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

package audit

import (
	"bytes"
	"context"
	"io"
	"time"

	"github.com/vivint/infectious"
	"github.com/zeebo/errs"
	"go.uber.org/zap"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/status"
	monkit "gopkg.in/spacemonkeygo/monkit.v2"

	"storj.io/storj/internal/memory"
	"storj.io/storj/pkg/auth/signing"
	"storj.io/storj/pkg/identity"
	"storj.io/storj/pkg/overlay"
	"storj.io/storj/pkg/pb"
	"storj.io/storj/pkg/pkcrypto"
	"storj.io/storj/pkg/storj"
	"storj.io/storj/pkg/transport"
	"storj.io/storj/satellite/metainfo"
	"storj.io/storj/satellite/orders"
	"storj.io/storj/storage"
	"storj.io/storj/uplink/piecestore"
)

var (
	mon = monkit.Package()

	// ErrNotEnoughShares is the errs class for when not enough shares are available to do an audit
	ErrNotEnoughShares = errs.Class("not enough shares for successful audit")
	// ErrSegmentDeleted is the errs class when the audited segment was deleted during the audit
	ErrSegmentDeleted = errs.Class("segment deleted during audit")
)

// Share represents required information about an audited share
type Share struct {
	Error    error
	PieceNum int
	NodeID   storj.NodeID
	Data     []byte
}

// Verifier helps verify the correctness of a given stripe
type Verifier struct {
	log                *zap.Logger
	metainfo           *metainfo.Service
	orders             *orders.Service
	auditor            *identity.PeerIdentity
	transport          transport.Client
	overlay            *overlay.Cache
	containment        Containment
	minBytesPerSecond  memory.Size
	minDownloadTimeout time.Duration
}

// NewVerifier creates a Verifier
func NewVerifier(log *zap.Logger, metainfo *metainfo.Service, transport transport.Client, overlay *overlay.Cache, containment Containment, orders *orders.Service, id *identity.FullIdentity, minBytesPerSecond memory.Size, minDownloadTimeout time.Duration) *Verifier {
	return &Verifier{
		log:                log,
		metainfo:           metainfo,
		orders:             orders,
		auditor:            id.PeerIdentity(),
		transport:          transport,
		overlay:            overlay,
		containment:        containment,
		minBytesPerSecond:  minBytesPerSecond,
		minDownloadTimeout: minDownloadTimeout,
	}
}

// Verify downloads shares then verifies the data correctness at the given stripe
func (verifier *Verifier) Verify(ctx context.Context, stripe *Stripe, skip map[storj.NodeID]bool) (report *Report, err error) {
	defer mon.Task()(&ctx)(&err)

	pointer := stripe.Segment
	shareSize := pointer.GetRemote().GetRedundancy().GetErasureShareSize()
	bucketID := createBucketID(stripe.SegmentPath)

	var offlineNodes storj.NodeIDList
	var failedNodes storj.NodeIDList
	containedNodes := make(map[int]storj.NodeID)
	sharesToAudit := make(map[int]Share)

	orderLimits, err := verifier.orders.CreateAuditOrderLimits(ctx, verifier.auditor, bucketID, pointer, skip)
	if err != nil {
		return nil, err
	}

	// note: offlineNodes here will include disqualified nodes
	offlineNodes = getOfflineNodes(stripe.Segment, orderLimits, skip)
	if len(offlineNodes) > 0 {
		verifier.log.Debug("Verify: order limits not created for some nodes (offline/disqualified)", zap.Strings("Node IDs", offlineNodes.Strings()))
	}

	shares, err := verifier.DownloadShares(ctx, orderLimits, stripe.Index, shareSize)
	if err != nil {
		return &Report{
			Offlines: offlineNodes,
		}, err
	}

	err = verifier.checkIfSegmentDeleted(ctx, stripe)
	if err != nil {
		return &Report{
			Offlines: offlineNodes,
		}, err
	}

