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fix: swapped modf outputs in circular variable::value() #438

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fix: swapped modf outputs in circular variable::value() #438

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5 changes: 3 additions & 2 deletions include/boost/histogram/axis/variable.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -210,8 +210,9 @@ class variable : public iterator_mixin<variable<Value, MetaData, Options, Alloca
if (options_type::test(option::circular)) {
auto shift = std::floor(i / size());
i -= shift * size();
double z;
const auto k = static_cast<index_type>(std::modf(i, &z));
double whole;
const double z = std::modf(i, &whole);
const auto k = static_cast<index_type>(whole);
const auto a = vec_[0];
const auto b = vec_[size()];
return (1.0 - z) * vec_[k] + z * vec_[k + 1] + shift * (b - a);
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15 changes: 15 additions & 0 deletions test/axis_variable_test.cpp
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Expand Up @@ -123,6 +123,21 @@ int main() {
BOOST_TEST_EQ(a.index(4), 1); // 4 - 3 = 1
}

// axis::variable circular with size > 2 and non-equidistant bins;
// the error in value() cancels for size == 2 and integer arguments
{
axis::variable<double, axis::null_type, op::circular_t> a{0, 1, 3, 4};
BOOST_TEST_EQ(a.value(-1), -1); // vec_[2] - period
BOOST_TEST_EQ(a.value(0), 0);
BOOST_TEST_EQ(a.value(1), 1);
BOOST_TEST_EQ(a.value(2), 3);
BOOST_TEST_EQ(a.value(3), 4);
BOOST_TEST_EQ(a.value(4), 5);
BOOST_TEST_EQ(a.value(0.5), 0.5);
BOOST_TEST_EQ(a.value(1.5), 2);
BOOST_TEST_EQ(a.value(2.5), 3.5);
}

// axis::regular with growth
{
using pii_t = std::pair<axis::index_type, axis::index_type>;
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