package mesh

import (
	"bytes"
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"hash/crc32"
	"io"
	"log"
	"net"
	"net/http"
	"os"
	"slices"
	"strconv"
	"strings"
	"sync"
	"time"
)

/*** Types & Config ***/

type Config struct {
	BindAddr         string   // e.g. ":9090"
	AdvertURL        string   // e.g. "http://10.0.0.5:9090"
	Seeds            []string // other peers' mesh base URLs
	ClusterSecret    string   // HMAC key
	EnableDiscovery  bool
	DiscoveryAddress string // "239.8.8.8:9898"
}

type Peer struct {
	URL       string    `json:"url"`
	LastSeen  time.Time `json:"lastSeen"`
	Self      bool      `json:"self"`
	OwnerHint int       `json:"ownerHint"` // optional
}

type Item struct {
	ID        string `json:"id"`
	Name      string `json:"name"`
	UpdatedAt int64  `json:"updatedAt"`
	Deleted   bool   `json:"deleted"` // <— NEU: Tombstone für Deletes
}

type Snapshot struct {
	Items []Item `json:"items"`
}

// Callbacks that your app provides
type Callbacks struct {
	GetSnapshot func(ctx context.Context) (Snapshot, error)
	ApplyRemote func(ctx context.Context, s Snapshot) error
	BlobOpen    func(ctx context.Context, id string) (io.ReadCloser, string, string, int64, error)
}

/*** Node ***/

type Node struct {
	cfg    Config
	cbs    Callbacks
	self   Peer
	mu     sync.RWMutex
	peers  map[string]*Peer
	client *http.Client
	srv    *http.Server
	stop   chan struct{}
	wg     sync.WaitGroup
}

func New(cfg Config, cbs Callbacks) (*Node, error) {
	if cfg.BindAddr == "" || cfg.AdvertURL == "" {
		return nil, errors.New("mesh: BindAddr and AdvertURL required")
	}
	if cfg.ClusterSecret == "" {
		return nil, errors.New("mesh: ClusterSecret required")
	}
	n := &Node{
		cfg:   cfg,
		cbs:   cbs,
		self:  Peer{URL: cfg.AdvertURL, LastSeen: time.Now(), Self: true},
		peers: make(map[string]*Peer),
		client: &http.Client{
			Timeout: 5 * time.Second,
		},
		stop: make(chan struct{}),
	}
	return n, nil
}

/*** HMAC helpers ***/

func (n *Node) sign(b []byte) string {
	m := hmac.New(sha256.New, []byte(n.cfg.ClusterSecret))
	m.Write(b)
	return hex.EncodeToString(m.Sum(nil))
}

func (n *Node) verify(b []byte, sig string) bool {
	want := n.sign(b)
	return hmac.Equal([]byte(want), []byte(sig))
}

/*** HTTP handlers (control plane) ***/

func (n *Node) helloHandler(w http.ResponseWriter, r *http.Request) {
	body, _ := io.ReadAll(r.Body)
	if !n.verify(body, r.Header.Get("X-Mesh-Sig")) {
		http.Error(w, "bad signature", http.StatusUnauthorized)
		return
	}
	var p Peer
	if err := json.Unmarshal(body, &p); err != nil {
		http.Error(w, "bad json", http.StatusBadRequest)
		return
	}
	p.LastSeen = time.Now()
	n.mu.Lock()
	if existing, ok := n.peers[p.URL]; ok {
		existing.LastSeen = p.LastSeen
	} else if p.URL != n.self.URL {
		cp := p
		n.peers[p.URL] = &cp
	}
	n.mu.Unlock()
	w.WriteHeader(http.StatusNoContent)
}

func (n *Node) peersHandler(w http.ResponseWriter, r *http.Request) {
	n.mu.RLock()
	defer n.mu.RUnlock()
	var list []Peer
	list = append(list, n.self)
	for _, p := range n.peers {
		list = append(list, *p)
	}
	writeJSON(w, http.StatusOK, list)
}

func (n *Node) syncHandler(w http.ResponseWriter, r *http.Request) {
	// verify signature
	body, _ := io.ReadAll(r.Body)
	if !n.verify(body, r.Header.Get("X-Mesh-Sig")) {
		http.Error(w, "bad signature", http.StatusUnauthorized)
		return
	}
	var s Snapshot
	if err := json.Unmarshal(body, &s); err != nil {
		http.Error(w, "bad json", http.StatusBadRequest)
		return
	}
	// apply
	if err := n.cbs.ApplyRemote(r.Context(), s); err != nil {
		http.Error(w, "apply error: "+err.Error(), http.StatusInternalServerError)
		return
	}
	w.WriteHeader(http.StatusNoContent)
}

func writeJSON(w http.ResponseWriter, code int, v any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(code)
	_ = json.NewEncoder(w).Encode(v)
}