	for pieceNum, share := range shares {
		if share.Error == nil {
			// no error -- share downloaded successfully
			sharesToAudit[pieceNum] = share
			continue
		}
		if transport.Error.Has(share.Error) {
			if errs.IsFunc(share.Error, func(err error) bool {
				return err == context.DeadlineExceeded
			}) {
				// dial timeout
				offlineNodes = append(offlineNodes, share.NodeID)
				verifier.log.Debug("Verify: dial timeout (offline)", zap.Stringer("Node ID", share.NodeID), zap.Error(share.Error))
				continue
			}
			if errs.IsFunc(share.Error, func(err error) bool {
				return status.Code(err) == codes.Unknown
			}) {
				// dial failed -- offline node
				offlineNodes = append(offlineNodes, share.NodeID)
				verifier.log.Debug("Verify: dial failed (offline)", zap.Stringer("Node ID", share.NodeID), zap.Error(share.Error))
				continue
			}
			// unknown transport error
			containedNodes[pieceNum] = share.NodeID
			verifier.log.Debug("Verify: unknown transport error (contained)", zap.Stringer("Node ID", share.NodeID), zap.Error(share.Error))
		}

		if errs.IsFunc(share.Error, func(err error) bool {
			return status.Code(err) == codes.NotFound
		}) {
			// missing share
			failedNodes = append(failedNodes, share.NodeID)
			verifier.log.Debug("Verify: piece not found (audit failed)", zap.Stringer("Node ID", share.NodeID), zap.Error(share.Error))
			continue
		}

		if errs.IsFunc(share.Error, func(err error) bool {
			return status.Code(err) == codes.DeadlineExceeded
		}) {
			// dial successful, but download timed out
			containedNodes[pieceNum] = share.NodeID
			verifier.log.Debug("Verify: download timeout (contained)", zap.Stringer("Node ID", share.NodeID), zap.Error(share.Error))
			continue
		}

		// unknown error
		containedNodes[pieceNum] = share.NodeID
		verifier.log.Debug("Verify: unknown error (contained)", zap.Stringer("Node ID", share.NodeID), zap.Error(share.Error))
	}

	required := int(pointer.Remote.Redundancy.GetMinReq())
	total := int(pointer.Remote.Redundancy.GetTotal())

	if len(sharesToAudit) < required {
		return &Report{
			Fails:    failedNodes,
			Offlines: offlineNodes,
		}, ErrNotEnoughShares.New("got %d, required %d", len(sharesToAudit), required)
	}

	pieceNums, correctedShares, err := auditShares(ctx, required, total, sharesToAudit)
	if err != nil {
		return &Report{
			Fails:    failedNodes,
			Offlines: offlineNodes,
		}, err
	}

	for _, pieceNum := range pieceNums {
		failedNodes = append(failedNodes, shares[pieceNum].NodeID)
	}

	successNodes := getSuccessNodes(ctx, shares, failedNodes, offlineNodes, containedNodes)

	totalInPointer := len(stripe.Segment.GetRemote().GetRemotePieces())
	numOffline := len(offlineNodes)
	numSuccessful := len(successNodes)
	numFailed := len(failedNodes)
	numContained := len(containedNodes)
	totalAudited := numSuccessful + numFailed + numOffline + numContained
	auditedPercentage := float64(totalAudited) / float64(totalInPointer)
	offlinePercentage := float64(0)
	successfulPercentage := float64(0)
	failedPercentage := float64(0)
	containedPercentage := float64(0)
	if totalAudited > 0 {
		offlinePercentage = float64(numOffline) / float64(totalAudited)
		successfulPercentage = float64(numSuccessful) / float64(totalAudited)
		failedPercentage = float64(numFailed) / float64(totalAudited)
		containedPercentage = float64(numContained) / float64(totalAudited)
	}

	mon.Meter("audit_success_nodes_global").Mark(numSuccessful)
	mon.Meter("audit_fail_nodes_global").Mark(numFailed)
	mon.Meter("audit_offline_nodes_global").Mark(numOffline)
	mon.Meter("audit_contained_nodes_global").Mark(numContained)
	mon.Meter("audit_total_nodes_global").Mark(totalAudited)
	mon.Meter("audit_total_pointer_nodes_global").Mark(totalInPointer)

