package metrics

import (
	"errors"
	"time"

	"github.com/go-kit/kit/metrics"
	"github.com/go-kit/kit/metrics/multi"
)

// Registry has to implemented by any system that wants to monitor and expose metrics.
type Registry interface {
	// IsEpEnabled shows whether metrics instrumentation is enabled on entry points.
	IsEpEnabled() bool
	// IsSvcEnabled shows whether metrics instrumentation is enabled on services.
	IsSvcEnabled() bool

	// server metrics
	ConfigReloadsCounter() metrics.Counter
	ConfigReloadsFailureCounter() metrics.Counter
	LastConfigReloadSuccessGauge() metrics.Gauge
	LastConfigReloadFailureGauge() metrics.Gauge

	// entry point metrics
	EntryPointReqsCounter() metrics.Counter
	EntryPointReqsTLSCounter() metrics.Counter
	EntryPointReqDurationHistogram() ScalableHistogram
	EntryPointOpenConnsGauge() metrics.Gauge

	// service metrics
	ServiceReqsCounter() metrics.Counter
	ServiceReqsTLSCounter() metrics.Counter
	ServiceReqDurationHistogram() ScalableHistogram
	ServiceOpenConnsGauge() metrics.Gauge
	ServiceRetriesCounter() metrics.Counter
	ServiceServerUpGauge() metrics.Gauge
}

// NewVoidRegistry is a noop implementation of metrics.Registry.
// It is used to avoid nil checking in components that do metric collections.
func NewVoidRegistry() Registry {
	return NewMultiRegistry([]Registry{})
}

// NewMultiRegistry is an implementation of metrics.Registry that wraps multiple registries.
// It handles the case when a registry hasn't registered some metric and returns nil.
// This allows for feature imparity between the different metric implementations.
func NewMultiRegistry(registries []Registry) Registry {
	var configReloadsCounter []metrics.Counter
	var configReloadsFailureCounter []metrics.Counter
	var lastConfigReloadSuccessGauge []metrics.Gauge
	var lastConfigReloadFailureGauge []metrics.Gauge
	var entryPointReqsCounter []metrics.Counter
	var entryPointReqsTLSCounter []metrics.Counter
	var entryPointReqDurationHistogram []ScalableHistogram
	var entryPointOpenConnsGauge []metrics.Gauge
	var serviceReqsCounter []metrics.Counter
	var serviceReqsTLSCounter []metrics.Counter
	var serviceReqDurationHistogram []ScalableHistogram
	var serviceOpenConnsGauge []metrics.Gauge
	var serviceRetriesCounter []metrics.Counter
	var serviceServerUpGauge []metrics.Gauge

	for _, r := range registries {
		if r.ConfigReloadsCounter() != nil {
			configReloadsCounter = append(configReloadsCounter, r.ConfigReloadsCounter())
		}
		if r.ConfigReloadsFailureCounter() != nil {
			configReloadsFailureCounter = append(configReloadsFailureCounter, r.ConfigReloadsFailureCounter())
		}
		if r.LastConfigReloadSuccessGauge() != nil {
			lastConfigReloadSuccessGauge = append(lastConfigReloadSuccessGauge, r.LastConfigReloadSuccessGauge())
		}
		if r.LastConfigReloadFailureGauge() != nil {
			lastConfigReloadFailureGauge = append(lastConfigReloadFailureGauge, r.LastConfigReloadFailureGauge())
		}
		if r.EntryPointReqsCounter() != nil {
			entryPointReqsCounter = append(entryPointReqsCounter, r.EntryPointReqsCounter())
		}
		if r.EntryPointReqsTLSCounter() != nil {
			entryPointReqsTLSCounter = append(entryPointReqsTLSCounter, r.EntryPointReqsTLSCounter())
		}
		if r.EntryPointReqDurationHistogram() != nil {
			entryPointReqDurationHistogram = append(entryPointReqDurationHistogram, r.EntryPointReqDurationHistogram())
		}
		if r.EntryPointOpenConnsGauge() != nil {
			entryPointOpenConnsGauge = append(entryPointOpenConnsGauge, r.EntryPointOpenConnsGauge())
		}
		if r.ServiceReqsCounter() != nil {
			serviceReqsCounter = append(serviceReqsCounter, r.ServiceReqsCounter())
		}
		if r.ServiceReqsTLSCounter() != nil {
			serviceReqsTLSCounter = append(serviceReqsTLSCounter, r.ServiceReqsTLSCounter())
		}
		if r.ServiceReqDurationHistogram() != nil {
			serviceReqDurationHistogram = append(serviceReqDurationHistogram, r.ServiceReqDurationHistogram())
		}
		if r.ServiceOpenConnsGauge() != nil {
			serviceOpenConnsGauge = append(serviceOpenConnsGauge, r.ServiceOpenConnsGauge())
		}
		if r.ServiceRetriesCounter() != nil {
			serviceRetriesCounter = append(serviceRetriesCounter, r.ServiceRetriesCounter())
		}
		if r.ServiceServerUpGauge() != nil {
			serviceServerUpGauge = append(serviceServerUpGauge, r.ServiceServerUpGauge())
		}
	}

