package checker

import (
	"fmt"
	"strings"
	"time"

	"github.com/intodns/backend/internal/resolver"
	"github.com/miekg/dns"
)

// checkNS runs the 17 nameserver checks.
func checkNS(domain string, r *resolver.Resolver) Category {
	cat := Category{Name: "nameservers", Title: "Nameservers"}

	domain = dns.Fqdn(domain)

	// Resolve the NS set via a recursive query.
	resp, err := r.Query(domain, "8.8.8.8", dns.TypeNS)
	if err != nil || resp == nil {
		cat.Checks = append(cat.Checks, CheckResult{
			ID: "ns-count", Title: "NS Count",
			Status: StatusFail, Message: fmt.Sprintf("Failed to query NS records: %v", err),
		})
		return cat
	}

	var nsNames []string
	for _, rr := range resp.Answer {
		if ns, ok := rr.(*dns.NS); ok {
			nsNames = appendUniqLower(nsNames, ns.Ns)
		}
	}

	// Resolve all NS to IPs.
	var nsInfos []nsEntry
	var allIPs []string
	for _, ns := range nsNames {
		ips := resolveNS(ns, r)
		nsInfos = append(nsInfos, nsEntry{Name: ns, IPs: ips})
		allIPs = append(allIPs, ips...)
	}

	// 1. ns-count
	cat.Checks = append(cat.Checks, checkNSCount(nsNames))

	// 2. ns-reachable — also builds a filtered list of reachable NS
	reachableCheck, reachableInfos := checkNSReachableFiltered(domain, nsInfos, r)
	cat.Checks = append(cat.Checks, reachableCheck)

	// Use reachable NS for checks that query NS directly to avoid timeouts.
	// Checks that only need names/IPs (no direct queries) still use full nsInfos.

	// 3. ns-auth
	cat.Checks = append(cat.Checks, checkNSAuth(domain, reachableInfos, r))

	// 4. ns-recursion
	cat.Checks = append(cat.Checks, checkNSRecursion(domain, reachableInfos, r))

	// 5. ns-identical
	cat.Checks = append(cat.Checks, checkNSIdentical(domain, reachableInfos, r))

	// 6. ns-ip-unique
	cat.Checks = append(cat.Checks, checkNSIPUnique(nsInfos))

	// 7. ns-subnet-diversity
	cat.Checks = append(cat.Checks, checkNSSubnetDiversity(allIPs))

	// 8. ns-as-diversity
	cat.Checks = append(cat.Checks, checkNSASDiversity(allIPs))

	// 9. ns-tcp
	cat.Checks = append(cat.Checks, checkNSTCP(domain, reachableInfos, r))

	// 10. ns-udp
	cat.Checks = append(cat.Checks, checkNSUDP(domain, reachableInfos, r))

	// 11. ns-no-cname
	cat.Checks = append(cat.Checks, checkNSNoCNAME(nsNames, r))

	// 12. ns-a-records
	cat.Checks = append(cat.Checks, checkNSARecords(nsInfos, r))

	// 13. ns-aaaa-records
	cat.Checks = append(cat.Checks, checkNSAAAARecords(nsNames, r))

	// 14. ns-version
	cat.Checks = append(cat.Checks, checkNSVersion(reachableInfos, r))

	// 15. ns-zone-transfer
	cat.Checks = append(cat.Checks, checkNSZoneTransfer(domain, reachableInfos, r))

	// 16. ns-lame
	cat.Checks = append(cat.Checks, checkNSLame(domain, reachableInfos, r))

	// 17. ns-response-size
	cat.Checks = append(cat.Checks, checkNSResponseSize(domain, reachableInfos, r))

	return cat
}

func checkNSCount(nsNames []string) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-count", Title: "NS Count"}
	defer func() { res.DurationMs = measureDuration(start) }()

	count := len(nsNames)
	switch {
	case count == 0:
		res.Status = StatusFail
		res.Message = "No NS records found"
	case count == 1:
		res.Status = StatusWarn
		res.Message = "Only 1 nameserver; at least 2 recommended"
		res.Details = nsNames
	default:
		res.Status = StatusPass
		res.Message = fmt.Sprintf("%d nameservers found", count)
		res.Details = nsNames
	}
	return res
}

type nsEntry struct {
	Name string
	IPs  []string
}

// checkNSReachableFiltered checks reachability and returns both the check result
// and a filtered list of only reachable NS entries (with only reachable IPs).
func checkNSReachableFiltered(domain string, infos []nsEntry, r *resolver.Resolver) (CheckResult, []nsEntry) {
	start := time.Now()
	res := CheckResult{ID: "ns-reachable", Title: "NS Reachability"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res, nil
	}