	mon.IntVal("audit_success_nodes").Observe(int64(numSuccessful))
	mon.IntVal("audit_fail_nodes").Observe(int64(numFailed))
	mon.IntVal("audit_offline_nodes").Observe(int64(numOffline))
	mon.IntVal("audit_contained_nodes").Observe(int64(numContained))
	mon.IntVal("audit_total_nodes").Observe(int64(totalAudited))
	mon.IntVal("audit_total_pointer_nodes").Observe(int64(totalInPointer))
	mon.FloatVal("audited_percentage").Observe(auditedPercentage)
	mon.FloatVal("audit_offline_percentage").Observe(offlinePercentage)
	mon.FloatVal("audit_successful_percentage").Observe(successfulPercentage)
	mon.FloatVal("audit_failed_percentage").Observe(failedPercentage)
	mon.FloatVal("audit_contained_percentage").Observe(containedPercentage)

	pendingAudits, err := createPendingAudits(ctx, containedNodes, correctedShares, stripe)
	if err != nil {
		return &Report{
			Successes: successNodes,
			Fails:     failedNodes,
			Offlines:  offlineNodes,
		}, err
	}

	return &Report{
		Successes:     successNodes,
		Fails:         failedNodes,
		Offlines:      offlineNodes,
		PendingAudits: pendingAudits,
	}, nil
}

// DownloadShares downloads shares from the nodes where remote pieces are located
func (verifier *Verifier) DownloadShares(ctx context.Context, limits []*pb.AddressedOrderLimit, stripeIndex int64, shareSize int32) (shares map[int]Share, err error) {
	defer mon.Task()(&ctx)(&err)

	shares = make(map[int]Share, len(limits))
	ch := make(chan *Share, len(limits))

	for i, limit := range limits {
		if limit == nil {
			ch <- nil
			continue
		}

		go func(i int, limit *pb.AddressedOrderLimit) {
			share, err := verifier.GetShare(ctx, limit, stripeIndex, shareSize, i)
			if err != nil {
				share = Share{
					Error:    err,
					PieceNum: i,
					NodeID:   limit.GetLimit().StorageNodeId,
					Data:     nil,
				}
			}
			ch <- &share
		}(i, limit)
	}

	for range limits {
		share := <-ch
		if share != nil {
			shares[share.PieceNum] = *share
		}
	}

	return shares, nil
}

// Reverify reverifies the contained nodes in the stripe
func (verifier *Verifier) Reverify(ctx context.Context, stripe *Stripe) (report *Report, err error) {
	defer mon.Task()(&ctx)(&err)

	// result status enum
	const (
		skipped = iota
		success
		offline
		failed
		contained
		erred
	)

	type result struct {
		nodeID       storj.NodeID
		status       int
		pendingAudit *PendingAudit
		err          error
	}

	pieces := stripe.Segment.GetRemote().GetRemotePieces()
	ch := make(chan result, len(pieces))
	var containedInSegment int64

	for _, piece := range pieces {
		pending, err := verifier.containment.Get(ctx, piece.NodeId)
		if err != nil {
			if ErrContainedNotFound.Has(err) {
				ch <- result{nodeID: piece.NodeId, status: skipped}
				continue
			}
			ch <- result{nodeID: piece.NodeId, status: erred, err: err}
			verifier.log.Debug("Reverify: error getting from containment db", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
			continue
		}
		containedInSegment++

		go func(pending *PendingAudit, piece *pb.RemotePiece) {
			limit, err := verifier.orders.CreateAuditOrderLimit(ctx, verifier.auditor, createBucketID(stripe.SegmentPath), pending.NodeID, pending.PieceID, pending.ShareSize)
			if err != nil {
				if overlay.ErrNodeDisqualified.Has(err) {
					_, errDelete := verifier.containment.Delete(ctx, piece.NodeId)
					if errDelete != nil {
						verifier.log.Debug("Error deleting disqualified node from containment db", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
						err = errs.Combine(err, errDelete)
					}
					ch <- result{nodeID: piece.NodeId, status: erred, err: err}
					verifier.log.Debug("Reverify: order limit not created (disqualified)", zap.Stringer("Node ID", piece.NodeId))
					return
				}
				if overlay.ErrNodeOffline.Has(err) {
					ch <- result{nodeID: piece.NodeId, status: offline}
					verifier.log.Debug("Reverify: order limit not created (offline)", zap.Stringer("Node ID", piece.NodeId))
					return
				}
				ch <- result{nodeID: piece.NodeId, status: erred, err: err}
				verifier.log.Debug("Reverify: error creating order limit", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
				return
			}

			share, err := verifier.GetShare(ctx, limit, pending.StripeIndex, pending.ShareSize, int(piece.PieceNum))

