package metrics

import (
	"context"
	"errors"
	"net/http"
	"reflect"

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

var ErrInvalidDependencies = errors.New("invalid metrics dependencies")

type ReadinessChecker interface {
	Ready(context.Context) error
}

type Dependencies struct {
	Gatherer  prometheus.Gatherer
	Readiness ReadinessChecker
}

func NewHandler(dependencies Dependencies) (http.Handler, error) {
	if nilInterface(dependencies.Gatherer) || nilInterface(dependencies.Readiness) {
		return nil, ErrInvalidDependencies
	}
	metricsHandler := promhttp.HandlerFor(dependencies.Gatherer, promhttp.HandlerOpts{})
	return http.HandlerFunc(func(response http.ResponseWriter, request *http.Request) {
		if request.Method != http.MethodGet {
			response.Header().Set("Allow", http.MethodGet)
			http.Error(response, "method not allowed", http.StatusMethodNotAllowed)
			return
		}
		switch request.URL.Path {
		case "/livez":
			writeStatus(response, http.StatusOK, "live")
		case "/readyz":
			if err := dependencies.Readiness.Ready(request.Context()); err != nil {
				writeStatus(response, http.StatusServiceUnavailable, "unavailable")
				return
			}
			writeStatus(response, http.StatusOK, "ready")
		case "/metrics":
			metricsHandler.ServeHTTP(response, request)
		default:
			http.NotFound(response, request)
		}
	}), nil
}

func writeStatus(response http.ResponseWriter, status int, value string) {
	response.Header().Set("Content-Type", "text/plain; charset=utf-8")
	response.Header().Set("X-Content-Type-Options", "nosniff")
	response.WriteHeader(status)
	_, _ = response.Write([]byte(value + "\n"))
}

func nilInterface(value any) bool {
	if value == nil {
		return true
	}
	reflected := reflect.ValueOf(value)
	switch reflected.Kind() {
	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
		return reflected.IsNil()
	default:
		return false
	}
}