// Copyright (C) 2019 Storj Labs, Inc.
// See LICENSE for copying information.

package metaloop

import (
	"context"
	"errors"
	"fmt"
	"time"

	"github.com/spacemonkeygo/monkit/v3"
	"github.com/zeebo/errs"
	"golang.org/x/time/rate"

	"storj.io/common/uuid"
	"storj.io/storj/satellite/metabase"
)

const batchsizeLimit = 2500

var (
	mon = monkit.Package()

	// Error is a standard error class for this component.
	Error = errs.Class("metainfo loop")
	// ErrClosed is a loop closed error.
	ErrClosed = Error.New("loop closed")
)

// Object is the object info passed to Observer by metainfo loop.
type Object metabase.LoopObjectEntry

// Expired checks if object expired relative to now.
func (object *Object) Expired(now time.Time) bool {
	return object.ExpiresAt != nil && object.ExpiresAt.Before(now)
}

// Segment is the segment info passed to Observer by metainfo loop.
type Segment struct {
	Location       metabase.SegmentLocation // tally, repair, graceful exit, audit
	ExpirationDate time.Time                // tally, repair

	metabase.LoopSegmentEntry
}

// Expired checks if segment is expired relative to now.
func (segment *Segment) Expired(now time.Time) bool {
	return !segment.ExpirationDate.IsZero() && segment.ExpirationDate.Before(now)
}

// Observer is an interface defining an observer that can subscribe to the metainfo loop.
//
// architecture: Observer
type Observer interface {
	Object(context.Context, *Object) error
	RemoteSegment(context.Context, *Segment) error
	InlineSegment(context.Context, *Segment) error
	LoopStarted(context.Context, LoopInfo) error
}

// LoopInfo contains information about the current loop.
type LoopInfo struct {
	Started time.Time
}

// NullObserver is an observer that does nothing. This is useful for joining
// and ensuring the metainfo loop runs once before you use a real observer.
type NullObserver struct{}

// Object implements the Observer interface.
func (NullObserver) Object(context.Context, *Object) error {
	return nil
}

// RemoteSegment implements the Observer interface.
func (NullObserver) RemoteSegment(context.Context, *Segment) error {
	return nil
}

// InlineSegment implements the Observer interface.
func (NullObserver) InlineSegment(context.Context, *Segment) error {
	return nil
}

// LoopStarted is called at each loop start.
func (NullObserver) LoopStarted(context.Context, LoopInfo) error {
	return nil
}

type observerContext struct {
	trigger  bool
	observer Observer

	ctx  context.Context
	done chan error

	object *monkit.DurationDist
	remote *monkit.DurationDist
	inline *monkit.DurationDist
}

func newObserverContext(ctx context.Context, obs Observer) *observerContext {
	name := fmt.Sprintf("%T", obs)
	key := monkit.NewSeriesKey("observer").WithTag("name", name)

	return &observerContext{
		observer: obs,

		ctx:  ctx,
		done: make(chan error),

		object: monkit.NewDurationDist(key.WithTag("pointer_type", "object")),
		inline: monkit.NewDurationDist(key.WithTag("pointer_type", "inline")),
		remote: monkit.NewDurationDist(key.WithTag("pointer_type", "remote")),
	}
}

func (observer *observerContext) Object(ctx context.Context, object *Object) error {
	start := time.Now()
	defer func() { observer.object.Insert(time.Since(start)) }()

	return observer.observer.Object(ctx, object)
}

func (observer *observerContext) RemoteSegment(ctx context.Context, segment *Segment) error {
	start := time.Now()
	defer func() { observer.remote.Insert(time.Since(start)) }()

	return observer.observer.RemoteSegment(ctx, segment)
}

func (observer *observerContext) InlineSegment(ctx context.Context, segment *Segment) error {
	start := time.Now()
	defer func() { observer.inline.Insert(time.Since(start)) }()

	return observer.observer.InlineSegment(ctx, segment)
}

func (observer *observerContext) HandleError(err error) bool {
	if err != nil {
		observer.done <- err
		observer.Finish()
		return true
	}
	return false
}

func (observer *observerContext) Finish() {
	close(observer.done)

	name := fmt.Sprintf("%T", observer.observer)
	stats := allObserverStatsCollectors.GetStats(name)
	stats.Observe(observer)
}

func (observer *observerContext) Wait() error {
	return <-observer.done
}

// Config contains configurable values for the metainfo loop.
type Config struct {
	CoalesceDuration time.Duration `help:"how long to wait for new observers before starting iteration" releaseDefault:"5s" devDefault:"5s" testDefault:"1s"`
	RateLimit        float64       `help:"rate limit (default is 0 which is unlimited segments per second)" default:"0"`
	ListLimit        int           `help:"how many items to query in a batch" default:"2500" testDefault:"10000"`

