package main

import (
	"encoding/json"
	"log/slog"
	"net/http"
	"os"
	"strconv"
	"time"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

var (
	reqTotal = promauto.NewCounterVec(prometheus.CounterOpts{
		Name: "http_requests_total",
		Help: "Total number of HTTP requests.",
	}, []string{"method", "path", "status"})

	reqDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
		Name:    "http_request_duration_seconds",
		Help:    "HTTP request latency in seconds.",
		Buckets: prometheus.DefBuckets,
	}, []string{"method", "path"})
)

func main() {
	env := getenv("APP_ENV", "dev")
	version := getenv("APP_VERSION", "unknown")
	port := getenv("PORT", "8080")

	logLevel := slog.LevelInfo
	if getenv("LOG_LEVEL", "info") == "debug" {
		logLevel = slog.LevelDebug
	}
	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: logLevel})))

	mux := http.NewServeMux()
	mux.Handle("/metrics", promhttp.Handler())
	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusOK)
	})
	mux.HandleFunc("/", instrument(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Content-Type", "application/json")
		json.NewEncoder(w).Encode(map[string]string{
			"env":     env,
			"version": version,
		})
	}))

	slog.Info("listening", "port", port, "env", env, "version", version)
	if err := http.ListenAndServe(":"+port, mux); err != nil {
		slog.Error("server failed", "err", err)
		os.Exit(1)
	}
}

func instrument(next http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		rw := &statusWriter{ResponseWriter: w, status: http.StatusOK}
		start := time.Now()
		next(rw, r)
		dur := time.Since(start).Seconds()
		status := strconv.Itoa(rw.status)
		reqTotal.WithLabelValues(r.Method, r.URL.Path, status).Inc()
		reqDuration.WithLabelValues(r.Method, r.URL.Path).Observe(dur)
		slog.Debug("request", "method", r.Method, "path", r.URL.Path, "status", rw.status, "duration", dur)
	}
}

type statusWriter struct {
	http.ResponseWriter
	status int
}

func (sw *statusWriter) WriteHeader(code int) {
	sw.status = code
	sw.ResponseWriter.WriteHeader(code)
}

func getenv(key, fallback string) string {
	if v := os.Getenv(key); v != "" {
		return v
	}
	return fallback
}