	var reachable []nsEntry
	allOK := true
	for _, ns := range infos {
		if len(ns.IPs) == 0 {
			allOK = false
			res.Details = append(res.Details, fmt.Sprintf("%s: no IPs resolved", ns.Name))
			continue
		}
		var reachableIPs []string
		for _, ip := range ns.IPs {
			_, err := r.QueryNoRecurse(domain, ip, dns.TypeSOA)
			if err == nil {
				reachableIPs = append(reachableIPs, ip)
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): OK", ns.Name, ip))
			} else {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): unreachable", ns.Name, ip))
			}
		}
		if len(reachableIPs) > 0 {
			reachable = append(reachable, nsEntry{Name: ns.Name, IPs: reachableIPs})
		} else {
			allOK = false
		}
	}

	if allOK {
		res.Status = StatusPass
		res.Message = "All nameservers respond to DNS queries"
	} else {
		var unreachableNames []string
		for _, ns := range infos {
			found := false
			for _, r := range reachable {
				if r.Name == ns.Name {
					found = true
					break
				}
			}
			if !found {
				unreachableNames = append(unreachableNames, ns.Name)
			}
		}
		res.Status = StatusFail
		res.Message = fmt.Sprintf("The following nameservers are not responding to DNS queries: %s. This means part of your DNS infrastructure is down. Check that these servers are running and accessible on port 53 (UDP/TCP).", strings.Join(unreachableNames, ", "))
	}
	return res, reachable
}

func checkNSAuth(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-auth", Title: "NS Authoritative"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res
	}

	allAuth := true
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			resp, err := r.QueryNoRecurse(domain, ip, dns.TypeSOA)
			if err != nil {
				allAuth = false
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): error %v", ns.Name, ip, err))
				continue
			}
			if resp.Authoritative {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): authoritative (AA=1)", ns.Name, ip))
			} else {
				allAuth = false
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): NOT authoritative (AA=0)", ns.Name, ip))
			}
			break // only check first reachable IP per NS
		}
	}

	if allAuth {
		res.Status = StatusPass
		res.Message = "All nameservers are authoritative"
	} else {
		res.Status = StatusFail
		res.Message = "Some nameservers are not authoritative"
	}
	return res
}

func checkNSRecursion(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-recursion", Title: "NS Recursion"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res
	}

	anyRecursive := false
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			resp, err := r.QueryNoRecurse(domain, ip, dns.TypeSOA)
			if err != nil {
				continue
			}
			if resp.RecursionAvailable {
				anyRecursive = true
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): recursion available (RA=1)", ns.Name, ip))
			} else {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): no recursion (RA=0)", ns.Name, ip))
			}
			break
		}
	}

	if anyRecursive {
		res.Status = StatusWarn
		res.Message = "Some nameservers offer recursion; this is a security risk"
	} else {
		res.Status = StatusPass
		res.Message = "No nameservers offer recursion"
	}
	return res
}

func checkNSIdentical(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-identical", Title: "NS Identical Sets"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res
	}

	var sets []string
	allSame := true
	var referenceSet string

	for _, ns := range infos {
		for _, ip := range ns.IPs {
			names := nsNamesFromAuthNS(domain, ip, r)
			sorted := sortedStrings(names)
			setStr := strings.Join(sorted, ",")
			sets = append(sets, setStr)
			res.Details = append(res.Details, fmt.Sprintf("%s: %s", ns.Name, strings.Join(sorted, ", ")))
			if referenceSet == "" {
				referenceSet = setStr
			} else if setStr != referenceSet {
				allSame = false
			}
			break
		}
	}

	if allSame {
		res.Status = StatusPass
		res.Message = "All nameservers return the same NS set"
	} else {
		res.Status = StatusFail
		res.Message = "Nameservers return different NS sets"
	}
	return res
}

func checkNSIPUnique(infos []nsEntry) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-ip-unique", Title: "NS IP Uniqueness"}
	defer func() { res.DurationMs = measureDuration(start) }()

	seen := make(map[string]string) // ip -> ns name
	duplicates := false
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			if prev, ok := seen[ip]; ok {
				duplicates = true
				res.Details = append(res.Details, fmt.Sprintf("Duplicate IP %s shared by %s and %s", ip, prev, ns.Name))
			} else {
				seen[ip] = ns.Name
			}
		}
	}

	if duplicates {
		res.Status = StatusWarn
		res.Message = "Some nameservers share IP addresses"
	} else {
		res.Status = StatusPass
		res.Message = "All nameserver IPs are unique"
	}
	return res
}

func checkNSSubnetDiversity(allIPs []string) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-subnet-diversity", Title: "NS Subnet Diversity"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(allIPs) == 0 {
		res.Status = StatusWarn
		res.Message = "No NS IPs to check"
		return res
	}

