file.go      | 29 ++++++++++++++++-------------
 file_test.go | 52 +++++++++++++++++++++++++++++++++++++++++++++++++---

diff --git a/file.go b/file.go
index 1c54f20c610d621301c9a826643a455d64cf7a85..f02424000971f46f65583864c7fd1809aac8b841 100644
--- a/file.go
+++ b/file.go
@@ -62,21 +62,24 @@ 	fileTorrentOffset int64,
 	fileLength int64,
 	torrentCompletedPieces bitmap.Bitmap,
 ) (left int64) {
-	fileEndPieceIndex--
-	bitmap.Flip(torrentCompletedPieces, fileFirstPieceIndex+1, fileEndPieceIndex).IterTyped(func(piece int) bool {
-		if piece >= fileEndPieceIndex {
-			return false
+	numPiecesSpanned := fileEndPieceIndex - fileFirstPieceIndex
+	switch numPiecesSpanned {
+	case 0:
+	case 1:
+		if !torrentCompletedPieces.Get(fileFirstPieceIndex) {
+			left += fileLength
+		}
+	default:
+		if !torrentCompletedPieces.Get(fileFirstPieceIndex) {
+			left += torrentUsualPieceSize - (fileTorrentOffset % torrentUsualPieceSize)
 		}
-		if piece > fileFirstPieceIndex {
-			left += torrentUsualPieceSize
+		if !torrentCompletedPieces.Get(fileEndPieceIndex - 1) {
+			left += fileTorrentOffset + fileLength - int64(fileEndPieceIndex-1)*torrentUsualPieceSize
 		}
-		return true
-	})
-	if !torrentCompletedPieces.Get(fileFirstPieceIndex) {
-		left += torrentUsualPieceSize - (fileTorrentOffset % torrentUsualPieceSize)
-	}
-	if !torrentCompletedPieces.Get(fileEndPieceIndex) {
-		left += (fileTorrentOffset + fileLength) % torrentUsualPieceSize
+		completedMiddlePieces := torrentCompletedPieces.Copy()
+		completedMiddlePieces.RemoveRange(0, fileFirstPieceIndex+1)
+		completedMiddlePieces.RemoveRange(fileEndPieceIndex-1, bitmap.ToEnd)
+		left += int64(numPiecesSpanned-2-completedMiddlePieces.Len()) * torrentUsualPieceSize
 	}
 	return
 }
diff --git a/file_test.go b/file_test.go
index b411126808ba922e8f48a4dcfa65cc59727e613a..0443cf3bd5cc63a9e9766f45a9d4a489f709c43f 100644
--- a/file_test.go
+++ b/file_test.go
@@ -40,19 +40,31 @@ 	})
 }
 
 func TestFileBytesLeft(t *testing.T) {
+
+	testFileBytesLeft{
+		usualPieceSize:  3,
+		firstPieceIndex: 1,
+		endPieceIndex:   1,
+		fileOffset:      1,
+		fileLength:      0,
+		expected:        0,
+		name:            "ZeroLengthFile",
+	}.Run(t)
+
 	testFileBytesLeft{
 		usualPieceSize:  2,
 		firstPieceIndex: 1,
-		endPieceIndex:   1,
+		endPieceIndex:   2,
 		fileOffset:      1,
 		fileLength:      1,
 		expected:        1,
+		name:            "EndOfSecondPiece",
 	}.Run(t)
 
 	testFileBytesLeft{
 		usualPieceSize:  3,
 		firstPieceIndex: 0,
-		endPieceIndex:   0,
+		endPieceIndex:   1,
 		fileOffset:      1,
 		fileLength:      1,
 		expected:        1,
@@ -62,10 +74,44 @@
 	testFileBytesLeft{
 		usualPieceSize:  3,
 		firstPieceIndex: 0,
-		endPieceIndex:   0,
+		endPieceIndex:   1,
 		fileOffset:      1,
 		fileLength:      1,
 		expected:        1,
 		name:            "LandLocked",
+	}.Run(t)
+
+	testFileBytesLeft{
+		usualPieceSize:  3,
+		firstPieceIndex: 1,
+		endPieceIndex:   3,
+		fileOffset:      4,
+		fileLength:      4,
+		expected:        4,
+		name:            "TwoPieces",
+	}.Run(t)
+
+	testFileBytesLeft{
+		usualPieceSize:  3,
+		firstPieceIndex: 1,
+		endPieceIndex:   4,
+		fileOffset:      5,
+		fileLength:      7,
+		expected:        7,
+		name:            "ThreePieces",
+	}.Run(t)
+
+	testFileBytesLeft{
+		usualPieceSize:  3,
+		firstPieceIndex: 1,
+		endPieceIndex:   4,
+		fileOffset:      5,
+		fileLength:      7,
+		expected:        0,
+		completedPieces: func() (ret bitmap.Bitmap) {
+			ret.AddRange(0, 5)
+			return
+		}(),
+		name: "ThreePiecesCompletedAll",
 	}.Run(t)
 }