diff --git a/R/AnsariBradley.R b/R/AnsariBradley.R
index f0091b4..c6d244a 100644
--- a/R/AnsariBradley.R
+++ b/R/AnsariBradley.R
@@ -89,4 +89,4 @@ AnsariBradley <- R6Class(
             private$.p_value <- get_p_continous(z, "norm", private$.side)
         }
     )
-)
+)
\ No newline at end of file
diff --git a/R/Friedman.R b/R/Friedman.R
index 3d82dcc..d5828d6 100644
--- a/R/Friedman.R
+++ b/R/Friedman.R
@@ -65,4 +65,4 @@ Friedman <- R6Class(
             )
         }
     )
-)
+)
\ No newline at end of file
diff --git a/R/KolmogorovSmirnov.R b/R/KolmogorovSmirnov.R
index 3773fea..f69a441 100644
--- a/R/KolmogorovSmirnov.R
+++ b/R/KolmogorovSmirnov.R
@@ -48,4 +48,4 @@ KolmogorovSmirnov <- R6Class(
             private$.side <- "r"
         }
     )
-)
+)
\ No newline at end of file
diff --git a/R/Page.R b/R/Page.R
index dcf77f9..ccde452 100644
--- a/R/Page.R
+++ b/R/Page.R
@@ -64,4 +64,4 @@ Page <- R6Class(
             private$.p_value <- get_p_continous(z, "norm", private$.side)
         }
     )
-)
+)
\ No newline at end of file
diff --git a/R/RCBDOneWay.R b/R/RCBDOneWay.R
index 882897c..d2edc8a 100644
--- a/R/RCBDOneWay.R
+++ b/R/RCBDOneWay.R
@@ -69,5 +69,4 @@ RCBDOneWay <- R6Class(
             )
         }
     )
-)
-
+)
\ No newline at end of file
diff --git a/R/do_call.R b/R/do_call.R
index b0fc51c..8b671fc 100644
--- a/R/do_call.R
+++ b/R/do_call.R
@@ -27,4 +27,4 @@ do_call <- function(func, default = list(), fixed = list(), ...) {
     # All arguments will be evaluated within the `args` environment.
     # https://stackoverflow.com/a/25371509/23137996
     eval(as.call(c(func, lapply(`names<-`(...args, ...args), as.name))), args)
-}
+}
\ No newline at end of file
diff --git a/R/pmt.R b/R/pmt.R
index 7e18e0c..7cdda23 100644
--- a/R/pmt.R
+++ b/R/pmt.R
@@ -164,8 +164,8 @@ pmts <- function(
 #' rcpp <- define_pmt(
 #'     inherit = "twosample", n_permu = 1e5,
 #'     statistic = "[](const auto& x, const auto& y) {
-#'         auto m = x.size();
-#'         auto n = y.size();
+#'         auto m = x.length();
+#'         auto n = y.length();
 #'         return [=](const auto& x, const auto& y) {
 #'             return sum(x) / m - sum(y) / n;
 #'         };
@@ -176,9 +176,9 @@ pmts <- function(
 #' # rcpp <- define_pmt(
 #' #     inherit = "twosample", n_permu = 1e5,
 #' #     statistic = "[](const NumericVector& x, const NumericVector& y) {
-#' #         R_xlen_t m = x.size();
-#' #         R_xlen_t n = y.size();
-#' #         return [=](const NumericVector& x, const NumericVector& y) -> double {
+#' #         R_xlen_t m = x.length();
+#' #         R_xlen_t n = y.length();
+#' #         return [m, n](const NumericVector& x, const NumericVector& y) -> double {
 #' #             return sum(x) / m - sum(y) / n;
 #' #         };
 #' #     }"
diff --git a/inst/include/pmt/permutation.hpp b/inst/include/pmt/permutation.hpp
index ba2df09..fba3b3d 100644
--- a/inst/include/pmt/permutation.hpp
+++ b/inst/include/pmt/permutation.hpp
@@ -3,6 +3,9 @@
 #include <algorithm>
 #include <iterator>
 
+template <typename T>
+using diff_t = typename std::iterator_traits<T>::difference_type;
+
 template <typename T>
 T rand_int(T n)
 {
@@ -12,12 +15,11 @@ T rand_int(T n)
 template <typename T>
 void random_shuffle(T first, T last)
 {
-    using diff_t = typename std::iterator_traits<T>::difference_type;
 
-    diff_t n = std::distance(first, last);
+    diff_t<T> n = std::distance(first, last);
 
-    for (diff_t i = 0; i < n - 1; i++) {
-        diff_t j = i + rand_int(n - i);
+    for (diff_t<T> i = 0; i < n - 1; i++) {
+        diff_t<T> j = i + rand_int(n - i);
         std::iter_swap(first + i, first + j);
     }
 }
@@ -33,7 +35,7 @@ double n_permutation(T first, T last)
 {
     double A = 1.0;
 
-    typename std::iterator_traits<T>::difference_type rep = 0;
+    diff_t<T> rep = 0;
 
     auto val = *first;
     for (auto it = first; it != last; it++) {
@@ -50,19 +52,19 @@ double n_permutation(T first, T last)
 }
 
 template <typename T>
-auto random_shuffle(T& v)
+auto random_shuffle(T&& v)
 {
     return random_shuffle(v.begin(), v.end());
 }
 
 template <typename T>
-auto next_permutation(T& v)
+auto next_permutation(T&& v)
 {
     return next_permutation(v.begin(), v.end());
 }
 
 template <typename T>
-auto n_permutation(const T& v)
+auto n_permutation(T&& v)
 {
     return n_permutation(v.begin(), v.end());
 }
\ No newline at end of file
diff --git a/man/pmt.Rd b/man/pmt.Rd
index 6da93ce..538badf 100644
--- a/man/pmt.Rd
+++ b/man/pmt.Rd
@@ -123,8 +123,8 @@ r <- define_pmt(
 rcpp <- define_pmt(
     inherit = "twosample", n_permu = 1e5,
     statistic = "[](const auto& x, const auto& y) {
-        auto m = x.size();
-        auto n = y.size();
+        auto m = x.length();
+        auto n = y.length();
         return [=](const auto& x, const auto& y) {
             return sum(x) / m - sum(y) / n;
         };
@@ -135,9 +135,9 @@ rcpp <- define_pmt(
 # rcpp <- define_pmt(
 #     inherit = "twosample", n_permu = 1e5,
 #     statistic = "[](const NumericVector& x, const NumericVector& y) {
-#         R_xlen_t m = x.size();
-#         R_xlen_t n = y.size();
-#         return [=](const NumericVector& x, const NumericVector& y) -> double {
+#         R_xlen_t m = x.length();
+#         R_xlen_t n = y.length();
+#         return [m, n](const NumericVector& x, const NumericVector& y) -> double {
 #             return sum(x) / m - sum(y) / n;
 #         };
 #     }"