diff --git a/pkgs/mruby/src/assert.c b/pkgs/mruby/src/assert.c
index 5dee060..2173637 100644
--- a/pkgs/mruby/src/assert.c
+++ b/pkgs/mruby/src/assert.c
@@ -41,12 +41,12 @@ void assert_cb(const char *const file, const int line, const char *const msg)
     if (mrb_nil_p(assert_cb_rb)) return;
 
     mrb_value file_rb = mrb_str_new_lit(mrb, "");
-    file_rb = mrb_str_cat(mrb, file_rb, file, strlen(file));
+    file_rb = mrb_str_cat_cstr(mrb, file_rb, file);
 
     mrb_value line_rb = mrb_fixnum_value(line);
 
     mrb_value msg_rb = mrb_str_new_lit(mrb, "");
-    msg_rb = mrb_str_cat(mrb, msg_rb, msg, strlen(msg));
+    msg_rb = mrb_str_cat_cstr(mrb, msg_rb, msg);
 
     mrb_funcall_id(
         mrb, assert_cb_rb, MRB_SYM(call), 3, file_rb, line_rb, msg_rb);
diff --git a/pkgs/mruby/src/main.c b/pkgs/mruby/src/main.c
index a05db33..444ba79 100644
--- a/pkgs/mruby/src/main.c
+++ b/pkgs/mruby/src/main.c
@@ -22,6 +22,7 @@ void mrb_mruby_kernaux_gem_init(mrb_state *const mrb)
 
     mrb_define_module_id(mrb, MRB_SYM(KernAux));
     init_assert(mrb);
+    init_ntoa(mrb);
 }
 
 void current_mrb_start(mrb_state *mrb)
diff --git a/pkgs/mruby/src/main.h b/pkgs/mruby/src/main.h
index 489a1e7..1eb543f 100644
--- a/pkgs/mruby/src/main.h
+++ b/pkgs/mruby/src/main.h
@@ -8,5 +8,6 @@ void current_mrb_finish(mrb_state *mrb);
 mrb_state *current_mrb_get();
 
 void init_assert(mrb_state *mrb);
+void init_ntoa(mrb_state *mrb);
 
