diff --git a/number/basis.cc b/number/basis.cc
new file mode 100644
index 0000000..7443be2
--- /dev/null
+++ b/number/basis.cc
@@ -0,0 +1,22 @@
+// in-place modification
+int basis(vector<ll>& a) {
+    int n = a.size();
+    int has = 0;
+    for (int i = 63; ~i and has < n; --i) {
+        for (int j = has; j < n; ++j) {
+            if (a[j] & (ll(1) << i)) {
+                swap(a[j], a[has]);
+                break;
+            }
+        }
+        if (not (a[has] & (ll(1) << i))) continue;
+        for (int j = 0; j < n; ++j) {
+            if (j == has) continue;
+            if (a[j] & (ll(1) << i)) {
+                a[j] ^= a[has];
+            }
+        }
+        ++has;
+    }
+    return has;
+}