	return &standardRegistry{
		epEnabled:                      len(entryPointReqsCounter) > 0 || len(entryPointReqDurationHistogram) > 0 || len(entryPointOpenConnsGauge) > 0,
		svcEnabled:                     len(serviceReqsCounter) > 0 || len(serviceReqDurationHistogram) > 0 || len(serviceOpenConnsGauge) > 0 || len(serviceRetriesCounter) > 0 || len(serviceServerUpGauge) > 0,
		configReloadsCounter:           multi.NewCounter(configReloadsCounter...),
		configReloadsFailureCounter:    multi.NewCounter(configReloadsFailureCounter...),
		lastConfigReloadSuccessGauge:   multi.NewGauge(lastConfigReloadSuccessGauge...),
		lastConfigReloadFailureGauge:   multi.NewGauge(lastConfigReloadFailureGauge...),
		entryPointReqsCounter:          multi.NewCounter(entryPointReqsCounter...),
		entryPointReqsTLSCounter:       multi.NewCounter(entryPointReqsTLSCounter...),
		entryPointReqDurationHistogram: NewMultiHistogram(entryPointReqDurationHistogram...),
		entryPointOpenConnsGauge:       multi.NewGauge(entryPointOpenConnsGauge...),
		serviceReqsCounter:             multi.NewCounter(serviceReqsCounter...),
		serviceReqsTLSCounter:          multi.NewCounter(serviceReqsTLSCounter...),
		serviceReqDurationHistogram:    NewMultiHistogram(serviceReqDurationHistogram...),
		serviceOpenConnsGauge:          multi.NewGauge(serviceOpenConnsGauge...),
		serviceRetriesCounter:          multi.NewCounter(serviceRetriesCounter...),
		serviceServerUpGauge:           multi.NewGauge(serviceServerUpGauge...),
	}
}

type standardRegistry struct {
	epEnabled                      bool
	svcEnabled                     bool
	configReloadsCounter           metrics.Counter
	configReloadsFailureCounter    metrics.Counter
	lastConfigReloadSuccessGauge   metrics.Gauge
	lastConfigReloadFailureGauge   metrics.Gauge
	entryPointReqsCounter          metrics.Counter
	entryPointReqsTLSCounter       metrics.Counter
	entryPointReqDurationHistogram ScalableHistogram
	entryPointOpenConnsGauge       metrics.Gauge
	serviceReqsCounter             metrics.Counter
	serviceReqsTLSCounter          metrics.Counter
	serviceReqDurationHistogram    ScalableHistogram
	serviceOpenConnsGauge          metrics.Gauge
	serviceRetriesCounter          metrics.Counter
	serviceServerUpGauge           metrics.Gauge
}

func (r *standardRegistry) IsEpEnabled() bool {
	return r.epEnabled
}

func (r *standardRegistry) IsSvcEnabled() bool {
	return r.svcEnabled
}

func (r *standardRegistry) ConfigReloadsCounter() metrics.Counter {
	return r.configReloadsCounter
}

func (r *standardRegistry) ConfigReloadsFailureCounter() metrics.Counter {
	return r.configReloadsFailureCounter
}

