package restorer

import (
	"context"
	"path/filepath"
	"sync"

	"golang.org/x/sync/errgroup"

	"github.com/restic/restic/internal/debug"
	"github.com/restic/restic/internal/errors"
	"github.com/restic/restic/internal/repository"
	"github.com/restic/restic/internal/restic"
	"github.com/restic/restic/internal/ui/restore"
)

// TODO if a blob is corrupt, there may be good blob copies in other packs
// TODO evaluate if it makes sense to split download and processing workers
//      pro: can (slowly) read network and decrypt/write files concurrently
//      con: each worker needs to keep one pack in memory

const (
	largeFileBlobCount = 25
)

// information about regular file being restored
type fileInfo struct {
	lock       sync.Mutex
	inProgress bool
	sparse     bool
	size       int64
	location   string      // file on local filesystem relative to restorer basedir
	blobs      interface{} // blobs of the file
}

type fileBlobInfo struct {
	id     restic.ID // the blob id
	offset int64     // blob offset in the file
}

// information about a data pack required to restore one or more files
type packInfo struct {
	id    restic.ID              // the pack id
	files map[*fileInfo]struct{} // set of files that use blobs from this pack
}

type blobsLoaderFn func(ctx context.Context, packID restic.ID, blobs []restic.Blob, handleBlobFn func(blob restic.BlobHandle, buf []byte, err error) error) error

// fileRestorer restores set of files
type fileRestorer struct {
	idx         func(restic.BlobType, restic.ID) []restic.PackedBlob
	blobsLoader blobsLoaderFn

	workerCount int
	filesWriter *filesWriter
	zeroChunk   restic.ID
	sparse      bool
	progress    *restore.Progress

	dst   string
	files []*fileInfo
	Error func(string, error) error
}

func newFileRestorer(dst string,
	blobsLoader blobsLoaderFn,
	idx func(restic.BlobType, restic.ID) []restic.PackedBlob,
	connections uint,
	sparse bool,
	progress *restore.Progress) *fileRestorer {

	// as packs are streamed the concurrency is limited by IO
	workerCount := int(connections)

	return &fileRestorer{
		idx:         idx,
		blobsLoader: blobsLoader,
		filesWriter: newFilesWriter(workerCount),
		zeroChunk:   repository.ZeroChunk(),
		sparse:      sparse,
		progress:    progress,
		workerCount: workerCount,
		dst:         dst,
		Error:       restorerAbortOnAllErrors,
	}
}

func (r *fileRestorer) addFile(location string, content restic.IDs, size int64) {
	r.files = append(r.files, &fileInfo{location: location, blobs: content, size: size})
}

func (r *fileRestorer) targetPath(location string) string {
	return filepath.Join(r.dst, location)
}

func (r *fileRestorer) forEachBlob(blobIDs []restic.ID, fn func(packID restic.ID, packBlob restic.Blob)) error {
	if len(blobIDs) == 0 {
		return nil
	}

	for _, blobID := range blobIDs {
		packs := r.idx(restic.DataBlob, blobID)
		if len(packs) == 0 {
			return errors.Errorf("Unknown blob %s", blobID.String())
		}
		fn(packs[0].PackID, packs[0].Blob)
	}

	return nil
}

func (r *fileRestorer) restoreFiles(ctx context.Context) error {

	packs := make(map[restic.ID]*packInfo) // all packs
	// Process packs in order of first access. While this cannot guarantee
	// that file chunks are restored sequentially, it offers a good enough
	// approximation to shorten restore times by up to 19% in some test.
	var packOrder restic.IDs

	// create packInfo from fileInfo
	for _, file := range r.files {
		fileBlobs := file.blobs.(restic.IDs)
		largeFile := len(fileBlobs) > largeFileBlobCount
		var packsMap map[restic.ID][]fileBlobInfo
		if largeFile {
			packsMap = make(map[restic.ID][]fileBlobInfo)
		}
		fileOffset := int64(0)
		err := r.forEachBlob(fileBlobs, func(packID restic.ID, blob restic.Blob) {
			if largeFile {
				packsMap[packID] = append(packsMap[packID], fileBlobInfo{id: blob.ID, offset: fileOffset})
				fileOffset += int64(blob.DataLength())
			}
			pack, ok := packs[packID]
			if !ok {
				pack = &packInfo{
					id:    packID,
					files: make(map[*fileInfo]struct{}),
				}
				packs[packID] = pack
				packOrder = append(packOrder, packID)
			}
			pack.files[file] = struct{}{}
			if blob.ID.Equal(r.zeroChunk) {
				file.sparse = r.sparse
			}
		})
		if len(fileBlobs) == 1 {
			// no need to preallocate files with a single block, thus we can always consider them to be sparse
			// in addition, a short chunk will never match r.zeroChunk which would prevent sparseness for short files
			file.sparse = r.sparse
		}

		if err != nil {
			// repository index is messed up, can't do anything
			return err
		}
		if largeFile {
			file.blobs = packsMap
		}
	}

	wg, ctx := errgroup.WithContext(ctx)
	downloadCh := make(chan *packInfo)

	worker := func() error {
		for pack := range downloadCh {
			if err := r.downloadPack(ctx, pack); err != nil {
				return err
			}
		}
		return nil
	}
	for i := 0; i < r.workerCount; i++ {
		wg.Go(worker)
	}