/*** Serve ***/

func (n *Node) Serve() error {
	mux := http.NewServeMux()
	mux.HandleFunc("/mesh/peers", n.peersHandler)
	mux.HandleFunc("/mesh/hello", n.helloHandler)
	mux.HandleFunc("/mesh/blob", n.blobHandler)
	mux.HandleFunc("/mesh/sync", n.syncHandler)
	n.srv = &http.Server{Addr: n.cfg.BindAddr, Handler: mux}

	// background loops
	n.wg.Add(1)
	go func() {
		defer n.wg.Done()
		n.loopSeeder()
	}()

	n.wg.Add(1)
	go func() { defer n.wg.Done(); n.loopPeerExchange() }()

	if n.cfg.EnableDiscovery && n.cfg.DiscoveryAddress != "" {
		n.wg.Add(2)
		go func() {
			defer n.wg.Done()
			n.loopBeaconSend()
		}()
		go func() {
			defer n.wg.Done()
			n.loopBeaconRecv()
		}()
	}

	n.wg.Add(1)
	go func() {
		defer n.wg.Done()
		n.loopAntiEntropy()
	}()

	// http server
	errc := make(chan error, 1)
	go func() {
		errc <- n.srv.ListenAndServe()
	}()
	select {
	case err := <-errc:
		return err
	case <-n.stop:
		return http.ErrServerClosed
	}
}

func (n *Node) Close(ctx context.Context) error {
	close(n.stop)
	if n.srv != nil {
		_ = n.srv.Shutdown(ctx)
	}
	n.wg.Wait()
	return nil
}

/*** Loops ***/

func (n *Node) loopPeerExchange() {
	t := time.NewTicker(30 * time.Second)
	defer t.Stop()
	for {
		select {
		case <-n.stop:
			return
		case <-t.C:
		}
		// Seeds abfragen
		for _, s := range n.cfg.Seeds {
			if strings.TrimSpace(s) == "" {
				continue
			}
			resp, err := n.client.Get(strings.TrimRight(s, "/") + "/mesh/peers")
			if err != nil {
				continue
			}
			var list []Peer
			if json.NewDecoder(resp.Body).Decode(&list) == nil {
				n.mu.Lock()
				for _, p := range list {
					if p.URL == "" || p.URL == n.self.URL {
						continue
					}
					if _, ok := n.peers[p.URL]; !ok {
						cp := p
						n.peers[p.URL] = &cp
					}
				}
				n.mu.Unlock()
			}
			resp.Body.Close()
		}
	}
}

func (n *Node) loopSeeder() {
	// attempt to hello known seeds every 5s at start, then every 30s
	backoff := 5 * time.Second
	for {
		select {
		case <-n.stop:
			return
		case <-time.After(backoff):
		}
		if len(n.cfg.Seeds) == 0 {
			backoff = 30 * time.Second
			continue
		}
		for _, s := range n.cfg.Seeds {
			if s == "" || s == n.self.URL {
				continue
			}
			_ = n.sendHello(s)
		}
		backoff = 30 * time.Second
	}
}

func (n *Node) loopAntiEntropy() {
	t := time.NewTicker(10 * time.Second)
	defer t.Stop()
	for {
		select {
		case <-n.stop:
			return
		case <-t.C:
			n.mu.RLock()
			targets := make([]string, 0, len(n.peers))
			for url := range n.peers {
				targets = append(targets, url)
			}
			n.mu.RUnlock()
			if len(targets) == 0 {
				continue
			}
			ctx, cancel := context.WithTimeout(context.Background(), 4*time.Second)
			snap, err := n.cbs.GetSnapshot(ctx)
			cancel()
			if err != nil {
				continue
			}
			for _, url := range targets {
				_ = n.sendSync(url, snap)
			}
		}
	}
}

