don't download pieces we already have

src/xd/lib/bittorrent/bitfield.go

@@ -167,13 +167,10 @@ func (bf *Bitfield) Completed() bool {
 type rareSet map[uint32]uint32
 
 // FindRarest finds the set bit we have that is rarest in others
-func (bf *Bitfield) FindRarest(others []*Bitfield, excluding []uint32) (idx uint32) {
+func (bf *Bitfield) FindRarest(others []*Bitfield, exclude func(uint32) bool) (idx uint32) {
 	bits := make(rareSet)
 	i := bf.Length
-	m := make(map[uint32]bool)
-	for _, k := range excluding {
-		m[k] = true
-	}
+
 	for i > 0 {
 		i--
 		bits[i] = 0
@@ -186,8 +183,7 @@ func (bf *Bitfield) FindRarest(others []*Bitfield, excluding []uint32) (idx uint
 
 	min := uint32(len(others) + 1)
 	for index, count := range bits {
-		_, exclude := m[index]
-		if exclude {
+		if exclude(index) {
 			continue
 		}
 		if count < min && bf.Has(index) {

src/xd/lib/bittorrent/swarm/torrent.go

@@ -151,7 +151,15 @@ func (t *Torrent) getRarestPiece(remote *bittorrent.Bitfield, exclude []uint32)
 			swarm = append(swarm, c.bf)
 		}
 	})
-	idx = remote.FindRarest(swarm, exclude)
+	m := make(map[uint32]bool)
+	for idx := range exclude {
+		m[exclude[idx]] = true
+	}
+	bt := t.st.Bitfield()
+	idx = remote.FindRarest(swarm, func(idx uint32) bool {
+		_, has := m[idx]
+		return bt.Has(idx) || has
+	})
 	return
 }