package resolver_test

import (
	"context"
	"errors"
	"fmt"
	"slices"
	"sort"
	"strings"
	"testing"

	"sneak.berlin/go/dnswatcher/internal/livedns"
	"sneak.berlin/go/dnswatcher/internal/resolver"
)

// ----------------------------------------------------------------
// Live DNS test support
// ----------------------------------------------------------------
//
// Every test in this package resolves against the real, live DNS —
// see TESTING.md. Each live operation below goes through
// livedns.Retry, which bounds how many resolutions are in flight at
// once and retries an operation that got no answer (see package
// livedns).
//
// Where an assertion spans several independent nameservers, a quorum
// is enough: a strict majority answering as expected. A server that
// fails to answer is tolerated, while a server that answers *wrongly*
// still fails the test.
//
// That tolerance is expressed as an ALLOWLIST of sanctioned statuses,
// never as a blocklist of known-bad ones. A blocklist bans the one
// wrong answer its author thought of and silently admits every other
// status, including any added to the resolver later; an allowlist
// fails on anything nobody explicitly sanctioned. Silence (timeout,
// error) is the only thing quorum exists to tolerate. A *wrong
// answer* — nxdomain for a name that exists, ok for one that does
// not, nodata for either — is never tolerated at any count.

// minNameservers is the smallest nameserver count a well-run zone is
// expected to publish.
const minNameservers = 2

// errLiveNoQuorum reports that too few of a domain's nameservers
// answered for a quorum assertion to be made.
var errLiveNoQuorum = errors.New("no nameserver quorum")

// liveQuorum is how many of total nameservers must agree for a
// multi-nameserver assertion to hold: a strict majority.
func liveQuorum(total int) int {
	if total < 1 {
		return 1
	}

	return total/2 + 1
}

// countStatus counts the responses carrying the given status.
func countStatus(
	results map[string]*resolver.NameserverResponse,
	status string,
) int {
	n := 0

	for _, resp := range results {
		if resp.Status == status {
			n++
		}
	}

	return n
}

// liveAnswerStatuses is the closed set of statuses that count as a
// nameserver having ANSWERED at all, whether or not the test agrees
// with the answer. It is deliberately an allowlist: a status added
// to the resolver later is treated as silence, so it can only ever
// cause a retry and then a loud failure, never a quiet pass.
func liveAnswerStatuses() []string {
	return []string{
		resolver.StatusOK,
		resolver.StatusNXDomain,
		resolver.StatusNoData,
	}
}

// answeredCount counts the nameservers that produced an answer of
// any kind, as opposed to failing or timing out.
func answeredCount(
	results map[string]*resolver.NameserverResponse,
) int {
	answers := liveAnswerStatuses()

	n := 0

	for _, resp := range results {
		if slices.Contains(answers, resp.Status) {
			n++
		}
	}

	return n
}

// unsanctionedStatuses returns "nameserver=status" for every result
// whose status the caller did not explicitly sanction, sorted for a
// stable failure message. Callers pass the full closed set they will
// accept — the expected answer plus whichever non-answers (timeout,
// error) quorum is allowed to tolerate — so that any status outside
// it fails the test by name.
func unsanctionedStatuses(
	results map[string]*resolver.NameserverResponse,
	allowed ...string,
) []string {
	offenders := make([]string, 0, len(results))

	for ns, resp := range results {
		if slices.Contains(allowed, resp.Status) {
			continue
		}

		offenders = append(
			offenders, fmt.Sprintf("%s=%s", ns, resp.Status),
		)
	}

	sort.Strings(offenders)

	return offenders
}

// describeStatuses renders per-nameserver statuses for use in
// assertion failure messages.
func describeStatuses(
	results map[string]*resolver.NameserverResponse,
) string {
	parts := make([]string, 0, len(results))
	for ns, resp := range results {
		parts = append(
			parts, fmt.Sprintf("%s=%s", ns, resp.Status),
		)
	}

	sort.Strings(parts)

	return strings.Join(parts, " ")
}

// ----------------------------------------------------------------
// Live operation wrappers
// ----------------------------------------------------------------

