/*
Technitium DNS Server
Copyright (C) 2024  Shreyas Zare (shreyas@technitium.com)

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

*/

using DnsServerCore.Dns.Zones;
using System;
using System.Collections.Generic;
using System.Threading;
using TechnitiumLibrary.Net.Dns.ResourceRecords;

namespace DnsServerCore.Dns.Trees
{
    class AuthZoneTree : ZoneTree<AuthZoneNode, SubDomainZone, ApexZone>
    {
        #region variables

        static readonly char[] _starPeriodTrimChars = new char[] { '*', '.' };

        #endregion

        #region private

        private static Node GetPreviousSubDomainZoneNode(byte[] key, Node currentNode, int baseDepth)
        {
            int k;

            NodeValue currentValue = currentNode.Value;
            if (currentValue is null)
            {
                //key value does not exists
                if (currentNode.Children is null)
                {
                    //no children available; move to previous sibling
                    k = currentNode.K - 1; //find previous node from sibling starting at k - 1
                    currentNode = currentNode.Parent;
                }
                else
                {
                    if (key.Length == currentNode.Depth)
                    {
                        //current node belongs to the key
                        k = currentNode.K - 1; //find previous node from sibling starting at k - 1
                        currentNode = currentNode.Parent;
                    }
                    else
                    {
                        //find the previous node for the given k in current node's children
                        k = key[currentNode.Depth];
                    }
                }
            }
            else
            {
                int x = DnsNSECRecordData.CanonicalComparison(currentValue.Key, key);
                if (x == 0)
                {
                    //current node value matches the key
                    k = currentNode.K - 1; //find previous node from sibling starting at k - 1
                    currentNode = currentNode.Parent;
                }
                else if (x > 0)
                {
                    //current node value is larger for the key
                    k = currentNode.K - 1; //find previous node from sibling starting at k - 1
                    currentNode = currentNode.Parent;
                }
                else
                {
                    //current node value is smaller for the key
                    if (currentNode.Children is null)
                    {
                        //the current node is previous node since no children exists and value is smaller for the key
                        return currentNode;
                    }
                    else
                    {
                        //find the previous node for the given k in current node's children
                        k = key[currentNode.Depth];
                    }
                }
            }

            //start reverse tree traversal
            while ((currentNode is not null) && (currentNode.Depth >= baseDepth))
            {
                Node[] children = currentNode.Children;
                if (children is not null)
                {
                    //find previous child node
                    Node child = null;

                    for (int i = k; i > -1; i--)
                    {
                        child = Volatile.Read(ref children[i]);
                        if (child is not null)
                        {
                            bool childNodeHasApexZone = false;

                            NodeValue childValue = child.Value;
                            if (childValue is not null)
                            {
                                AuthZoneNode authZoneNode = childValue.Value;
                                if (authZoneNode is not null)
                                {
                                    if (authZoneNode.ApexZone is not null)
                                        childNodeHasApexZone = true; //must stop checking children of the apex of the sub zone
                                }
                            }

                            if (!childNodeHasApexZone && child.Children is not null)
                                break; //child has further children so check them first

                            if (childValue is not null)
                            {
                                AuthZoneNode authZoneNode = childValue.Value;
                                if (authZoneNode is not null)
                                {
                                    if (authZoneNode.ParentSideZone is not null)
                                    {
                                        //is sub domain zone
                                        return child; //child has value so return it
                                    }

                                    if (authZoneNode.ApexZone is not null)
                                    {
                                        //is apex zone
                                        //skip to next child to avoid listing this auth zone's sub domains
                                        child = null; //set null to avoid child being set as current after the loop
                                    }
                                }
                            }
                        }
                    }

                    if (child is not null)
                    {
                        //make found child as current
                        k = children.Length - 1;
                        currentNode = child;
                        continue; //start over
                    }
                }

                //no child node available; check for current node value
                {
                    NodeValue value = currentNode.Value;
                    if (value is not null)
                    {
                        AuthZoneNode authZoneNode = value.Value;
                        if (authZoneNode is not null)
                        {
                            if ((authZoneNode.ApexZone is not null) && (currentNode.Depth == baseDepth))
                            {
                                //current node contains apex zone for the base depth i.e. current zone; return it
                                return currentNode;
                            }

                            if (authZoneNode.ParentSideZone is not null)
                            {
                                //current node contains sub domain zone; return it
                                return currentNode;
                            }
                        }
                    }
                }

