package checker

import (
	"context"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"strings"

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

// Rule returns the DANE/TLSA matching rule.
func Rule() sdk.CheckRule {
	return &daneRule{}
}

type daneRule struct{}

func (r *daneRule) Name() string { return "dane_tlsa_match" }

func (r *daneRule) Description() string {
	return "Verifies each TLSA record matches the certificate chain presented by the corresponding TLS endpoint."
}

// Evaluate walks each target, fetches the related checker-tls probe keyed on
// the same Ref we emitted in DiscoverEntries, and compares every TLSA record
// against the chain. It returns one CheckState per target so the UI can
// surface per-endpoint status; unmatched records aggregate into a single
// critical state with the first non-matching record's detail.
func (r *daneRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, opts sdk.CheckerOptions) []sdk.CheckState {
	var data DANEData
	if err := obs.Get(ctx, ObservationKeyDANE, &data); err != nil {
		return []sdk.CheckState{{
			Status:  sdk.StatusError,
			Message: fmt.Sprintf("Failed to read %s: %v", ObservationKeyDANE, err),
			Code:    "dane_observation_error",
		}}
	}
	if len(data.Targets) == 0 {
		return []sdk.CheckState{{
			Status:  sdk.StatusOK,
			Message: "No TLSA records declared on this service.",
			Code:    "dane_no_records",
		}}
	}

	// Index related TLS probes by Ref so we can pair them to targets.
	probes, warn := relatedTLSProbes(ctx, obs)

	out := make([]sdk.CheckState, 0, len(data.Targets))
	for _, t := range data.Targets {
		out = append(out, evaluateTarget(t, probes))
	}
	if warn != "" {
		out = append([]sdk.CheckState{{
			Status:  sdk.StatusError,
			Message: warn,
			Code:    "dane_observation_warning",
		}}, out...)
	}
	return out
}

// evaluateTarget matches a single target's TLSA records against the cached
// TLS probe and distills the outcome into a CheckState. The matched/unmatched
// split mirrors what DANE deployers care about most:
//
//   - match_ok: at least one TLSA record in the RRset matches (DANE uses
//     OR semantics per RFC 6698 §2.1).
//   - no_match: no record matched, usually a certificate rotation without
//     TLSA rollover, which is the single most common DANE outage cause.
//   - no_probe: checker-tls has not yet probed this endpoint; rule should
//     not flap red on the first run after publication.
func evaluateTarget(t TargetResult, probes map[string]*tls.TLSProbe) sdk.CheckState {
	subject := fmt.Sprintf("%s:%d (%s)", t.Host, t.Port, t.Proto)
	meta := map[string]any{
		"host":     t.Host,
		"port":     t.Port,
		"proto":    t.Proto,
		"owner":    t.Owner,
		"starttls": t.STARTTLS,
		"records":  len(t.Records),
	}

	probe := probes[t.Ref]
	if probe == nil {
		return sdk.CheckState{
			Status:  sdk.StatusUnknown,
			Code:    "dane_no_probe",
			Subject: subject,
			Message: "No TLS probe available yet for this endpoint; re-evaluate after the next checker-tls cycle.",
			Meta:    meta,
		}
	}
	if probe.Error != "" || len(probe.Chain) == 0 {
		return sdk.CheckState{
			Status:  sdk.StatusCrit,
			Code:    "dane_handshake_failed",
			Subject: subject,
			Message: "TLS handshake failed, cannot validate DANE: " + probe.Error,
			Meta:    meta,
		}
	}

	// Per-record classification: matched / not_matched / unsupported.
	var matched, unmatched int
	var firstUnmatchedIdx = -1
	var firstUnmatchedReason string
	usages := map[uint8]int{}
	for i, rec := range t.Records {
		usages[rec.Usage]++
		ok, reason := matchRecord(rec, probe)
		if ok {
			matched++
			continue
		}
		unmatched++
		if firstUnmatchedIdx < 0 {
			firstUnmatchedIdx = i
			firstUnmatchedReason = reason
		}
	}

	// PKIX-dependent usages (0/1) require a valid public chain; call it
	// out explicitly so users can tell "chain invalid" apart from "hash
	// mismatch".
	pkixRequired := usages[UsagePKIXTA]+usages[UsagePKIXEE] > 0
	if pkixRequired && (probe.ChainValid == nil || !*probe.ChainValid) {
		return sdk.CheckState{
			Status:  sdk.StatusCrit,
			Code:    "dane_pkix_chain_invalid",
			Subject: subject,
			Message: "Usage 0/1 requires a publicly-trusted chain, but the certificate chain did not validate against system roots.",
			Meta:    meta,
		}
	}

	meta["matched"] = matched
	meta["unmatched"] = unmatched

	switch {
	case matched > 0:
		return sdk.CheckState{
			Status:  sdk.StatusOK,
			Code:    "dane_match_ok",
			Subject: subject,
			Message: fmt.Sprintf("%d/%d TLSA record(s) match the presented certificate chain.", matched, matched+unmatched),
			Meta:    meta,
		}
	default:
		msg := "No TLSA record matches the presented certificate chain."
		if firstUnmatchedReason != "" {
			msg += " " + firstUnmatchedReason
		}
		meta["first_unmatched_index"] = firstUnmatchedIdx
		return sdk.CheckState{
			Status:  sdk.StatusCrit,
			Code:    "dane_no_match",
			Subject: subject,
			Message: msg,
			Meta:    meta,
		}
	}
}

