package checker

import (
	"context"
	"encoding/json"
	"fmt"
	"net"
	"regexp"
	"strings"

	"github.com/miekg/dns"

	sdk "git.happydns.org/checker-sdk-go/checker"
)

// Collect gathers raw PTR observation data. It does NOT judge: no severity,
// no pass/fail, no pre-derived findings. CheckRule implementations turn the
// raw fields into CheckStates.
func (p *ptrProvider) Collect(ctx context.Context, opts sdk.CheckerOptions) (any, error) {
	owner, declaredTarget, declaredTTL, err := resolvePTRInputs(opts)
	if err != nil {
		return nil, err
	}

	data := &PTRData{
		OwnerName:      owner,
		DeclaredTarget: declaredTarget,
		DeclaredTTL:    declaredTTL,
	}

	// Structural classification: is this a reverse-arpa name, and can we
	// decode an IP from it?
	data.InReverseArpa = isReverseArpa(owner)
	data.IsIPv6 = strings.HasSuffix(strings.TrimSuffix(lowerFQDN(owner), "."), ".ip6.arpa")
	ip := reverseNameToIP(owner)
	if ip != nil {
		data.ReverseIP = ip.String()
	} else if data.InReverseArpa {
		data.OwnerDecodeFailed = true
	}

	// Reverse zone location.
	zone, servers, zerr := findReverseZone(ctx, owner)
	data.ReverseZone = zone
	data.ReverseNS = servers
	if zerr != nil {
		data.ZoneLookupError = zerr.Error()
	}

	// PTR query at authoritative servers (fall back to the system resolver).
	observed, observedTTL, rcode, qerr := queryPTR(ctx, owner, servers)
	data.Rcode = rcode
	data.ObservedTargets = observed
	data.ObservedTTL = observedTTL
	if qerr != nil {
		data.QueryError = qerr.Error()
	}

	// Effective target for hostname hygiene / FCrDNS: prefer observed,
	// fall back to declared.
	declNorm := lowerFQDN(declaredTarget)
	normalizedObserved := make([]string, len(observed))
	for i, o := range observed {
		normalizedObserved[i] = lowerFQDN(o)
	}
	target := declNorm
	if len(normalizedObserved) > 0 {
		target = normalizedObserved[0]
	}
	data.EffectiveTarget = target

	if target != "" {
		_, syntaxOK := dns.IsDomainName(strings.TrimSuffix(target, "."))
		data.TargetSyntaxValid = syntaxOK
		if ip != nil {
			data.TargetLooksGeneric = looksGeneric(target, ip)
		}
	}

	// Forward-Confirmed Reverse DNS: resolve target and compare with
	// ReverseIP.
	if target != "" && ip != nil {
		addrs, tResolves := resolveForward(ctx, target)
		data.ForwardAddresses = addrs
		data.TargetResolves = tResolves

		for _, a := range addrs {
			if ipEqual(a.Address, ip) {
				data.ForwardMatch = true
				break
			}
		}
	}

	return data, nil
}

// resolvePTRInputs extracts the PTR owner, declared target and TTL from the
// auto-filled options.
func resolvePTRInputs(opts sdk.CheckerOptions) (owner, target string, ttl uint32, err error) {
	if svcMsg, ok := sdk.GetOption[serviceMessage](opts, "service"); ok && len(svcMsg.Service) > 0 {
		if svcMsg.Type != "" && svcMsg.Type != "svcs.PTR" {
			return "", "", 0, fmt.Errorf("service is %s, expected svcs.PTR", svcMsg.Type)
		}
		var s ptrService
		if err := json.Unmarshal(svcMsg.Service, &s); err == nil && s.Record != nil {
			ownerName := s.Record.Hdr.Name
			if ownerName == "" || ownerName == "@" {
				ownerName = svcMsg.Domain
			} else if !strings.HasSuffix(ownerName, ".") {
				if svcMsg.Domain != "" {
					ownerName = ownerName + "." + strings.TrimSuffix(svcMsg.Domain, ".")
				}
			}
			declared := ""
			if s.Record.Ptr != "" {
				declared = lowerFQDN(s.Record.Ptr)
			}
			return lowerFQDN(ownerName), declared, s.Record.Hdr.Ttl, nil
		}
	}

	parent, _ := sdk.GetOption[string](opts, "domain_name")
	sub, _ := sdk.GetOption[string](opts, "subdomain")
	if parent == "" {
		return "", "", 0, fmt.Errorf("missing 'service' and 'domain_name' options")
	}
	expected, _ := sdk.GetOption[string](opts, "expected_target")
	expected = strings.TrimSpace(expected)
	declared := ""
	if expected != "" {
		declared = lowerFQDN(expected)
	}
	parent = strings.TrimSuffix(parent, ".")
	if sub == "" || sub == "@" {
		return lowerFQDN(parent), declared, 0, nil
	}
	sub = strings.TrimSuffix(sub, ".")
	return lowerFQDN(sub + "." + parent), declared, 0, nil
}