                //move up to parent node for previous sibling
                k = currentNode.K - 1;
                currentNode = currentNode.Parent;
            }

            return null;
        }

        private static Node GetNextSubDomainZoneNode(byte[] key, Node currentNode, int baseDepth)
        {
            int k;

            NodeValue currentValue = currentNode.Value;
            if (currentValue is null)
            {
                //key value does not exists
                if (currentNode.Children is null)
                {
                    //no children available; move to next sibling
                    k = currentNode.K + 1; //find next node from sibling starting at k + 1
                    currentNode = currentNode.Parent;
                }
                else
                {
                    if (key.Length == currentNode.Depth)
                    {
                        //current node belongs to the key
                        k = 0; //find next node from first child of current node
                    }
                    else
                    {
                        //find next node for the given k in current node's children
                        k = key[currentNode.Depth];
                    }
                }
            }
            else
            {
                //check if node contains apex zone
                bool foundApexZone = false;

                if (currentNode.Depth > baseDepth)
                {
                    AuthZoneNode authZoneNode = currentValue.Value;
                    if (authZoneNode is not null)
                    {
                        ApexZone apexZone = authZoneNode.ApexZone;
                        if (apexZone is not null)
                            foundApexZone = true;
                    }
                }

                if (foundApexZone)
                {
                    //current contains apex for a sub zone; move up to parent node
                    k = currentNode.K + 1; //find next node from sibling starting at k + 1
                    currentNode = currentNode.Parent;
                }
                else
                {
                    int x = DnsNSECRecordData.CanonicalComparison(currentValue.Key, key);
                    if (x == 0)
                    {
                        //current node value matches the key
                        k = 0; //find next node from children starting at k
                    }
                    else if (x > 0)
                    {
                        //current node value is larger for the key thus current is the next node
                        return currentNode;
                    }
                    else
                    {
                        //current node value is smaller for the key
                        k = key[currentNode.Depth]; //find next node from children starting at k = key[depth]
                    }
                }
            }

            //start tree traversal
            while ((currentNode is not null) && (currentNode.Depth >= baseDepth))
            {
                Node[] children = currentNode.Children;
                if (children is not null)
                {
                    //find next child node
                    Node child = null;

                    for (int i = k; i < children.Length; i++)
                    {
                        child = Volatile.Read(ref children[i]);
                        if (child is not null)
                        {
                            NodeValue childValue = child.Value;
                            if (childValue is not null)
                            {
                                AuthZoneNode authZoneNode = childValue.Value;
                                if (authZoneNode is not null)
                                {
                                    if (authZoneNode.ParentSideZone is not null)
                                    {
                                        //is sub domain zone
                                        return child; //child has value so return it
                                    }

                                    if (authZoneNode.ApexZone is not null)
                                    {
                                        //is apex zone
                                        //skip to next child to avoid listing this auth zone's sub domains
                                        child = null; //set null to avoid child being set as current after the loop
                                        continue;
                                    }
                                }
                            }

                            if (child.Children is not null)
                                break;
                        }
                    }

                    if (child is not null)
                    {
                        //make found child as current
                        k = 0;
                        currentNode = child;
                        continue; //start over
                    }
                }

                //no child nodes available; move up to parent node
                k = currentNode.K + 1;
                currentNode = currentNode.Parent;
            }

            return null;
        }

        private static bool SubDomainExists(byte[] key, Node currentNode)
        {
            Node[] children = currentNode.Children;
            if (children is not null)
            {
                Node child = Volatile.Read(ref children[1]); //[*]
                if (child is not null)
                    return true; //wildcard exists so subdomain name exists: RFC 4592 section 4.9
            }

            Node nextSubDomain = GetNextSubDomainZoneNode(key, currentNode, currentNode.Depth);
            if (nextSubDomain is null)
                return false;

            NodeValue value = nextSubDomain.Value;
            if (value is null)
                return false;

            return IsKeySubDomain(key, value.Key, false);
        }

        private static AuthZone GetAuthZoneFromNode(Node node, string zoneName)
        {
            NodeValue value = node.Value;
            if (value is not null)
            {
                AuthZoneNode authZoneNode = value.Value;
                if (authZoneNode is not null)
                    return authZoneNode.GetAuthZone(zoneName);
            }

            return null;
        }

        private void RemoveAllSubDomains(string domain, Node currentNode)
        {
            //remove all sub domains under current zone
            Node current = currentNode;
            byte[] currentKey = ConvertToByteKey(domain);

            do
            {
                current = GetNextSubDomainZoneNode(currentKey, current, currentNode.Depth);
                if (current is null)
                    break;

