ByteArray and String extension to add hexadecimal methods in Kotlin

ByteArray.kt

private val HEX_CHARS = "0123456789ABCDEF".toCharArray()

fun ByteArray.toHex() : String{
    val result = StringBuffer()

    forEach {
        val octet = it.toInt()
        val firstIndex = (octet and 0xF0).ushr(4)
        val secondIndex = octet and 0x0F
        result.append(HEX_CHARS[firstIndex])
        result.append(HEX_CHARS[secondIndex])
    }

    return result.toString()
}

String.kt

private val HEX_CHARS = "0123456789ABCDEF"

fun String.hexStringToByteArray() : ByteArray {

    val result = ByteArray(length / 2)

    for (i in 0 until length step 2) {
        val firstIndex = HEX_CHARS.indexOf(this[i]);
        val secondIndex = HEX_CHARS.indexOf(this[i + 1]);

        val octet = firstIndex.shl(4).or(secondIndex)
        result.set(i.shr(1), octet.toByte())
    }

    return result
}

For people commenting alternatives to the gist, bear in mind that the gist is in Kotlin. The API does not have String.format, as this is from the Kotlin JVM lib. The posted examples won't work in Kotlin Native.

source.joinToString("") { it.toInt().and(0xff).toString(16).padStart(2, '0') }

works with Kotlin/Native at least for the targets JVM, Android, iOS & macOS as expected