	subnets := make(map[string]bool)
	for _, ip := range allIPs {
		subnets[subnet24(ip)] = true
	}

	for s := range subnets {
		res.Details = append(res.Details, s)
	}

	if len(subnets) >= 2 {
		res.Status = StatusPass
		res.Message = fmt.Sprintf("Nameservers span %d /24 subnets", len(subnets))
	} else {
		res.Status = StatusWarn
		res.Message = "All nameservers are in the same /24 subnet"
	}
	return res
}

func checkNSASDiversity(allIPs []string) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-as-diversity", Title: "NS AS Diversity"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(allIPs) == 0 {
		res.Status = StatusInfo
		res.Message = "No NS IPs to check"
		return res
	}

	networks := make(map[string]bool)
	for _, ip := range allIPs {
		networks[subnet16(ip)] = true
	}

	for n := range networks {
		res.Details = append(res.Details, n)
	}

	if len(networks) >= 2 {
		res.Status = StatusInfo
		res.Message = fmt.Sprintf("Nameservers appear to span %d /16 networks (rough AS diversity proxy)", len(networks))
	} else {
		res.Status = StatusInfo
		res.Message = "All nameservers appear to be in the same /16 network"
	}
	return res
}

func checkNSTCP(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-tcp", Title: "NS TCP Support"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res
	}

	allOK := true
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			_, err := r.QueryTCP(domain, ip, dns.TypeSOA)
			if err != nil {
				allOK = false
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): TCP failed: %v", ns.Name, ip, err))
			} else {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): TCP OK", ns.Name, ip))
			}
			break
		}
	}

	if allOK {
		res.Status = StatusPass
		res.Message = "All nameservers respond over TCP"
	} else {
		res.Status = StatusFail
		res.Message = "Some nameservers do not respond over TCP"
	}
	return res
}

func checkNSUDP(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-udp", Title: "NS UDP Support"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res
	}

	allOK := true
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			_, err := r.QueryNoRecurse(domain, ip, dns.TypeSOA)
			if err != nil {
				allOK = false
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): UDP failed: %v", ns.Name, ip, err))
			} else {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): UDP OK", ns.Name, ip))
			}
			break
		}
	}

	if allOK {
		res.Status = StatusPass
		res.Message = "All nameservers respond over UDP"
	} else {
		res.Status = StatusFail
		res.Message = "Some nameservers do not respond over UDP"
	}
	return res
}

func checkNSNoCNAME(nsNames []string, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-no-cname", Title: "NS No CNAME"}
	defer func() { res.DurationMs = measureDuration(start) }()

	hasCNAME := false
	for _, ns := range nsNames {
		resp, err := r.Query(ns, "8.8.8.8", dns.TypeCNAME)
		if err != nil {
			continue
		}
		for _, rr := range resp.Answer {
			if cname, ok := rr.(*dns.CNAME); ok {
				hasCNAME = true
				res.Details = append(res.Details, fmt.Sprintf("%s is a CNAME to %s", ns, cname.Target))
			}
		}
	}

	if hasCNAME {
		res.Status = StatusFail
		res.Message = "Some NS names resolve to CNAMEs (RFC violation)"
	} else {
		res.Status = StatusPass
		res.Message = "No NS names resolve to CNAMEs"
	}
	return res
}

func checkNSARecords(infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-a-records", Title: "NS A Records"}
	defer func() { res.DurationMs = measureDuration(start) }()

	allHaveA := true
	for _, ns := range infos {
		resp, err := r.Query(ns.Name, "8.8.8.8", dns.TypeA)
		if err != nil {
			allHaveA = false
			res.Details = append(res.Details, fmt.Sprintf("%s: query error %v", ns.Name, err))
			continue
		}
		found := false
		for _, rr := range resp.Answer {
			if a, ok := rr.(*dns.A); ok {
				found = true
				res.Details = append(res.Details, fmt.Sprintf("%s: %s", ns.Name, a.A.String()))
			}
		}
		if !found {
			allHaveA = false
			res.Details = append(res.Details, fmt.Sprintf("%s: no A record", ns.Name))
		}
	}

	if allHaveA {
		res.Status = StatusPass
		res.Message = "All nameservers have A records"
	} else {
		res.Status = StatusWarn
		res.Message = "Some nameservers lack A records"
	}
	return res
}

func checkNSAAAARecords(nsNames []string, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-aaaa-records", Title: "NS AAAA Records"}
	defer func() { res.DurationMs = measureDuration(start) }()