			// check if the pending audit was deleted while downloading the share
			_, getErr := verifier.containment.Get(ctx, piece.NodeId)
			if getErr != nil {
				if ErrContainedNotFound.Has(getErr) {
					ch <- result{nodeID: piece.NodeId, status: skipped}
					verifier.log.Debug("Reverify: pending audit deleted during reverification", zap.Stringer("Node ID", piece.NodeId), zap.Error(getErr))
					return
				}
				ch <- result{nodeID: piece.NodeId, status: erred, err: getErr}
				verifier.log.Debug("Reverify: error getting from containment db", zap.Stringer("Node ID", piece.NodeId), zap.Error(getErr))
				return
			}

			// analyze the error from GetShare
			if err != nil {
				if transport.Error.Has(err) {
					if errs.IsFunc(err, func(err error) bool {
						return err == context.DeadlineExceeded
					}) {
						// dial timeout
						ch <- result{nodeID: piece.NodeId, status: offline}
						verifier.log.Debug("Reverify: dial timeout (offline)", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
						return
					}
					if errs.IsFunc(err, func(err error) bool {
						return status.Code(err) == codes.Unknown
					}) {
						// dial failed -- offline node
						verifier.log.Debug("Reverify: dial failed (offline)", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
						ch <- result{nodeID: piece.NodeId, status: offline}
						return
					}
					// unknown transport error
					ch <- result{nodeID: piece.NodeId, status: contained, pendingAudit: pending}
					verifier.log.Debug("Reverify: unknown transport error (contained)", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
					return
				}

				if errs.IsFunc(err, func(err error) bool {
					return status.Code(err) == codes.NotFound
				}) {
					// missing share
					ch <- result{nodeID: piece.NodeId, status: failed}
					verifier.log.Debug("Reverify: piece not found (audit failed)", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
					return
				}

				if errs.IsFunc(err, func(err error) bool {
					return status.Code(err) == codes.DeadlineExceeded
				}) {
					// dial successful, but download timed out
					ch <- result{nodeID: piece.NodeId, status: contained, pendingAudit: pending}
					verifier.log.Debug("Reverify: download timeout (contained)", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
					return
				}

				// unknown error
				ch <- result{nodeID: piece.NodeId, status: contained, pendingAudit: pending}
				verifier.log.Debug("Reverify: unknown error (contained)", zap.Stringer("Node ID", piece.NodeId), zap.Error(err))
				return
			}

			downloadedHash := pkcrypto.SHA256Hash(share.Data)
			if bytes.Equal(downloadedHash, pending.ExpectedShareHash) {
				ch <- result{nodeID: piece.NodeId, status: success}
				verifier.log.Debug("Reverify: hashes match (audit success)", zap.Stringer("Node ID", piece.NodeId))
			} else {
				ch <- result{nodeID: piece.NodeId, status: failed}
				verifier.log.Debug("Reverify: hashes mismatch (audit failed)", zap.Stringer("Node ID", piece.NodeId),
					zap.Binary("expected hash", pending.ExpectedShareHash), zap.Binary("downloaded hash", downloadedHash))
			}
		}(pending, piece)
	}

	report = &Report{}
	for range pieces {
		result := <-ch
		switch result.status {
		case success:
			report.Successes = append(report.Successes, result.nodeID)
		case offline:
			report.Offlines = append(report.Offlines, result.nodeID)
		case failed:
			report.Fails = append(report.Fails, result.nodeID)
		case contained:
			report.PendingAudits = append(report.PendingAudits, result.pendingAudit)
		case erred:
			err = errs.Combine(err, result.err)
		}
	}

	mon.Meter("reverify_successes_global").Mark(len(report.Successes))
	mon.Meter("reverify_offlines_global").Mark(len(report.Offlines))
	mon.Meter("reverify_fails_global").Mark(len(report.Fails))
	mon.Meter("reverify_contained_global").Mark(len(report.PendingAudits))

