package metrics

import (
	"net/http"
	"sync"

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

var (
	registry = prometheus.NewRegistry()
	// registeredDescriptionsMtx protects collectors slice.
	// It should not be acessed concurrently, but we can easily forget this in future, thus this mutex.
	registeredDescriptionsMtx sync.Mutex
	registeredDescriptions    []Description
)

func init() {
	registry.MustRegister(collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}))
	registry.MustRegister(collectors.NewGoCollector())
}

// Register registers custom collectors to registry.
// Should be used with metrics from other packages.
func Register(customCollectors ...prometheus.Collector) {
	registry.MustRegister(customCollectors...)
}

func MustRegister(c prometheus.Collector, descs ...Description) {
	registry.MustRegister(c)
	registeredDescriptionsMtx.Lock()
	defer registeredDescriptionsMtx.Unlock()
	registeredDescriptions = append(registeredDescriptions, descs...)
}

// Handler returns an http.Handler for the local registry.
func Handler() http.Handler {
	promhttp.Handler()
	return promhttp.InstrumentMetricHandler(
		registry,
		promhttp.HandlerFor(registry, promhttp.HandlerOpts{}))
}