diff --git a/cmd/torrent-verify/main.go b/cmd/torrent-verify/main.go
new file mode 100644
index 00000000..7259f0e2
--- /dev/null
+++ b/cmd/torrent-verify/main.go
@@ -0,0 +1,75 @@
+package main
+
+import (
+	"bitbucket.org/anacrolix/go.torrent"
+	"bytes"
+	"crypto/sha1"
+	"flag"
+	metainfo "github.com/nsf/libtorgo/torrent"
+	"io"
+	"launchpad.net/gommap"
+	"log"
+	"os"
+	"path/filepath"
+)
+
+var (
+	filePath = flag.String("torrent", "/path/to/the.torrent", "path of the torrent file")
+	dirPath  = flag.String("path", "/torrent/data", "path of the torrent data")
+)
+
+func init() {
+	flag.Parse()
+}
+
+func main() {
+	metaInfo, err := metainfo.LoadFromFile(*filePath)
+	if err != nil {
+		log.Fatal(err)
+	}
+	devZero, err := os.Open("/dev/zero")
+	if err != nil {
+		log.Print(err)
+	}
+	defer devZero.Close()
+	var mmapSpan torrent.MmapSpan
+	for _, file := range metaInfo.Files {
+		filename := filepath.Join(append([]string{*dirPath, metaInfo.Name}, file.Path...)...)
+		osFile, err := os.Open(filename)
+		mmapFd := osFile.Fd()
+		if err != nil {
+			if pe, ok := err.(*os.PathError); ok && pe.Err.Error() == "no such file or directory" {
+				mmapFd = devZero.Fd()
+			} else {
+				log.Fatal(err)
+			}
+		}
+		goMMap, err := gommap.MapRegion(mmapFd, 0, file.Length, gommap.PROT_READ, gommap.MAP_PRIVATE)
+		if err != nil {
+			log.Fatal(err)
+		}
+		if int64(len(goMMap)) != file.Length {
+			log.Printf("file mmap has wrong size: %#v", filename)
+		}
+		osFile.Close()
+		mmapSpan = append(mmapSpan, torrent.Mmap{goMMap})
+	}
+	log.Println(len(metaInfo.Files))
+	log.Println(mmapSpan.Size())
+	log.Println(len(metaInfo.Pieces))
+	for piece := int64(0); ; piece++ {
+		expectedHash := metaInfo.Pieces[sha1.Size*piece : sha1.Size*(piece+1)]
+		if len(expectedHash) == 0 {
+			break
+		}
+		hash := sha1.New()
+		n, err := io.Copy(hash, io.NewSectionReader(mmapSpan, piece*metaInfo.PieceLength, metaInfo.PieceLength))
+		if n != metaInfo.PieceLength {
+			panic("oh no")
+		}
+		if err != nil {
+			log.Fatal(err)
+		}
+		log.Println(piece, bytes.Equal(hash.Sum(nil), expectedHash))
+	}
+}