// FDN-08 — /metrics im Prometheus-Textformat, ohne Fremdabhängigkeit.
//
// Zugriff: bewusst OHNE Login (Scrape-Clients haben keine Session), dafür
// IP-beschränkt. Default ist loopback-only; weitere Scraper werden über
// config api.metrics_allowed_ips (IP oder CIDR) freigeschaltet. Es werden
// ausschließlich aggregierte Zähler ausgegeben — keine Tenant-Daten, keine
// Pfadsegmente mit IDs oder Tokens (siehe normalizeRoute).
package api

import (
	"context"
	"fmt"
	"net"
	"net/http"
	"runtime"
	"sort"
	"strings"
	"time"
)

// handleMetrics rendert die Registry im Prometheus-Textformat.
func (s *Server) handleMetrics(w http.ResponseWriter, r *http.Request) {
	if !s.metricsAllowed(r) {
		writeError(w, http.StatusForbidden, "metrics endpoint not allowed from this address")
		return
	}

	routes, inFlight, panics := s.metrics.snapshot()

	w.Header().Set("Content-Type", "text/plain; version=0.0.4; charset=utf-8")
	w.WriteHeader(http.StatusOK)

	var b strings.Builder

	fmt.Fprintf(&b, "# HELP archivdms_build_info Statische Build-Information.\n")
	fmt.Fprintf(&b, "# TYPE archivdms_build_info gauge\n")
	fmt.Fprintf(&b, "archivdms_build_info{version=\"%s\"} 1\n", escapeLabel(s.appVersion))

	fmt.Fprintf(&b, "# HELP archivdms_uptime_seconds Laufzeit des Prozesses in Sekunden.\n")
	fmt.Fprintf(&b, "# TYPE archivdms_uptime_seconds gauge\n")
	fmt.Fprintf(&b, "archivdms_uptime_seconds %.3f\n", time.Since(s.startTime).Seconds())

	fmt.Fprintf(&b, "# HELP archivdms_goroutines Aktuelle Anzahl Goroutinen.\n")
	fmt.Fprintf(&b, "# TYPE archivdms_goroutines gauge\n")
	fmt.Fprintf(&b, "archivdms_goroutines %d\n", runtime.NumGoroutine())

	fmt.Fprintf(&b, "# HELP archivdms_http_requests_in_flight Aktuell laufende HTTP-Anfragen.\n")
	fmt.Fprintf(&b, "# TYPE archivdms_http_requests_in_flight gauge\n")
	fmt.Fprintf(&b, "archivdms_http_requests_in_flight %d\n", inFlight)

	fmt.Fprintf(&b, "# HELP archivdms_panics_total Zentral abgefangene Panics (unbehandelte Fehler).\n")
	fmt.Fprintf(&b, "# TYPE archivdms_panics_total counter\n")
	fmt.Fprintf(&b, "archivdms_panics_total %d\n", panics)

	// Requests + Latenz je Route/Status.
	keys := make([]routeKey, 0, len(routes))
	for k := range routes {
		keys = append(keys, k)
	}
	sort.Slice(keys, func(i, j int) bool {
		if keys[i].route != keys[j].route {
			return keys[i].route < keys[j].route
		}
		if keys[i].method != keys[j].method {
			return keys[i].method < keys[j].method
		}
		return keys[i].status < keys[j].status
	})

	fmt.Fprintf(&b, "# HELP archivdms_http_requests_total Anzahl HTTP-Anfragen je Route und Status.\n")
	fmt.Fprintf(&b, "# TYPE archivdms_http_requests_total counter\n")
	for _, k := range keys {
		st := routes[k]
		fmt.Fprintf(&b, "archivdms_http_requests_total{method=\"%s\",route=\"%s\",status=\"%d\"} %d\n",
			escapeLabel(k.method), escapeLabel(k.route), k.status, st.count)
	}

	fmt.Fprintf(&b, "# HELP archivdms_http_request_duration_seconds Latenz der HTTP-Anfragen.\n")
	fmt.Fprintf(&b, "# TYPE archivdms_http_request_duration_seconds histogram\n")
	for _, k := range keys {
		st := routes[k]
		labels := fmt.Sprintf("method=\"%s\",route=\"%s\",status=\"%d\"",
			escapeLabel(k.method), escapeLabel(k.route), k.status)
		for i, ub := range latencyBuckets {
			fmt.Fprintf(&b, "archivdms_http_request_duration_seconds_bucket{%s,le=\"%g\"} %d\n",
				labels, ub, st.bucketCount[i])
		}
		fmt.Fprintf(&b, "archivdms_http_request_duration_seconds_bucket{%s,le=\"+Inf\"} %d\n", labels, st.count)
		fmt.Fprintf(&b, "archivdms_http_request_duration_seconds_sum{%s} %.6f\n", labels, st.sumSeconds)
		fmt.Fprintf(&b, "archivdms_http_request_duration_seconds_count{%s} %d\n", labels, st.count)
	}

	// Queue-Länge (Akzeptanzkriterium 2). Fehler hier dürfen den Scrape nicht
	// scheitern lassen — dann fehlt die Metrik einfach für diesen Durchlauf.
	if s.store != nil {
		ctx, cancel := context.WithTimeout(r.Context(), 3*time.Second)
		defer cancel()
		counts, err := s.store.CountProcessingJobsByStatus(ctx)
		if err != nil {
			s.reqLog(r.Context()).Warn("metrics: queue length query failed", "err", err)
		} else {
			fmt.Fprintf(&b, "# HELP archivdms_processing_jobs Länge der Verarbeitungswarteschlange je Status.\n")
			fmt.Fprintf(&b, "# TYPE archivdms_processing_jobs gauge\n")
			statuses := make([]string, 0, len(counts))
			for st := range counts {
				statuses = append(statuses, st)
			}
			sort.Strings(statuses)
			for _, st := range statuses {
				fmt.Fprintf(&b, "archivdms_processing_jobs{status=\"%s\"} %d\n", escapeLabel(st), counts[st])
			}
		}
	}

	w.Write([]byte(b.String()))
}

// metricsAllowed prüft die Herkunft des Scrape-Requests: loopback immer,
// sonst nur konfigurierte IPs/CIDRs (config api.metrics_allowed_ips).
func (s *Server) metricsAllowed(r *http.Request) bool {
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		host = r.RemoteAddr
	}
	ip := net.ParseIP(strings.TrimSpace(host))
	if ip == nil {
		return false
	}
	if ip.IsLoopback() {
		return true
	}
	for _, entry := range s.cfg.MetricsAllowedIPs {
		entry = strings.TrimSpace(entry)
		if entry == "" {
			continue
		}
		if strings.Contains(entry, "/") {
			if _, cidr, err := net.ParseCIDR(entry); err == nil && cidr.Contains(ip) {
				return true
			}
			continue
		}
		if parsed := net.ParseIP(entry); parsed != nil && parsed.Equal(ip) {
			return true
		}
	}
	return false
}

// escapeLabel entschärft Anführungszeichen/Backslashes/Zeilenumbrüche in
// Prometheus-Labelwerten.
func escapeLabel(v string) string {
	r := strings.NewReplacer(`\`, `\\`, `"`, `\"`, "\n", `\n`)
	return r.Replace(v)
}