Avoid frequent allocation of piece request order iterator

internal/pieceordering/pieceordering.go

@@ -89,7 +89,7 @@ func (me *Instance) DeletePiece(piece int) {
 }
 
 // Returns the piece with the lowest key.
-func (me Instance) First() Element {
+func (me *Instance) First() Element {
 	i := me.sl.SeekToFirst()
 	if i == nil {
 		return nil
@@ -97,6 +97,10 @@ func (me Instance) First() Element {
 	return &element{i, i.Value().([]int)}
 }
 
+func (me *Instance) Empty() bool {
+	return me.sl.Len() == 0
+}
+
 type Element interface {
 	Piece() int
 	Next() Element
@@ -120,6 +124,6 @@ func (e *element) Next() Element {
 	return e
 }
 
-func (e element) Piece() int {
+func (e *element) Piece() int {
 	return e.sl[0]
 }

internal/pieceordering/pieceordering_test.go

@@ -3,6 +3,9 @@ package pieceordering
 import (
 	"sort"
 	"testing"
+
+	"github.com/bradfitz/iter"
+	"github.com/stretchr/testify/assert"
 )
 
 func instanceSlice(i *Instance) (sl []int) {
@@ -26,7 +29,7 @@ func sameContents(a, b []int) bool {
 	return true
 }
 
-func checkOrder(t *testing.T, i *Instance, ppp ...[]int) {
+func checkOrder(t testing.TB, i *Instance, ppp ...[]int) {
 	fatal := func() {
 		t.Fatalf("have %v, expected %v", instanceSlice(i), ppp)
 	}
@@ -46,9 +49,11 @@ func checkOrder(t *testing.T, i *Instance, ppp ...[]int) {
 	}
 }
 
-func TestPieceOrdering(t *testing.T) {
+func testPieceOrdering(t testing.TB) {
 	i := New()
+	assert.True(t, i.Empty())
 	i.SetPiece(0, 1)
+	assert.False(t, i.Empty())
 	i.SetPiece(1, 0)
 	checkOrder(t, i, []int{1, 0})
 	i.SetPiece(1, 2)
@@ -59,8 +64,10 @@ func TestPieceOrdering(t *testing.T) {
 	i.DeletePiece(1)
 	checkOrder(t, i, []int{0})
 	i.DeletePiece(0)
+	assert.True(t, i.Empty())
 	checkOrder(t, i, nil)
 	i.SetPiece(2, 1)
+	assert.False(t, i.Empty())
 	i.SetPiece(1, 1)
 	i.SetPiece(3, 1)
 	checkOrder(t, i, []int{3, 1, 2})
@@ -70,9 +77,33 @@ func TestPieceOrdering(t *testing.T) {
 	i.DeletePiece(2)
 	i.DeletePiece(3)
 	i.DeletePiece(1)
+	assert.True(t, i.Empty())
 	checkOrder(t, i, nil)
+	// Deleting pieces that aren't present.
 	i.DeletePiece(2)
 	i.DeletePiece(3)
 	i.DeletePiece(1)
+	assert.True(t, i.Empty())
 	checkOrder(t, i, nil)
 }
+
+func TestPieceOrdering(t *testing.T) {
+	testPieceOrdering(t)
+}
+
+func BenchmarkPieceOrdering(b *testing.B) {
+	for range iter.N(b.N) {
+		testPieceOrdering(b)
+	}
+}
+
+func BenchmarkIteration(b *testing.B) {
+	for range iter.N(b.N) {
+		i := New()
+		for p := range iter.N(500) {
+			i.SetPiece(p, p)
+		}
+		for e := i.First(); e != nil; e = e.Next() {
+		}
+	}
+}

torrent.go

@@ -738,7 +738,7 @@ func (t *torrent) wantPiece(index int) bool {
 }
 
 func (t *torrent) connHasWantedPieces(c *connection) bool {
-	return c.pieceRequestOrder != nil && c.pieceRequestOrder.First() != nil
+	return c.pieceRequestOrder != nil && !c.pieceRequestOrder.Empty()
 }
 
 func (t *torrent) extentPieces(off, _len int64) (pieces []int) {