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a*p % m may overflow, do not perform naive multiplication in unit tests or undefined behavior may result. [CI SKIP]

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This commit is contained in:
Nick Thompson
2018-10-26 11:19:43 -06:00
parent 3f1603938c
commit 2d463f3ee7
4 changed files with 35 additions and 34 deletions
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21
test/mod_inverse_test.cpp
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@ -19,10 +19,15 @@ template<class Z>
void test_mod_inverse()
{
std::cout << "Testing the modular multiplicative inverse on type " << boost::typeindex::type_id<Z>().pretty_name() << "\n";
//Z max_arg = std::numeric_limits<Z>::max();
Z max_arg = 500;
for (Z modulus = 2; modulus < max_arg; ++modulus)
{
for (Z a = 1; a < max_arg; ++a)
if (modulus % 1000 == 0)
{
std::cout << "Testing all inverses modulo " << modulus << std::endl;
}
for (Z a = 1; a < modulus; ++a)
{
Z gcdam = gcd(a, modulus);
boost::optional<Z> inv_a = mod_inverse(a, modulus);
@ -34,7 +39,11 @@ void test_mod_inverse()
else
{
BOOST_CHECK(inv_a.value() > 0);
Z outta_be_one = (inv_a.value()*a) % modulus;
// Cast to a bigger type so the multiplication won't overflow.
int256_t a_inv = inv_a.value();
int256_t big_a = a;
int256_t m = modulus;
int256_t outta_be_one = (a_inv*big_a) % m;
BOOST_CHECK_EQUAL(outta_be_one, 1);
}
}
@ -43,10 +52,8 @@ void test_mod_inverse()
BOOST_AUTO_TEST_CASE(mod_inverse_test)
{
test_mod_inverse<short int>();
test_mod_inverse<int>();
test_mod_inverse<long>();
test_mod_inverse<long long>();
test_mod_inverse<int16_t>();
test_mod_inverse<int32_t>();
test_mod_inverse<int64_t>();
test_mod_inverse<int128_t>();
test_mod_inverse<int256_t>();
}