package automc

import (
	"context"
	"embed"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"net/http"
	"sort"
	"time"

	"github.com/itzg/mc-router/server"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/sirupsen/logrus"
)

//go:embed static/*
var staticFS embed.FS

// startUI starts a separate HTTP server on uiBinding serving the operator
// dashboard (embedded index.html), an SSE log feed, and a JSON snapshot of
// the current route table. The upstream JSON API on API_BINDING is left
// untouched so existing tooling keeps working.
func startUI(ctx context.Context, uiBinding string, bus *LogBus) {
	mux := http.NewServeMux()

	sub, err := fs.Sub(staticFS, "static")
	if err != nil {
		logrus.WithError(err).Error("automc ui: embed misconfigured")
		return
	}
	mux.Handle("GET /", http.FileServer(http.FS(sub)))
	mux.HandleFunc("GET /api/routes", routesSnapshotHandler)
	mux.HandleFunc("GET /api/logs", sseLogsHandler(bus))

	srv := &http.Server{
		Addr:              uiBinding,
		Handler:           mux,
		ReadHeaderTimeout: 5 * time.Second,
	}
	go func() {
		<-ctx.Done()
		shutCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
		defer cancel()
		_ = srv.Shutdown(shutCtx)
	}()
	go func() {
		logrus.WithField("binding", uiBinding).Info("automc: ui server listening")
		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
			logrus.WithError(err).Error("automc ui server failed")
		}
	}()
}

// RouteSnapshot is one row of the routes table the UI renders. Same shape as
// the upstream /routes JSON but flatter — the UI doesn't need both backend
// and scalingTarget shown separately. ActiveConnections is the live gauge
// value scraped from the Prometheus registry (-1 if the metric isn't
// registered, which can happen briefly at startup).
type RouteSnapshot struct {
	ServerAddress     string `json:"server_address"`
	Backend           string `json:"backend"`
	ActiveConnections int    `json:"active_connections"`
}

func routesSnapshotHandler(w http.ResponseWriter, _ *http.Request) {
	mappings := server.Routes.GetMappings()
	perRoute, total := scrapeActiveConnections()

	out := make([]RouteSnapshot, 0, len(mappings))
	for addr, backend := range mappings {
		conns, ok := perRoute[addr]
		if !ok {
			conns = 0
		}
		out = append(out, RouteSnapshot{ServerAddress: addr, Backend: backend, ActiveConnections: conns})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].ServerAddress < out[j].ServerAddress })
	w.Header().Set("Content-Type", "application/json")
	_ = json.NewEncoder(w).Encode(map[string]any{
		"routes":             out,
		"total_connections":  total,
		"at":                 time.Now(),
	})
}

// scrapeActiveConnections walks the Prometheus default registry to extract
// two gauges that mc-router exposes:
//
//	mc_router_active_connections                          (no labels)
//	mc_router_server_active_connections{server_address=…} (per route)
//
// Returns (perServerAddress, total). On any gathering error returns zero
// values silently — the UI shows 0 rather than blocking on a metrics issue.
func scrapeActiveConnections() (map[string]int, int) {
	per := map[string]int{}
	total := 0
	families, err := prometheus.DefaultGatherer.Gather()
	if err != nil {
		return per, 0
	}
	for _, mf := range families {
		switch mf.GetName() {
		case "mc_router_active_connections":
			for _, m := range mf.GetMetric() {
				if g := m.GetGauge(); g != nil {
					total = int(g.GetValue())
				}
			}
		case "mc_router_server_active_connections":
			for _, m := range mf.GetMetric() {
				var addr string
				for _, lp := range m.GetLabel() {
					if lp.GetName() == "server_address" {
						addr = lp.GetValue()
						break
					}
				}
				if addr == "" {
					continue
				}
				if g := m.GetGauge(); g != nil {
					per[addr] = int(g.GetValue())
				}
			}
		}
	}
	return per, total
}

func sseLogsHandler(bus *LogBus) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		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 {
			http.Error(w, "streaming unsupported", http.StatusInternalServerError)
			return
		}
		for _, e := range bus.Backlog() {
			writeSSEEvent(w, e)
		}
		flusher.Flush()

		ch := bus.Subscribe()
		defer bus.Unsubscribe(ch)

		hb := time.NewTicker(30 * time.Second)
		defer hb.Stop()
		for {
			select {
			case <-r.Context().Done():
				return
			case e := <-ch:
				writeSSEEvent(w, e)
				flusher.Flush()
			case <-hb.C:
				_, _ = io.WriteString(w, ":hb\n\n")
				flusher.Flush()
			}
		}
	}
}

func writeSSEEvent(w io.Writer, e LogEntry) {
	fmt.Fprintf(w, "data: {\"time\":%q,\"level\":%q,\"msg\":%q,\"attrs\":%q}\n\n",
		e.Time.Format(time.RFC3339Nano), e.Level, e.Msg, e.Attrs)
}