package api

import (
	"context"
	"crypto/rand"
	"encoding/json"
	"fmt"
	"io/fs"
	"net/http"
	"sync"
	"time"

	"github.com/hits/ExploreDNS/internal/config"
	idns "github.com/hits/ExploreDNS/internal/dns"
	"github.com/hits/ExploreDNS/internal/traverse"
)

// Job status values.
const (
	statusRunning  = "running"
	statusComplete = "complete"
	statusError    = "error"
)

// jobTTL is how long completed jobs are retained in memory.
const jobTTL = time.Hour

// TraverseRequest is the JSON body for POST /api/traverse.
type TraverseRequest struct {
	Domain   string `json:"domain"`
	Type     string `json:"type"`
	AllRoots bool   `json:"all_roots"`
}

// TraverseStartResponse is returned by POST /api/traverse.
type TraverseStartResponse struct {
	ID     string `json:"id"`
	Status string `json:"status"`
}

// ProgressEvent carries a single traversal hook event.
type ProgressEvent struct {
	Stage     string `json:"stage"`
	Depth     int    `json:"depth"`
	Name      string `json:"name"`
	QType     string `json:"qtype"`
	Server    string `json:"server,omitempty"`
	Bailiwick string `json:"bailiwick,omitempty"`
	IsResolve bool   `json:"is_resolve,omitempty"`
}

// ResultItem is a single traversal step result for API consumers.
type ResultItem struct {
	Depth        int      `json:"depth"`
	Probability  float64  `json:"probability"`
	ResponseType string   `json:"response_type"`
	Server       string   `json:"server,omitempty"`
	Answers      []string `json:"answers,omitempty"`
	CNAMEChain   []string `json:"cname_chain,omitempty"`
}

// TraversalJob holds all state for a single asynchronous traversal.
type TraversalJob struct {
	ID        string          `json:"id"`
	Status    string          `json:"status"`
	Domain    string          `json:"domain"`
	QueryType string          `json:"query_type"`
	Results   []ResultItem    `json:"results,omitempty"`
	Progress  []ProgressEvent `json:"progress,omitempty"`
	Error     string          `json:"error,omitempty"`
	StartedAt time.Time       `json:"started_at"`
	DoneAt    *time.Time      `json:"done_at,omitempty"`

	mu     sync.RWMutex
	subs   []chan ProgressEvent
	cancel context.CancelFunc
}

// subscribe returns a channel that receives future progress events.
// The channel is closed when the job finishes.
func (j *TraversalJob) subscribe() <-chan ProgressEvent {
	ch := make(chan ProgressEvent, 32)
	j.mu.Lock()
	j.subs = append(j.subs, ch)
	j.mu.Unlock()
	return ch
}

// publish sends an event to all current subscribers.
func (j *TraversalJob) publish(ev ProgressEvent) {
	j.mu.Lock()
	defer j.mu.Unlock()
	for _, ch := range j.subs {
		select {
		case ch <- ev:
		default:
			// subscriber too slow — drop rather than block traversal
		}
	}
}

// closeSubscribers drains and closes all subscriber channels.
func (j *TraversalJob) closeSubscribers() {
	j.mu.Lock()
	defer j.mu.Unlock()
	for _, ch := range j.subs {
		close(ch)
	}
	j.subs = nil
}

// store is a thread-safe in-memory job registry.
type store struct {
	mu   sync.RWMutex
	jobs map[string]*TraversalJob
}

func newStore() *store {
	return &store{jobs: make(map[string]*TraversalJob)}
}

func (s *store) get(id string) (*TraversalJob, bool) {
	s.mu.RLock()
	defer s.mu.RUnlock()
	j, ok := s.jobs[id]
	return j, ok
}

func (s *store) set(j *TraversalJob) {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.jobs[j.ID] = j
}

// cleanup removes completed jobs older than jobTTL.
func (s *store) cleanup() {
	cutoff := time.Now().Add(-jobTTL)
	s.mu.Lock()
	defer s.mu.Unlock()
	for id, j := range s.jobs {
		j.mu.RLock()
		done := j.DoneAt != nil && j.DoneAt.Before(cutoff)
		j.mu.RUnlock()
		if done {
			delete(s.jobs, id)
		}
	}
}

// Handler wires together the HTTP routes and the job store.
type Handler struct {
	st  *store
	mux *http.ServeMux
}

func newHandler() *Handler {
	h := &Handler{
		st:  newStore(),
		mux: http.NewServeMux(),
	}

	h.mux.HandleFunc("POST /api/traverse", h.startTraversal)
	h.mux.HandleFunc("GET /api/traverse/{id}/stream", h.streamTraversal)
	h.mux.HandleFunc("GET /api/traverse/{id}", h.getTraversal)
	h.mux.HandleFunc("GET /api/health", h.health)

