added modulo exponentiation

src/ModuloExponentiation.cpp

@@ -0,0 +1,32 @@
+#include <cstdint>
+using ll = intmax_t;
+using ull = uintmax_t;
+
+constexpr ll MODULO = 1000000007;
+
+// exponent is very large
+// modulo is prime (1e9 + 7)
+// base is always 2
+
+/// @brief Binary Exponentiation with modulo
+/// @param expo exponent
+/// @param base base (default: 2)
+/// @param modulo modulo
+template <bool USE_FERMAT_LITTLE_THEOREM = false>
+constexpr auto bin_mod_exp(ll expo, ll base = 2, ll modulo = MODULO) -> ll {
+
+  if constexpr (USE_FERMAT_LITTLE_THEOREM) {
+    expo %= (modulo - 1);
+  }
+
+  ll res = 1;
+
+  while (expo > 0) {
+    if (expo & 1LL) {
+      res = (res * base) % modulo;
+    }
+    base = (base * base) % modulo;
+    expo >>= 1LL;
+  }
+  return res;
+}