func (r *standardRegistry) LastConfigReloadSuccessGauge() metrics.Gauge {
	return r.lastConfigReloadSuccessGauge
}

func (r *standardRegistry) LastConfigReloadFailureGauge() metrics.Gauge {
	return r.lastConfigReloadFailureGauge
}

func (r *standardRegistry) EntryPointReqsCounter() metrics.Counter {
	return r.entryPointReqsCounter
}

func (r *standardRegistry) EntryPointReqsTLSCounter() metrics.Counter {
	return r.entryPointReqsTLSCounter
}

func (r *standardRegistry) EntryPointReqDurationHistogram() ScalableHistogram {
	return r.entryPointReqDurationHistogram
}

func (r *standardRegistry) EntryPointOpenConnsGauge() metrics.Gauge {
	return r.entryPointOpenConnsGauge
}

func (r *standardRegistry) ServiceReqsCounter() metrics.Counter {
	return r.serviceReqsCounter
}

func (r *standardRegistry) ServiceReqsTLSCounter() metrics.Counter {
	return r.serviceReqsTLSCounter
}

func (r *standardRegistry) ServiceReqDurationHistogram() ScalableHistogram {
	return r.serviceReqDurationHistogram
}

func (r *standardRegistry) ServiceOpenConnsGauge() metrics.Gauge {
	return r.serviceOpenConnsGauge
}

func (r *standardRegistry) ServiceRetriesCounter() metrics.Counter {
	return r.serviceRetriesCounter
}

func (r *standardRegistry) ServiceServerUpGauge() metrics.Gauge {
	return r.serviceServerUpGauge
}

// ScalableHistogram is a Histogram with a predefined time unit,
// used when producing observations without explicitly setting the observed value.
type ScalableHistogram interface {
	With(labelValues ...string) ScalableHistogram
	Observe(v float64)
	ObserveFromStart(start time.Time)
}

// HistogramWithScale is a histogram that will convert its observed value to the specified unit.
type HistogramWithScale struct {
	histogram metrics.Histogram
	unit      time.Duration
}

// With implements ScalableHistogram.
func (s *HistogramWithScale) With(labelValues ...string) ScalableHistogram {
	h, _ := NewHistogramWithScale(s.histogram.With(labelValues...), s.unit)
	return h
}

// ObserveFromStart implements ScalableHistogram.
func (s *HistogramWithScale) ObserveFromStart(start time.Time) {
	if s.unit <= 0 {
		return
	}

	d := float64(time.Since(start).Nanoseconds()) / float64(s.unit)
	if d < 0 {
		d = 0
	}
	s.histogram.Observe(d)
}

// Observe implements ScalableHistogram.
func (s *HistogramWithScale) Observe(v float64) {
	s.histogram.Observe(v)
}

// NewHistogramWithScale returns a ScalableHistogram. It returns an error if the given unit is <= 0.
func NewHistogramWithScale(histogram metrics.Histogram, unit time.Duration) (ScalableHistogram, error) {
	if unit <= 0 {
		return nil, errors.New("invalid time unit")
	}
	return &HistogramWithScale{
		histogram: histogram,
		unit:      unit,
	}, nil
}

// MultiHistogram collects multiple individual histograms and treats them as a unit.
type MultiHistogram []ScalableHistogram

// NewMultiHistogram returns a multi-histogram, wrapping the passed histograms.
func NewMultiHistogram(h ...ScalableHistogram) MultiHistogram {
	return MultiHistogram(h)
}

// ObserveFromStart implements ScalableHistogram.
func (h MultiHistogram) ObserveFromStart(start time.Time) {
	for _, histogram := range h {
		histogram.ObserveFromStart(start)
	}
}

// Observe implements ScalableHistogram.
func (h MultiHistogram) Observe(v float64) {
	for _, histogram := range h {
		histogram.Observe(v)
	}
}

// With implements ScalableHistogram.
func (h MultiHistogram) With(labelValues ...string) ScalableHistogram {
	next := make(MultiHistogram, len(h))
	for i := range h {
		next[i] = h[i].With(labelValues...)
	}
	return next
}