package metrics

import (
	"math"
	"sync"
	"sync/atomic"
	"time"
)

type MeterSnapshot interface {
	Count() int64
	Rate1() float64
	Rate5() float64
	Rate15() float64
	RateMean() float64
}

// Meters count events to produce exponentially-weighted moving average rates
// at one-, five-, and fifteen-minutes and a mean rate.
type Meter interface {
	Mark(int64)
	Snapshot() MeterSnapshot
	Stop()
}

// GetOrRegisterMeter returns an existing Meter or constructs and registers a
// new StandardMeter.
// Be sure to unregister the meter from the registry once it is of no use to
// allow for garbage collection.
func GetOrRegisterMeter(name string, r Registry) Meter {
	if nil == r {
		r = DefaultRegistry
	}
	return r.GetOrRegister(name, NewMeter).(Meter)
}

// NewMeter constructs a new StandardMeter and launches a goroutine.
// Be sure to call Stop() once the meter is of no use to allow for garbage collection.
func NewMeter() Meter {
	if !Enabled {
		return NilMeter{}
	}
	m := newStandardMeter()
	arbiter.Lock()
	defer arbiter.Unlock()
	arbiter.meters[m] = struct{}{}
	if !arbiter.started {
		arbiter.started = true
		go arbiter.tick()
	}
	return m
}

// NewInactiveMeter returns a meter but does not start any goroutines. This
// method is mainly intended for testing.
func NewInactiveMeter() Meter {
	if !Enabled {
		return NilMeter{}
	}
	m := newStandardMeter()
	return m
}

// NewRegisteredMeter constructs and registers a new StandardMeter
// and launches a goroutine.
// Be sure to unregister the meter from the registry once it is of no use to
// allow for garbage collection.
func NewRegisteredMeter(name string, r Registry) Meter {
	return GetOrRegisterMeter(name, r)
}

// meterSnapshot is a read-only copy of the meter's internal values.
type meterSnapshot struct {
	count                          int64
	rate1, rate5, rate15, rateMean float64
}

// Count returns the count of events at the time the snapshot was taken.
func (m *meterSnapshot) Count() int64 { return m.count }

// Rate1 returns the one-minute moving average rate of events per second at the
// time the snapshot was taken.
func (m *meterSnapshot) Rate1() float64 { return m.rate1 }

// Rate5 returns the five-minute moving average rate of events per second at
// the time the snapshot was taken.
func (m *meterSnapshot) Rate5() float64 { return m.rate5 }

// Rate15 returns the fifteen-minute moving average rate of events per second
// at the time the snapshot was taken.
func (m *meterSnapshot) Rate15() float64 { return m.rate15 }

// RateMean returns the meter's mean rate of events per second at the time the
// snapshot was taken.
func (m *meterSnapshot) RateMean() float64 { return m.rateMean }

// NilMeter is a no-op Meter.
type NilMeter struct{}

func (NilMeter) Count() int64            { return 0 }
func (NilMeter) Mark(n int64)            {}
func (NilMeter) Snapshot() MeterSnapshot { return (*emptySnapshot)(nil) }
func (NilMeter) Stop()                   {}

// StandardMeter is the standard implementation of a Meter.
type StandardMeter struct {
	count     atomic.Int64
	uncounted atomic.Int64 // not yet added to the EWMAs
	rateMean  atomic.Uint64

	a1, a5, a15 EWMA
	startTime   time.Time
	stopped     atomic.Bool
}

func newStandardMeter() *StandardMeter {
	return &StandardMeter{
		a1:        NewEWMA1(),
		a5:        NewEWMA5(),
		a15:       NewEWMA15(),
		startTime: time.Now(),
	}
}

// Stop stops the meter, Mark() will be a no-op if you use it after being stopped.
func (m *StandardMeter) Stop() {
	if stopped := m.stopped.Swap(true); !stopped {
		arbiter.Lock()
		delete(arbiter.meters, m)
		arbiter.Unlock()
	}
}

// Mark records the occurrence of n events.
func (m *StandardMeter) Mark(n int64) {
	m.uncounted.Add(n)
}

// Snapshot returns a read-only copy of the meter.
func (m *StandardMeter) Snapshot() MeterSnapshot {
	return &meterSnapshot{
		count:    m.count.Load() + m.uncounted.Load(),
		rate1:    m.a1.Snapshot().Rate(),
		rate5:    m.a5.Snapshot().Rate(),
		rate15:   m.a15.Snapshot().Rate(),
		rateMean: math.Float64frombits(m.rateMean.Load()),
	}
}

func (m *StandardMeter) tick() {
	// Take the uncounted values, add to count
	n := m.uncounted.Swap(0)
	count := m.count.Add(n)
	m.rateMean.Store(math.Float64bits(float64(count) / time.Since(m.startTime).Seconds()))
	// Update the EWMA's internal state
	m.a1.Update(n)
	m.a5.Update(n)
	m.a15.Update(n)
	// And trigger them to calculate the rates
	m.a1.Tick()
	m.a5.Tick()
	m.a15.Tick()
}

// meterArbiter ticks meters every 5s from a single goroutine.
// meters are references in a set for future stopping.
type meterArbiter struct {
	sync.RWMutex
	started bool
	meters  map[*StandardMeter]struct{}
	ticker  *time.Ticker
}

var arbiter = meterArbiter{ticker: time.NewTicker(5 * time.Second), meters: make(map[*StandardMeter]struct{})}

// tick meters on the scheduled interval
func (ma *meterArbiter) tick() {
	for range ma.ticker.C {
		ma.tickMeters()
	}
}

func (ma *meterArbiter) tickMeters() {
	ma.RLock()
	defer ma.RUnlock()
	for meter := range ma.meters {
		meter.tick()
	}
}