	// periodic cleanup
	go func() {
		t := time.NewTicker(10 * time.Minute)
		defer t.Stop()
		for range t.C {
			h.st.cleanup()
		}
	}()

	return h
}

// registerStatic adds SPA static file serving to the mux.
func (h *Handler) registerStatic(sub fs.FS) {
	fileServer := http.FileServer(http.FS(sub))
	h.mux.Handle("/", spaHandler{fileServer: fileServer, fs: sub})
}

// health handles GET /api/health.
func (h *Handler) health(w http.ResponseWriter, _ *http.Request) {
	writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
}

// startTraversal handles POST /api/traverse.
func (h *Handler) startTraversal(w http.ResponseWriter, r *http.Request) {
	var req TraverseRequest
	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
		writeError(w, http.StatusBadRequest, "invalid request body: "+err.Error())
		return
	}
	if req.Domain == "" {
		writeError(w, http.StatusBadRequest, "domain is required")
		return
	}
	queryType := req.Type
	if queryType == "" {
		queryType = "A"
	}
	qtype, err := config.ParseQueryType(queryType)
	if err != nil {
		writeError(w, http.StatusBadRequest, err.Error())
		return
	}

	id := newUUID()
	ctx, cancel := context.WithCancel(context.Background())

	job := &TraversalJob{
		ID:        id,
		Status:    statusRunning,
		Domain:    req.Domain,
		QueryType: queryType,
		StartedAt: time.Now(),
		cancel:    cancel,
	}
	h.st.set(job)

	go h.runTraversal(ctx, job, req.Domain, qtype, req.AllRoots)

	writeJSON(w, http.StatusAccepted, TraverseStartResponse{ID: id, Status: statusRunning})
}

// getTraversal handles GET /api/traverse/{id}.
func (h *Handler) getTraversal(w http.ResponseWriter, r *http.Request) {
	id := r.PathValue("id")
	job, ok := h.st.get(id)
	if !ok {
		writeError(w, http.StatusNotFound, "traversal not found")
		return
	}

	job.mu.RLock()
	// Copy the fields we need while holding the read lock.
	snapshot := struct {
		ID        string          `json:"id"`
		Status    string          `json:"status"`
		Domain    string          `json:"domain"`
		QueryType string          `json:"query_type"`
		Results   []ResultItem    `json:"results,omitempty"`
		Progress  []ProgressEvent `json:"progress,omitempty"`
		Error     string          `json:"error,omitempty"`
		StartedAt time.Time       `json:"started_at"`
		DoneAt    *time.Time      `json:"done_at,omitempty"`
	}{
		ID:        job.ID,
		Status:    job.Status,
		Domain:    job.Domain,
		QueryType: job.QueryType,
		Results:   job.Results,
		Progress:  job.Progress,
		Error:     job.Error,
		StartedAt: job.StartedAt,
		DoneAt:    job.DoneAt,
	}
	job.mu.RUnlock()

	writeJSON(w, http.StatusOK, snapshot)
}

// streamTraversal handles GET /api/traverse/{id}/stream (SSE).
func (h *Handler) streamTraversal(w http.ResponseWriter, r *http.Request) {
	id := r.PathValue("id")
	job, ok := h.st.get(id)
	if !ok {
		writeError(w, http.StatusNotFound, "traversal not found")
		return
	}

	// Set SSE headers before any write.
	w.Header().Set("Content-Type", "text/event-stream")
	w.Header().Set("Cache-Control", "no-cache")
	w.Header().Set("Connection", "keep-alive")
	w.Header().Set("X-Accel-Buffering", "no")

	flusher, ok := w.(http.Flusher)
	if !ok {
		writeError(w, http.StatusInternalServerError, "streaming not supported")
		return
	}

	// Subscribe before snapshotting progress so we don't miss events between
	// the two operations.
	sub := job.subscribe()

	// Replay events already recorded.
	job.mu.RLock()
	past := make([]ProgressEvent, len(job.Progress))
	copy(past, job.Progress)
	alreadyDone := job.Status != statusRunning
	job.mu.RUnlock()

	sendSSE := func(ev ProgressEvent) bool {
		b, err := json.Marshal(ev)
		if err != nil {
			return false
		}
		_, err = fmt.Fprintf(w, "data: %s\n\n", b)
		flusher.Flush()
		return err == nil
	}

	for _, ev := range past {
		if !sendSSE(ev) {
			return
		}
	}

	if alreadyDone {
		fmt.Fprintf(w, "event: done\ndata: {}\n\n")
		flusher.Flush()
		return
	}

	ctx := r.Context()
	for {
		select {
		case <-ctx.Done():
			return
		case ev, open := <-sub:
			if !open {
				fmt.Fprintf(w, "event: done\ndata: {}\n\n")
				flusher.Flush()
				return
			}
			if !sendSSE(ev) {
				return
			}
		}
	}
}

