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From: Benjamin Barenblat <bbaren@google.com>
Subject: Round a double multiplication before casting it to integer
Forwarded: yes
Applied-Upstream: https://github.com/abseil/abseil-cpp/commit/60be12ed9822078970f05f3c560324184302df6b
The code
static_cast<int>(x * y)
(for double x and y) performs a double multiplication into a temporary
that, by standard, may have excess precision. The subsequent cast to int
discards the excess precision. However, the cast may examine the excess
precision during conversion, producing surprising results like
static_cast<int>(1.7 * 10) == 16
on certain systems. Correct this case by explicitly rounding 1.7 * 10
before casting it.
The author works at Google. Upstream applied this patch as Piper
revision 378922064 and exported it to GitHub; the Applied-Upstream URL
above points to the exported commit.
--- a/absl/random/mocking_bit_gen_test.cc
+++ b/absl/random/mocking_bit_gen_test.cc
@@ -15,6 +15,7 @@
//
#include "absl/random/mocking_bit_gen.h"
+#include <cmath>
#include <numeric>
#include <random>
@@ -328,8 +329,9 @@ TEST(BasicMocking, WillByDefaultWithArgs) {
absl::MockingBitGen gen;
ON_CALL(absl::MockPoisson<int>(), Call(gen, _))
- .WillByDefault(
- [](double lambda) { return static_cast<int>(lambda * 10); });
+ .WillByDefault([](double lambda) {
+ return static_cast<int>(std::rint(lambda * 10));
+ });
EXPECT_EQ(absl::Poisson<int>(gen, 1.7), 17);
EXPECT_EQ(absl::Poisson<int>(gen, 0.03), 0);
}
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