From e4dec03a329e3ef2fde131d421aae376cada776d Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Wed, 4 Mar 2015 13:07:11 +1100
Subject: [PATCH] Change the way readahead pieces are calculated

---
 client.go      |  6 +++++-
 client_test.go | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/client.go b/client.go
index 78269d61..9a427faf 100644
--- a/client.go
+++ b/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
diff --git a/client_test.go b/client_test.go
index 5dc8a4d3..48eddfec 100644
--- a/client_test.go
+++ b/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_)
+		}
+	}
+}