package telemetry

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"time"

	"github.com/hashicorp/go-metrics"
	"github.com/hashicorp/go-metrics/datadog"
	metricsprom "github.com/hashicorp/go-metrics/prometheus"
	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/common/expfmt"
)

// globalTelemetryEnabled is a private variable that stores the telemetry enabled state.
// It is set on initialization and does not change for the lifetime of the program.
var globalTelemetryEnabled bool

// IsTelemetryEnabled provides controlled access to check if telemetry is enabled.
func IsTelemetryEnabled() bool {
	return globalTelemetryEnabled
}

// EnableTelemetry allows for the global telemetry enabled state to be set.
func EnableTelemetry() {
	globalTelemetryEnabled = true
}

// globalLabels defines the set of global labels that will be applied to all
// metrics emitted using the telemetry package function wrappers.
var globalLabels = []metrics.Label{}

// Metrics supported format types.
const (
	FormatDefault    = ""
	FormatPrometheus = "prometheus"
	FormatText       = "text"
	ContentTypeText  = `text/plain; version=` + expfmt.TextVersion + `; charset=utf-8`

	MetricSinkInMem      = "mem"
	MetricSinkStatsd     = "statsd"
	MetricSinkDogsStatsd = "dogstatsd"
)

// DisplayableSink is an interface that defines a method for displaying metrics.
type DisplayableSink interface {
	DisplayMetrics(resp http.ResponseWriter, req *http.Request) (any, error)
}

// Config defines the configuration options for application telemetry.
type Config struct {
	// Prefixed with keys to separate services
	ServiceName string `mapstructure:"service-name"`

	// Enabled enables the application telemetry functionality. When enabled,
	// an in-memory sink is also enabled by default. Operators may also enabled
	// other sinks such as Prometheus.
	Enabled bool `mapstructure:"enabled"`

	// Enable prefixing gauge values with hostname
	EnableHostname bool `mapstructure:"enable-hostname"`

	// Enable adding hostname to labels
	EnableHostnameLabel bool `mapstructure:"enable-hostname-label"`

	// Enable adding service to labels
	EnableServiceLabel bool `mapstructure:"enable-service-label"`

	// PrometheusRetentionTime, when positive, enables a Prometheus metrics sink.
	// It defines the retention duration in seconds.
	PrometheusRetentionTime int64 `mapstructure:"prometheus-retention-time"`

	// GlobalLabels defines a global set of name/value label tuples applied to all
	// metrics emitted using the wrapper functions defined in telemetry package.
	//
	// Example:
	// [["chain_id", "cosmoshub-1"]]
	GlobalLabels [][]string `mapstructure:"global-labels"`

	// MetricsSink defines the type of metrics backend to use.
	MetricsSink string `mapstructure:"metrics-sink" default:"mem"`

	// StatsdAddr defines the address of a statsd server to send metrics to.
	// Only utilized if MetricsSink is set to "statsd" or "dogstatsd".
	StatsdAddr string `mapstructure:"statsd-addr"`

	// DatadogHostname defines the hostname to use when emitting metrics to
	// Datadog. Only utilized if MetricsSink is set to "dogstatsd".
	DatadogHostname string `mapstructure:"datadog-hostname"`
}

// Metrics defines a wrapper around application telemetry functionality. It allows
// metrics to be gathered at any point in time. When creating a Metrics object,
// internally, a global metrics is registered with a set of sinks as configured
// by the operator. In addition to the sinks, when a process gets a SIGUSR1, a
// dump of formatted recent metrics will be sent to STDERR.
type Metrics struct {
	sink              metrics.MetricSink
	prometheusEnabled bool
}

// GatherResponse is the response type of registered metrics
type GatherResponse struct {
	Metrics     []byte
	ContentType string
}

