package de.data_it_solution.netdiag

import java.io.ByteArrayOutputStream
import java.net.DatagramPacket
import java.net.DatagramSocket
import java.net.InetAddress

/**
 * Minimaler SNMP-v2c-GET-Client.
 *
 * Reicht für das Auslesen einzelner OIDs (Link-Speed, Fehlerzähler) von
 * gemanagten Switches. Implementiert nur so viel BER-Kodierung wie nötig.
 */
object Snmp {

    /**
     * Eine OID per SNMP v2c GET abfragen.
     *
     * @return Wert als String oder null bei Fehler/Timeout
     */
    fun get(host: String, community: String, oid: String): String? {
        return try {
            val request = buildGetRequest(community, oid)
            val socket = DatagramSocket()
            socket.soTimeout = 2500
            socket.use {
                it.send(DatagramPacket(request, request.size, InetAddress.getByName(host), 161))
                val buf = ByteArray(2048)
                val resp = DatagramPacket(buf, buf.size)
                it.receive(resp)
                parseFirstValue(buf, resp.length)
            }
        } catch (e: Exception) {
            null
        }
    }

    /* ---- BER-Kodierung ---- */

    private fun tlv(tag: Int, value: ByteArray): ByteArray {
        val out = ByteArrayOutputStream()
        out.write(tag)
        when {
            value.size < 0x80 -> out.write(value.size)
            value.size < 0x100 -> { out.write(0x81); out.write(value.size) }
            else -> { out.write(0x82); out.write(value.size shr 8); out.write(value.size and 0xFF) }
        }
        out.write(value)
        return out.toByteArray()
    }

    private fun integer(v: Int): ByteArray {
        val bytes = when {
            v == 0 -> byteArrayOf(0)
            v < 0x80 -> byteArrayOf(v.toByte())
            v < 0x8000 -> byteArrayOf((v shr 8).toByte(), v.toByte())
            else -> byteArrayOf((v shr 24).toByte(), (v shr 16).toByte(), (v shr 8).toByte(), v.toByte())
        }
        return tlv(0x02, bytes)
    }

    private fun octetString(s: String): ByteArray = tlv(0x04, s.toByteArray())

    private fun oid(o: String): ByteArray {
        val parts = o.trim().trimStart('.').split('.').map { it.toInt() }
        val out = ByteArrayOutputStream()
        out.write(parts[0] * 40 + parts[1]) // erste zwei Subidentifier zusammengefasst
        for (i in 2 until parts.size) {
            var v = parts[i]
            if (v < 0x80) {
                out.write(v)
            } else {
                val stack = ArrayDeque<Int>()
                stack.addFirst(v and 0x7F)
                v = v shr 7
                while (v > 0) { stack.addFirst((v and 0x7F) or 0x80); v = v shr 7 }
                stack.forEach { out.write(it) }
            }
        }
        return tlv(0x06, out.toByteArray())
    }

    private fun buildGetRequest(community: String, oidStr: String): ByteArray {
        val varbind = tlv(0x30, oid(oidStr) + tlv(0x05, ByteArray(0))) // OID + NULL
        val varbindList = tlv(0x30, varbind)
        val requestId = (System.currentTimeMillis() and 0x7FFF).toInt()
        val pdu = tlv(
            0xA0, // GetRequest-PDU
            integer(requestId) + integer(0) + integer(0) + varbindList,
        )
        val message = tlv(
            0x30,
            integer(1) + octetString(community) + pdu, // version 1 = SNMPv2c
        )
        return message
    }

    /* ---- Antwort parsen: ersten Variablen-Wert herausziehen ---- */

    private fun parseFirstValue(buf: ByteArray, len: Int): String? {
        var i = 0
        // Durch die Struktur navigieren bis zum ersten primitiven Wert nach einer OID
        var lastWasOid = false
        while (i < len) {
            val tag = buf[i].toInt() and 0xFF
            i++
            if (i >= len) break
            var l = buf[i].toInt() and 0xFF
            i++
            if (l and 0x80 != 0) {
                val n = l and 0x7F
                l = 0
                for (k in 0 until n) { l = (l shl 8) or (buf[i].toInt() and 0xFF); i++ }
            }
            // Konstruierte Typen (SEQUENCE, PDU) aufsteigen
            if (tag == 0x30 || tag == 0xA2 || tag == 0xA0) continue
            if (tag == 0x06) { lastWasOid = true; i += l; continue }
            if (lastWasOid) {
                return when (tag) {
                    0x02, 0x41, 0x42, 0x43, 0x44, 0x46 -> { // INTEGER, Counter, Gauge, TimeTicks ...
                        var v = 0L
                        for (k in 0 until l) v = (v shl 8) or (buf[i + k].toLong() and 0xFF)
                        v.toString()
                    }
                    0x04 -> String(buf, i, l) // OCTET STRING
                    0x05 -> "" // NULL -> kein Wert
                    else -> {
                        val sb = StringBuilder()
                        for (k in 0 until l) sb.append("%02X".format(buf[i + k]))
                        sb.toString()
                    }
                }
            }
            i += l
        }
        return null
    }
}