	mon.IntVal("reverify_successes").Observe(int64(len(report.Successes)))
	mon.IntVal("reverify_offlines").Observe(int64(len(report.Offlines)))
	mon.IntVal("reverify_fails").Observe(int64(len(report.Fails)))
	mon.IntVal("reverify_contained").Observe(int64(len(report.PendingAudits)))

	mon.IntVal("reverify_contained_in_segment").Observe(containedInSegment)
	mon.IntVal("reverify_total_in_segment").Observe(int64(len(pieces)))

	return report, err
}

// GetShare use piece store client to download shares from nodes
func (verifier *Verifier) GetShare(ctx context.Context, limit *pb.AddressedOrderLimit, stripeIndex int64, shareSize int32, pieceNum int) (share Share, err error) {
	defer mon.Task()(&ctx)(&err)

	bandwidthMsgSize := shareSize

	// determines number of seconds allotted for receiving data from a storage node
	timedCtx := ctx
	if verifier.minBytesPerSecond > 0 {
		maxTransferTime := time.Duration(int64(time.Second) * int64(bandwidthMsgSize) / verifier.minBytesPerSecond.Int64())
		if maxTransferTime < verifier.minDownloadTimeout {
			maxTransferTime = verifier.minDownloadTimeout
		}
		var cancel func()
		timedCtx, cancel = context.WithTimeout(ctx, maxTransferTime)
		defer cancel()
	}

	storageNodeID := limit.GetLimit().StorageNodeId

	conn, err := verifier.transport.DialNode(timedCtx, &pb.Node{
		Id:      storageNodeID,
		Address: limit.GetStorageNodeAddress(),
	})
	if err != nil {
		return Share{}, err
	}
	ps := piecestore.NewClient(
		verifier.log.Named(storageNodeID.String()),
		signing.SignerFromFullIdentity(verifier.transport.Identity()),
		conn,
		piecestore.DefaultConfig,
	)
	defer func() {
		err := ps.Close()
		if err != nil {
			verifier.log.Error("audit verifier failed to close conn to node: %+v", zap.Error(err))
		}
	}()

	offset := int64(shareSize) * stripeIndex

	downloader, err := ps.Download(timedCtx, limit.GetLimit(), offset, int64(shareSize))
	if err != nil {
		return Share{}, err
	}
	defer func() { err = errs.Combine(err, downloader.Close()) }()

	buf := make([]byte, shareSize)
	_, err = io.ReadFull(downloader, buf)
	if err != nil {
		return Share{}, err
	}

	return Share{
		Error:    nil,
		PieceNum: pieceNum,
		NodeID:   storageNodeID,
		Data:     buf,
	}, nil
}

// checkIfSegmentDeleted checks if stripe's pointer has been deleted since stripe was selected.
func (verifier *Verifier) checkIfSegmentDeleted(ctx context.Context, stripe *Stripe) (err error) {
	defer mon.Task()(&ctx)(&err)

	pointer, err := verifier.metainfo.Get(ctx, stripe.SegmentPath)
	if err != nil {
		if storage.ErrKeyNotFound.Has(err) {
			return ErrSegmentDeleted.New(stripe.SegmentPath)
		}
		return err
	}

	if pointer.GetCreationDate().GetSeconds() != stripe.Segment.GetCreationDate().GetSeconds() ||
		pointer.GetCreationDate().GetNanos() != stripe.Segment.GetCreationDate().GetNanos() {
		return ErrSegmentDeleted.New(stripe.SegmentPath)
	}

	return nil
}

// auditShares takes the downloaded shares and uses infectious's Correct function to check that they
// haven't been altered. auditShares returns a slice containing the piece numbers of altered shares,
// and a slice of the corrected shares.
func auditShares(ctx context.Context, required, total int, originals map[int]Share) (pieceNums []int, corrected []infectious.Share, err error) {
	defer mon.Task()(&ctx)(&err)
	f, err := infectious.NewFEC(required, total)
	if err != nil {
		return nil, nil, err
	}

	copies, err := makeCopies(ctx, originals)
	if err != nil {
		return nil, nil, err
	}

	err = f.Correct(copies)
	if err != nil {
		return nil, nil, err
	}

	for _, share := range copies {
		if !bytes.Equal(originals[share.Number].Data, share.Data) {
			pieceNums = append(pieceNums, share.Number)
		}
	}
	return pieceNums, copies, nil
}