func (n *Node) loopBeaconSend() {
	addr, err := net.ResolveUDPAddr("udp", n.cfg.DiscoveryAddress)
	if err != nil {
		log.Printf("mesh beacon send resolve: %v", err)
		return
	}
	conn, err := net.DialUDP("udp", nil, addr)
	if err != nil {
		log.Printf("mesh beacon send dial: %v", err)
		return
	}
	defer conn.Close()
	type beacon struct {
		URL string `json:"url"`
	}
	t := time.NewTicker(5 * time.Second)
	defer t.Stop()
	for {
		select {
		case <-n.stop:
			return
		case <-t.C:
			b, _ := json.Marshal(beacon{URL: n.self.URL})
			_, _ = conn.Write(b)
		}
	}
}

func (n *Node) loopBeaconRecv() {
	addr, err := net.ResolveUDPAddr("udp", n.cfg.DiscoveryAddress)
	if err != nil {
		log.Printf("mesh beacon recv resolve: %v", err)
		return
	}
	// enable multicast receive
	l, err := net.ListenMulticastUDP("udp", nil, addr)
	if err != nil {
		log.Printf("mesh beacon recv listen: %v", err)
		return
	}
	defer l.Close()
	_ = l.SetReadBuffer(1 << 16)

	buf := make([]byte, 2048)
	for {
		select {
		case <-n.stop:
			return
		default:
		}
		_ = l.SetDeadline(time.Now().Add(6 * time.Second))
		nr, _, err := l.ReadFromUDP(buf)
		if err != nil {
			if ne, ok := err.(net.Error); ok && ne.Timeout() {
				continue
			}
			continue
		}
		var msg struct{ URL string }
		if err := json.Unmarshal(buf[:nr], &msg); err == nil {
			if msg.URL != "" && msg.URL != n.self.URL {
				_ = n.sendHello(msg.URL)
			}
		}
	}
}

/*** Outgoing ***/

func (n *Node) sendHello(url string) error {
	p := n.self
	b, _ := json.Marshal(p)
	req, _ := http.NewRequest(http.MethodPost, strings.TrimRight(url, "/")+"/mesh/hello", bytes.NewReader(b))
	req.Header.Set("X-Mesh-Sig", n.sign(b))
	resp, err := n.client.Do(req)
	if err == nil {
		io.Copy(io.Discard, resp.Body)
		resp.Body.Close()
	}
	return err
}

func (n *Node) sendSync(url string, s Snapshot) error {
	b, _ := json.Marshal(s)
	req, _ := http.NewRequest(http.MethodPost, strings.TrimRight(url, "/")+"/mesh/sync", bytes.NewReader(b))
	req.Header.Set("X-Mesh-Sig", n.sign(b))
	resp, err := n.client.Do(req)
	if err == nil {
		io.Copy(io.Discard, resp.Body)
		resp.Body.Close()
	}
	return err
}

// PeerList liefert eine Kopie der bekannten Peers inkl. Self.
func (n *Node) PeerList() []Peer {
	n.mu.RLock()
	defer n.mu.RUnlock()
	out := make([]Peer, 0, len(n.peers)+1)
	out = append(out, n.self)
	for _, p := range n.peers {
		cp := *p
		out = append(out, cp)
	}
	return out
}

// SyncNow verschickt sofort den aktuellen Snapshot an alle bekannten Peers.
func (n *Node) SyncNow(ctx context.Context) error {
	snap, err := n.cbs.GetSnapshot(ctx)
	if err != nil {
		return err
	}
	n.mu.RLock()
	targets := make([]string, 0, len(n.peers))
	for url := range n.peers {
		targets = append(targets, url)
	}
	n.mu.RUnlock()
	for _, u := range targets {
		_ = n.sendSync(u, snap)
	}
	return nil
}

/*** Utilities ***/

// OwnerHint is a simple, optional mapping to distribute responsibility.
func OwnerHint(id string, peers []string) int {
	if len(peers) == 0 {
		return 0
	}
	h := crc32.ChecksumIEEE([]byte(id))
	return int(h % uint32(len(peers)))
}