// New creates a new instance of Metrics
func New(cfg Config) (_ *Metrics, rerr error) {
	globalTelemetryEnabled = cfg.Enabled
	if !cfg.Enabled {
		return nil, nil
	}

	if numGlobalLabels := len(cfg.GlobalLabels); numGlobalLabels > 0 {
		parsedGlobalLabels := make([]metrics.Label, numGlobalLabels)
		for i, gl := range cfg.GlobalLabels {
			parsedGlobalLabels[i] = NewLabel(gl[0], gl[1])
		}
		globalLabels = parsedGlobalLabels
	}

	metricsConf := metrics.DefaultConfig(cfg.ServiceName)
	metricsConf.EnableHostname = cfg.EnableHostname
	metricsConf.EnableHostnameLabel = cfg.EnableHostnameLabel

	var (
		sink metrics.MetricSink
		err  error
	)
	switch cfg.MetricsSink {
	case MetricSinkStatsd:
		sink, err = metrics.NewStatsdSink(cfg.StatsdAddr)
	case MetricSinkDogsStatsd:
		sink, err = datadog.NewDogStatsdSink(cfg.StatsdAddr, cfg.DatadogHostname)
	default:
		memSink := metrics.NewInmemSink(10*time.Second, time.Minute)
		sink = memSink
		inMemSig := metrics.DefaultInmemSignal(memSink)
		defer func() {
			if rerr != nil {
				inMemSig.Stop()
			}
		}()
	}

	if err != nil {
		return nil, err
	}

	m := &Metrics{sink: sink}
	fanout := metrics.FanoutSink{sink}

	if cfg.PrometheusRetentionTime > 0 {
		m.prometheusEnabled = true
		prometheusOpts := metricsprom.PrometheusOpts{
			Expiration: time.Duration(cfg.PrometheusRetentionTime) * time.Second,
		}

		promSink, err := metricsprom.NewPrometheusSinkFrom(prometheusOpts)
		if err != nil {
			return nil, err
		}

		fanout = append(fanout, promSink)
	}

	if _, err := metrics.NewGlobal(metricsConf, fanout); err != nil {
		return nil, err
	}

	return m, nil
}

// Gather collects all registered metrics and returns a GatherResponse where the
// metrics are encoded depending on the type. Metrics are either encoded via
// Prometheus or JSON if in-memory.
func (m *Metrics) Gather(format string) (GatherResponse, error) {
	switch format {
	case FormatPrometheus:
		return m.gatherPrometheus()

	case FormatText:
		return m.gatherGeneric()

	case FormatDefault:
		return m.gatherGeneric()

	default:
		return GatherResponse{}, fmt.Errorf("unsupported metrics format: %s", format)
	}
}

// gatherPrometheus collects Prometheus metrics and returns a GatherResponse.
// If Prometheus metrics are not enabled, it returns an error.
func (m *Metrics) gatherPrometheus() (GatherResponse, error) {
	if !m.prometheusEnabled {
		return GatherResponse{}, errors.New("prometheus metrics are not enabled")
	}

	metricsFamilies, err := prometheus.DefaultGatherer.Gather()
	if err != nil {
		return GatherResponse{}, fmt.Errorf("failed to gather prometheus metrics: %w", err)
	}

	buf := &bytes.Buffer{}
	defer buf.Reset()

	e := expfmt.NewEncoder(buf, expfmt.NewFormat(expfmt.TypeTextPlain))

	for _, mf := range metricsFamilies {
		if err := e.Encode(mf); err != nil {
			return GatherResponse{}, fmt.Errorf("failed to encode prometheus metrics: %w", err)
		}
	}

	return GatherResponse{ContentType: ContentTypeText, Metrics: buf.Bytes()}, nil
}

// gatherGeneric collects generic metrics and returns a GatherResponse.
func (m *Metrics) gatherGeneric() (GatherResponse, error) {
	gm, ok := m.sink.(DisplayableSink)
	if !ok {
		return GatherResponse{}, errors.New("non in-memory metrics sink does not support generic format")
	}

	summary, err := gm.DisplayMetrics(nil, nil)
	if err != nil {
		return GatherResponse{}, fmt.Errorf("failed to gather in-memory metrics: %w", err)
	}

	content, err := json.Marshal(summary)
	if err != nil {
		return GatherResponse{}, fmt.Errorf("failed to encode in-memory metrics: %w", err)
	}

	return GatherResponse{ContentType: "application/json", Metrics: content}, nil
}