// Package metricsserver provides a standalone HTTP server for exposing Prometheus metrics.
//
// This package implements a dedicated metrics server that exposes application metrics
// via HTTP. It uses a custom Prometheus registry to avoid conflicts with other metric
// collectors and provides graceful shutdown capabilities.
//
// # Usage
//
// Create a new metrics server and register your metrics with its Registry():
//
//	m := metricsserver.New()
//	myCounter := prometheus.NewCounter(...)
//	m.Registry().MustRegister(myCounter)
//
// When you need a Gatherer (for example, to pass to other libraries), use the Gatherer() method
// instead of prometheus.DefaultGatherer to ensure your custom metrics are collected:
//
//	gatherer := m.Gatherer()  // Returns the custom registry as a Gatherer
//
// To use the default Prometheus gatherer alongside your custom registry:
//
//	m := metricsserver.NewWithDefaultGatherer()
//	m.Gatherer()  // Returns prometheus.DefaultGatherer
package metricsserver

import (
	"context"
	"net/http"
	"strconv"
	"time"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
	"golang.org/x/sync/errgroup"

	"go.ntppool.org/common/logger"
)

// Metrics provides a custom Prometheus registry and HTTP handlers for metrics exposure.
// It isolates application metrics from the default global registry.
type Metrics struct {
	r                  *prometheus.Registry
	useDefaultGatherer bool
}

// New creates a new Metrics instance with a custom Prometheus registry.
// Use this when you want isolated metrics that don't interfere with the default registry.
func New() *Metrics {
	r := prometheus.NewRegistry()

	m := &Metrics{
		r:                  r,
		useDefaultGatherer: false,
	}

	return m
}

// NewWithDefaultGatherer creates a new Metrics instance that uses the default Prometheus gatherer.
// This is useful when you want to expose metrics from the default registry alongside your custom metrics.
// The custom registry will still be available via Registry() but Gatherer() will return prometheus.DefaultGatherer.
func NewWithDefaultGatherer() *Metrics {
	r := prometheus.NewRegistry()

	m := &Metrics{
		r:                  r,
		useDefaultGatherer: true,
	}

	return m
}

// Registry returns the custom Prometheus registry.
// Use this to register your application's metrics collectors.
func (m *Metrics) Registry() *prometheus.Registry {
	return m.r
}

// Gatherer returns the Prometheus gatherer to use for collecting metrics.
// This returns the custom registry's Gatherer by default, ensuring your registered
// metrics are collected. If the instance was created with NewWithDefaultGatherer(),
// this returns prometheus.DefaultGatherer instead.
//
// Use this method when you need a prometheus.Gatherer interface, for example when
// configuring other libraries that need to collect metrics.
//
// IMPORTANT: Do not use prometheus.DefaultGatherer directly if you want to collect
// metrics registered with this instance's Registry(). Always use this Gatherer() method.
func (m *Metrics) Gatherer() prometheus.Gatherer {
	if m.useDefaultGatherer {
		return prometheus.DefaultGatherer
	}
	return m.r
}

// Handler returns an HTTP handler for the /metrics endpoint with OpenMetrics support.
func (m *Metrics) Handler() http.Handler {
	log := logger.NewStdLog("prom http", false, nil)

	return promhttp.HandlerFor(m.r, promhttp.HandlerOpts{
		ErrorLog:          log,
		Registry:          m.r,
		EnableOpenMetrics: true,
	})
}

// ListenAndServe starts a metrics server on the specified port and blocks until ctx is done.
// The server exposes the metrics handler and shuts down gracefully when the context is cancelled.
func (m *Metrics) ListenAndServe(ctx context.Context, port int) error {
	log := logger.Setup()

	srv := &http.Server{
		Addr:         ":" + strconv.Itoa(port),
		ReadTimeout:  10 * time.Second,
		WriteTimeout: 20 * time.Second,
		IdleTimeout:  120 * time.Second,
		Handler:      m.Handler(),
	}

	g, ctx := errgroup.WithContext(ctx)

	g.Go(func() error {
		err := srv.ListenAndServe()
		if err != nil && err != http.ErrServerClosed {
			log.Warn("metrics server done listening", "err", err)
			return err
		}
		return nil
	})

	<-ctx.Done()

	if err := srv.Shutdown(ctx); err != nil {
		log.Error("metrics server shutdown failed", "err", err)
		return err
	}

	return g.Wait()
}