storage/boltPieceCompletion.go  |  2 +-
 storage/boltpc_test.go          |  2 +-
 storage/completion.go           |  8 ++++----
 storage/completion_piece_map.go |  6 +++++-
 storage/file.go                 | 16 ++++++++++++----
 storage/mmap.go                 | 12 ++++++++----

diff --git a/storage/boltPieceCompletion.go b/storage/boltPieceCompletion.go
index 1b5e18a1955e3af33bfb2ad49908f527578e5967..510e283459063e6914b310d18a7d7b882cf91630 100644
--- a/storage/boltPieceCompletion.go
+++ b/storage/boltPieceCompletion.go
@@ -18,7 +18,7 @@ type boltPieceCompletion struct {
 	db *bolt.DB
 }
 
-func newBoltPieceCompletion(dir string) (ret *boltPieceCompletion, err error) {
+func NewBoltPieceCompletion(dir string) (ret PieceCompletion, err error) {
 	p := filepath.Join(dir, ".torrent.bolt.db")
 	db, err := bolt.Open(p, 0660, &bolt.Options{
 		Timeout: time.Second,
diff --git a/storage/boltpc_test.go b/storage/boltpc_test.go
index e1be2579bdf95b469f92499604d226e6268e1202..3f5572e711ae97316fe5c5c4a230679975a5b270 100644
--- a/storage/boltpc_test.go
+++ b/storage/boltpc_test.go
@@ -16,7 +16,7 @@ 	td, err := ioutil.TempDir("", "")
 	require.NoError(t, err)
 	defer os.RemoveAll(td)
 
-	pc, err := newBoltPieceCompletion(td)
+	pc, err := NewBoltPieceCompletion(td)
 	require.NoError(t, err)
 	defer pc.Close()
 
diff --git a/storage/completion.go b/storage/completion.go
index 7804351036a238c25b9a3b0ddf06ae312d492dcc..c464a0a7b5e06dfad2aaa1a0c852012a251bf67e 100644
--- a/storage/completion.go
+++ b/storage/completion.go
@@ -7,17 +7,17 @@ 	"github.com/anacrolix/torrent/metainfo"
 )
 
 // Implementations track the completion of pieces. It must be concurrent-safe.
-type pieceCompletion interface {
+type PieceCompletion interface {
 	Get(metainfo.PieceKey) (bool, error)
 	Set(metainfo.PieceKey, bool) error
 	Close() error
 }
 
-func pieceCompletionForDir(dir string) (ret pieceCompletion) {
-	ret, err := newBoltPieceCompletion(dir)
+func pieceCompletionForDir(dir string) (ret PieceCompletion) {
+	ret, err := NewBoltPieceCompletion(dir)
 	if err != nil {
 		log.Printf("couldn't open piece completion db in %q: %s", dir, err)
-		ret = new(mapPieceCompletion)
+		ret = NewMapPieceCompletion()
 	}
 	return
 }
diff --git a/storage/completion_piece_map.go b/storage/completion_piece_map.go
index f4b4e28582d0744b7e46e2c913436c1d323b8187..ebd98259edebbaeb5a7472758b63059a759027ba 100644
--- a/storage/completion_piece_map.go
+++ b/storage/completion_piece_map.go
@@ -11,7 +11,11 @@ 	mu sync.Mutex
 	m  map[metainfo.PieceKey]struct{}
 }
 
-func (mapPieceCompletion) Close() error { return nil }
+func NewMapPieceCompletion() PieceCompletion {
+	return &mapPieceCompletion{m: make(map[metainfo.PieceKey]struct{})}
+}
+
+func (*mapPieceCompletion) Close() error { return nil }
 
 func (me *mapPieceCompletion) Get(pk metainfo.PieceKey) (bool, error) {
 	me.mu.Lock()
diff --git a/storage/file.go b/storage/file.go
index b0a9ca1034ec933973cd989fe9b72f6f49e925b0..f4ab0913c4aa6d7b17130d822c138e3253e7991c 100644
--- a/storage/file.go
+++ b/storage/file.go
@@ -15,7 +15,7 @@ // torrent.
 type fileClientImpl struct {
 	baseDir   string
 	pathMaker func(baseDir string, info *metainfo.Info, infoHash metainfo.Hash) string
-	pc        pieceCompletion
+	pc        PieceCompletion
 }
 
 // The Default path maker just returns the current path
@@ -29,7 +29,11 @@ }
 
 // All Torrent data stored in this baseDir
 func NewFile(baseDir string) ClientImpl {
-	return NewFileWithCustomPathMaker(baseDir, nil)
+	return NewFileWithCompletion(baseDir, pieceCompletionForDir(baseDir))
+}
+
+func NewFileWithCompletion(baseDir string, completion PieceCompletion) ClientImpl {
+	return newFileWithCustomPathMakerAndCompletion(baseDir, nil, completion)
 }
 
 // All Torrent data stored in subdirectorys by infohash
@@ -39,13 +43,17 @@ }
 
 // Allows passing a function to determine the path for storing torrent data
 func NewFileWithCustomPathMaker(baseDir string, pathMaker func(baseDir string, info *metainfo.Info, infoHash metainfo.Hash) string) ClientImpl {
+	return newFileWithCustomPathMakerAndCompletion(baseDir, pathMaker, pieceCompletionForDir(baseDir))
+}
+
+func newFileWithCustomPathMakerAndCompletion(baseDir string, pathMaker func(baseDir string, info *metainfo.Info, infoHash metainfo.Hash) string, completion PieceCompletion) ClientImpl {
 	if pathMaker == nil {
 		pathMaker = defaultPathMaker
 	}
 	return &fileClientImpl{
 		baseDir:   baseDir,
 		pathMaker: pathMaker,
-		pc:        pieceCompletionForDir(baseDir),
+		pc:        completion,
 	}
 }
 
@@ -72,7 +80,7 @@ type fileTorrentImpl struct {
 	dir        string
 	info       *metainfo.Info
 	infoHash   metainfo.Hash
-	completion pieceCompletion
+	completion PieceCompletion
 }
 
 func (fts *fileTorrentImpl) Piece(p metainfo.Piece) PieceImpl {
diff --git a/storage/mmap.go b/storage/mmap.go
index 8cc5a7d573341d386ae4f07344c63b66dac7810d..bbc9ed03f3938d424ae134bc1be6972dc9ec66c1 100644
--- a/storage/mmap.go
+++ b/storage/mmap.go
@@ -15,13 +15,17 @@ )
 
 type mmapStorage struct {
 	baseDir string
-	pc      pieceCompletion
+	pc      PieceCompletion
 }
 
 func NewMMap(baseDir string) ClientImpl {
+	return NewMMapWithCompletion(baseDir, pieceCompletionForDir(baseDir))
+}
+
+func NewMMapWithCompletion(baseDir string, completion PieceCompletion) ClientImpl {
 	return &mmapStorage{
 		baseDir: baseDir,
-		pc:      pieceCompletionForDir(baseDir),
+		pc:      completion,
 	}
 }
 
@@ -40,7 +44,7 @@ }
 
 type mmapTorrentStorage struct {
 	span mmap_span.MMapSpan
-	pc   pieceCompletion
+	pc   PieceCompletion
 }
 
 func (ts *mmapTorrentStorage) Piece(p metainfo.Piece) PieceImpl {
@@ -58,7 +62,7 @@ 	return ts.span.Close()
 }
 
 type mmapStoragePiece struct {
-	pc pieceCompletion
+	pc PieceCompletion
 	p  metainfo.Piece
 	ih metainfo.Hash
 	io.ReaderAt