From 0ecfeec242bf9ebabe4ae30a982187757e966b49 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Wed, 20 Mar 2024 14:02:41 +1100
Subject: [PATCH] Include piece layers in dynamically generated metainfo

---
 metainfo/metainfo.go |  2 +-
 torrent.go           | 34 +++++++++++++++++++++++++++++++++-
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/metainfo/metainfo.go b/metainfo/metainfo.go
index b20a9efd..6a5ea1cd 100644
--- a/metainfo/metainfo.go
+++ b/metainfo/metainfo.go
@@ -24,7 +24,7 @@ type MetaInfo struct {
 	CreatedBy    string  `bencode:"created by,omitempty"`
 	Encoding     string  `bencode:"encoding,omitempty"`
 	UrlList      UrlList `bencode:"url-list,omitempty"` // BEP 19 WebSeeds
-	// BEP 52 (BitTorrent v2): Keys are file merkle roots (pieces root?), and the values are the
+	// BEP 52 (BitTorrent v2): Keys are file merkle roots ("pieces root"s), and the values are the
 	// concatenated hashes of the merkle tree layer that corresponds to the piece length.
 	PieceLayers map[string]string `bencode:"piece layers,omitempty"`
 }
diff --git a/torrent.go b/torrent.go
index e2375a44..b952e3ce 100644
--- a/torrent.go
+++ b/torrent.go
@@ -922,7 +922,7 @@ func (t *Torrent) newMetaInfo() metainfo.MetaInfo {
 	return metainfo.MetaInfo{
 		CreationDate: time.Now().Unix(),
 		Comment:      "dynamic metainfo from client",
-		CreatedBy:    "go.torrent",
+		CreatedBy:    "https://github.com/anacrolix/torrent",
 		AnnounceList: t.metainfo.UpvertedAnnounceList().Clone(),
 		InfoBytes: func() []byte {
 			if t.haveInfo() {
@@ -938,6 +938,7 @@ func (t *Torrent) newMetaInfo() metainfo.MetaInfo {
 			}
 			return ret
 		}(),
+		PieceLayers: t.pieceLayers(),
 	}
 }
 
@@ -3169,3 +3170,34 @@ func (t *Torrent) getFileByPiecesRoot(hash [32]byte) *File {
 	}
 	return nil
 }
+
+func (t *Torrent) pieceLayers() (pieceLayers map[string]string) {
+	if t.files == nil {
+		return
+	}
+	files := *t.files
+	g.MakeMapWithCap(&pieceLayers, len(files))
+file:
+	for _, f := range files {
+		if !f.piecesRoot.Ok {
+			continue
+		}
+		key := f.piecesRoot.Value
+		var value strings.Builder
+		for i := f.BeginPieceIndex(); i < f.EndPieceIndex(); i++ {
+			hashOpt := t.piece(i).hashV2
+			if !hashOpt.Ok {
+				// All hashes must be present. This implementation should handle missing files, so move on to the next file.
+				continue file
+			}
+			value.Write(hashOpt.Value[:])
+		}
+		if value.Len() == 0 {
+			// Non-empty files are not recorded in piece layers.
+			continue
+		}
+		// If multiple files have the same root that shouldn't matter.
+		pieceLayers[string(key[:])] = value.String()
+	}
+	return
+}
-- 
2.44.0