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

package metainfo

import (
	"context"
	"fmt"
	"time"

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

	"storj.io/common/pb"
	"storj.io/common/storj"
	"storj.io/common/uuid"
	"storj.io/storj/storage"
)

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

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

// 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, ScopedPath, *pb.Pointer) error {
	return nil
}

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

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

// ScopedPath contains full expanded information about the path.
type ScopedPath struct {
	ProjectID           uuid.UUID
	ProjectIDString     string
	Segment             string
	BucketName          string
	EncryptedObjectPath string

	// TODO: should these be a []byte?

	// Raw is the same path as pointerDB is using.
	Raw storj.Path
}

type observerContext struct {
	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, path ScopedPath, pointer *pb.Pointer) error {
	start := time.Now()
	defer func() { observer.object.Insert(time.Since(start)) }()

	return observer.observer.Object(ctx, path, pointer)
}

func (observer *observerContext) RemoteSegment(ctx context.Context, path ScopedPath, pointer *pb.Pointer) error {
	start := time.Now()
	defer func() { observer.remote.Insert(time.Since(start)) }()

	return observer.observer.RemoteSegment(ctx, path, pointer)
}

func (observer *observerContext) InlineSegment(ctx context.Context, path ScopedPath, pointer *pb.Pointer) error {
	start := time.Now()
	defer func() { observer.inline.Insert(time.Since(start)) }()

	return observer.observer.InlineSegment(ctx, path, pointer)
}

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
}

// LoopConfig contains configurable values for the metainfo loop.
type LoopConfig struct {
	CoalesceDuration time.Duration `help:"how long to wait for new observers before starting iteration" releaseDefault:"5s" devDefault:"5s"`
	RateLimit        float64       `help:"metainfo loop rate limit (default is 0 which is unlimited segments per second)" default:"0"`
}

// Loop is a metainfo loop service.
//
// architecture: Service
type Loop struct {
	config LoopConfig
	db     PointerDB
	join   chan *observerContext
	done   chan struct{}
}

// NewLoop creates a new metainfo loop service.
func NewLoop(config LoopConfig, db PointerDB) *Loop {
	return &Loop{
		db:     db,
		config: config,
		join:   make(chan *observerContext),
		done:   make(chan struct{}),
	}
}

// Join 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 *Loop) Join(ctx context.Context, observer Observer) (err error) {
	defer mon.Task()(&ctx)(&err)

	obsContext := newObserverContext(ctx, observer)

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

	return obsContext.Wait()
}

// Run starts the looping service.
// It can only be called once, otherwise a panic will occur.
func (loop *Loop) 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 *Loop) Close() (err error) {
	close(loop.done)
	return nil
}

// runOnce goes through metainfo one time and sends information to observers.
func (loop *Loop) runOnce(ctx context.Context) (err error) {
	defer mon.Task()(&ctx)(&err)

	var observers []*observerContext

	// wait for the first observer, or exit because context is canceled
	select {
	case observer := <-loop.join:
		observers = append(observers, observer)
	case <-ctx.Done():
		return ctx.Err()
	}

	// after the first observer is found, set timer for CoalesceDuration and add any observers that try to join before the timer is up
	timer := time.NewTimer(loop.config.CoalesceDuration)
waitformore:
	for {
		select {
		case observer := <-loop.join:
			observers = append(observers, observer)
		case <-timer.C:
			break waitformore
		case <-ctx.Done():
			finishObservers(observers)
			return ctx.Err()
		}
	}
	return iterateDatabase(ctx, loop.db, observers, rate.NewLimiter(rate.Limit(loop.config.RateLimit), 1))
}

// IterateDatabase iterates over PointerDB and notifies specified observers about results.
func IterateDatabase(ctx context.Context, rateLimit float64, db PointerDB, observers ...Observer) error {
	obsContexts := make([]*observerContext, len(observers))
	for i, observer := range observers {
		obsContexts[i] = newObserverContext(ctx, observer)
	}
	return iterateDatabase(ctx, db, obsContexts, rate.NewLimiter(rate.Limit(rateLimit), 1))
}

// handlePointer deals with a pointer for a single observer
// if there is some error on the observer, handles the error and returns false. Otherwise, returns true.
func handlePointer(ctx context.Context, observer *observerContext, path ScopedPath, isLastSegment bool, pointer *pb.Pointer) bool {
	switch pointer.GetType() {
	case pb.Pointer_REMOTE:
		if observer.HandleError(observer.RemoteSegment(ctx, path, pointer)) {
			return false
		}
	case pb.Pointer_INLINE:
		if observer.HandleError(observer.InlineSegment(ctx, path, pointer)) {
			return false
		}
	default:
		return false
	}
	if isLastSegment {
		if observer.HandleError(observer.Object(ctx, path, pointer)) {
			return false
		}
	}

	select {
	case <-observer.ctx.Done():
		observer.HandleError(observer.ctx.Err())
		return false
	default:
	}

	return true
}

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

func iterateDatabase(ctx context.Context, db PointerDB, observers []*observerContext, rateLimiter *rate.Limiter) (err error) {
	defer func() {
		if err != nil {
			for _, observer := range observers {
				observer.HandleError(err)
			}
			return
		}
		finishObservers(observers)
	}()

	err = db.Iterate(ctx, storage.IterateOptions{Recurse: true},
		func(ctx context.Context, it storage.Iterator) error {
			var item storage.ListItem

			// iterate over every segment in metainfo
		nextSegment:
			for it.Next(ctx, &item) {
				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.
					return LoopError.Wrap(err)
				}

				rawPath := item.Key.String()
				pointer := &pb.Pointer{}

				err := proto.Unmarshal(item.Value, pointer)
				if err != nil {
					return LoopError.New("unexpected error unmarshalling pointer %s", err)
				}

				pathElements := storj.SplitPath(rawPath)

				if len(pathElements) < 4 {
					// We skip this path because it belongs to bucket metadata, not to an
					// actual object
					continue nextSegment
				}

				isLastSegment := pathElements[1] == "l"

				path := ScopedPath{
					Raw:                 rawPath,
					ProjectIDString:     pathElements[0],
					Segment:             pathElements[1],
					BucketName:          pathElements[2],
					EncryptedObjectPath: storj.JoinPaths(pathElements[3:]...),
				}

				projectID, err := uuid.Parse(path.ProjectIDString)
				if err != nil {
					return LoopError.Wrap(err)
				}
				path.ProjectID = *projectID

				nextObservers := observers[:0]
				for _, observer := range observers {
					keepObserver := handlePointer(ctx, observer, path, isLastSegment, pointer)
					if keepObserver {
						nextObservers = append(nextObservers, observer)
					}
				}

				observers = nextObservers
				if len(observers) == 0 {
					return nil
				}

				// if context has been canceled exit. Otherwise, continue
				select {
				case <-ctx.Done():
					return ctx.Err()
				default:
				}
			}
			return nil
		})
	return err
}

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