 #endif
diff --git a/pkgs/mruby/src/ntoa.c b/pkgs/mruby/src/ntoa.c
new file mode 100644
index 0000000..b42838b
--- /dev/null
+++ b/pkgs/mruby/src/ntoa.c
@@ -0,0 +1,98 @@
+#include "main.h"
+
+#include <stdint.h>
+
+#include <kernaux.h>
+
+#include <mruby.h>
+#include <mruby/presym.h>
+#include <mruby/string.h>
+
+static mrb_value rb_KernAux_utoa10(mrb_state *mrb, mrb_value self);
+static mrb_value rb_KernAux_itoa10(mrb_state *mrb, mrb_value self);
+static mrb_value rb_KernAux_utoa16(mrb_state *mrb, mrb_value self);
+static mrb_value rb_KernAux_itoa16(mrb_state *mrb, mrb_value self);
+
+void init_ntoa(mrb_state *const mrb)
+{
+    struct RClass *const rb_KernAux = mrb_module_get_id(mrb, MRB_SYM(KernAux));
+
+    mrb_define_class_method(mrb, rb_KernAux, "utoa10",
+                            rb_KernAux_utoa10, MRB_ARGS_REQ(1));
+    mrb_define_class_method(mrb, rb_KernAux, "itoa10",
+                            rb_KernAux_itoa10, MRB_ARGS_REQ(1));
+    mrb_define_class_method(mrb, rb_KernAux, "utoa16",
+                            rb_KernAux_utoa16, MRB_ARGS_REQ(1));
+    mrb_define_class_method(mrb, rb_KernAux, "itoa16",
+                            rb_KernAux_itoa16, MRB_ARGS_REQ(1));
+}
+
+mrb_value rb_KernAux_utoa10(mrb_state *mrb, mrb_value self)
+{
+    mrb_int value = 0;
+    mrb_get_args(mrb, "i", &value);
+
+    if (value < 0) {
+        mrb_raise(mrb, E_RANGE_ERROR,
+                  "can't convert negative number to uint64_t");
+    }
+
+    char buffer[KERNAUX_UTOA10_BUFFER_SIZE];
+    current_mrb_start(mrb);
+    kernaux_utoa10(value, buffer);
+    current_mrb_finish(mrb);
+
+    mrb_value result = mrb_str_new_lit(mrb, "");
+    result = mrb_str_cat_cstr(mrb, result, buffer);
+    return result;
+}
+
+mrb_value rb_KernAux_itoa10(mrb_state *mrb, mrb_value self)
+{
+    mrb_int value = 0;
+    mrb_get_args(mrb, "i", &value);
+
+    char buffer[KERNAUX_ITOA10_BUFFER_SIZE];
+    current_mrb_start(mrb);
+    kernaux_itoa10(value, buffer);
+    current_mrb_finish(mrb);
+
+    mrb_value result = mrb_str_new_lit(mrb, "");
+    result = mrb_str_cat_cstr(mrb, result, buffer);
+    return result;
+}
+
+mrb_value rb_KernAux_utoa16(mrb_state *mrb, mrb_value self)
+{
+    mrb_int value = 0;
+    mrb_get_args(mrb, "i", &value);
+
+    if (value < 0) {
+        mrb_raise(mrb, E_RANGE_ERROR,
+                  "can't convert negative number to uint64_t");
+    }
+
+    char buffer[KERNAUX_UTOA16_BUFFER_SIZE];
+    current_mrb_start(mrb);
+    kernaux_utoa16(value, buffer);
+    current_mrb_finish(mrb);
+
+    mrb_value result = mrb_str_new_lit(mrb, "");
+    result = mrb_str_cat_cstr(mrb, result, buffer);
+    return result;
+}
+
+mrb_value rb_KernAux_itoa16(mrb_state *mrb, mrb_value self)
+{
+    mrb_int value = 0;
+    mrb_get_args(mrb, "i", &value);
+
+    char buffer[KERNAUX_ITOA16_BUFFER_SIZE];
+    current_mrb_start(mrb);
+    kernaux_itoa16(value, buffer);
+    current_mrb_finish(mrb);
+
+    mrb_value result = mrb_str_new_lit(mrb, "");
+    result = mrb_str_cat_cstr(mrb, result, buffer);
+    return result;
+}
diff --git a/pkgs/mruby/test/ntoa.rb b/pkgs/mruby/test/ntoa.rb
new file mode 100644
index 0000000..8d8d352
--- /dev/null
+++ b/pkgs/mruby/test/ntoa.rb
@@ -0,0 +1,33 @@
+assert 'KernAux.utoa10' do
+  assert_equal '0', KernAux.utoa10(0)
+  assert_equal '1', KernAux.utoa10(1)
+  assert_equal '123', KernAux.utoa10(123)
+  assert_equal (2**32 - 1).to_s, KernAux.utoa10(2**32 - 1)
+end
+
+assert 'KernAux.itoa10' do
+  assert_equal '0', KernAux.itoa10(0)
+  assert_equal '1', KernAux.itoa10(1)
+  assert_equal '-1', KernAux.itoa10(-1)
+  assert_equal '123', KernAux.itoa10(123)
+  assert_equal '-123', KernAux.itoa10(-123)
+  assert_equal (2**31 - 1).to_s, KernAux.itoa10(2**31 - 1)
+  assert_equal (-2**31).to_s, KernAux.itoa10(-2**31)
+end
+
+assert 'KernAux.utoa16' do
+  assert_equal '0', KernAux.utoa16(0)
+  assert_equal '1', KernAux.utoa16(1)
+  assert_equal '123', KernAux.utoa16(0x123)
+  assert_equal (2**32 - 1).to_s(16), KernAux.utoa16(2**32 - 1)
+end
+
+assert 'KernAux.itoa16' do
+  assert_equal '0', KernAux.itoa16(0)
+  assert_equal '1', KernAux.itoa16(1)
+  assert_equal '-1', KernAux.itoa16(-1)
+  assert_equal '123', KernAux.itoa16(0x123)
+  assert_equal '-123', KernAux.itoa16(-0x123)
+  assert_equal (2**31 - 1).to_s(16), KernAux.itoa16(2**31 - 1)
+  assert_equal (-2**31).to_s(16), KernAux.itoa16(-2**31)
+end