libutil: add checked arithmetic tools

This is in preparation for adding checked arithmetic to the evaluator.

Change-Id: I6e115ce8f5411feda1706624977a4dcd5efd4d13

tests/unit/libutil/checked-arithmetic.cc

@@ -0,0 +1,158 @@
+#include <cstdint>
+#include <gtest/gtest.h>
+#include <limits>
+#include <rapidcheck/Assertions.h>
+#include <rapidcheck/gtest.h>
+#include <rapidcheck/gen/Arbitrary.hpp>
+
+#include <checked-arithmetic.hh>
+
+#include "tests/gtest-with-params.hh"
+
+namespace rc {
+using namespace nix;
+
+template<std::integral T>
+struct Arbitrary<nix::checked::Checked<T>>
+{
+    static Gen<nix::checked::Checked<T>> arbitrary()
+    {
+        return gen::arbitrary<T>();
+    }
+};
+
+}
+
+namespace nix::checked {
+
+// Pointer to member function! Mildly gross.
+template<std::integral T>
+using Oper = Checked<T>::Result (Checked<T>::*)(T const other) const;
+
+template<std::integral T>
+using ReferenceOper = T (*)(T a, T b);
+
+/**
+ * Checks that performing an operation that overflows into an inaccurate result
+ * has the desired behaviour.
+ *
+ * TBig is a type large enough to represent all results of TSmall operations.
+ */
+template<std::integral TSmall, std::integral TBig>
+void checkType(TSmall a_, TSmall b, Oper<TSmall> oper, ReferenceOper<TBig> reference)
+{
+    // Sufficient to fit all values
+    TBig referenceResult = reference(a_, b);
+    constexpr const TSmall minV = std::numeric_limits<TSmall>::min();
+    constexpr const TSmall maxV = std::numeric_limits<TSmall>::max();
+
+    Checked<TSmall> a{a_};
+    auto result = (a.*(oper))(b);
+
+    // Just truncate it to get the in-range result
+    RC_ASSERT(result.valueWrapping() == static_cast<TSmall>(referenceResult));
+
+    if (referenceResult > maxV || referenceResult < minV) {
+        RC_ASSERT(result.overflowed());
+        RC_ASSERT(!result.valueChecked().has_value());
+    } else {
+        RC_ASSERT(!result.overflowed());
+        RC_ASSERT(result.valueChecked().has_value());
+        RC_ASSERT(*result.valueChecked() == referenceResult);
+    }
+}
+
+/**
+ * Checks that performing an operation that overflows into an inaccurate result
+ * has the desired behaviour.
+ *
+ * TBig is a type large enough to represent all results of TSmall operations.
+ */
+template<std::integral TSmall, std::integral TBig>
+void checkDivision(TSmall a_, TSmall b)
+{
+    // Sufficient to fit all values
+    constexpr const TSmall minV = std::numeric_limits<TSmall>::min();
+
+    Checked<TSmall> a{a_};
+    auto result = a / b;
+
+    if (std::is_signed<TSmall>() && a_ == minV && b == -1) {
+        // This is the only possible overflow condition
+        RC_ASSERT(result.valueWrapping() == minV);
+        RC_ASSERT(result.overflowed());
+    } else if (b == 0) {
+        RC_ASSERT(result.divideByZero());
+        RC_ASSERT_THROWS_AS(result.valueWrapping(), nix::checked::DivideByZero);
+        RC_ASSERT(result.valueChecked() == std::nullopt);
+    } else {
+        TBig referenceResult = a_ / b;
+        auto result_ = result.valueChecked();
+        RC_ASSERT(result_.has_value());
+        RC_ASSERT(*result_ == referenceResult);
+        RC_ASSERT(result.valueWrapping() == referenceResult);
+    }
+}
+
+/** Creates parameters that perform a more adequate number of checks to validate
+ * extremely cheap tests such as arithmetic tests */
+static rc::detail::TestParams makeParams()
+{
+    auto const & conf = rc::detail::configuration();
+    auto newParams = conf.testParams;
+    newParams.maxSuccess = 10000;
+    return newParams;
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, add_unsigned, makeParams, (uint16_t a, uint16_t b))
+{
+    checkType<uint16_t, int32_t>(a, b, &Checked<uint16_t>::operator+, [](int32_t a, int32_t b) { return a + b; });
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, add_signed, makeParams, (int16_t a, int16_t b))
+{
+    checkType<int16_t, int32_t>(a, b, &Checked<int16_t>::operator+, [](int32_t a, int32_t b) { return a + b; });
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, sub_unsigned, makeParams, (uint16_t a, uint16_t b))
+{
+    checkType<uint16_t, int32_t>(a, b, &Checked<uint16_t>::operator-, [](int32_t a, int32_t b) { return a - b; });
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, sub_signed, makeParams, (int16_t a, int16_t b))
+{
+    checkType<int16_t, int32_t>(a, b, &Checked<int16_t>::operator-, [](int32_t a, int32_t b) { return a - b; });
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, mul_unsigned, makeParams, (uint16_t a, uint16_t b))
+{
+    checkType<uint16_t, int64_t>(a, b, &Checked<uint16_t>::operator*, [](int64_t a, int64_t b) { return a * b; });
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, mul_signed, makeParams, (int16_t a, int16_t b))
+{
+    checkType<int16_t, int64_t>(a, b, &Checked<int16_t>::operator*, [](int64_t a, int64_t b) { return a * b; });
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, div_unsigned, makeParams, (uint16_t a, uint16_t b))
+{
+    checkDivision<uint16_t, int64_t>(a, b);
+}
+
+RC_GTEST_PROP_WITH_PARAMS(Checked, div_signed, makeParams, (int16_t a, int16_t b))
+{
+    checkDivision<int16_t, int64_t>(a, b);
+}
+
+// Make absolutely sure that we check the special cases if the proptest
+// generator does not come up with them. This one is especially important
+// because it has very specific pairs required for the edge cases unlike the
+// others.
+TEST(Checked, div_signed_special_cases)
+{
+    checkDivision<int16_t, int64_t>(std::numeric_limits<int16_t>::min(), -1);
+    checkDivision<int16_t, int64_t>(std::numeric_limits<int16_t>::min(), 0);
+    checkDivision<int16_t, int64_t>(0, 0);
+}
+
+}