                NodeValue v = current.Value;
                if (v is not null)
                {
                    AuthZoneNode z = v.Value;
                    if (z is not null)
                    {
                        if (z.ApexZone is null)
                        {
                            //no apex zone at this node; remove complete zone node
                            current.RemoveNodeValue(v.Key, out _); //remove node value
                            current.CleanThisBranch();
                        }
                        else
                        {
                            //apex node exists; remove parent size sub domain
                            z.TryRemove(out SubDomainZone _);
                        }
                    }

                    currentKey = v.Key;
                }
            }
            while (true);
        }

        #endregion

        #region protected

        protected override void GetClosestValuesForZone(AuthZoneNode zoneValue, out SubDomainZone closestSubDomain, out SubDomainZone closestDelegation, out ApexZone closestAuthority)
        {
            ApexZone apexZone = zoneValue.ApexZone;
            if (apexZone is not null)
            {
                //hosted primary/secondary/stub/forwarder zone found
                closestSubDomain = null;
                closestDelegation = zoneValue.ParentSideZone;
                closestAuthority = apexZone;
            }
            else
            {
                //hosted sub domain
                SubDomainZone subDomainZone = zoneValue.ParentSideZone;

                if (subDomainZone.ContainsNameServerRecords())
                {
                    //delegated sub domain found
                    closestSubDomain = null;
                    closestDelegation = subDomainZone;
                }
                else
                {
                    closestSubDomain = subDomainZone;
                    closestDelegation = null;
                }

                closestAuthority = null;
            }
        }

        #endregion

        #region public

        public bool TryAdd(ApexZone zone)
        {
            AuthZoneNode zoneNode = GetOrAdd(zone.Name, delegate (string key)
            {
                return new AuthZoneNode(null, zone);
            });

            if (ReferenceEquals(zoneNode.ApexZone, zone))
                return true; //added successfully

            return zoneNode.TryAdd(zone);
        }

        public bool TryGet(string zoneName, string domain, out AuthZone authZone)
        {
            if (TryGet(domain, out AuthZoneNode authZoneNode))
            {
                authZone = authZoneNode.GetAuthZone(zoneName);
                return authZone is not null;
            }

            authZone = null;
            return false;
        }

        public bool TryGet(string zoneName, out ApexZone apexZone)
        {
            if (TryGet(zoneName, out AuthZoneNode authZoneNode) && (authZoneNode.ApexZone is not null))
            {
                apexZone = authZoneNode.ApexZone;
                return true;
            }

            apexZone = null;
            return false;
        }

        public bool TryRemove(string domain, out ApexZone apexZone)
        {
            if (!TryGet(domain, out AuthZoneNode authZoneNode, out Node currentNode) || (authZoneNode.ApexZone is null))
            {
                apexZone = null;
                return false;
            }

            apexZone = authZoneNode.ApexZone;

            if (authZoneNode.ParentSideZone is null)
            {
                //remove complete zone node
                if (!base.TryRemove(domain, out AuthZoneNode _))
                {
                    apexZone = null;
                    return false;
                }
            }
            else
            {
                //parent side sub domain exists; remove only apex zone from zone node
                if (!authZoneNode.TryRemove(out ApexZone _))
                {
                    apexZone = null;
                    return false;
                }
            }

            //remove all sub domains under current apex zone
            RemoveAllSubDomains(domain, currentNode);

            currentNode.CleanThisBranch();
            return true;
        }

        public bool TryRemove(string domain, out SubDomainZone subDomainZone, bool removeAllSubDomains = false)
        {
            if (!TryGet(domain, out AuthZoneNode zoneNode, out Node currentNode) || (zoneNode.ParentSideZone is null))
            {
                subDomainZone = null;
                return false;
            }

            subDomainZone = zoneNode.ParentSideZone;

            if (zoneNode.ApexZone is null)
            {
                //remove complete zone node
                if (!base.TryRemove(domain, out AuthZoneNode _))
                {
                    subDomainZone = null;
                    return false;
                }
            }
            else
            {
                //apex zone exists; remove only parent side sub domain from zone node
                if (!zoneNode.TryRemove(out SubDomainZone _))
                {
                    subDomainZone = null;
                    return false;
                }
            }

