metainfo/info.go |  3 ---
 piece.go         | 10 +++++-----
 torrent.go       | 23 +++++++++++------------

diff --git a/metainfo/info.go b/metainfo/info.go
index 7fbc88e9546bfa79a2918dd6a76c683406139006..a7e23a7f07843360a2f92eff81bd278d86305ca4 100644
--- a/metainfo/info.go
+++ b/metainfo/info.go
@@ -126,9 +126,6 @@ 	return
 }
 
 func (info *Info) NumPieces() int {
-	if len(info.Pieces)%20 != 0 {
-		panic(len(info.Pieces))
-	}
 	return len(info.Pieces) / 20
 }
 
diff --git a/piece.go b/piece.go
index 7a0c2f1789f4b169c694724a9e981db478717b06..e09e00c8da5160dbcd14ffb92e53b7b88ce8c6fd 100644
--- a/piece.go
+++ b/piece.go
@@ -128,11 +128,11 @@ 	return chunkIndexSpec(chunk, p.length(), p.chunkSize())
 }
 
 func (p *Piece) numDirtyBytes() (ret pp.Integer) {
-	defer func() {
-		if ret > p.length() {
-			panic("too many dirty bytes")
-		}
-	}()
+	// defer func() {
+	// 	if ret > p.length() {
+	// 		panic("too many dirty bytes")
+	// 	}
+	// }()
 	numRegularDirtyChunks := p.numDirtyChunks()
 	if p.chunkIndexDirty(p.numChunks() - 1) {
 		numRegularDirtyChunks--
diff --git a/torrent.go b/torrent.go
index 84691af65ca0a8ee10699981269841566cc53427..3374778b2c12bedd0b362fdc691faf14a896a846 100644
--- a/torrent.go
+++ b/torrent.go
@@ -558,9 +558,11 @@ 	return t.bytesLeft()
 }
 
 func (t *Torrent) bytesLeft() (left int64) {
-	for i := 0; i < t.numPieces(); i++ {
-		left += int64(t.pieces[i].bytesLeft())
-	}
+	bitmap.Flip(t.completedPieces, 0, t.numPieces()).IterTyped(func(piece int) bool {
+		p := t.pieces[piece]
+		left += int64(p.length() - p.numDirtyBytes())
+		return true
+	})
 	return
 }
 
@@ -658,17 +660,14 @@ 	// Peer is known to support encryption.
 	SupportsEncryption bool
 }
 
-func (t *Torrent) pieceLength(piece int) (len_ pp.Integer) {
-	if piece < 0 || piece >= t.info.NumPieces() {
-		return
-	}
+func (t *Torrent) pieceLength(piece int) pp.Integer {
 	if piece == t.numPieces()-1 {
-		len_ = pp.Integer(t.length % t.info.PieceLength)
+		ret := pp.Integer(t.length % t.info.PieceLength)
+		if ret != 0 {
+			return ret
+		}
 	}
-	if len_ == 0 {
-		len_ = pp.Integer(t.info.PieceLength)
-	}
-	return
+	return pp.Integer(t.info.PieceLength)
 }
 
 func (t *Torrent) hashPiece(piece int) (ret metainfo.Hash) {