// Package node — Server-Sent Events hub for the block explorer.
//
// Clients connect to GET /api/events and receive a real-time stream of:
//
//	event: block        — every committed block
//	event: tx           — every confirmed transaction (synthetic BLOCK_REWARD excluded)
//	event: contract_log — every log entry written by a smart contract
//
// The stream uses the standard text/event-stream format so the browser's
// native EventSource API works without any library.
package node

import (
	"encoding/json"
	"fmt"
	"net/http"
	"sync"
	"time"

	"go-blockchain/blockchain"
)

// ── event payload types ──────────────────────────────────────────────────────

// SSEBlockEvent is emitted when a block is committed.
type SSEBlockEvent struct {
	Index     uint64 `json:"index"`
	Hash      string `json:"hash"`
	TxCount   int    `json:"tx_count"`
	Validator string `json:"validator"`
	Timestamp string `json:"timestamp"`
}

// SSETxEvent is emitted for each confirmed transaction.
type SSETxEvent struct {
	ID     string                `json:"id"`
	TxType blockchain.EventType  `json:"tx_type"`
	From   string                `json:"from"`
	To     string                `json:"to,omitempty"`
	Amount uint64                `json:"amount,omitempty"`
	Fee    uint64                `json:"fee,omitempty"`
}

// SSEContractLogEvent is emitted each time a contract calls env.log().
type SSEContractLogEvent = blockchain.ContractLogEntry

// ── hub ───────────────────────────────────────────────────────────────────────

// SSEHub manages all active SSE client connections.
// It is safe for concurrent use from multiple goroutines.
type SSEHub struct {
	mu      sync.RWMutex
	clients map[chan string]struct{}
}

// NewSSEHub returns an initialised hub ready to accept connections.
func NewSSEHub() *SSEHub {
	return &SSEHub{clients: make(map[chan string]struct{})}
}

// Clients returns the current number of connected SSE clients.
func (h *SSEHub) Clients() int {
	h.mu.RLock()
	defer h.mu.RUnlock()
	return len(h.clients)
}

func (h *SSEHub) subscribe() chan string {
	ch := make(chan string, 64) // buffered: drop events for slow clients
	h.mu.Lock()
	h.clients[ch] = struct{}{}
	h.mu.Unlock()
	return ch
}

func (h *SSEHub) unsubscribe(ch chan string) {
	h.mu.Lock()
	delete(h.clients, ch)
	h.mu.Unlock()
	close(ch)
}

// emit serialises data and broadcasts an SSE message to all subscribers.
func (h *SSEHub) emit(eventName string, data any) {
	payload, err := json.Marshal(data)
	if err != nil {
		return
	}
	// SSE wire format: "event: <name>\ndata: <json>\n\n"
	msg := fmt.Sprintf("event: %s\ndata: %s\n\n", eventName, payload)
	h.mu.RLock()
	for ch := range h.clients {
		select {
		case ch <- msg:
		default: // drop silently — client is too slow
		}
	}
	h.mu.RUnlock()
}

// ── public emit methods ───────────────────────────────────────────────────────

// EmitBlock broadcasts a "block" event for the committed block b.
func (h *SSEHub) EmitBlock(b *blockchain.Block) {
	h.emit("block", SSEBlockEvent{
		Index:     b.Index,
		Hash:      b.HashHex(),
		TxCount:   len(b.Transactions),
		Validator: b.Validator,
		Timestamp: b.Timestamp.UTC().Format(time.RFC3339),
	})
}

// EmitTx broadcasts a "tx" event for each confirmed transaction.
// Synthetic BLOCK_REWARD records are skipped.
func (h *SSEHub) EmitTx(tx *blockchain.Transaction) {
	if tx.Type == blockchain.EventBlockReward {
		return
	}
	h.emit("tx", SSETxEvent{
		ID:     tx.ID,
		TxType: tx.Type,
		From:   tx.From,
		To:     tx.To,
		Amount: tx.Amount,
		Fee:    tx.Fee,
	})
}

// EmitContractLog broadcasts a "contract_log" event.
func (h *SSEHub) EmitContractLog(entry blockchain.ContractLogEntry) {
	h.emit("contract_log", entry)
}

// EmitBlockWithTxs calls EmitBlock then EmitTx for each transaction in b.
func (h *SSEHub) EmitBlockWithTxs(b *blockchain.Block) {
	h.EmitBlock(b)
	for _, tx := range b.Transactions {
		h.EmitTx(tx)
	}
}

// ── HTTP handler ──────────────────────────────────────────────────────────────

// ServeHTTP handles GET /api/events.
// The response is a text/event-stream that streams block, tx, and
// contract_log events until the client disconnects.
func (h *SSEHub) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	flusher, ok := w.(http.Flusher)
	if !ok {
		http.Error(w, "streaming unsupported by this server", http.StatusInternalServerError)
		return
	}

	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") // tell nginx not to buffer

	ch := h.subscribe()
	defer h.unsubscribe(ch)

	// Opening comment — confirms the connection to the client.
	fmt.Fprintf(w, ": connected to DChain event stream\n\n")
	flusher.Flush()

	// Keepalive comments every 20 s prevent proxy/load-balancer timeouts.
	keepalive := time.NewTicker(20 * time.Second)
	defer keepalive.Stop()

	for {
		select {
		case msg, ok := <-ch:
			if !ok {
				return
			}
			fmt.Fprint(w, msg)
			flusher.Flush()
		case <-keepalive.C:
			fmt.Fprintf(w, ": keepalive\n\n")
			flusher.Flush()
		case <-r.Context().Done():
			return
		}
	}
}