	MaxAsOfSystemDuration time.Duration `help:"limits how old can AS OF SYSTEM TIME query be" releaseDefault:"5m" devDefault:"5m"`
}

// MetabaseDB contains iterators for the metabase data.
type MetabaseDB interface {
	// Now returns the time on the database.
	Now(ctx context.Context) (time.Time, error)
	// IterateLoopObjects iterates through all objects in metabase for metainfo loop purpose.
	IterateLoopObjects(ctx context.Context, opts metabase.IterateLoopObjects, fn func(context.Context, metabase.LoopObjectsIterator) error) (err error)
	// IterateLoopStreams iterates through all streams passed in as arguments.
	IterateLoopStreams(ctx context.Context, opts metabase.IterateLoopStreams, handleStream func(ctx context.Context, streamID uuid.UUID, next metabase.SegmentIterator) error) (err error)
}

// Service is a metainfo loop service.
//
// architecture: Service
type Service struct {
	config     Config
	metabaseDB MetabaseDB
	join       chan *observerContext
	done       chan struct{}
}

// New creates a new metainfo loop service.
func New(config Config, metabaseDB MetabaseDB) *Service {
	return &Service{
		metabaseDB: metabaseDB,
		config:     config,
		join:       make(chan *observerContext),
		done:       make(chan struct{}),
	}
}

// Join will join the looper for one full cycle until completion and then returns.
// Joining will trigger a new iteration after coalesce duration.
// On ctx cancel the observer will return without completely finishing.
// Only on full complete iteration it will return nil.
// Safe to be called concurrently.
func (loop *Service) Join(ctx context.Context, observer Observer) (err error) {
	return loop.joinObserver(ctx, true, observer)
}

// Monitor will join the looper for one full cycle until completion and then returns.
// Joining with monitoring won't trigger after coalesce duration.
// On ctx cancel the observer will return without completely finishing.
// Only on full complete iteration it will return nil.
// Safe to be called concurrently.
func (loop *Service) Monitor(ctx context.Context, observer Observer) (err error) {
	return loop.joinObserver(ctx, false, observer)
}

// joinObserver will join the looper for one full cycle until completion and then returns.
// On ctx cancel the observer will return without completely finishing.
// Only on full complete iteration it will return nil.
// Safe to be called concurrently.
func (loop *Service) joinObserver(ctx context.Context, trigger bool, obs Observer) (err error) {
	defer mon.Task()(&ctx)(&err)

	obsctx := newObserverContext(ctx, obs)
	obsctx.trigger = trigger

	select {
	case loop.join <- obsctx:
	case <-ctx.Done():
		return ctx.Err()
	case <-loop.done:
		return ErrClosed
	}

	return obsctx.Wait()
}

// Run starts the looping service.
// It can only be called once, otherwise a panic will occur.
func (loop *Service) Run(ctx context.Context) (err error) {
	defer mon.Task()(&ctx)(&err)

	for {
		err := loop.RunOnce(ctx)
		if err != nil {
			return err
		}
	}
}

// Close closes the looping services.
func (loop *Service) Close() (err error) {
	close(loop.done)
	return nil
}

// monMetainfo is to preserve the monitoring names.
var monMetainfo = monkit.ScopeNamed("storj.io/storj/satellite/metainfo/metaloop")

// RunOnce goes through metainfo one time and sends information to observers.
//
// It is not safe to call this concurrently with Run.
func (loop *Service) RunOnce(ctx context.Context) (err error) {
	defer monMetainfo.Task()(&ctx)(&err) //mon:locked

	coalesceTimer := time.NewTimer(loop.config.CoalesceDuration)
	defer coalesceTimer.Stop()
	stopTimer(coalesceTimer)

	earlyExit := make(chan *observerContext)
	earlyExitDone := make(chan struct{})
	monitorEarlyExit := func(obs *observerContext) {
		select {
		case <-obs.ctx.Done():
			select {
			case <-earlyExitDone:
			case earlyExit <- obs:
			}
		case <-earlyExitDone:
		}
	}

	timerStarted := false
	observers := []*observerContext{}

waitformore:
	for {
		select {
		// when the coalesce timer hits, we have waited enough for observers to join.
		case <-coalesceTimer.C:
			break waitformore

