// Package p2p — peer version discovery via gossipsub.
//
// What this solves
// ────────────────
// A decentralized node fleet has no registry telling each operator what
// version everyone else is running. Without that knowledge:
//
//   • We can't decide when it's safe to activate a new feature-flag tx
//     (§5.2 of UPDATE_STRATEGY.md) — activation must wait until ≥N% of
//     the network has the new binary.
//   • Operators can't see at a glance "am I the one holding back an
//     upgrade?" — because their node's Explorer had no way to ask peers.
//   • Clients can't warn the user "this node is running a pre-channels
//     build" without making N extra HTTP round-trips.
//
// How it works
// ────────────
// A small gossipsub topic — `dchain/version/v1` — carries a JSON blob from
// each node:
//
//	{
//	  "peer_id":          "12D3KooW…",
//	  "tag":              "v0.5.1",
//	  "commit":           "abc1234…",
//	  "protocol_version": 1,
//	  "timestamp":        1715000000
//	}
//
// Every node:
//   1. Publishes its own blob every 60 seconds.
//   2. Subscribes to the topic and keeps a bounded in-memory map
//      peer.ID → latest announce.
//   3. Evicts entries older than 15 minutes (peer disconnect / stale).
//
// Messages are unsigned and advisory — a peer lying about its version is
// detectable when their blocks/txs use unsupported fields (consensus will
// reject), so we don't add a signature layer here. The map is pure UX.
//
// Memory budget: ~200 bytes per peer × bounded by connected peer count.
// Topic traffic: ~300 bytes every 60s per peer → trivial for a libp2p fleet.
package p2p

import (
	"context"
	"encoding/json"
	"log"
	"sync"
	"time"

	pubsub "github.com/libp2p/go-libp2p-pubsub"
	"github.com/libp2p/go-libp2p/core/peer"

	"go-blockchain/node/version"
)

// TopicVersion is the gossipsub topic for peer-version announces.
const TopicVersion = "dchain/version/v1"

const (
	versionGossipInterval = 60 * time.Second
	versionGossipTTL      = 15 * time.Minute
)

// PeerVersion is one peer's self-reported identity.
type PeerVersion struct {
	PeerID          string    `json:"peer_id"`
	Tag             string    `json:"tag"`
	Commit          string    `json:"commit"`
	ProtocolVersion int       `json:"protocol_version"`
	Timestamp       int64     `json:"timestamp"`
	ReceivedAt      time.Time `json:"received_at,omitempty"`
}

// versionAnnouncer is wired into Host via StartVersionGossip. Holds the
// publish topic + subscription + the latest-seen map under its own mutex,
// so read path (PeerVersions) is lock-free against publish.
type versionAnnouncer struct {
	h         *Host
	topic     *pubsub.Topic
	sub       *pubsub.Subscription
	protoVer  int
	mu        sync.RWMutex
	latest    map[peer.ID]PeerVersion
}

// StartVersionGossip joins the version topic, spawns the publisher loop and
// the subscriber loop, and returns. Both goroutines run until ctx is done.
//
// Call exactly once per Host. protocolVersion should be node.ProtocolVersion
// (the compile-time wire-protocol const) — threaded through as an int to
// avoid an import cycle (p2p → node would be circular; node → p2p already
// exists via the host injection).
func (n *Host) StartVersionGossip(ctx context.Context, protocolVersion int) error {
	topic, err := n.ps.Join(TopicVersion)
	if err != nil {
		return err
	}
	sub, err := topic.Subscribe()
	if err != nil {
		return err
	}
	va := &versionAnnouncer{
		h:        n,
		topic:    topic,
		sub:      sub,
		protoVer: protocolVersion,
		latest:   make(map[peer.ID]PeerVersion),
	}
	n.versionAnnouncer = va
	go va.publishLoop(ctx)
	go va.subscribeLoop(ctx)
	go va.evictLoop(ctx)
	return nil
}

// PeerVersions returns a snapshot of every peer's last-known version.
// Result is a copy — caller can iterate without a lock.
func (n *Host) PeerVersions() map[string]PeerVersion {
	if n.versionAnnouncer == nil {
		return nil
	}
	va := n.versionAnnouncer
	va.mu.RLock()
	defer va.mu.RUnlock()
	out := make(map[string]PeerVersion, len(va.latest))
	for pid, v := range va.latest {
		out[pid.String()] = v
	}
	return out
}

func (va *versionAnnouncer) publishLoop(ctx context.Context) {
	// First publish immediately so peers who just joined learn our version
	// without a minute of lag.
	va.publishOnce(ctx)
	t := time.NewTicker(versionGossipInterval)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			va.publishOnce(ctx)
		}
	}
}

func (va *versionAnnouncer) publishOnce(ctx context.Context) {
	msg := PeerVersion{
		PeerID:          va.h.h.ID().String(),
		Tag:             version.Tag,
		Commit:          version.Commit,
		ProtocolVersion: va.protoVer,
		Timestamp:       time.Now().Unix(),
	}
	b, err := json.Marshal(msg)
	if err != nil {
		log.Printf("[P2P] version gossip marshal: %v", err)
		return
	}
	pubCtx, cancel := context.WithTimeout(ctx, 5*time.Second)
	defer cancel()
	if err := va.topic.Publish(pubCtx, b); err != nil {
		log.Printf("[P2P] version gossip publish: %v", err)
	}
}

func (va *versionAnnouncer) subscribeLoop(ctx context.Context) {
	for {
		m, err := va.sub.Next(ctx)
		if err != nil {
			if ctx.Err() != nil {
				return
			}
			log.Printf("[P2P] version gossip recv: %v", err)
			continue
		}
		// Skip our own broadcasts — gossipsub delivers them back to us by
		// default. Without this we'd overwrite our own "received" timestamp
		// every minute and clutter metrics.
		if m.ReceivedFrom == va.h.h.ID() {
			continue
		}
		var pv PeerVersion
		if err := json.Unmarshal(m.Data, &pv); err != nil {
			log.Printf("[P2P] version gossip bad msg from %s: %v", m.ReceivedFrom, err)
			continue
		}
		// Source validation: the peer ID inside the message must match the
		// peer that sent it. Otherwise a node could spoof "version" rows
		// for peers it doesn't control, confusing the UX.
		if pv.PeerID != m.ReceivedFrom.String() {
			continue
		}
		pv.ReceivedAt = time.Now()
		va.mu.Lock()
		va.latest[m.ReceivedFrom] = pv
		va.mu.Unlock()
	}
}

func (va *versionAnnouncer) evictLoop(ctx context.Context) {
	t := time.NewTicker(versionGossipTTL / 3)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case now := <-t.C:
			cutoff := now.Add(-versionGossipTTL)
			va.mu.Lock()
			for pid, v := range va.latest {
				if v.ReceivedAt.Before(cutoff) {
					delete(va.latest, pid)
				}
			}
			va.mu.Unlock()
		}
	}
}