diff --git a/refcode/1-ds/pbds.cpp b/refcode/1-ds/pbds.cpp
new file mode 100644
index 0000000..b9f0b7f
--- /dev/null
+++ b/refcode/1-ds/pbds.cpp
@@ -0,0 +1,27 @@
+#include "../common/common.hpp"
+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/tree_policy.hpp>
+using namespace __gnu_pbds;
+#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
+int main() {
+  ordered_set os; // empty
+  // basic usage
+  os.insert(1); // 1
+  os.insert(1); // 1
+  os.insert(4); // 1 4
+  os.insert(3); // 1 3 4
+  os.insert(2); // 1 2 3 4
+  os.insert(4); // 1 2 3 4
+  os.erase(2);  // 1 3 4
+  os.erase(2);  // 1 3 4
+  os.insert(1); // 1 3 4
+  os.erase(1);  // 3 4
+  // order_of_key() (Assume os = {13, 32, 34, 45, 48, 59, 61, 68, 71, 78, 87, 128})
+  cout << os.order_of_key(10) << '\n'; // 0
+  cout << os.order_of_key(49) << '\n'; // 5
+  cout << os.order_of_key(48) << '\n'; // 4
+  // find_by_order() (Assume os = {13, 32, 34, 45, 48, 59, 61, 68, 71, 78, 87, 128})
+  cout << *(os.find_by_order(0)) << '\n';   // 13
+  cout << *(os.find_by_order(7)) << '\n';   // 68
+  cout << *(os.find_by_order(100)) << '\n'; // 0
+}
\ No newline at end of file
diff --git a/scripts/format.sh b/scripts/format.sh
new file mode 100755
index 0000000..e1f5d2d
--- /dev/null
+++ b/scripts/format.sh
@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+root="$(cd "$(dirname "$0")/.." && pwd)"
+target="$root/refcode"
+mapfile -t files < <(find "$target" -type f \( -name '*.cpp' -o -name '*.hpp' \))
+((${#files[@]})) || { echo "no .cpp/.hpp files in $target"; exit 0; }
+declare -A before
+for f in "${files[@]}"; do before["$f"]="$(sha1sum "$f" | cut -d' ' -f1)"; done
+clang-format -i "${files[@]}"
+changed=()
+for f in "${files[@]}"; do [[ ${before["$f"]} != "$(sha1sum "$f" | cut -d' ' -f1)" ]] && changed+=("$f"); done
+echo "formatted ${#files[@]} file(s) in $target"
+((${#changed[@]})) && printf 'changed %d file(s):\n%s\n' "${#changed[@]}" "$(printf '%s\n' "${changed[@]}")" || echo "no files changed"