            if (removeAllSubDomains)
                RemoveAllSubDomains(domain, currentNode); //remove all sub domains under current subdomain zone

            currentNode.CleanThisBranch();
            return true;
        }

        public override bool TryRemove(string key, out AuthZoneNode authZoneNode)
        {
            throw new InvalidOperationException();
        }

        public IReadOnlyList<AuthZone> GetApexZoneWithSubDomainZones(string zoneName)
        {
            List<AuthZone> zones = new List<AuthZone>();

            byte[] key = ConvertToByteKey(zoneName);

            NodeValue nodeValue = _root.FindNodeValue(key, out Node currentNode);
            if (nodeValue is not null)
            {
                AuthZoneNode authZoneNode = nodeValue.Value;
                if (authZoneNode is not null)
                {
                    ApexZone apexZone = authZoneNode.ApexZone;
                    if (apexZone is not null)
                    {
                        zones.Add(apexZone);

                        Node current = currentNode;
                        byte[] currentKey = key;

                        do
                        {
                            current = GetNextSubDomainZoneNode(currentKey, current, currentNode.Depth);
                            if (current is null)
                                break;

                            NodeValue value = current.Value;
                            if (value is not null)
                            {
                                authZoneNode = value.Value;
                                if (authZoneNode is not null)
                                    zones.Add(authZoneNode.ParentSideZone);

                                currentKey = value.Key;
                            }
                        }
                        while (true);
                    }
                }
            }

            return zones;
        }

        public IReadOnlyList<AuthZone> GetSubDomainZoneWithSubDomainZones(string domain)
        {
            List<AuthZone> zones = new List<AuthZone>();

            byte[] key = ConvertToByteKey(domain);

            NodeValue nodeValue = _root.FindNodeValue(key, out Node currentNode);
            if (nodeValue is not null)
            {
                AuthZoneNode authZoneNode = nodeValue.Value;
                if (authZoneNode is not null)
                {
                    SubDomainZone subDomainZone = authZoneNode.ParentSideZone;
                    if (subDomainZone is not null)
                    {
                        zones.Add(subDomainZone);

                        Node current = currentNode;
                        byte[] currentKey = key;

                        do
                        {
                            current = GetNextSubDomainZoneNode(currentKey, current, currentNode.Depth);
                            if (current is null)
                                break;

                            NodeValue value = current.Value;
                            if (value is not null)
                            {
                                authZoneNode = value.Value;
                                if (authZoneNode is not null)
                                    zones.Add(authZoneNode.ParentSideZone);

                                currentKey = value.Key;
                            }
                        }
                        while (true);
                    }
                }
            }

            return zones;
        }

        public AuthZone GetOrAddSubDomainZone(string zoneName, string domain, Func<SubDomainZone> valueFactory)
        {
            bool isApex = zoneName.Equals(domain, StringComparison.OrdinalIgnoreCase);

            AuthZoneNode authZoneNode = GetOrAdd(domain, delegate (string key)
            {
                if (isApex)
                    throw new DnsServerException("Zone was not found for domain: " + key);

                return new AuthZoneNode(valueFactory(), null);
            });

            if (isApex)
            {
                if (authZoneNode.ApexZone is null)
                    throw new DnsServerException("Zone was not found: " + zoneName);

                return authZoneNode.ApexZone;
            }
            else
            {
                return authZoneNode.GetOrAddParentSideZone(valueFactory);
            }
        }

        public AuthZone GetAuthZone(string zoneName, string domain)
        {
            if (TryGet(domain, out AuthZoneNode authZoneNode))
                return authZoneNode.GetAuthZone(zoneName);

            return null;
        }

        public ApexZone GetApexZone(string zoneName)
        {
            if (TryGet(zoneName, out AuthZoneNode authZoneNode))
                return authZoneNode.ApexZone;

            return null;
        }

        public AuthZone FindZone(string domain, out SubDomainZone closest, out SubDomainZone delegation, out ApexZone authority, out bool hasSubDomains)
        {
            byte[] key = ConvertToByteKey(domain);