// Helpers to load from ENV quickly
func FromEnv() Config {
	return Config{
		BindAddr:         getenvDefault("MESH_BIND", ":9090"),
		AdvertURL:        os.Getenv("MESH_ADVERT"),
		Seeds:            splitCSV(os.Getenv("MESH_SEEDS")),
		ClusterSecret:    os.Getenv("MESH_CLUSTER_SECRET"),
		EnableDiscovery:  os.Getenv("MESH_ENABLE_DISCOVERY") == "true",
		DiscoveryAddress: getenvDefault("MESH_DISCOVERY_ADDR", "239.8.8.8:9898"),
	}
}

func splitCSV(s string) []string {
	if strings.TrimSpace(s) == "" {
		return nil
	}
	parts := strings.Split(s, ",")
	for i := range parts {
		parts[i] = strings.TrimSpace(parts[i])
	}
	// dedup
	out := make([]string, 0, len(parts))
	for _, p := range parts {
		if p == "" || slices.Contains(out, p) {
			continue
		}
		out = append(out, p)
	}
	return out
}

func getenvDefault(k, def string) string {
	v := os.Getenv(k)
	if v == "" {
		return def
	}
	return v
}

// POST /mesh/blob  (Body: {"id":<int64>}) -> streamt den Blob
func (n *Node) blobHandler(w http.ResponseWriter, r *http.Request) {
	body, _ := io.ReadAll(r.Body)
	if !n.verify(body, r.Header.Get("X-Mesh-Sig")) {
		http.Error(w, "bad signature", http.StatusUnauthorized)
		return
	}
	var req struct {
		ID string `json:"id"`
	}
	if err := json.Unmarshal(body, &req); err != nil {
		http.Error(w, "bad json", http.StatusBadRequest)
		return
	}
	if n.cbs.BlobOpen == nil {
		http.Error(w, "blob unavailable", http.StatusNotFound)
		return
	}
	rc, name, ct, size, err := n.cbs.BlobOpen(r.Context(), req.ID)
	if err != nil {
		http.Error(w, "not found", http.StatusNotFound)
		return
	}
	defer rc.Close()
	if ct == "" {
		ct = "application/octet-stream"
	}
	if size > 0 {
		w.Header().Set("Content-Length", strconv.FormatInt(size, 10))
	}
	w.Header().Set("Content-Type", ct)
	w.Header().Set("X-Blob-Name", name)
	_, _ = io.Copy(w, rc)
}

// interner Helper: signierter Blob-Request an einen Peer
func (n *Node) sendBlobRequest(url string, id string) (*http.Response, error) {
	b, _ := json.Marshal(struct {
		ID string `json:"id"`
	}{ID: id})
	req, _ := http.NewRequest(http.MethodPost, strings.TrimRight(url, "/")+"/mesh/blob", bytes.NewReader(b))
	req.Header.Set("X-Mesh-Sig", n.sign(b))
	return n.client.Do(req)
}

// Öffentliche Methode: versuche Blob bei irgendeinem Peer zu holen
func (n *Node) FetchBlobAny(ctx context.Context, id string) (io.ReadCloser, string, string, int64, error) {
	n.mu.RLock()
	targets := make([]string, 0, len(n.peers))
	for url := range n.peers {
		targets = append(targets, url)
	}
	n.mu.RUnlock()
	if len(targets) == 0 {
		// Fallback: Seeds probieren
		targets = append(targets, n.cfg.Seeds...)
	}
	for _, u := range targets {
		if strings.TrimSpace(u) == "" || u == n.self.URL {
			continue
		}
		resp, err := n.sendBlobRequest(u, id)
		if err != nil {
			continue
		}
		if resp.StatusCode == http.StatusOK {
			name := resp.Header.Get("X-Blob-Name")
			ct := resp.Header.Get("Content-Type")
			var size int64 = -1
			if cl := resp.Header.Get("Content-Length"); cl != "" {
				if s, err := strconv.ParseInt(cl, 10, 64); err == nil {
					size = s
				}
			}
			// Caller muss resp.Body schließen
			return resp.Body, name, ct, size, nil
		}
		io.Copy(io.Discard, resp.Body)
		resp.Body.Close()
	}
	return nil, "", "", 0, fmt.Errorf("blob %s not found on peers", id)
}