import java.math.BigInteger
import kotlin.math.pow

/**
 * An outgoing 9P message with the given type, tag, and fields. The message size is calculated automatically.
 *
 * Important note: the field names in [fieldValuesInt], [fieldValuesStr], and [fieldValuesRaw] (i.e. the keys of their
 * maps) must be mutually exclusive and the union of these two maps' keys must result in a subset of (or a set equal to)
 * [fieldNames]. Calling [write] when these conditions are not met throws an exception.
 *
 * @param type The 9P message type.
 * @param tag The tag given to the message.
 * @param fieldNames The names of the message fields, in the same order they are expected to be sent.
 * @param fieldValuesInt A map of each field name into its value. This map only stores integer values.
 * @param fieldValuesStr A map of each field name into its value. This map only stores string values.
 */
class OutMessage(val type: NinePMessageType, val tag: UShort, val fieldNames: List<String>, val fieldValuesInt: Map<String, BigInteger>, val fieldValuesStr: Map<String, String>, val fieldValuesRaw: Map<String, Array<UByte>>, val maxSize: UInt) {
    /**
     * Intersection between [fieldNames] and [fieldValuesInt]. In other words: the integer fields that are going to be
     * used when writing the message.
     */
    private val insecInts = fieldNames.intersect(fieldValuesInt.keys)

    /**
     * Intersection between [fieldNames] and [fieldValuesStr]. In other words: the string fields that are going to be
     * used when writing the message.
     */
    private val insecStrs = fieldNames.intersect(fieldValuesStr.keys)

    /**
     * Intersection between [fieldNames] and [fieldValuesRaw]. In other words: the raw fields that are going to be used
     * when writing the message.
     */
    private val insecRaws = fieldNames.intersect(fieldValuesRaw.keys)

    /**
     * Send the message using the given networking API.
     *
     * @param tl The networking API.
     * @throws IllegalArgumentException if [fieldNames], [fieldValuesInt], and [fieldValuesStr] are incoherent or the
     *         final size of the message exceeds the negotiated value.
     */
    fun write(tl: TransportLayer) {
        // check that names in fieldNames exist as keys in either fieldValuesInt or fieldValuesStr but not both
        require(fieldNames.size == insecInts.size + insecStrs.size + insecRaws.size)

        val totalSize = size()
        if (totalSize > this.maxSize) {
            throw IllegalArgumentException("Message size exceeded.")
        }
        writeMessageSizeType(tl, totalSize, type)
        writeInteger(tl, tag.toInt().toBigInteger())
        for (field in fieldNames) {
            if (field in insecInts) {
                writeInteger(tl, fieldValuesInt[field]!!)
            } else if (field in insecStrs) {
                writeString(tl, fieldValuesStr[field]!!)
            } else {
                tl.transmit(fieldValuesRaw[field]!!)
            }
        }
    }

    /**
     * Write the message size and type.
     *
     * @param tl The networking API.
     * @param size The total message size, including the 4 bytes of this parameter and the type's byte.
     * @param type The 9P message type as a [NinePMessageType] constant.
     */
    private fun writeMessageSizeType(tl: TransportLayer, size: UInt, type: NinePMessageType) {
        writeInteger(tl, size.toInt().toBigInteger())
        writeInteger(tl, type.value.toInt().toBigInteger())
    }

    /**
     * Calculate the expected size of the message.
     */
    fun size(): UInt {
        return 4u + 1u + 2u + this.insecInts.sumOf { this.fieldValuesInt[it]!!.bitLength().toUInt() } + this.insecStrs.sumOf { 2u + this.fieldValuesStr[it]!!.length.toUInt() } + this.insecRaws.sumOf { this.fieldValuesRaw[it]!!.size.toUInt() }
    }

    companion object {
        // TODO: Add size that the value is required to fit in
        /**
         * Write an integer number to the connection.
         *
         * In 9P, binary numbers (non-textual) are specified in little-endian order (least significant byte first).
         *
         * @param tl The networking API.
         * @param value The number's value.
         */
        fun writeInteger(tl: TransportLayer, value: BigInteger) {
            val bytes = value.toByteArray()
            tl.transmit(Array(bytes.size) { i -> bytes[i].toUByte() })
        }

        /**
         * Write a string to 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.
         *
         * @param tl The networking API.
         * @param value The string.
         * @throws IllegalArgumentException if the value of the string's size does not fit into 2 bytes.
         */
        fun writeString(tl: TransportLayer, value: String) {
            require(value.length <= 2.0.pow(16.0) - 1)
            writeInteger(tl, value.length.toBigInteger())
            val bytes = value.toByteArray()
            tl.transmit(Array(bytes.size) { i -> bytes[i].toUByte() })
        }
    }
}