            AuthZoneNode authZoneNode = FindZoneNode(key, true, out Node currentNode, out Node closestSubDomainNode, out _, out SubDomainZone closestSubDomain, out SubDomainZone closestDelegation, out ApexZone closestAuthority);
            if (authZoneNode is null)
            {
                //zone not found
                closest = closestSubDomain;
                delegation = closestDelegation;
                authority = closestAuthority;

                if (authority is null)
                {
                    //no authority so no sub domains
                    hasSubDomains = false;
                }
                else if ((closestSubDomainNode is not null) && !closestSubDomainNode.HasChildren)
                {
                    //closest sub domain node does not have any children so no sub domains
                    hasSubDomains = false;
                }
                else
                {
                    //check if current node has sub domains
                    hasSubDomains = SubDomainExists(key, currentNode);
                }

                return null;
            }
            else
            {
                //zone found
                AuthZone zone;

                ApexZone apexZone = authZoneNode.ApexZone;
                if (apexZone is not null)
                {
                    zone = apexZone;
                    closest = null;
                    delegation = authZoneNode.ParentSideZone;
                    authority = apexZone;
                }
                else
                {
                    SubDomainZone subDomainZone = authZoneNode.ParentSideZone;

                    zone = subDomainZone;

                    if (zone == closestSubDomain)
                        closest = null;
                    else
                        closest = closestSubDomain;

                    if (closestDelegation is not null)
                        delegation = closestDelegation;
                    else if (subDomainZone.ContainsNameServerRecords())
                        delegation = subDomainZone;
                    else
                        delegation = null;

                    authority = closestAuthority;
                }

                if (zone.Disabled)
                {
                    if ((closestSubDomainNode is not null) && !closestSubDomainNode.HasChildren)
                    {
                        //closest sub domain node does not have any children so no sub domains
                        hasSubDomains = false;
                    }
                    else
                    {
                        //check if current node has sub domains
                        hasSubDomains = SubDomainExists(key, currentNode);
                    }
                }
                else
                {
                    //since zone is found, it does not matter if subdomain exists or not
                    hasSubDomains = false;
                }

                return zone;
            }
        }

        public AuthZone FindPreviousSubDomainZone(string zoneName, string domain)
        {
            byte[] key = ConvertToByteKey(domain);

            AuthZoneNode authZoneNode = FindZoneNode(key, false, out Node currentNode, out _, out Node closestAuthorityNode, out _, out _, out _);
            if (authZoneNode is not null)
            {
                //zone exists
                ApexZone apexZone = authZoneNode.ApexZone;
                SubDomainZone parentSideZone = authZoneNode.ParentSideZone;

                if ((apexZone is not null) && (parentSideZone is not null))
                {
                    //found ambiguity between apex zone and sub domain zone
                    if (!apexZone.Name.Equals(zoneName, StringComparison.OrdinalIgnoreCase))
                    {
                        //zone name does not match with apex zone and thus not match with closest authority node
                        //find the closest authority zone for given zone name
                        if (!TryGet(zoneName, out _, out Node closestNodeForZoneName))
                            throw new InvalidOperationException();

                        closestAuthorityNode = closestNodeForZoneName;
                    }
                }
            }

            Node previousNode = GetPreviousSubDomainZoneNode(key, currentNode, closestAuthorityNode.Depth);
            if (previousNode is not null)
            {
                AuthZone authZone = GetAuthZoneFromNode(previousNode, zoneName);
                if (authZone is not null)
                    return authZone;
            }

            return null;
        }

        public AuthZone FindNextSubDomainZone(string zoneName, string domain)
        {
            byte[] key = ConvertToByteKey(domain);

            AuthZoneNode authZoneNode = FindZoneNode(key, false, out Node currentNode, out _, out Node closestAuthorityNode, out _, out _, out _);
            if (authZoneNode is not null)
            {
                //zone exists
                ApexZone apexZone = authZoneNode.ApexZone;
                SubDomainZone parentSideZone = authZoneNode.ParentSideZone;

                if ((apexZone is not null) && (parentSideZone is not null))
                {
                    //found ambiguity between apex zone and sub domain zone
                    if (!apexZone.Name.Equals(zoneName, StringComparison.OrdinalIgnoreCase))
                    {
                        //zone name does not match with apex zone and thus not match with closest authority node
                        //find the closest authority zone for given zone name
                        if (!TryGet(zoneName, out _, out Node closestNodeForZoneName))
                            throw new InvalidOperationException();

                        closestAuthorityNode = closestNodeForZoneName;
                    }
                }
            }

