package checker

import (
	"context"
	"fmt"
	"sort"

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

// consensusRule emits one state per RRset summarising how much of the probed
// resolver set agrees on its answer. It covers the "partial propagation"
// case (several distinct NOERROR signatures observed).
type consensusRule struct{}

func (r *consensusRule) Name() string { return "resolver_propagation.consensus" }
func (r *consensusRule) Description() string {
	return "Checks that public resolvers agree on a single answer for each probed RRset."
}
func (r *consensusRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
	data, errSt := loadData(ctx, obs)
	if errSt != nil {
		return []sdk.CheckState{*errSt}
	}
	if len(data.Resolvers) == 0 || len(data.RRsets) == 0 {
		return []sdk.CheckState{{Status: sdk.StatusUnknown,
			Code:    "resolver_propagation.consensus.skipped",
			Message: "no resolver probes available"}}
	}

	keys := sortedRRsetKeys(data)
	var states []sdk.CheckState
	for _, key := range keys {
		v := data.RRsets[key]

		unfilteredNOERRORSigs := map[string]bool{}
		for _, g := range v.Groups {
			if g.Rcode != "NOERROR" {
				continue
			}
			for _, rid := range g.Resolvers {
				rv := data.Resolvers[rid]
				if rv != nil && !rv.Filtered {
					unfilteredNOERRORSigs[g.Signature] = true
					break
				}
			}
		}

		switch {
		case v.ConsensusSig == "" && len(v.Groups) == 0:
			states = append(states, infoState("resolver_propagation.consensus.no_data", key,
				fmt.Sprintf("no resolver returned a usable answer for %s", key)))
		case len(unfilteredNOERRORSigs) > 1:
			states = append(states, warnState(CodePartialPropagation, key,
				fmt.Sprintf("%d distinct answers seen across public resolvers for %s, change is mid-propagation",
					len(unfilteredNOERRORSigs), key)))
		default:
			states = append(states, sdk.CheckState{
				Status:  sdk.StatusOK,
				Code:    "resolver_propagation.consensus.ok",
				Subject: key,
				Message: fmt.Sprintf("all %d probed resolver(s) agree on %s", len(v.Agreeing), key),
			})
		}
	}
	return states
}

// authoritativeMatchRule checks the consensus against the answer served by
// the zone's own authoritative servers.
type authoritativeMatchRule struct{}

func (r *authoritativeMatchRule) Name() string { return "resolver_propagation.matches_authoritative" }
func (r *authoritativeMatchRule) Description() string {
	return "Checks that the public consensus matches the answer served by the zone's authoritative nameservers."
}
func (r *authoritativeMatchRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
	data, errSt := loadData(ctx, obs)
	if errSt != nil {
		return []sdk.CheckState{*errSt}
	}

	var states []sdk.CheckState
	anyExpected := false
	for _, key := range sortedRRsetKeys(data) {
		v := data.RRsets[key]
		if v.Expected == "" {
			continue
		}
		anyExpected = true
		switch {
		case v.ConsensusSig == "":
			states = append(states, critState("resolver_propagation.matches_authoritative.no_consensus", key,
				fmt.Sprintf("no public resolver returned a usable answer for %s (authoritative answer is known)", key)))
		case !v.MatchesExpected:
			states = append(states, critState(CodeAnswerDrift, key,
				fmt.Sprintf("consensus of public resolvers for %s differs from the authoritative answer, wait for TTL expiry or force a flush", key)))
		default:
			states = append(states, sdk.CheckState{
				Status: sdk.StatusOK, Code: "resolver_propagation.matches_authoritative.ok", Subject: key,
				Message: fmt.Sprintf("public consensus for %s matches the authoritative answer", key),
			})
		}
	}
	if !anyExpected {
		return []sdk.CheckState{{Status: sdk.StatusUnknown,
			Code:    "resolver_propagation.matches_authoritative.skipped",
			Message: "authoritative nameservers were unreachable; cannot compare consensus to ground truth"}}
	}
	return states
}

