Don't copy unmodified files from source to dest

file.go

@@ -39,6 +39,10 @@ type file struct {
 }
 
 func (f *file) export(dstPath string) error {
+	if f.reader == nil && fileCached(f.asset, dstPath) {
+		return nil
+	}
+
 	if err := os.MkdirAll(path.Dir(dstPath), 0755); err != nil {
 		return err
 	}

util.go

@@ -66,3 +66,17 @@ func scanDir(root string, files, dirs chan string) {
 		return nil
 	})
 }
+
+func fileCached(srcPath, dstPath string) bool {
+	srcStat, err := os.Stat(srcPath)
+	if err != nil {
+		return false
+	}
+
+	dstStat, err := os.Stat(dstPath)
+	if err != nil {
+		return false
+	}
+
+	return dstStat.ModTime().Unix() >= srcStat.ModTime().Unix()
+}