package traverse

import (
	"context"
	"fmt"
	"net"
	"sort"
	"strings"

	"gitea.hansenits.com.au/hits/ExploreDNS/internal/dns"
	miekgdns "github.com/miekg/dns"
)

// TraverserConfig configures the behaviour of a Traverser.
type TraverserConfig struct {
	// MaxDepth is the maximum referral depth (non-zero refid components)
	// before a "Maxdepth N exceeded" exception is injected.
	MaxDepth int
	// QueryType is the DNS record type to query (e.g. dns.TypeA).
	QueryType uint16
	// RootConfig controls how root servers are discovered.
	RootConfig *dns.RootDiscoveryConfig
	// QueryConfig controls per-query transport parameters.
	QueryConfig *dns.QueryConfig
	// RootAddrs is an optional pre-seeded list of root server IP addresses.
	// When non-empty, root discovery via RootConfig is skipped and each
	// address becomes one root (named by its address).
	RootAddrs []net.IP
	// Hooks provides optional callbacks for traversal events.
	Hooks *TraverserHooks
	// Fast enables the completed-referral memo (traverser.rb @answered):
	// a referral identical to an earlier completed one (same qname/qclass/
	// qtype/server and per-IP weights) is replaced by it instead of being
	// walked again. Non-fast mode re-walks every branch.
	Fast bool
}

func DefaultTraverserConfig() *TraverserConfig {
	return &TraverserConfig{
		MaxDepth:  DefaultMaxDepth,
		QueryType: dns.TypeA,
		Fast:      true,
	}
}

// Traverser drives the traversal: it owns the packet-cached query client,
// the fast-mode memo and the explicit stack loop (traverser.rb).
type Traverser struct {
	config   *TraverserConfig
	client   *dns.Client
	exchange dns.ExchangeFunc
	// answered is the fast-mode memo of completed referrals.
	answered map[string]*Referral
	// seen maps every server name encountered to its IP addresses.
	seen map[string][]string
	// roots memoises root discovery so Roots() and Run() share one lookup.
	roots []StartServer
}

func NewTraverser(cfg *TraverserConfig) *Traverser {
	if cfg == nil {
		cfg = DefaultTraverserConfig()
	}
	if cfg.MaxDepth <= 0 {
		cfg.MaxDepth = DefaultMaxDepth
	}
	if cfg.QueryType == 0 {
		cfg.QueryType = dns.TypeA
	}
	return &Traverser{
		config:   cfg,
		client:   dns.NewClient(cfg.QueryConfig, nil),
		answered: make(map[string]*Referral),
		seen:     make(map[string][]string),
	}
}

// SetExchange injects a mock wire exchange into the single query path (both
// traversal queries and root discovery); tests use this so no packets leave
// the process.
func (t *Traverser) SetExchange(fn dns.ExchangeFunc) {
	t.exchange = fn
	t.client = dns.NewClient(t.config.QueryConfig, fn)
}

func (t *Traverser) SetHooks(hooks *TraverserHooks) {
	t.config.Hooks = hooks
}

// ServersEncountered returns every server name seen during the run mapped to
// its known IP addresses (traverser.rb servers_encountered).
func (t *Traverser) ServersEncountered() map[string][]string {
	return t.seen
}

// Roots performs (and memoises) root discovery, returning the start servers
// the traversal will begin from. Callers may use it before Run to report the
// initial root; Run reuses the memoised result.
func (t *Traverser) Roots(ctx context.Context) ([]StartServer, error) {
	if t.roots == nil {
		roots, err := t.rootStartServers(ctx)
		if err != nil {
			return nil, fmt.Errorf("root discovery: %w", err)
		}
		t.roots = roots
	}
	return t.roots, nil
}

// Run traverses the DNS for name and returns the synthetic rootroot node
// (never displayed) whose Stats aggregate every leaf outcome; the per-leaf
// probabilities sum to 1.0.
func (t *Traverser) Run(ctx context.Context, name string) (*Referral, error) {
	roots, err := t.Roots(ctx)
	if err != nil {
		return nil, err
	}

	cache := NewInfoCache(nil)
	cache.AddHints("", roots)

	root := &Referral{
		RefID:         "",
		Qname:         canonicalName(toASCII(name)),
		Qclass:        miekgdns.ClassINET,
		Qtype:         t.config.QueryType,
		NSAType:       dns.TypeA,
		Server:        "",
		Bailiwick:     "",
		InfoCache:     cache,
		Status:        RefStatusNormal,
		Responses:     make(map[string]*ServerResponse),
		Children:      make(map[string][]*Referral),
		ServerWeights: make(map[string]float64),
		client:        t.client,
		maxdepth:      t.config.MaxDepth,
	}
	t.config.Hooks.emit(StageNew, root, "")

	if err := t.run(ctx, root); err != nil {
		return root, err
	}
	return root, nil
}