	anyAAAA := false
	for _, ns := range nsNames {
		resp, err := r.Query(ns, "8.8.8.8", dns.TypeAAAA)
		if err != nil {
			res.Details = append(res.Details, fmt.Sprintf("%s: query error", ns))
			continue
		}
		found := false
		for _, rr := range resp.Answer {
			if aaaa, ok := rr.(*dns.AAAA); ok {
				found = true
				anyAAAA = true
				res.Details = append(res.Details, fmt.Sprintf("%s: %s", ns, aaaa.AAAA.String()))
			}
		}
		if !found {
			res.Details = append(res.Details, fmt.Sprintf("%s: no AAAA record", ns))
		}
	}

	if anyAAAA {
		res.Status = StatusInfo
		res.Message = "Some nameservers have AAAA records (IPv6)"
	} else {
		res.Status = StatusInfo
		res.Message = "No nameservers have AAAA records"
	}
	return res
}

func checkNSVersion(infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-version", Title: "NS Version"}
	defer func() { res.DurationMs = measureDuration(start) }()

	for _, ns := range infos {
		for _, ip := range ns.IPs {
			ver, err := r.QueryVersionBind(ip)
			if err != nil || ver == "" {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): not disclosed", ns.Name, ip))
			} else {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): %s", ns.Name, ip, ver))
			}
			break
		}
	}

	res.Status = StatusInfo
	res.Message = "Version information (version.bind)"
	return res
}

func checkNSZoneTransfer(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-zone-transfer", Title: "NS Zone Transfer (AXFR)"}
	defer func() { res.DurationMs = measureDuration(start) }()

	anyAllowed := false
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			allowed, _ := r.QueryAXFR(domain, ip)
			if allowed {
				anyAllowed = true
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): AXFR ALLOWED (dangerous!)", ns.Name, ip))
			} else {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): AXFR refused (good)", ns.Name, ip))
			}
			break
		}
	}

	if anyAllowed {
		res.Status = StatusFail
		res.Message = "Zone transfer (AXFR) is allowed on some nameservers"
	} else {
		res.Status = StatusPass
		res.Message = "Zone transfer (AXFR) is refused on all nameservers"
	}
	return res
}

func checkNSLame(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-lame", Title: "NS Lame Delegation"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res
	}

	anyLame := false
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			resp, err := r.QueryNoRecurse(domain, ip, dns.TypeSOA)
			if err != nil {
				anyLame = true
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): lame (unreachable)", ns.Name, ip))
				break
			}
			if !resp.Authoritative {
				anyLame = true
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): lame (not authoritative)", ns.Name, ip))
			} else {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): OK (authoritative)", ns.Name, ip))
			}
			break
		}
	}

	if anyLame {
		res.Status = StatusFail
		res.Message = "Lame delegation detected"
	} else {
		res.Status = StatusPass
		res.Message = "No lame delegations"
	}
	return res
}

func checkNSResponseSize(domain string, infos []nsEntry, r *resolver.Resolver) CheckResult {
	start := time.Now()
	res := CheckResult{ID: "ns-response-size", Title: "NS Response Size"}
	defer func() { res.DurationMs = measureDuration(start) }()

	if len(infos) == 0 {
		res.Status = StatusFail
		res.Message = "No NS to check"
		return res
	}

	allOK := true
	for _, ns := range infos {
		for _, ip := range ns.IPs {
			// Try a normal query and check the response size.
			resp, err := r.QueryNoRecurse(domain, ip, dns.TypeNS)
			if err != nil {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): error", ns.Name, ip))
				break
			}
			packed, err := resp.Pack()
			if err != nil {
				break
			}
			size := len(packed)
			if size <= 512 {
				res.Details = append(res.Details, fmt.Sprintf("%s (%s): %d bytes (fits in 512)", ns.Name, ip, size))
			} else {
				// Check EDNS support.
				ednsResp, ednsErr := r.QueryEDNS(domain, ip, dns.TypeNS, 4096)
				if ednsErr != nil {
					allOK = false
					res.Details = append(res.Details, fmt.Sprintf("%s (%s): %d bytes, no EDNS support", ns.Name, ip, size))
				} else {
					opt := ednsResp.IsEdns0()
					if opt != nil {
						res.Details = append(res.Details, fmt.Sprintf("%s (%s): %d bytes, EDNS supported (bufsize %d)", ns.Name, ip, size, opt.UDPSize()))
					} else {
						allOK = false
						res.Details = append(res.Details, fmt.Sprintf("%s (%s): %d bytes, EDNS not in response", ns.Name, ip, size))
					}
				}
			}
			break
		}
	}

	if allOK {
		res.Status = StatusPass
		res.Message = "Responses fit in 512 bytes or EDNS is supported"
	} else {
		res.Status = StatusWarn
		res.Message = "Some responses exceed 512 bytes without EDNS support"
	}
	return res
}