2021 Day 16

class Packet private constructor(binary: String) {
    private val version = binary.substring(0..2).toInt(2)
    private val type = binary.substring(3..5).toInt(2)
    private var subPackets = emptyList<Packet>()
    private var value = 0L
    private val length: Int

    init {
        length = when {
            type == 4 -> {
                var pointer = 1
                var binaryValue = ""
                do {
                    pointer += 5
                    binaryValue += binary.substring(pointer + 1..pointer + 4)
                } while (binary[pointer] == '1')
                value = binaryValue.toLong(2)
                pointer + 5
            }
            binary[6] == '1' -> {
                var pointer = 18
                val numberOfPackages = binary.substring(7 until pointer).toInt(2)
                repeat(numberOfPackages) { pointer += extractSubPacket(binary.substring(pointer)) }
                pointer
            }
            else -> {
                var pointer = 22
                val lengthOfPackges = binary.substring(7 until pointer).toInt(2)
                while (pointer - 21 < lengthOfPackges) {
                    pointer += extractSubPacket(binary.substring(pointer))
                }
                pointer
            }
        }
    }

    private fun extractSubPacket(binary: String): Int {
        val subPacket = Packet(binary)
        subPackets = subPackets + subPacket
        return subPacket.length
    }

    fun versionSum(): Int = version + subPackets.sumBy { it.versionSum() }

    fun calculate(): Long {
        val values = subPackets.map { it.calculate() }
        return when (type) {
            0 -> values.sum()
            1 -> values.fold(1L) { product, it -> product * it }
            2 -> values.min() ?: 0L
            3 -> values.max() ?: 0L
            4 -> value
            5 -> if (values[0] > values[1]) 1L else 0L
            6 -> if (values[0] < values[1]) 1L else 0L
            7 -> if (values[0] == values[1]) 1L else 0L
            else -> 0L
        }
    }

    companion object HexPacket {
        operator fun invoke(hexadecimal: String) = Packet(hexadecimal.toBinary())
    }
}

private fun String.toBinary() = this.toCharArray().joinToString("") { hex ->
    when (hex) {
        '0' -> "0000"
        '1' -> "0001"
        '2' -> "0010"
        '3' -> "0011"
        '4' -> "0100"
        '5' -> "0101"
        '6' -> "0110"
        '7' -> "0111"
        '8' -> "1000"
        '9' -> "1001"
        'A' -> "1010"
        'B' -> "1011"
        'C' -> "1100"
        'D' -> "1101"
        'E' -> "1110"
        'F' -> "1111"
        else -> ""
    }
}