package cache

import (
	"context"

	"github.com/docker/distribution"
	dcontext "github.com/docker/distribution/context"
	prometheus "github.com/docker/distribution/metrics"
	"github.com/opencontainers/go-digest"
)

// Metrics is used to hold metric counters
// related to the number of times a cache was
// hit or missed.
type Metrics struct {
	Requests uint64
	Hits     uint64
	Misses   uint64
}

// Logger can be provided on the MetricsTracker to log errors.
//
// Usually, this is just a proxy to dcontext.GetLogger.
type Logger interface {
	Errorf(format string, args ...interface{})
}

// MetricsTracker represents a metric tracker
// which simply counts the number of hits and misses.
type MetricsTracker interface {
	Hit()
	Miss()
	Metrics() Metrics
	Logger(context.Context) Logger
}

type cachedBlobStatter struct {
	cache   distribution.BlobDescriptorService
	backend distribution.BlobDescriptorService
	tracker MetricsTracker
}

var (
	// cacheCount is the number of total cache request received/hits/misses
	cacheCount = prometheus.StorageNamespace.NewLabeledCounter("cache", "The number of cache request received", "type")
)

// NewCachedBlobStatter creates a new statter which prefers a cache and
// falls back to a backend.
func NewCachedBlobStatter(cache distribution.BlobDescriptorService, backend distribution.BlobDescriptorService) distribution.BlobDescriptorService {
	return &cachedBlobStatter{
		cache:   cache,
		backend: backend,
	}
}

// NewCachedBlobStatterWithMetrics creates a new statter which prefers a cache and
// falls back to a backend. Hits and misses will send to the tracker.
func NewCachedBlobStatterWithMetrics(cache distribution.BlobDescriptorService, backend distribution.BlobDescriptorService, tracker MetricsTracker) distribution.BlobStatter {
	return &cachedBlobStatter{
		cache:   cache,
		backend: backend,
		tracker: tracker,
	}
}

func (cbds *cachedBlobStatter) Stat(ctx context.Context, dgst digest.Digest) (distribution.Descriptor, error) {
	cacheCount.WithValues("Request").Inc(1)

	// try getting from cache
	desc, cacheErr := cbds.cache.Stat(ctx, dgst)
	if cacheErr == nil {
		cacheCount.WithValues("Hit").Inc(1)
		if cbds.tracker != nil {
			cbds.tracker.Hit()
		}
		return desc, nil
	}

	// couldn't get from cache; get from backend
	desc, err := cbds.backend.Stat(ctx, dgst)
	if err != nil {
		return desc, err
	}

	if cacheErr == distribution.ErrBlobUnknown {
		// cache doesn't have info. update it with info got from backend
		cacheCount.WithValues("Miss").Inc(1)
		if cbds.tracker != nil {
			cbds.tracker.Miss()
		}
		if err := cbds.cache.SetDescriptor(ctx, dgst, desc); err != nil {
			dcontext.GetLoggerWithField(ctx, "blob", dgst).WithError(err).Error("error from cache setting desc")
		}
		// we don't need to return cache error upstream if any. continue returning value from backend
		return desc, nil
	}

	// unknown error from cache. just log and error. do not store cache as it may be trigger many set calls
	dcontext.GetLoggerWithField(ctx, "blob", dgst).WithError(cacheErr).Error("error from cache stat(ing) blob")
	cacheCount.WithValues("Error").Inc(1)

	return desc, nil
}

func (cbds *cachedBlobStatter) Clear(ctx context.Context, dgst digest.Digest) error {
	err := cbds.cache.Clear(ctx, dgst)
	if err != nil {
		return err
	}

	err = cbds.backend.Clear(ctx, dgst)
	if err != nil {
		return err
	}
	return nil
}

func (cbds *cachedBlobStatter) SetDescriptor(ctx context.Context, dgst digest.Digest, desc distribution.Descriptor) error {
	if err := cbds.cache.SetDescriptor(ctx, dgst, desc); err != nil {
		dcontext.GetLoggerWithField(ctx, "blob", dgst).WithError(err).Error("error from cache setting desc")
	}
	return nil
}

func logErrorf(ctx context.Context, tracker MetricsTracker, format string, args ...interface{}) {
	if tracker == nil {
		return
	}

	logger := tracker.Logger(ctx)
	if logger == nil {
		return
	}
	logger.Errorf(format, args...)
}