		// wait for a new observer to join.
		case obsctx := <-loop.join:
			// when the observer triggers the loop and it's the first one,
			// then start the coalescing timer.
			if obsctx.trigger {
				if !timerStarted {
					coalesceTimer.Reset(loop.config.CoalesceDuration)
					timerStarted = true
				}
			}
			observers = append(observers, obsctx)
			go monitorEarlyExit(obsctx)

		// remove an observer from waiting when it's canceled before the loop starts.
		case obsctx := <-earlyExit:
			for i, obs := range observers {
				if obs == obsctx {
					observers = append(observers[:i], observers[i+1:]...)
					break
				}
			}

			obsctx.HandleError(obsctx.ctx.Err())

			// reevalute, whether we acually need to start the loop.
			timerShouldRun := false
			for _, obs := range observers {
				timerShouldRun = timerShouldRun || obs.trigger
			}

			if !timerShouldRun && timerStarted {
				stopTimer(coalesceTimer)
			}

		// when ctx done happens we can finish all the waiting observers.
		case <-ctx.Done():
			close(earlyExitDone)
			errorObservers(observers, ctx.Err())
			return ctx.Err()
		}
	}
	close(earlyExitDone)

	return loop.iterateDatabase(ctx, observers)
}

func stopTimer(t *time.Timer) {
	t.Stop()
	// drain if it contains something
	select {
	case <-t.C:
	default:
	}
}

// Wait waits for run to be finished.
// Safe to be called concurrently.
func (loop *Service) Wait() {
	<-loop.done
}

var errNoObservers = errs.New("no observers")

func (loop *Service) iterateDatabase(ctx context.Context, observers []*observerContext) (err error) {
	defer func() {
		if err != nil {
			errorObservers(observers, err)
			return
		}
		finishObservers(observers)
	}()

	observers, err = loop.iterateObjects(ctx, observers)
	if errors.Is(err, errNoObservers) {
		return nil
	}
	if err != nil {
		return Error.Wrap(err)
	}

	return nil
}

func (loop *Service) iterateObjects(ctx context.Context, observers []*observerContext) (_ []*observerContext, err error) {
	defer mon.Task()(&ctx)(&err)

	limit := loop.config.ListLimit
	if limit <= 0 || limit > batchsizeLimit {
		limit = batchsizeLimit
	}

	rateLimiter := rate.NewLimiter(rate.Limit(loop.config.RateLimit), 1)

	startingTime, err := loop.metabaseDB.Now(ctx)
	if err != nil {
		return observers, Error.Wrap(err)
	}

	observers = withObservers(ctx, observers, func(ctx context.Context, observer *observerContext) bool {
		err := observer.observer.LoopStarted(ctx, LoopInfo{Started: startingTime})
		return !observer.HandleError(err)
	})

	currentAsOfSystemTime := startingTime

	if len(observers) == 0 {
		return observers, errNoObservers
	}
	// TODO we may consider keeping only expiration time as its
	// only thing we need to handle segments
	objectsMap := make(map[uuid.UUID]metabase.LoopObjectEntry)
	ids := make([]uuid.UUID, 0, limit)

	var objectsProcessed, segmentsProcessed int64

	processBatch := func(ctx context.Context) (err error) {
		defer mon.TaskNamed("processBatch")(&ctx)(&err)

		if len(objectsMap) == 0 {
			return nil
		}

		if time.Since(currentAsOfSystemTime) > loop.config.MaxAsOfSystemDuration {
			currentAsOfSystemTime = time.Now().Add(-loop.config.MaxAsOfSystemDuration)
		}

		err = loop.metabaseDB.IterateLoopStreams(ctx, metabase.IterateLoopStreams{
			StreamIDs:      ids,
			AsOfSystemTime: currentAsOfSystemTime,
		}, func(ctx context.Context, streamID uuid.UUID, next metabase.SegmentIterator) (err error) {
			defer mon.TaskNamed("iterateLoopStreamsCB")(&ctx, "objs", objectsProcessed, "segs", segmentsProcessed)(&err)

			if err := ctx.Err(); err != nil {
				return err
			}

			obj, ok := objectsMap[streamID]
			if !ok {
				return Error.New("unable to find corresponding object: %v", streamID)
			}
			delete(objectsMap, streamID)

