diff --git a/cmd/torrent-verify/main.go b/cmd/torrent-verify/main.go
index ba64d801..4e5774d9 100644
--- a/cmd/torrent-verify/main.go
+++ b/cmd/torrent-verify/main.go
@@ -3,13 +3,13 @@ package main
 import (
 	"bytes"
 	"crypto/sha1"
-	"flag"
 	"fmt"
 	"io"
 	"log"
 	"os"
 	"path/filepath"
 
+	"github.com/anacrolix/tagflag"
 	"github.com/bradfitz/iter"
 	"github.com/edsrzf/mmap-go"
 
@@ -17,35 +17,56 @@ import (
 	"github.com/anacrolix/torrent/mmap_span"
 )
 
-var (
-	torrentPath = flag.String("torrent", "/path/to/the.torrent", "path of the torrent file")
-	dataPath    = flag.String("path", "/torrent/data", "path of the torrent data")
-)
-
-func fileToMmap(filename string, length int64) mmap.MMap {
-	osFile, err := os.Open(filename)
-	if os.IsNotExist(err) {
-		return nil
-	}
+func mmapFile(name string) (mm mmap.MMap, err error) {
+	f, err := os.Open(name)
 	if err != nil {
-		log.Fatal(err)
+		return
 	}
-	goMMap, err := mmap.MapRegion(osFile, int(length), mmap.RDONLY, mmap.COPY, 0)
+	defer f.Close()
+	fi, err := f.Stat()
 	if err != nil {
-		log.Fatal(err)
+		return
 	}
-	if int64(len(goMMap)) != length {
-		log.Printf("file mmap has wrong size: %#v", filename)
+	if fi.Size() == 0 {
+		return
 	}
-	osFile.Close()
+	return mmap.MapRegion(f, -1, mmap.RDONLY, mmap.COPY, 0)
+}
 
-	return goMMap
+func verifyTorrent(info *metainfo.Info, root string) error {
+	span := new(mmap_span.MMapSpan)
+	for _, file := range info.UpvertedFiles() {
+		filename := filepath.Join(append([]string{root, info.Name}, file.Path...)...)
+		mm, err := mmapFile(filename)
+		if err != nil {
+			return err
+		}
+		if int64(len(mm)) != file.Length {
+			return fmt.Errorf("file %q has wrong length", filename)
+		}
+		span.Append(mm)
+	}
+	for i := range iter.N(info.NumPieces()) {
+		p := info.Piece(i)
+		hash := sha1.New()
+		_, err := io.Copy(hash, io.NewSectionReader(span, p.Offset(), p.Length()))
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("%d: %x: %v\n", i, p.Hash(), bytes.Equal(hash.Sum(nil), p.Hash().Bytes()))
+	}
+	return nil
 }
 
 func main() {
 	log.SetFlags(log.Flags() | log.Lshortfile)
-	flag.Parse()
-	metaInfo, err := metainfo.LoadFromFile(*torrentPath)
+	var flags = struct {
+		DataDir string
+		tagflag.StartPos
+		TorrentFile string
+	}{}
+	tagflag.Parse(&flags)
+	metaInfo, err := metainfo.LoadFromFile(flags.TorrentFile)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -53,27 +74,8 @@ func main() {
 	if err != nil {
 		log.Fatalf("error unmarshalling info: %s", err)
 	}
-	mMapSpan := &mmap_span.MMapSpan{}
-	if len(info.Files) > 0 {
-		for _, file := range info.Files {
-			filename := filepath.Join(append([]string{*dataPath, info.Name}, file.Path...)...)
-			goMMap := fileToMmap(filename, file.Length)
-			mMapSpan.Append(goMMap)
-		}
-		log.Println(len(info.Files))
-	} else {
-		goMMap := fileToMmap(*dataPath, info.Length)
-		mMapSpan.Append(goMMap)
-	}
-	log.Println(mMapSpan.Size())
-	log.Println(len(info.Pieces))
-	for i := range iter.N(info.NumPieces()) {
-		p := info.Piece(i)
-		hash := sha1.New()
-		_, err := io.Copy(hash, io.NewSectionReader(mMapSpan, p.Offset(), p.Length()))
-		if err != nil {
-			log.Fatal(err)
-		}
-		fmt.Printf("%d: %x: %v\n", i, p.Hash(), bytes.Equal(hash.Sum(nil), p.Hash().Bytes()))
+	err = verifyTorrent(&info, flags.DataDir)
+	if err != nil {
+		log.Fatalf("torrent failed verification: %s", err)
 	}
 }