// nxdomainRule flags RRsets returning NXDOMAIN on some (but not all) resolvers.
type nxdomainRule struct{}

func (r *nxdomainRule) Name() string { return "resolver_propagation.nxdomain" }
func (r *nxdomainRule) Description() string {
	return "Flags RRsets for which some resolvers return NXDOMAIN while others return NOERROR."
}
func (r *nxdomainRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
	data, errSt := loadData(ctx, obs)
	if errSt != nil {
		return []sdk.CheckState{*errSt}
	}
	var states []sdk.CheckState
	for _, key := range sortedRRsetKeys(data) {
		v := data.RRsets[key]
		var nxList []string
		for _, g := range v.Groups {
			if g.Rcode == "NXDOMAIN" {
				nxList = append(nxList, g.Resolvers...)
			}
		}
		if len(nxList) > 0 && len(nxList) < len(data.Resolvers) {
			states = append(states, critState(CodeUnexpectedNXDOMAIN, key,
				fmt.Sprintf("%s resolved as NXDOMAIN on %d resolver(s): %s", key, len(nxList), firstN(nxList, 6))))
		}
	}
	if len(states) == 0 {
		return []sdk.CheckState{passState("resolver_propagation.nxdomain.ok",
			"No resolver unexpectedly returns NXDOMAIN.")}
	}
	return states
}

// servfailRule flags RRsets returning SERVFAIL on any resolver.
type servfailRule struct{}

func (r *servfailRule) Name() string { return "resolver_propagation.servfail" }
func (r *servfailRule) Description() string {
	return "Flags RRsets for which any resolver returns SERVFAIL (usually DNSSEC or reachability failure)."
}
func (r *servfailRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
	data, errSt := loadData(ctx, obs)
	if errSt != nil {
		return []sdk.CheckState{*errSt}
	}
	var states []sdk.CheckState
	for _, key := range sortedRRsetKeys(data) {
		v := data.RRsets[key]
		var sfList []string
		for _, g := range v.Groups {
			if g.Rcode == "SERVFAIL" {
				sfList = append(sfList, g.Resolvers...)
			}
		}
		if len(sfList) > 0 {
			states = append(states, critState(CodeUnexpectedSERVFAIL, key,
				fmt.Sprintf("%s returned SERVFAIL on %d resolver(s): %s", key, len(sfList), firstN(sfList, 6))))
		}
	}
	if len(states) == 0 {
		return []sdk.CheckState{passState("resolver_propagation.servfail.ok",
			"No resolver returns SERVFAIL.")}
	}
	return states
}

// regionalSplitRule flags regions in which all resolvers agree on an answer
// that diverges from the global consensus.
type regionalSplitRule struct{}

func (r *regionalSplitRule) Name() string { return "resolver_propagation.regional_split" }
func (r *regionalSplitRule) Description() string {
	return "Flags regions in which every resolver agrees on an answer that differs from the global consensus."
}
func (r *regionalSplitRule) Evaluate(ctx context.Context, obs sdk.ObservationGetter, _ sdk.CheckerOptions) []sdk.CheckState {
	data, errSt := loadData(ctx, obs)
	if errSt != nil {
		return []sdk.CheckState{*errSt}
	}

	var states []sdk.CheckState
	for _, key := range sortedRRsetKeys(data) {
		v := data.RRsets[key]
		region2sig := map[string]map[string]int{}
		for _, g := range v.Groups {
			for _, rid := range g.Resolvers {
				rv := data.Resolvers[rid]
				if rv == nil || rv.Filtered {
					continue
				}
				if region2sig[rv.Region] == nil {
					region2sig[rv.Region] = map[string]int{}
				}
				region2sig[rv.Region][g.Signature]++
			}
		}
		regions := make([]string, 0, len(region2sig))
		for r := range region2sig {
			regions = append(regions, r)
		}
		sort.Strings(regions)
		for _, region := range regions {
			sigs := region2sig[region]
			if len(sigs) != 1 {
				continue
			}
			var only string
			for s := range sigs {
				only = s
			}
			if only != "" && only != v.ConsensusSig {
				states = append(states, warnState(CodeRegionalSplit, region+" "+key,
					fmt.Sprintf("all %s resolvers agree on an answer that differs from the global consensus for %s",
						regionLabel(region), key)))
			}
		}
	}
	if len(states) == 0 {
		return []sdk.CheckState{passState("resolver_propagation.regional_split.ok",
			"No region is split from the global consensus.")}
	}
	return states
}

func sortedRRsetKeys(data *ResolverPropagationData) []string {
	keys := make([]string, 0, len(data.RRsets))
	for k := range data.RRsets {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	return keys
}