// queryPTR asks for the PTR RRset at owner. It uses the supplied authoritative
// servers when available; otherwise it falls back to the system resolver.
func queryPTR(ctx context.Context, owner string, authServers []string) ([]string, uint32, string, error) {
	q := dns.Question{Name: dns.Fqdn(owner), Qtype: dns.TypePTR, Qclass: dns.ClassINET}

	var r *dns.Msg
	var err error
	if len(authServers) > 0 {
		r, _, err = queryAtAuth(ctx, authServers, q)
	} else {
		r, err = dnsExchange(ctx, "", systemResolver(), q, true)
	}
	if err != nil {
		return nil, 0, "", err
	}

	rcode := rcodeText(r.Rcode)
	var targets []string
	var ttl uint32
	for _, rr := range r.Answer {
		if ptr, ok := rr.(*dns.PTR); ok && strings.EqualFold(dns.Fqdn(ptr.Hdr.Name), dns.Fqdn(owner)) {
			targets = append(targets, lowerFQDN(ptr.Ptr))
			if ttl == 0 || ptr.Hdr.Ttl < ttl {
				ttl = ptr.Hdr.Ttl
			}
		}
	}
	return targets, ttl, rcode, nil
}

// resolveForward runs the forward lookup of name via the system resolver.
func resolveForward(ctx context.Context, name string) ([]ForwardAddress, bool) {
	var resolver net.Resolver
	var out []ForwardAddress

	ips, err := resolver.LookupIP(ctx, "ip", strings.TrimSuffix(name, "."))
	if err != nil || len(ips) == 0 {
		// Fall back to direct DNS queries (system resolver may filter AAAA).
		for _, qt := range []uint16{dns.TypeA, dns.TypeAAAA} {
			q := dns.Question{Name: dns.Fqdn(name), Qtype: qt, Qclass: dns.ClassINET}
			r, rerr := dnsExchange(ctx, "", systemResolver(), q, true)
			if rerr != nil || r == nil {
				continue
			}
			for _, rr := range r.Answer {
				switch v := rr.(type) {
				case *dns.A:
					out = append(out, ForwardAddress{Type: "A", Address: v.A.String(), TTL: v.Hdr.Ttl})
				case *dns.AAAA:
					out = append(out, ForwardAddress{Type: "AAAA", Address: v.AAAA.String(), TTL: v.Hdr.Ttl})
				}
			}
		}
		return out, len(out) > 0
	}

	for _, ip := range ips {
		if v4 := ip.To4(); v4 != nil {
			out = append(out, ForwardAddress{Type: "A", Address: v4.String()})
		} else {
			out = append(out, ForwardAddress{Type: "AAAA", Address: ip.String()})
		}
	}
	return out, true
}

// ipEqual compares an address string with a net.IP (normalising IPv4-in-IPv6).
func ipEqual(addr string, ip net.IP) bool {
	parsed := net.ParseIP(addr)
	if parsed == nil {
		return false
	}
	return parsed.Equal(ip)
}

// looksGeneric reports whether hostname embeds the dotted/hyphenated IP or
// matches the common ISP auto-generated patterns that mail filters penalise.
//
// The pattern requires a "-" or "." separator before the digit run so legitimate
// names like "host1.example.com" or "static-www" do not match; auto-generated
// PTRs almost always look like "dhcp-1-2-3-4", "pool.10.20", "dyn-203" etc.
var genericHints = regexp.MustCompile(`(?i)\b(dhcp|dyn(amic)?|dsl|cable|ppp|pool|client|broadband|static|user|host|ip)[-.]\d+([-.]\d+){1,3}\b`)

func looksGeneric(hostname string, ip net.IP) bool {
	h := strings.ToLower(hostname)

	if v4 := ip.To4(); v4 != nil {
		ipStr := v4.String()
		if strings.Contains(h, ipStr) {
			return true
		}
		if strings.Contains(h, strings.ReplaceAll(ipStr, ".", "-")) {
			return true
		}
	} else if v6 := ip.To16(); v6 != nil {
		// Build the 32-nibble hex form, then check the common embedded
		// shapes: continuous ("20010db8…"), dash-grouped ("2001-0db8-…"),
		// dot-grouped ("2001.0db8.…"), and full nibble-by-nibble ("2.0.0.1.0.d.b.8.…").
		var hex [32]byte
		const hexdigits = "0123456789abcdef"
		for i, b := range v6 {
			hex[i*2] = hexdigits[b>>4]
			hex[i*2+1] = hexdigits[b&0x0f]
		}
		flat := string(hex[:])
		if strings.Contains(h, flat) {
			return true
		}
		groups := []string{
			flat[0:4], flat[4:8], flat[8:12], flat[12:16],
			flat[16:20], flat[20:24], flat[24:28], flat[28:32],
		}
		// At least four consecutive groups must be present to claim the
		// hostname embeds the address (avoids false positives on short
		// hex-looking labels).
		for _, sep := range []string{"-", "."} {
			for start := 0; start <= 4; start++ {
				probe := strings.Join(groups[start:start+4], sep)
				if strings.Contains(h, probe) {
					return true
				}
			}
		}
		// Nibble-per-label form, as appears in some ISP PTRs.
		nibbles := make([]string, 32)
		for i, c := range flat {
			nibbles[i] = string(c)
		}
		for start := 0; start <= 32-16; start++ {
			probe := strings.Join(nibbles[start:start+16], ".")
			if strings.Contains(h, probe) {
				return true
			}
		}
	}
	return genericHints.MatchString(h)
}