import java.io.IOException
import java.math.BigInteger

/**
 * This class represents a 9P connection. It provides a practical implementation with networking to the 9P methods
 * described in [NinePTranslator]. Details about methods related to 9P can be found in [NinePTranslator]. Remember to
 * disconnect using [disconnect] after use.
 *
 * Details about network-related topics can be found in [NetworkPacketTransporter] and the implementation of choice.
 *
 * Details about 9P messages and methods can be found in [NinePTranslator].
 *
 * @param netPackTrans The networking API backend of choice.
 *
 * @throws UnresolvableHostException if the host resolution made by [netPackTrans] failed.
 */
class NinePConnection(netPackTrans: NetworkPacketTransporter) : NinePTranslator {
    /**
     * Networking API.
     */
    val npt: NetworkPacketTransporter = netPackTrans

    /**
     * Disconnect from the remote host,
     *
     * @throws IOException if an I/O error occurred while closing the socket.
     */
    fun disconnect() {
        this.npt.close()
    }

    /**
     * Read an [nBytes]-long unsigned integer number from the connection.
     *
     * In 9P, binary numbers (non-textual) are specified in little-endian order (least significant byte first).
     *
     * @param nBytes The length of the integer number in bytes. It only works with unsigned numbers strictly greater
     *               than zero (zero is excluded).
     * @return The number's value.
     * @throws IllegalArgumentException if [nBytes] is zero or negative.
     */
    private fun readInteger(nBytes: Int): BigInteger {
        require(nBytes > 0)
        val bytes = this.npt.receiveFixed(nBytes.toULong())
        var number: BigInteger = BigInteger.valueOf(0)
        for (i in 0..<bytes.size) {
            number += bytes[i].toInt().toBigInteger().shl(i)
        }

        return number
    }

    /**
     * Read a string from the connection.
     *
     * In 9P, strings are represented as a 2-byte integer (the string's size) followed by the actual UTF-8 string. The
     * null terminator is forbidden in 9P messages.
     *
     * @return The string.
     */
    private fun readString(): String {
        val length = readInteger(2).toInt()
        return String(this.npt.receiveFixed(length.toULong()).toByteArray())
    }

    /**
     * Read a message size, type, and tag.
     *
     * @return A triple in which the first element is the message size in bytes, the second is the message type as a
     * [NinePMessageType] constant, and the third element is the message tag.
     */
    private fun readSizeTypeTag(): Triple<UInt, NinePMessageType, UInt> {
        return Triple(
            readInteger(4).toInt().toUInt(),
            NinePMessageType.fromByte(readInteger(1).toByte()),
            readInteger(2).toInt().toUInt()
        )
    }

    // TODO: implement methods from NinePTranslator
    /**
     * Wait for a 9P message with the same tag from the server.
     *
     * All messages prior to the returned one are discarded.
     *
     * @param tag The tag to wait for.
     * @return A pair of (1) the size and (2) the type of the message that matches the given tag.
     */
    private fun waitForTag(tag: UInt): Pair<UInt, NinePMessageType> {
        var s = 0u
        var ty: NinePMessageType? = null
        var ta = 0u
        var found = false
        while (!found) {
            val stt = readSizeTypeTag()
            s = stt.first
            ty = stt.second
            ta = stt.third

            if (ta == tag) {
                found = true
            }
        }
        return Pair(s, ty ?: NinePMessageType.RERROR)
    }

    /**
     * Read a 9P message of type Rerror, after the message size and type have already been read.
     *
     * @return A pair of: (1) the message tag and (2) the error message
     */
    private fun readError(): Pair<SizedMessageField, String> {
        val tag = readInteger(2)
        val error = readString()
        return Pair(SizedMessageField(2, tag), error)
    }
}