Precompute File.DisplayPath

This is an optimization for dir lookups in torrentfs.

file.go

@@ -1,8 +1,6 @@
 package torrent
 
 import (
-	"strings"
-
 	"github.com/anacrolix/missinggo/v2/bitmap"
 
 	"github.com/anacrolix/torrent/metainfo"
@@ -15,6 +13,7 @@ type File struct {
 	offset      int64
 	length      int64
 	fi          metainfo.FileInfo
+	displayPath string
 	prio        piecePriority
 }
 
@@ -89,13 +88,9 @@ func (f *File) bytesLeft() (left int64) {
 }
 
 // The relative file path for a multi-file torrent, and the torrent name for a
-// single-file torrent.
+// single-file torrent. Dir separators are '/'.
 func (f *File) DisplayPath() string {
-	fip := f.FileInfo().Path
-	if len(fip) == 0 {
-		return f.t.info.Name
-	}
-	return strings.Join(fip, "/")
+	return f.displayPath
 }
 
 // The download status of a piece that comprises part of a File.

t.go

@@ -203,12 +203,17 @@ func (t *Torrent) initFiles() {
 		} else {
 			path = fi.Path
 		}
+		dp := t.info.Name
+		if len(fi.Path) != 0 {
+			dp = strings.Join(fi.Path, "/")
+		}
 		*t.files = append(*t.files, &File{
 			t,
 			strings.Join(append([]string{t.info.Name}, path...), "/"),
 			offset,
 			fi.Length,
 			fi,
+			dp,
 			PiecePriorityNone,
 		})
 		offset += fi.Length