From 13b339cf750351316d436d0f7918620177156772 Mon Sep 17 00:00:00 2001
From: Matt Joiner <anacrolix@gmail.com>
Date: Wed, 28 Feb 2024 00:15:40 +1100
Subject: [PATCH] Fix metainfo.Piece.Length for v2 torrents

---
 metainfo/fileinfo.go | 12 +-----------
 metainfo/info.go     |  2 +-
 metainfo/piece.go    | 27 +++++++++++++++++++++++++--
 3 files changed, 27 insertions(+), 14 deletions(-)

diff --git a/metainfo/fileinfo.go b/metainfo/fileinfo.go
index bf472156..82e1c94f 100644
--- a/metainfo/fileinfo.go
+++ b/metainfo/fileinfo.go
@@ -28,17 +28,7 @@ func (fi *FileInfo) DisplayPath(info *Info) string {
 	}
 }
 
-func (me FileInfo) Offset(info *Info) (ret int64) {
-	for _, fi := range info.UpvertedFiles() {
-		if me.DisplayPath(info) == fi.DisplayPath(info) {
-			return
-		}
-		ret += fi.Length
-	}
-	panic("not found")
-}
-
-func (fi FileInfo) BestPath() []string {
+func (fi *FileInfo) BestPath() []string {
 	if len(fi.PathUtf8) != 0 {
 		return fi.PathUtf8
 	}
diff --git a/metainfo/info.go b/metainfo/info.go
index 2798fd59..d1b54d44 100644
--- a/metainfo/info.go
+++ b/metainfo/info.go
@@ -172,7 +172,7 @@ func (info *Info) UpvertedFiles() (files []FileInfo) {
 }
 
 func (info *Info) Piece(index int) Piece {
-	return Piece{info, pieceIndex(index)}
+	return Piece{info, index}
 }
 
 func (info *Info) BestName() string {
diff --git a/metainfo/piece.go b/metainfo/piece.go
index d8895384..c7377f5d 100644
--- a/metainfo/piece.go
+++ b/metainfo/piece.go
@@ -8,7 +8,30 @@ type Piece struct {
 type pieceIndex = int
 
 func (p Piece) Length() int64 {
-	if int(p.i) == p.Info.NumPieces()-1 {
+	if p.Info.HasV2() {
+		var offset int64
+		pieceLength := p.Info.PieceLength
+		lastFileEnd := int64(0)
+		done := false
+		p.Info.FileTree.UpvertedFiles(nil, func(fi FileInfo) {
+			if done {
+				return
+			}
+			fileStartPiece := int(offset / pieceLength)
+			if fileStartPiece > p.i {
+				done = true
+				return
+			}
+			lastFileEnd = offset + fi.Length
+			offset = (lastFileEnd + pieceLength - 1) / pieceLength * pieceLength
+		})
+		ret := min(lastFileEnd-int64(p.i)*pieceLength, pieceLength)
+		if ret <= 0 {
+			panic(ret)
+		}
+		return ret
+	}
+	if p.i == p.Info.NumPieces()-1 {
 		return p.Info.TotalLength() - int64(p.i)*p.Info.PieceLength
 	}
 	return p.Info.PieceLength
@@ -23,6 +46,6 @@ func (p Piece) Hash() (ret Hash) {
 	return
 }
 
-func (p Piece) Index() pieceIndex {
+func (p Piece) Index() int {
 	return p.i
 }
-- 
2.44.0