// runTraversal executes a traversal in the background and updates the job.
func (h *Handler) runTraversal(ctx context.Context, job *TraversalJob, domain string, qtype uint16, allRoots bool) {
	defer func() {
		if r := recover(); r != nil {
			now := time.Now()
			job.mu.Lock()
			job.Status = statusError
			job.Error = fmt.Sprintf("panic: %v", r)
			job.DoneAt = &now
			job.mu.Unlock()
		}
		job.cancel()
		job.closeSubscribers()
	}()

	cfg := traverse.DefaultTraverserConfig()
	cfg.QueryType = qtype
	if allRoots {
		cfg.RootConfig = &idns.RootDiscoveryConfig{AllRoots: true}
	}
	cfg.Hooks = &traverse.TraverserHooks{
		OnEvent: func(event traverse.TraversalEvent) {
			ref := event.Result.Referral
			if ref == nil {
				return
			}
			stage := "start"
			if event.Stage == traverse.EventComplete {
				stage = "complete"
			}
			server := ""
			if event.Result.Response != nil && event.Result.Response.Server != nil {
				server = event.Result.Response.Server.String()
			}
			ev := ProgressEvent{
				Stage:     stage,
				Depth:     ref.Depth,
				Name:      trimFQDN(ref.Name),
				QType:     idns.QNameType(ref.Qtype),
				Server:    server,
				Bailiwick: trimFQDN(ref.Bailiwick),
				IsResolve: event.IsResolve,
			}

			job.mu.Lock()
			job.Progress = append(job.Progress, ev)
			job.mu.Unlock()

			job.publish(ev)
		},
	}

	tr := traverse.NewTraverser(cfg)
	rawResults, err := tr.Traverse(ctx, domain)

	now := time.Now()
	job.mu.Lock()
	defer job.mu.Unlock()

	job.DoneAt = &now

	if err != nil && ctx.Err() == nil {
		job.Status = statusError
		job.Error = err.Error()
		return
	}

	items := make([]ResultItem, 0, len(rawResults))
	for _, r := range rawResults {
		items = append(items, toResultItem(r))
	}
	job.Results = items
	job.Status = statusComplete
}

// toResultItem converts a TraversalResult to its API representation.
func toResultItem(r traverse.TraversalResult) ResultItem {
	item := ResultItem{}
	if r.Referral != nil {
		item.Depth = r.Referral.Depth
		item.Probability = r.Referral.Prob
	}
	if r.Response != nil {
		item.ResponseType = r.Response.Type.String()
		if r.Response.Server != nil {
			item.Server = r.Response.Server.String()
		}
		if r.Response.Decoded != nil {
			for _, rr := range r.Response.Decoded.Answers {
				item.Answers = append(item.Answers, idns.FormatRecord(rr))
			}
			item.CNAMEChain = append(item.CNAMEChain, r.Response.Decoded.CNAMEChain...)
		}
	}
	return item
}

// trimFQDN removes a trailing dot from an FQDN for nicer output.
func trimFQDN(s string) string {
	if len(s) > 0 && s[len(s)-1] == '.' {
		return s[:len(s)-1]
	}
	return s
}

// spaHandler serves static files and falls back to index.html for unknown paths.
type spaHandler struct {
	fileServer http.Handler
	fs         fs.FS
}

func (h spaHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	// If the file exists, serve it directly.
	if _, err := fs.Stat(h.fs, r.URL.Path[1:]); err == nil {
		h.fileServer.ServeHTTP(w, r)
		return
	}
	// Serve index.html for SPA client-side routing.
	r2 := r.Clone(r.Context())
	r2.URL.Path = "/"
	h.fileServer.ServeHTTP(w, r2)
}

// writeJSON encodes v as JSON and writes it to w with the given status code.
func writeJSON(w http.ResponseWriter, status int, v any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	_ = json.NewEncoder(w).Encode(v)
}

// writeError writes a JSON error response.
func writeError(w http.ResponseWriter, status int, msg string) {
	writeJSON(w, status, map[string]string{"error": msg})
}

// newUUID returns a random UUID v4 string.
func newUUID() string {
	var b [16]byte
	_, _ = rand.Read(b[:])
	b[6] = (b[6] & 0x0f) | 0x40 // version 4
	b[8] = (b[8] & 0x3f) | 0x80 // variant bits
	return fmt.Sprintf("%08x-%04x-%04x-%04x-%012x", b[0:4], b[4:6], b[6:8], b[8:10], b[10:])
}