            Node nextNode = GetNextSubDomainZoneNode(key, currentNode, closestAuthorityNode.Depth);
            if (nextNode is not null)
            {
                AuthZone authZone = GetAuthZoneFromNode(nextNode, zoneName);
                if (authZone is not null)
                    return authZone;
            }

            return null;
        }

        public bool SubDomainExistsFor(string zoneName, string domain)
        {
            AuthZone nextAuthZone = FindNextSubDomainZone(zoneName, domain);
            if (nextAuthZone is null)
                return false;

            return nextAuthZone.Name.EndsWith("." + domain);
        }

        #endregion

        #region DNSSEC

        public IReadOnlyList<DnsResourceRecord> FindNSecProofOfNonExistenceNxDomain(string domain, bool isWildcardAnswer)
        {
            List<DnsResourceRecord> nsecRecords = new List<DnsResourceRecord>(2 * 2);

            void AddProofOfCoverFor(string domain)
            {
                byte[] key = ConvertToByteKey(domain);

                AuthZoneNode authZoneNode = FindZoneNode(key, false, out Node currentNode, out _, out Node closestAuthorityNode, out _, out _, out ApexZone closestAuthority);
                if (authZoneNode is not null)
                    throw new InvalidOperationException(); //domain exists! cannot prove non-existence

                Node previousNode = GetPreviousSubDomainZoneNode(key, currentNode, closestAuthorityNode.Depth);
                if (previousNode is not null)
                {
                    AuthZone authZone = GetAuthZoneFromNode(previousNode, closestAuthority.Name);
                    if (authZone is not null)
                    {
                        IReadOnlyList<DnsResourceRecord> proofOfCoverRecords = authZone.QueryRecords(DnsResourceRecordType.NSEC, true);

                        foreach (DnsResourceRecord proofOfCoverRecord in proofOfCoverRecords)
                        {
                            if (!nsecRecords.Contains(proofOfCoverRecord))
                                nsecRecords.Add(proofOfCoverRecord);
                        }
                    }
                }
            }

            //add proof of cover for domain
            AddProofOfCoverFor(domain);

            if (isWildcardAnswer)
                return nsecRecords;

            //add proof of cover for wildcard
            if (nsecRecords.Count > 0)
            {
                //add wildcard proof to prove that a wildcard expansion was not possible
                DnsResourceRecord nsecRecord = nsecRecords[0];
                DnsNSECRecordData nsec = nsecRecord.RDATA as DnsNSECRecordData;
                string wildcardName = DnsNSECRecordData.GetWildcardFor(nsecRecord, domain);

                if (!DnsNSECRecordData.IsDomainCovered(nsecRecord.Name, nsec.NextDomainName, wildcardName))
                    AddProofOfCoverFor(wildcardName);
            }

            return nsecRecords;
        }

        public IReadOnlyList<DnsResourceRecord> FindNSec3ProofOfNonExistenceNxDomain(string domain, bool isWildcardAnswer, CancellationToken cancellationToken)
        {
            List<DnsResourceRecord> nsec3Records = new List<DnsResourceRecord>(3 * 2);

            void AddProofOfCoverFor(string hashedOwnerName, string zoneName)
            {
                //find previous NSEC3 for the hashed owner name
                IReadOnlyList<DnsResourceRecord> proofOfCoverRecords = null;
                string currentOwnerName = hashedOwnerName;

                while (true)
                {
                    cancellationToken.ThrowIfCancellationRequested();

                    AuthZone previousNSec3Zone = FindPreviousSubDomainZone(zoneName, currentOwnerName);
                    if (previousNSec3Zone is null)
                        break;

                    IReadOnlyList<DnsResourceRecord> previousNSec3Records = previousNSec3Zone.QueryRecords(DnsResourceRecordType.NSEC3, true);
                    if (previousNSec3Records.Count > 0)
                    {
                        proofOfCoverRecords = previousNSec3Records;
                        break;
                    }

                    currentOwnerName = previousNSec3Zone.Name;
                }

                if (proofOfCoverRecords is null)
                {
                    //didnt find previous NSEC3; find the last NSEC3
                    currentOwnerName = hashedOwnerName;

                    while (true)
                    {
                        cancellationToken.ThrowIfCancellationRequested();