// matchRecord returns true when rec matches some certificate at the chain
// slot implied by rec.Usage. reason explains the miss on a false return.
//
// Slot selection:
//
//   - Usage 1 (PKIX-EE) and 3 (DANE-EE): leaf only.
//   - Usage 0 (PKIX-TA) and 2 (DANE-TA): intermediates + the root the
//     server presented (if any). We match against every non-leaf cert the
//     server sent, because some deployments publish the root and some the
//     intermediate; either is a valid TA reference for the connection's
//     path.
func matchRecord(rec TLSARecord, p *tls.TLSProbe) (bool, string) {
	if len(p.Chain) == 0 {
		return false, "no certificates observed on the endpoint"
	}
	var slots []tls.CertInfo
	switch rec.Usage {
	case UsagePKIXEE, UsageDANEEE:
		slots = p.Chain[:1]
	case UsagePKIXTA, UsageDANETA:
		if len(p.Chain) > 1 {
			slots = p.Chain[1:]
		} else {
			// Self-signed / bundle with only a leaf: allow matching against
			// the leaf as a degenerate TA so the user gets a hash comparison
			// rather than a silent "no slot".
			slots = p.Chain[:1]
		}
	default:
		return false, fmt.Sprintf("unsupported TLSA usage %d", rec.Usage)
	}

	for _, c := range slots {
		got, err := recordCandidate(rec, c)
		if err != nil {
			return false, err.Error()
		}
		if strings.EqualFold(got, rec.Certificate) {
			return true, ""
		}
	}
	return false, fmt.Sprintf("expected %s, got none matching in chain", truncHex(rec.Certificate))
}

// recordCandidate returns the hex value the TLSA record should match for
// the (selector, matching_type) pair against this certificate slot. For
// matching_type 0 (Full), both sides are compared as hex-encoded DER.
func recordCandidate(rec TLSARecord, c tls.CertInfo) (string, error) {
	var source string
	switch rec.Selector {
	case SelectorCert:
		switch rec.MatchingType {
		case MatchingFull:
			der, err := base64.StdEncoding.DecodeString(c.DERBase64)
			if err != nil {
				return "", fmt.Errorf("decode cert DER: %w", err)
			}
			source = hex.EncodeToString(der)
		case MatchingSHA256:
			source = c.CertSHA256
		case MatchingSHA512:
			source = c.CertSHA512
		default:
			return "", fmt.Errorf("unsupported matching type %d", rec.MatchingType)
		}
	case SelectorSPKI:
		switch rec.MatchingType {
		case MatchingFull:
			spki, err := base64.StdEncoding.DecodeString(c.SPKIDERBase64)
			if err != nil {
				return "", fmt.Errorf("decode SPKI DER: %w", err)
			}
			source = hex.EncodeToString(spki)
		case MatchingSHA256:
			source = c.SPKISHA256
		case MatchingSHA512:
			source = c.SPKISHA512
		default:
			return "", fmt.Errorf("unsupported matching type %d", rec.MatchingType)
		}
	default:
		return "", fmt.Errorf("unsupported selector %d", rec.Selector)
	}
	return source, nil
}

// parseTLSProbeMap decodes one related-observation payload into its constituent
// probes, keyed by endpoint Ref. Returns nil on decode error (caller skips).
func parseTLSProbeMap(data []byte) map[string]tls.TLSProbe {
	var payload struct {
		Probes map[string]tls.TLSProbe `json:"probes"`
	}
	if err := json.Unmarshal(data, &payload); err != nil {
		return nil
	}
	return payload.Probes
}

// relatedTLSProbes indexes TLS probes fetched via GetRelated by endpoint Ref.
func relatedTLSProbes(ctx context.Context, obs sdk.ObservationGetter) (map[string]*tls.TLSProbe, string) {
	related, err := obs.GetRelated(ctx, tls.ObservationKeyTLSProbes)
	if err != nil {
		return nil, "related TLS probes unavailable: " + err.Error()
	}
	out := map[string]*tls.TLSProbe{}
	for _, ro := range related {
		for k, v := range parseTLSProbeMap(ro.Data) {
			out[k] = &v
		}
	}
	return out, ""
}

func truncHex(s string) string {
	if len(s) > 12 {
		return s[:12] + "…"
	}
	return s
}