteArray())
+    }
+}
+fun hexStringToByteArrayUsingForLoop(hex: String): ByteArray {
+    val length = hex.length
+    val byteArray = ByteArray(length / 2)
+    for (i in byteArray.indices) {
+        val index = i * 2
+        val byte = hex.substring(index, index + 2).toInt(16).toByte()
+        byteArray[i] = byte
+    }
+    return byteArray
+}
+fun hexStringToByteArrayUsingBigInteger(hexString: String): ByteArray {
+    val bigInteger = BigInteger(hexString, 16)
+    return bigInteger.toByteArray()
+}
+
+fun hexStringToByteArrayUsingStandardLibraryMethods(hex: String): ByteArray {
+    return hex.chunked(2)
+        .map { it.toInt(16).toByte() }
+        .toByteArray()
+}