			observers = withObservers(ctx, observers, func(ctx context.Context, observer *observerContext) bool {
				object := Object(obj)
				return !observer.HandleError(handleObject(ctx, observer, &object))
			})
			if len(observers) == 0 {
				return errNoObservers
			}

			objectsProcessed++
			monMetainfo.IntVal("objectsProcessed").Observe(objectsProcessed) //mon:locked

			for {
				// if context has been canceled exit. Otherwise, continue
				if err := ctx.Err(); err != nil {
					return err
				}

				var segment metabase.LoopSegmentEntry
				if !next(&segment) {
					break
				}

				location := metabase.SegmentLocation{
					ProjectID:  obj.ProjectID,
					BucketName: obj.BucketName,
					ObjectKey:  obj.ObjectKey,
					Position:   segment.Position,
				}

				observers = withObservers(ctx, observers, func(ctx context.Context, observer *observerContext) bool {
					return !observer.HandleError(handleSegment(ctx, observer, location, segment, obj.ExpiresAt))
				})
				if len(observers) == 0 {
					return errNoObservers
				}

				segmentsProcessed++
				monMetainfo.IntVal("segmentsProcessed").Observe(segmentsProcessed) //mon:locked

			}

			return nil
		})
		if err != nil {
			return Error.Wrap(err)
		}

		if len(objectsMap) > 0 {
			return Error.New("unhandled objects %#v", objectsMap)
		}

		return nil
	}

	var objectsIterated int64

	segmentsInBatch := int32(0)
	err = loop.metabaseDB.IterateLoopObjects(ctx, metabase.IterateLoopObjects{
		BatchSize:      limit,
		AsOfSystemTime: startingTime,
	}, func(ctx context.Context, it metabase.LoopObjectsIterator) (err error) {
		defer mon.TaskNamed("iterateLoopObjectsCB")(&ctx)(&err)
		var entry metabase.LoopObjectEntry
		for it.Next(ctx, &entry) {
			timer := mon.Timer("iterateLoopObjectsRateLimit").Start()
			if err := rateLimiter.Wait(ctx); err != nil {
				// We don't really execute concurrent batches so we should never
				// exceed the burst size of 1 and this should never happen.
				// We can also enter here if the context is cancelled.
				timer.Stop()
				return err
			}
			timer.Stop()

			monMetainfo.IntVal("objectsIterated").Observe(objectsIterated) //mon:locked
			objectsIterated++

			objectsMap[entry.StreamID] = entry
			ids = append(ids, entry.StreamID)

			// add +1 to reduce risk of crossing limit
			segmentsInBatch += entry.SegmentCount + 1

			if segmentsInBatch >= int32(limit) {
				err := processBatch(ctx)
				if err != nil {
					return err
				}

				if len(objectsMap) > 0 {
					return errs.New("objects map is not empty")
				}

				ids = ids[:0]
				segmentsInBatch = 0
			}
		}

		return processBatch(ctx)
	})

	return observers, err
}

func withObservers(ctx context.Context, observers []*observerContext, handleObserver func(ctx context.Context, observer *observerContext) bool) []*observerContext {
	defer mon.Task()(&ctx)(nil)
	nextObservers := observers[:0]
	for _, observer := range observers {
		keepObserver := handleObserver(ctx, observer)
		if keepObserver {
			nextObservers = append(nextObservers, observer)
		}
	}
	return nextObservers
}

func handleObject(ctx context.Context, observer *observerContext, object *Object) (err error) {
	defer mon.Task()(&ctx)(&err)

	if err := observer.Object(ctx, object); err != nil {
		return err
	}

	return observer.ctx.Err()
}

func handleSegment(ctx context.Context, observer *observerContext, location metabase.SegmentLocation, segment metabase.LoopSegmentEntry, expirationDate *time.Time) (err error) {
	defer mon.Task()(&ctx)(&err)

	loopSegment := &Segment{
		Location:         location,
		LoopSegmentEntry: segment,
	}

	if expirationDate != nil {
		loopSegment.ExpirationDate = *expirationDate
	}

	if loopSegment.Inline() {
		if err := observer.InlineSegment(ctx, loopSegment); err != nil {
			return err
		}
	} else {
		if err := observer.RemoteSegment(ctx, loopSegment); err != nil {
			return err
		}
	}

	return observer.ctx.Err()
}

func finishObservers(observers []*observerContext) {
	for _, observer := range observers {
		observer.Finish()
	}
}

func errorObservers(observers []*observerContext, err error) {
	for _, observer := range observers {
		observer.HandleError(err)
	}
}