// stack markers mirroring Ruby's :calc_resolve / :calc_answer placeholders:
// the referral is revisited after its resolves/children finished, giving
// post-order statistics calculation without recursion.
type stackMarker int

const (
	markerNone stackMarker = iota
	markerCalcResolve
	markerCalcAnswer
)

type stackEntry struct {
	ref    *Referral
	marker stackMarker
}

func (t *Traverser) run(ctx context.Context, root *Referral) error {
	stack := []stackEntry{{ref: root}}
	pop := func() stackEntry {
		e := stack[len(stack)-1]
		stack = stack[:len(stack)-1]
		return e
	}

	for len(stack) > 0 {
		select {
		case <-ctx.Done():
			return fmt.Errorf("traversal cancelled: %w", ctx.Err())
		default:
		}

		e := pop()
		r := e.ref

		switch e.marker {
		case markerCalcResolve:
			r.resolveCalculate()
			t.config.Hooks.emit(StageResolve, r, "")
			stack = append(stack, stackEntry{ref: r}) // now needs processing
			continue
		case markerCalcAnswer:
			r.answerCalculate()
			t.config.Hooks.emit(StageAnswer, r, "")
			if t.config.Fast && r.Status == RefStatusNormal && !hasLameResponse(r) {
				t.answered[fastKey(r)] = r
			}
			if !r.IsRootRoot() {
				t.recordSeen(r)
			}
			continue
		}

		// A new item. Fast mode: an identical completed referral replaces
		// this one wholesale. noglue/loop nodes are excluded because their
		// stats carry node-specific attributes and are cheap to recreate.
		if t.config.Fast && r.Parent != nil {
			if memo, ok := t.answered[fastKey(r)]; ok && !r.isNoGlue() && !r.isLoop() {
				r.Parent.replaceChild(r, memo)
				t.config.Hooks.emit(StageAnswerFast, r, memo.RefID)
				continue
			}
		}

		t.config.Hooks.emit(StageStart, r, "")

		if !r.Resolved() {
			// Push the resolve subtree with a calc_resolve placeholder so the
			// weights are folded in once every resolve leaf completed.
			stack = append(stack, stackEntry{ref: r, marker: markerCalcResolve})
			resolves, err := r.resolve()
			if err != nil {
				return err
			}
			for _, c := range resolves {
				t.config.Hooks.emit(StageNew, c, "")
			}
			for i := len(resolves) - 1; i >= 0; i-- {
				stack = append(stack, stackEntry{ref: resolves[i]})
			}
			continue
		}

		stack = append(stack, stackEntry{ref: r, marker: markerCalcAnswer})
		childrenSets, err := r.process(ctx)
		if err != nil {
			return err
		}

		seenParentIP := make(map[string]bool)
		var flat []*Referral
		for _, set := range childrenSets {
			for _, c := range set {
				if len(childrenSets) > 1 && !seenParentIP[c.ParentIP] {
					t.config.Hooks.emit(StageNewReferralSet, c, "")
					seenParentIP[c.ParentIP] = true
				}
				stage, earlier := StageNew, ""
				if t.config.Fast {
					if memo, ok := t.answered[fastKey(c)]; ok {
						stage, earlier = StageNewFast, memo.RefID
					}
				}
				t.config.Hooks.emit(stage, c, earlier)
				flat = append(flat, c)
			}
		}
		for i := len(flat) - 1; i >= 0; i-- {
			stack = append(stack, stackEntry{ref: flat[i]})
		}
	}
	return nil
}

// fastKey is the fast-mode memo key (traverser.rb): qname/qclass/qtype/
// server plus the per-IP weights, lowercased.
func fastKey(r *Referral) string {
	return strings.ToLower(fmt.Sprintf("%s:%s:%s:%s:%s",
		r.Qname, ClassToString(r.Qclass), TypeToString(r.Qtype), r.Server, r.TxtIPsVerbose()))
}

func hasLameResponse(r *Referral) bool {
	for _, resp := range r.Responses {
		if resp.Status == StatusReferralLame {
			return true
		}
	}
	return false
}

func (t *Traverser) recordSeen(r *Referral) {
	name := strings.ToLower(r.Server)
	existing := t.seen[name]
	for _, ip := range r.IPsAsArray() {
		found := false
		for _, have := range existing {
			if have == ip {
				found = true
				break
			}
		}
		if !found {
			existing = append(existing, ip)
		}
	}
	t.seen[name] = existing
}

// rootStartServers returns the root servers as start-server hints: either
// the pre-seeded RootAddrs or the servers found via root discovery (one by
// default, all of them with AllRoots). IPv4 only, like the reference.
func (t *Traverser) rootStartServers(ctx context.Context) ([]StartServer, error) {
	if len(t.config.RootAddrs) > 0 {
		var out []StartServer
		for _, ip := range t.config.RootAddrs {
			if v4 := ip.To4(); v4 != nil {
				out = append(out, StartServer{Name: v4.String(), IPs: []string{v4.String()}})
			}
		}
		if len(out) == 0 {
			return nil, fmt.Errorf("no usable IPv4 root addresses")
		}
		return out, nil
	}

	rootCfg := t.config.RootConfig
	if t.exchange != nil {
		var cp dns.RootDiscoveryConfig
		if rootCfg != nil {
			cp = *rootCfg
		}
		cp.Exchange = t.exchange
		rootCfg = &cp
	}

	servers, err := dns.DiscoverRoots(ctx, rootCfg)
	if err != nil {
		return nil, err
	}

	var out []StartServer
	for _, srv := range servers {
		var ips []string
		for _, ip := range srv.IPv4 {
			ips = append(ips, ip.String())
		}
		if len(ips) == 0 {
			continue
		}
		out = append(out, StartServer{Name: canonicalName(srv.Name), IPs: ips})
	}
	if len(out) == 0 {
		return nil, fmt.Errorf("no root servers with IPv4 addresses")
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
	return out, nil
}