Fix bytesLeft for files (#348)

* Refactor (*File).bytesLeft

* Always use the same piece size

file.go

@@ -6,7 +6,6 @@ import (
 	"github.com/anacrolix/missinggo/bitmap"
 
 	"github.com/anacrolix/torrent/metainfo"
-	pp "github.com/anacrolix/torrent/peer_protocol"
 )
 
 // Provides access to regions of torrent data that correspond to its files.
@@ -56,30 +55,24 @@ func (f *File) bytesCompleted() int64 {
 }
 
 func (f *File) bytesLeft() (left int64) {
+	pieceSize := int64(f.t.usualPieceSize())
 	firstPieceIndex := f.firstPieceIndex()
-	endPieceIndex := f.endPieceIndex()
-	bitmap.Flip(f.t.completedPieces, firstPieceIndex, endPieceIndex+1).IterTyped(func(piece int) bool {
-		p := &f.t.pieces[piece]
-		left += int64(p.length() - p.numDirtyBytes())
-		return true
-	})
-	startPiece := f.t.piece(firstPieceIndex)
-	endChunk := int(f.offset%f.t.info.PieceLength) * int(startPiece.numChunks()) / int(startPiece.length())
-	bitmap.Flip(startPiece.dirtyChunks, 0, endChunk).IterTyped(func(chunk int) bool {
-		left -= int64(startPiece.chunkSize())
-		return true
-	})
-	endPiece := f.t.piece(endPieceIndex)
-	startChunk := int((f.offset+f.length)%f.t.info.PieceLength) * int(endPiece.numChunks()) / int(endPiece.length())
-	lastChunkIndex := int(endPiece.lastChunkIndex())
-	bitmap.Flip(endPiece.dirtyChunks, startChunk, int(endPiece.numChunks())).IterTyped(func(chunk int) bool {
-		if chunk == lastChunkIndex {
-			left -= int64(endPiece.chunkIndexSpec(pp.Integer(chunk)).Length)
-		} else {
-			left -= int64(endPiece.chunkSize())
+	endPieceIndex := f.endPieceIndex() - 1
+	bitmap.Flip(f.t.completedPieces, firstPieceIndex+1, endPieceIndex).IterTyped(func(piece int) bool {
+		if piece >= endPieceIndex {
+			return false
+		}
+		if piece > firstPieceIndex {
+			left += pieceSize
 		}
 		return true
 	})
+	if !f.t.pieceComplete(firstPieceIndex) {
+		left += pieceSize - (f.offset % pieceSize)
+	}
+	if !f.t.pieceComplete(endPieceIndex) {
+		left += (f.offset + f.length) % pieceSize
+	}
 	return
 }