Change the way readahead pieces are calculated

client.go

@@ -305,6 +305,10 @@ func dataReadAt(d Data, b []byte, off int64) (n int, err error) {
 	panic(fmt.Sprintf("can't read from %T", d))
 }
 
+func readaheadPieces(readahead, pieceLength int64) int {
+	return int((readahead+pieceLength-1)/pieceLength - 1)
+}
+
 func (cl *Client) readRaisePiecePriorities(t *torrent, off int64) {
 	index := int(off / int64(t.usualPieceSize()))
 	cl.raisePiecePriority(t, index, piecePriorityNow)
@@ -313,7 +317,7 @@ func (cl *Client) readRaisePiecePriorities(t *torrent, off int64) {
 		return
 	}
 	cl.raisePiecePriority(t, index, piecePriorityNext)
-	for i := 0; i < t.numConnsUnchoked()/2; i++ {
+	for i := 0; i < readaheadPieces(5*1024*1024, t.Info.PieceLength); i++ {
 		index++
 		if index >= t.numPieces() {
 			break

client_test.go

@@ -275,3 +275,21 @@ func TestClientTransfer(t *testing.T) {
 		t.Fatal(":(")
 	}
 }
+
+func TestReadaheadPieces(t *testing.T) {
+	for _, case_ := range []struct {
+		readaheadBytes, pieceLength int64
+		readaheadPieces             int
+	}{
+		{5 * 1024 * 1024, 256 * 1024, 19},
+		{5 * 1024 * 1024, 5 * 1024 * 1024, 0},
+		{5*1024*1024 - 1, 5 * 1024 * 1024, 0},
+		{5 * 1024 * 1024, 5*1024*1024 - 1, 1},
+		{0, 5 * 1024 * 1024, -1},
+		{5 * 1024 * 1024, 1048576, 4},
+	} {
+		if readaheadPieces(case_.readaheadBytes, case_.pieceLength) != case_.readaheadPieces {
+			t.Fatalf("case failed: %s", case_)
+		}
+	}
+}