package expvar

import (
	"github.com/elastic/beats/libbeat/common"
	"github.com/elastic/beats/libbeat/common/cfgwarn"
	"github.com/elastic/beats/metricbeat/helper"
	"github.com/elastic/beats/metricbeat/mb"
	"github.com/elastic/beats/metricbeat/mb/parse"
	"github.com/elastic/beats/metricbeat/module/golang"
)

const (
	defaultScheme = "http"
	defaultPath   = "/debug/vars"
)

var (
	hostParser = parse.URLHostParserBuilder{
		DefaultScheme: defaultScheme,
		DefaultPath:   defaultPath,
		PathConfigKey: "expvar.path",
	}.Build()
)

// init registers the MetricSet with the central registry.
// The New method will be called after the setup of the module and before starting to fetch data
func init() {
	if err := mb.Registry.AddMetricSet("golang", "expvar", New, hostParser); err != nil {
		panic(err)
	}
}

// MetricSet type defines all fields of the MetricSet
// As a minimum it must inherit the mb.BaseMetricSet fields, but can be extended with
// additional entries. These variables can be used to persist data or configuration between
// multiple fetch calls.
type MetricSet struct {
	mb.BaseMetricSet
	http      *helper.HTTP
	namespace string
}

// New create a new instance of the MetricSet
// Part of new is also setting up the configuration by processing additional
// configuration entries if needed.
func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
	cfgwarn.Experimental("The golang expvar metricset is experimental")

	config := struct {
		Namespace string `config:"expvar.namespace" validate:"required"`
	}{}

	if err := base.Module().UnpackConfig(&config); err != nil {
		return nil, err
	}

	return &MetricSet{
		BaseMetricSet: base,
		http:          helper.NewHTTP(base),
		namespace:     config.Namespace,
	}, nil
}

// Fetch methods implements the data gathering and data conversion to the right format
// It returns the event which is then forward to the output. In case of an error, a
// descriptive error must be returned.
func (m *MetricSet) Fetch() (common.MapStr, error) {
	json, err := m.http.FetchJSON()

	if err != nil {
		return nil, err
	}

	//flatten cmdline
	json["cmdline"] = golang.GetCmdStr(json["cmdline"])

	//set namespace
	json[mb.NamespaceKey] = m.namespace

	return json, nil
}