[ci skip] Use less verbose naming. Add asserts as verfication of algorithms is a negligible fraction of total runtime. Use boost::multiprecision::powm and boost::multiprecision::sqrt rather than one-offs.

test/mod_inverse_test.cpp

@@ -0,0 +1,48 @@
+/*
+ *  (C) Copyright Nick Thompson 2018.
+ *  Use, modification and distribution are subject to the
+ *  Boost Software License, Version 1.0. (See accompanying file
+ *  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+#define BOOST_TEST_MODULE modular_multiplicative_inverse_test
+#include <boost/test/included/unit_test.hpp>
+#include <boost/multiprecision/cpp_int.hpp>
+#include <boost/integer/common_factor.hpp>
+#include <boost/integer/mod_inverse.hpp>
+
+using boost::multiprecision::int128_t;
+using boost::integer::mod_inverse;
+using boost::integer::gcd;
+
+template<class Z>
+void test_mod_inverse()
+{
+    Z max_arg = 1000;
+    for (Z modulus = 2; modulus < max_arg; ++modulus)
+    {
+        for (Z a = 1; a < max_arg; ++a)
+        {
+            Z gcdam = gcd(a, modulus);
+            boost::optional<Z> inv_a = mod_inverse(a, modulus);
+            // Should fail if gcd(a, mod) != 1:
+            if (gcdam > 1)
+            {
+                BOOST_CHECK(!inv_a);
+            }
+            else
+            {
+                BOOST_CHECK(inv_a.value() > 0);
+                Z outta_be_one = (inv_a.value()*a) % modulus;
+                BOOST_CHECK_EQUAL(outta_be_one, 1);
+            }
+        }
+    }
+}
+
+BOOST_AUTO_TEST_CASE(extended_euclidean_test)
+{
+    test_mod_inverse<int>();
+    test_mod_inverse<long>();
+    test_mod_inverse<long long>();
+    test_mod_inverse<int128_t>();
+}