	// the main restore loop
	wg.Go(func() error {
		for _, id := range packOrder {
			pack := packs[id]
			// allow garbage collection of packInfo
			delete(packs, id)
			select {
			case <-ctx.Done():
				return ctx.Err()
			case downloadCh <- pack:
				debug.Log("Scheduled download pack %s", pack.id.Str())
			}
		}
		close(downloadCh)
		return nil
	})

	return wg.Wait()
}

type blobToFileOffsetsMapping map[restic.ID]struct {
	files map[*fileInfo][]int64 // file -> offsets (plural!) of the blob in the file
	blob  restic.Blob
}

func (r *fileRestorer) downloadPack(ctx context.Context, pack *packInfo) error {
	// calculate blob->[]files->[]offsets mappings
	blobs := make(blobToFileOffsetsMapping)
	for file := range pack.files {
		addBlob := func(blob restic.Blob, fileOffset int64) {
			blobInfo, ok := blobs[blob.ID]
			if !ok {
				blobInfo.files = make(map[*fileInfo][]int64)
				blobInfo.blob = blob
				blobs[blob.ID] = blobInfo
			}
			blobInfo.files[file] = append(blobInfo.files[file], fileOffset)
		}
		if fileBlobs, ok := file.blobs.(restic.IDs); ok {
			fileOffset := int64(0)
			err := r.forEachBlob(fileBlobs, func(packID restic.ID, blob restic.Blob) {
				if packID.Equal(pack.id) {
					addBlob(blob, fileOffset)
				}
				fileOffset += int64(blob.DataLength())
			})
			if err != nil {
				// restoreFiles should have caught this error before
				panic(err)
			}
		} else if packsMap, ok := file.blobs.(map[restic.ID][]fileBlobInfo); ok {
			for _, blob := range packsMap[pack.id] {
				idxPacks := r.idx(restic.DataBlob, blob.id)
				for _, idxPack := range idxPacks {
					if idxPack.PackID.Equal(pack.id) {
						addBlob(idxPack.Blob, blob.offset)
						break
					}
				}
			}
		}
	}

	// track already processed blobs for precise error reporting
	processedBlobs := restic.NewBlobSet()
	for _, entry := range blobs {
		occurrences := 0
		for _, offsets := range entry.files {
			occurrences += len(offsets)
		}
		// With a maximum blob size of 8MB, the normal blob streaming has to write
		// at most 800MB for a single blob. This should be short enough to avoid
		// network connection timeouts. Based on a quick test, a limit of 100 only
		// selects a very small number of blobs (the number of references per blob
		// - aka. `count` - seem to follow a expontential distribution)
		if occurrences > 100 {
			// process frequently referenced blobs first as these can take a long time to write
			// which can cause backend connections to time out
			delete(blobs, entry.blob.ID)
			partialBlobs := blobToFileOffsetsMapping{entry.blob.ID: entry}
			err := r.downloadBlobs(ctx, pack.id, partialBlobs, processedBlobs)
			if err := r.reportError(blobs, processedBlobs, err); err != nil {
				return err
			}
		}
	}

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

	err := r.downloadBlobs(ctx, pack.id, blobs, processedBlobs)
	return r.reportError(blobs, processedBlobs, err)
}

func (r *fileRestorer) sanitizeError(file *fileInfo, err error) error {
	if err != nil {
		err = r.Error(file.location, err)
	}
	return err
}

func (r *fileRestorer) reportError(blobs blobToFileOffsetsMapping, processedBlobs restic.BlobSet, err error) error {
	if err == nil {
		return nil
	}

	// only report error for not yet processed blobs
	affectedFiles := make(map[*fileInfo]struct{})
	for _, entry := range blobs {
		if processedBlobs.Has(entry.blob.BlobHandle) {
			continue
		}
		for file := range entry.files {
			affectedFiles[file] = struct{}{}
		}
	}

	for file := range affectedFiles {
		if errFile := r.sanitizeError(file, err); errFile != nil {
			return errFile
		}
	}
	return nil
}

func (r *fileRestorer) downloadBlobs(ctx context.Context, packID restic.ID,
	blobs blobToFileOffsetsMapping, processedBlobs restic.BlobSet) error {

	blobList := make([]restic.Blob, 0, len(blobs))
	for _, entry := range blobs {
		blobList = append(blobList, entry.blob)
	}
	return r.blobsLoader(ctx, packID, blobList,
		func(h restic.BlobHandle, blobData []byte, err error) error {
			processedBlobs.Insert(h)
			blob := blobs[h.ID]
			if err != nil {
				for file := range blob.files {
					if errFile := r.sanitizeError(file, err); errFile != nil {
						return errFile
					}
				}
				return nil
			}
			for file, offsets := range blob.files {
				for _, offset := range offsets {
					writeToFile := func() error {
						// this looks overly complicated and needs explanation
						// two competing requirements:
						// - must create the file once and only once
						// - should allow concurrent writes to the file
						// so write the first blob while holding file lock
						// write other blobs after releasing the lock
						createSize := int64(-1)
						file.lock.Lock()
						if file.inProgress {
							file.lock.Unlock()
						} else {
							defer file.lock.Unlock()
							file.inProgress = true
							createSize = file.size
						}
						writeErr := r.filesWriter.writeToFile(r.targetPath(file.location), blobData, offset, createSize, file.sparse)

						if r.progress != nil {
							r.progress.AddProgress(file.location, uint64(len(blobData)), uint64(file.size))
						}

						return writeErr
					}
					err := r.sanitizeError(file, writeToFile())
					if err != nil {
						return err
					}
				}
			}
			return nil
		})
}