                        AuthZone nextNSec3Zone = GetAuthZone(zoneName, currentOwnerName);
                        if (nextNSec3Zone is null)
                        {
                            nextNSec3Zone = FindNextSubDomainZone(zoneName, currentOwnerName);
                            if (nextNSec3Zone is null)
                                break;
                        }

                        IReadOnlyList<DnsResourceRecord> nextNSec3Records = nextNSec3Zone.QueryRecords(DnsResourceRecordType.NSEC3, true);
                        if (nextNSec3Records.Count > 0)
                        {
                            proofOfCoverRecords = nextNSec3Records;
                            DnsResourceRecord previousNSec3Record = nextNSec3Records[0];

                            string nextHashedOwnerNameString = (previousNSec3Record.RDATA as DnsNSEC3RecordData).NextHashedOwnerName + (zoneName.Length > 0 ? "." + zoneName : "");
                            if (DnsNSECRecordData.CanonicalComparison(previousNSec3Record.Name, nextHashedOwnerNameString) >= 0)
                                break; //found last NSEC3

                            //jump to next hashed owner
                            currentOwnerName = nextHashedOwnerNameString;
                        }
                        else
                        {
                            currentOwnerName = nextNSec3Zone.Name;
                        }
                    }
                }

                if (proofOfCoverRecords is null)
                    throw new InvalidOperationException();

                foreach (DnsResourceRecord proofOfCoverRecord in proofOfCoverRecords)
                {
                    if (!nsec3Records.Contains(proofOfCoverRecord))
                        nsec3Records.Add(proofOfCoverRecord);
                }
            }

            byte[] key = ConvertToByteKey(domain);
            string closestEncloser;

            AuthZoneNode authZoneNode = FindZoneNode(key, isWildcardAnswer, out _, out _, out _, out SubDomainZone closestSubDomain, out _, out ApexZone closestAuthority);
            if (authZoneNode is not null)
            {
                if (isWildcardAnswer && (closestSubDomain is not null) && closestSubDomain.Name.StartsWith('*'))
                {
                    closestEncloser = closestSubDomain.Name.TrimStart(_starPeriodTrimChars);
                }
                else
                {
                    //subdomain that contains only NSEC3 record does not really exists: RFC5155 section 7.2.8    
                    if ((authZoneNode.ApexZone is not null) || ((authZoneNode.ParentSideZone is not null) && !authZoneNode.ParentSideZone.HasOnlyNSec3Records()))
                        throw new InvalidOperationException(); //domain exists! cannot prove non-existence

                    //continue to prove non-existence of this nsec3 owner name
                    closestEncloser = closestAuthority.Name;
                }
            }
            else
            {
                if (closestSubDomain is not null)
                    closestEncloser = closestSubDomain.Name;
                else if (closestAuthority is not null)
                    closestEncloser = closestAuthority.Name;
                else
                    throw new InvalidOperationException(); //cannot find closest encloser
            }

            IReadOnlyList<DnsResourceRecord> nsec3ParamRecords = closestAuthority.GetRecords(DnsResourceRecordType.NSEC3PARAM);
            if (nsec3ParamRecords.Count == 0)
                throw new InvalidOperationException("Zone does not have NSEC3 deployed.");

            DnsNSEC3PARAMRecordData nsec3Param = nsec3ParamRecords[0].RDATA as DnsNSEC3PARAMRecordData;

            //find correct closest encloser
            string hashedNextCloserName;

            while (true)
            {
                cancellationToken.ThrowIfCancellationRequested();

                string nextCloserName = DnsNSEC3RecordData.GetNextCloserName(domain, closestEncloser);
                hashedNextCloserName = nsec3Param.ComputeHashedOwnerNameBase32HexString(nextCloserName) + (closestAuthority.Name.Length > 0 ? "." + closestAuthority.Name : "");

                AuthZone nsec3Zone = GetAuthZone(closestAuthority.Name, hashedNextCloserName);
                if (nsec3Zone is null)
                    break; //next closer name does not exists

                //next closer name exists as an ENT
                closestEncloser = nextCloserName;

                if (domain.Equals(closestEncloser, StringComparison.OrdinalIgnoreCase))
                {
                    //domain exists as an ENT; return no data proof
                    return FindNSec3ProofOfNonExistenceNoData(nsec3Zone);
                }
            }

            if (isWildcardAnswer)
            {
                //add proof of cover for the domain to prove non-existence (wildcard)
                AddProofOfCoverFor(hashedNextCloserName, closestAuthority.Name);
            }
            else
            {
                //add closest encloser proof
                string hashedClosestEncloser = nsec3Param.ComputeHashedOwnerNameBase32HexString(closestEncloser) + (closestAuthority.Name.Length > 0 ? "." + closestAuthority.Name : "");

