When Reader position changes only update piece priorities that may be affected

Seems to significantly reduce CPU load reading from torrents with huge piece counts.

reader.go

@@ -202,7 +202,7 @@ func (r *Reader) readOnceAt(b []byte, pos int64, ctxErr *error) (n int, err erro
 		log.Printf("error reading torrent %q piece %d offset %d, %d bytes: %s", r.t, pi, po, len(b1), err)
 		r.t.cl.mu.Lock()
 		r.t.updateAllPieceCompletions()
-		r.t.updatePiecePriorities()
+		r.t.updateAllPiecePriorities()
 		r.t.cl.mu.Unlock()
 	}
 }
@@ -214,12 +214,13 @@ func (r *Reader) Close() error {
 }
 
 func (r *Reader) posChanged() {
-	p := r.piecesUncached()
+	to := r.piecesUncached()
-	if p == r.pieces {
+	from := r.pieces
+	if to == from {
 		return
 	}
-	r.pieces = p
+	r.pieces = to
-	r.t.readersChanged()
+	r.t.readerPosChanged(from, to)
 }
 
 func (r *Reader) Seek(off int64, whence int) (ret int64, err error) {

t.go

@@ -132,7 +132,7 @@ func (t *Torrent) addReader(r *Reader) {
 		t.readers = make(map[*Reader]struct{})
 	}
 	t.readers[r] = struct{}{}
-	t.readersChanged()
+	r.posChanged()
 }
 
 func (t *Torrent) deleteReader(r *Reader) {

torrent.go

@@ -835,8 +835,36 @@ func (t *Torrent) forUrgentPieces(f func(piece int) (again bool)) (all bool) {
 }
 
 func (t *Torrent) readersChanged() {
+	t.updateReaderPieces()
+	t.updateAllPiecePriorities()
+}
+
+func (t *Torrent) updateReaderPieces() {
 	t.readerNowPieces, t.readerReadaheadPieces = t.readerPiecePriorities()
-	t.updatePiecePriorities()
+}
+
+func (t *Torrent) readerPosChanged(from, to pieceRange) {
+	if from == to {
+		return
+	}
+	t.updateReaderPieces()
+	// Order the ranges, high and low.
+	l, h := from, to
+	if l.begin > h.begin {
+		l, h = h, l
+	}
+	if l.end < h.begin {
+		// Two distinct ranges.
+		t.updatePiecePriorities(l.begin, l.end)
+		t.updatePiecePriorities(h.begin, h.end)
+	} else {
+		// Ranges overlap.
+		end := l.end
+		if h.end > end {
+			end = h.end
+		}
+		t.updatePiecePriorities(l.begin, end)
+	}
 }
 
 func (t *Torrent) maybeNewConns() {
@@ -863,10 +891,14 @@ func (t *Torrent) updatePiecePriority(piece int) {
 	t.piecePriorityChanged(piece)
 }
 
+func (t *Torrent) updateAllPiecePriorities() {
+	t.updatePiecePriorities(0, len(t.pieces))
+}
+
 // Update all piece priorities in one hit. This function should have the same
 // output as updatePiecePriority, but across all pieces.
-func (t *Torrent) updatePiecePriorities() {
+func (t *Torrent) updatePiecePriorities(begin, end int) {
-	for i := range t.pieces {
+	for i := begin; i < end; i++ {
 		t.updatePiecePriority(i)
 	}
 }

torrent_test.go

@@ -92,7 +92,7 @@ func BenchmarkUpdatePiecePriorities(b *testing.B) {
 		t.completedPieces.Set(i, true)
 	}
 	for range iter.N(b.N) {
-		t.updatePiecePriorities()
+		t.updateAllPiecePriorities()
 	}
 }