feat: bt fetcher reuse (#142)

internal/protocol/bt/file_misc.go

@@ -0,0 +1,36 @@
+// modify from github.com/anacrolix/torrent/storage/file_misc.go
+
+package bt
+
+import "github.com/anacrolix/torrent/metainfo"
+
+type requiredLength struct {
+	fileIndex int
+	length    int64
+}
+
+func extentCompleteRequiredLengths(info *metainfo.Info, off, n int64) (ret []requiredLength) {
+	if n == 0 {
+		return
+	}
+	for i, fi := range info.UpvertedFiles() {
+		if off >= fi.Length {
+			off -= fi.Length
+			continue
+		}
+		n1 := n
+		if off+n1 > fi.Length {
+			n1 = fi.Length - off
+		}
+		ret = append(ret, requiredLength{
+			fileIndex: i,
+			length:    off + n1,
+		})
+		n -= n1
+		if n == 0 {
+			return
+		}
+		off = 0
+	}
+	return
+}