                AuthZone nsec3Zone = GetAuthZone(closestAuthority.Name, hashedClosestEncloser);
                if (nsec3Zone is null)
                    throw new InvalidOperationException();

                IReadOnlyList<DnsResourceRecord> closestEncloserProofRecords = nsec3Zone.QueryRecords(DnsResourceRecordType.NSEC3, true);
                if (closestEncloserProofRecords.Count == 0)
                    throw new InvalidOperationException();

                nsec3Records.AddRange(closestEncloserProofRecords);

                DnsResourceRecord closestEncloserProofRecord = closestEncloserProofRecords[0];
                DnsNSEC3RecordData closestEncloserProof = closestEncloserProofRecord.RDATA as DnsNSEC3RecordData;

                //add proof of cover for the next closer name
                if (!DnsNSECRecordData.IsDomainCovered(closestEncloserProofRecord.Name, closestEncloserProof.NextHashedOwnerName + (closestAuthority.Name.Length > 0 ? "." + closestAuthority.Name : ""), hashedNextCloserName))
                    AddProofOfCoverFor(hashedNextCloserName, closestAuthority.Name);

                //add proof of cover to prove that a wildcard expansion was not possible
                string wildcardDomain = closestEncloser.Length > 0 ? "*." + closestEncloser : "*";
                string hashedWildcardDomainName = nsec3Param.ComputeHashedOwnerNameBase32HexString(wildcardDomain) + (closestAuthority.Name.Length > 0 ? "." + closestAuthority.Name : "");

                if (!DnsNSECRecordData.IsDomainCovered(closestEncloserProofRecord.Name, closestEncloserProof.NextHashedOwnerName + (closestAuthority.Name.Length > 0 ? "." + closestAuthority.Name : ""), hashedWildcardDomainName))
                    AddProofOfCoverFor(hashedWildcardDomainName, closestAuthority.Name);
            }

            return nsec3Records;
        }

        public static IReadOnlyList<DnsResourceRecord> FindNSecProofOfNonExistenceNoData(AuthZone zone)
        {
            IReadOnlyList<DnsResourceRecord> nsecRecords = zone.QueryRecords(DnsResourceRecordType.NSEC, true);
            if (nsecRecords.Count == 0)
                throw new InvalidOperationException("Zone does not have NSEC deployed correctly.");

            return nsecRecords;
        }

        public IReadOnlyList<DnsResourceRecord> FindNSec3ProofOfNonExistenceNoData(AuthZone zone, ApexZone apexZone, CancellationToken cancellationToken)
        {
            IReadOnlyList<DnsResourceRecord> nsec3ParamRecords = apexZone.GetRecords(DnsResourceRecordType.NSEC3PARAM);
            if (nsec3ParamRecords.Count == 0)
                throw new InvalidOperationException("Zone does not have NSEC3 deployed.");

            DnsNSEC3PARAMRecordData nsec3Param = nsec3ParamRecords[0].RDATA as DnsNSEC3PARAMRecordData;
            string hashedOwnerName = nsec3Param.ComputeHashedOwnerNameBase32HexString(zone.Name) + (apexZone.Name.Length > 0 ? "." + apexZone.Name : "");

            AuthZone nsec3Zone = GetAuthZone(apexZone.Name, hashedOwnerName);
            if (nsec3Zone is null)
            {
                //this is probably since the domain in request is for an nsec3 record owner name
                return FindNSec3ProofOfNonExistenceNxDomain(zone.Name, false, cancellationToken);
            }

            return FindNSec3ProofOfNonExistenceNoData(nsec3Zone);
        }

        private static IReadOnlyList<DnsResourceRecord> FindNSec3ProofOfNonExistenceNoData(AuthZone nsec3Zone)
        {
            IReadOnlyList<DnsResourceRecord> nsec3Records = nsec3Zone.QueryRecords(DnsResourceRecordType.NSEC3, true);
            if (nsec3Records.Count > 0)
                return nsec3Records;

            return Array.Empty<DnsResourceRecord>();
        }

        #endregion
    }
}