package meta

import (
	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
	"go.etcd.io/bbolt"
)

// GetChildren returns parent -> children map.
// If an object has no children, then map will contain addr -> empty slice value.
func (db *DB) GetChildren(addresses []oid.Address) (map[oid.Address][]oid.Address, error) {
	db.modeMtx.RLock()
	defer db.modeMtx.RUnlock()

	if db.mode.NoMetabase() {
		return nil, ErrDegradedMode
	}

	result := make(map[oid.Address][]oid.Address, len(addresses))

	buffer := make([]byte, bucketKeySize)
	err := db.boltDB.View(func(tx *bbolt.Tx) error {
		for _, addr := range addresses {
			if _, found := result[addr]; found {
				continue
			}

			result[addr] = []oid.Address{}
			bkt := tx.Bucket(parentBucketName(addr.Container(), buffer))
			if bkt == nil {
				continue
			}

			binObjIDs, err := decodeList(bkt.Get(objectKey(addr.Object(), buffer)))
			if err != nil {
				return err
			}

			for _, binObjID := range binObjIDs {
				var id oid.ID
				if err = id.Decode(binObjID); err != nil {
					return err
				}
				var resultAddress oid.Address
				resultAddress.SetContainer(addr.Container())
				resultAddress.SetObject(id)
				result[addr] = append(result[addr], resultAddress)
			}
		}
		return nil
	})

	if err != nil {
		return nil, err
	}

	return result, nil
}