package com.infendro.mac

import com.infendro.bytes.bytearray.xorInto
import com.infendro.hash.HashFunction

class HMAC(
    val function: HashFunction,
) : MAC {
    override val bytes: Int
        get() = function.bytes

    override fun hashInto(key: ByteArray, value: ByteArray, destination: ByteArray, destinationOffset: Int) {
        val paddedKey = pad(key)
        val inner = ByteArray(function.block + value.size).also {
            paddedKey.xorInto(0x36, it)
            value.copyInto(it, function.block)
        }
        val outer = ByteArray(function.block + function.bytes).also {
            paddedKey.xorInto(0x5C, it)
            function.hashInto(inner, it, function.block)
        }
        function.hashInto(outer, destination, destinationOffset)
    }

    private fun pad(key: ByteArray): ByteArray =
        ByteArray(function.block).also {
            when {
                key.size > function.block -> function.hashInto(key, it)
                else -> key.copyInto(it)
            }
        }
}