Kotlin convert hex string to ByteArray

user_15245005 0
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I have this string:

val s = "00b44a0bc6303782b729a7f9b44a3611b247ddf1e544f8b1420e2aae976003219175461d2bd7" +
        "6e64ba657d7c9dff6ed7b17980778ec0cbf75fc16e52463e2d784f5f20c1691f17cdc597d7a514108" +
        "0809a38c635b2a62082e310aa963ca15953894221ad54c6b30aea10f4dd88a66c55ab9c413eae49c0b" +
        "28e6a3981e0021a7dcb0759af34b095ce3efce78938f2a2bed70939ba47591b88f908db1eadf237a7a" +
        "7100ac87130b6119d7ae41b35fd27ff6021ac928273c20f0b3a01df1e6a070b8e2e93b5220ad0210400" +
        "0c0c1e82e17fd00f6ac16ef37c3b6153d348e470843a84f25473a51f040a42671cd94ffc989eb27fd42" +
        "b817f8173bfa95bdfa17a2ae22fd5c89dab2822bcc973b5b90f8fadc9b074cca8f9365b1e8994ff0bda48" +            "b1f7498cce02d4e794915f8a4208de3eaf9fbff5"

Which is hexadecimal notation of bytes, hardcoded in as string format. I need this thing as a bytearray, importantly, not the ASCII representation of it, actually the hexadecimal values that is represents.

All the kotlin methods I can find, such as:

val b = s.encodeToByteArray()

Seem to be taking the actual ASCII value of the string, and converting it to a bytearray.

How do I create a bytearray directly from the values in this string?

kotlin
2 Answers

You can handle it like this:

fun String.decodeHex(): ByteArray {
    check(length % 2 == 0) { "Must have an even length" }

    return chunked(2)
        .map { it.toInt(16).toByte() }
        .toByteArray()
}
  1. Split the string into 2-character pairs, representing each byte.
  2. Parse each hex pair to their integer values.
  3. Convert the parsed Ints to Bytes.
user_12256170

My other answer is the simplest way, but it creates two intermediate lists - a list of strings and a list of bytes - before it creates the byte array. Here are two slightly more complex versions that are more efficient.

This version uses sequences to take advantage of lazy evaluation. It still produces a string for every byte, but uses no intermediate lists.

fun String.decodeHex(): ByteArray {
    check(length % 2 == 0) { "Must have an even length" }

    val byteIterator = chunkedSequence(2)
        .map { it.toInt(16).toByte() }
        .iterator()

    return ByteArray(length / 2) { byteIterator.next() }
}

This version uses the JDK's java.lang.Integer.parseInt function. It creates the ByteArray directly with no intermediate data-structures.

fun String.decodeHex(): ByteArray {
    check(length % 2 == 0) { "Must have an even length" }

    return ByteArray(length / 2) {
        Integer.parseInt(this, it * 2, (it + 1) * 2, 16).toByte()
    }
}
user_12256170
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