// liveFindAuthoritative resolves a domain's authoritative
// nameservers, retrying until the delegation chain can be walked.
func liveFindAuthoritative(
	t *testing.T,
	r *resolver.Resolver,
	domain string,
) []string {
	t.Helper()

	var out []string

	livedns.Retry(
		t,
		"FindAuthoritativeNameservers("+domain+")",
		func(ctx context.Context) error {
			ns, err := r.FindAuthoritativeNameservers(ctx, domain)
			if err != nil {
				return err
			}

			if len(ns) == 0 {
				return fmt.Errorf(
					"%w: %s has no nameservers",
					livedns.ErrNoAnswer, domain,
				)
			}

			out = ns

			return nil
		},
	)

	return out
}

// liveLookupNS is liveFindAuthoritative through the LookupNS entry
// point, so that both entry points stay independently exercised.
func liveLookupNS(
	t *testing.T,
	r *resolver.Resolver,
	domain string,
) []string {
	t.Helper()

	var out []string

	livedns.Retry(
		t,
		"LookupNS("+domain+")",
		func(ctx context.Context) error {
			ns, err := r.LookupNS(ctx, domain)
			if err != nil {
				return err
			}

			if len(ns) == 0 {
				return fmt.Errorf(
					"%w: %s has no nameservers",
					livedns.ErrNoAnswer, domain,
				)
			}

			out = ns

			return nil
		},
	)

	return out
}

// liveQueryNameserver queries one nameserver, retrying while that
// nameserver fails to answer. NXDOMAIN and NODATA are answers and
// are returned to the caller to assert on.
func liveQueryNameserver(
	t *testing.T,
	r *resolver.Resolver,
	nameserver string,
	hostname string,
) *resolver.NameserverResponse {
	t.Helper()

	what := fmt.Sprintf(
		"QueryNameserver(%s, %s)", nameserver, hostname,
	)

	var out *resolver.NameserverResponse

	livedns.Retry(
		t,
		what,
		func(ctx context.Context) error {
			resp, err := r.QueryNameserver(
				ctx, nameserver, hostname,
			)
			if err != nil {
				return err
			}

			if resp.Status == resolver.StatusTimeout ||
				resp.Status == resolver.StatusError {
				return fmt.Errorf(
					"%w: %s returned %s: %s",
					livedns.ErrNoAnswer, nameserver,
					resp.Status, resp.Error,
				)
			}

			out = resp

			return nil
		},
	)

	return out
}

// liveQueryAllNameservers queries every authoritative nameserver
// for a hostname, retrying until a quorum of them has answered.
// Individual nameservers that stay silent are left in the result
// for the caller to account for.
func liveQueryAllNameservers(
	t *testing.T,
	r *resolver.Resolver,
	hostname string,
) map[string]*resolver.NameserverResponse {
	t.Helper()

	var out map[string]*resolver.NameserverResponse

	livedns.Retry(
		t,
		"QueryAllNameservers("+hostname+")",
		func(ctx context.Context) error {
			results, err := r.QueryAllNameservers(ctx, hostname)
			if err != nil {
				return err
			}

			if len(results) == 0 {
				return fmt.Errorf(
					"%w: no nameservers queried for %s",
					livedns.ErrNoAnswer, hostname,
				)
			}

			answered := answeredCount(results)
			if answered < liveQuorum(len(results)) {
				return fmt.Errorf(
					"%w: %d of %d answered: %s",
					errLiveNoQuorum, answered,
					len(results), describeStatuses(results),
				)
			}

			out = results

			return nil
		},
	)

	return out
}

// liveResolveIPs resolves a hostname that is expected to have
// addresses, retrying until at least one is returned.
func liveResolveIPs(
	t *testing.T,
	r *resolver.Resolver,
	hostname string,
) []string {
	t.Helper()

	var out []string

	livedns.Retry(
		t,
		"ResolveIPAddresses("+hostname+")",
		func(ctx context.Context) error {
			ips, err := r.ResolveIPAddresses(ctx, hostname)
			if err != nil {
				return err
			}

			if len(ips) == 0 {
				return fmt.Errorf(
					"%w: no addresses for %s",
					livedns.ErrNoAnswer, hostname,
				)
			}

			out = ips

			return nil
		},
	)

	return out
}

// liveResolveIPsAllowingEmpty resolves a hostname that may legitimately
// have no addresses, so the empty result is returned rather than
// retried. Used for names that must not exist; the corresponding
// QueryAllNameservers test is what proves the nameservers actively
// said NXDOMAIN rather than merely staying silent.
func liveResolveIPsAllowingEmpty(
	t *testing.T,
	r *resolver.Resolver,
	hostname string,
) []string {
	t.Helper()

	var out []string

	livedns.Retry(
		t,
		"ResolveIPAddresses("+hostname+")",
		func(ctx context.Context) error {
			ips, err := r.ResolveIPAddresses(ctx, hostname)
			if err != nil {
				return err
			}

			out = ips

			return nil
		},
	)

	return out
}