// makeCopies takes in a map of audit Shares and deep copies their data to a slice of infectious Shares
func makeCopies(ctx context.Context, originals map[int]Share) (copies []infectious.Share, err error) {
	defer mon.Task()(&ctx)(&err)
	copies = make([]infectious.Share, 0, len(originals))
	for _, original := range originals {
		copies = append(copies, infectious.Share{
			Data:   append([]byte{}, original.Data...),
			Number: original.PieceNum})
	}
	return copies, nil
}

// getOfflines nodes returns these storage nodes from pointer which have no order limit
func getOfflineNodes(pointer *pb.Pointer, limits []*pb.AddressedOrderLimit, skip map[storj.NodeID]bool) storj.NodeIDList {
	var offlines storj.NodeIDList

	nodesWithLimit := make(map[storj.NodeID]bool, len(limits))
	for _, limit := range limits {
		if limit != nil {
			nodesWithLimit[limit.GetLimit().StorageNodeId] = true
		}
	}

	for _, piece := range pointer.GetRemote().GetRemotePieces() {
		if !nodesWithLimit[piece.NodeId] && !skip[piece.NodeId] {
			offlines = append(offlines, piece.NodeId)
		}
	}

	return offlines
}

// getSuccessNodes uses the failed nodes, offline nodes and contained nodes arrays to determine which nodes passed the audit
func getSuccessNodes(ctx context.Context, shares map[int]Share, failedNodes, offlineNodes storj.NodeIDList, containedNodes map[int]storj.NodeID) (successNodes storj.NodeIDList) {
	defer mon.Task()(&ctx)(nil)
	fails := make(map[storj.NodeID]bool)
	for _, fail := range failedNodes {
		fails[fail] = true
	}
	for _, offline := range offlineNodes {
		fails[offline] = true
	}
	for _, contained := range containedNodes {
		fails[contained] = true
	}

	for _, share := range shares {
		if !fails[share.NodeID] {
			successNodes = append(successNodes, share.NodeID)
		}
	}

	return successNodes
}

func createBucketID(path storj.Path) []byte {
	comps := storj.SplitPath(path)
	if len(comps) < 3 {
		return nil
	}
	// project_id/bucket_name
	return []byte(storj.JoinPaths(comps[0], comps[2]))
}

func createPendingAudits(ctx context.Context, containedNodes map[int]storj.NodeID, correctedShares []infectious.Share, stripe *Stripe) (pending []*PendingAudit, err error) {
	defer mon.Task()(&ctx)(&err)

	if len(containedNodes) == 0 {
		return nil, nil
	}

	redundancy := stripe.Segment.GetRemote().GetRedundancy()
	required := int(redundancy.GetMinReq())
	total := int(redundancy.GetTotal())
	shareSize := redundancy.GetErasureShareSize()

	fec, err := infectious.NewFEC(required, total)
	if err != nil {
		return nil, Error.Wrap(err)
	}

	stripeData, err := rebuildStripe(ctx, fec, correctedShares, int(shareSize))
	if err != nil {
		return nil, Error.Wrap(err)
	}

	for pieceNum, nodeID := range containedNodes {
		share := make([]byte, shareSize)
		err = fec.EncodeSingle(stripeData, share, pieceNum)
		if err != nil {
			return nil, Error.Wrap(err)
		}
		pending = append(pending, &PendingAudit{
			NodeID:            nodeID,
			PieceID:           stripe.Segment.GetRemote().RootPieceId,
			StripeIndex:       stripe.Index,
			ShareSize:         shareSize,
			ExpectedShareHash: pkcrypto.SHA256Hash(share),
		})
	}

	return pending, nil
}

func rebuildStripe(ctx context.Context, fec *infectious.FEC, corrected []infectious.Share, shareSize int) (_ []byte, err error) {
	defer mon.Task()(&ctx)(&err)
	stripe := make([]byte, fec.Required()*shareSize)
	err = fec.Rebuild(corrected, func(share infectious.Share) {
		copy(stripe[share.Number*shareSize:], share.Data)
	})
	if err != nil {
		return nil, err
	}
	return stripe, nil
}