fix of reduce when histogram lacks flow bins

reducing histograms with axes that lacked flow bins failed before this patch

include/boost/histogram/algorithm/reduce.hpp

@@ -263,26 +263,44 @@ decltype(auto) reduce(const Histogram& hist, const Iterable& options) {
     ++iaxis;
   });
 
-  auto storage = detail::make_default(unsafe_access::storage(hist));
+  auto idx = detail::make_stack_buffer<int>(axes);
-  auto result = Histogram(std::move(axes), std::move(storage));
+  auto irange = detail::make_stack_buffer<std::pair<int, int>>(axes);
-
+  detail::for_each_axis(axes, [it = irange.begin()](const auto& a) mutable {
-  auto idx = detail::make_stack_buffer<int>(unsafe_access::axes(result));
+    using A = std::decay_t<decltype(a)>;
+    it->first = axis::traits::static_options<A>::test(axis::option::underflow) ? -1 : 0;
+    it->second = axis::traits::static_options<A>::test(axis::option::overflow)
+                     ? a.size() + 1
+                     : a.size();
+    ++it;
+  });
+  auto result =
+      Histogram(std::move(axes), detail::make_default(unsafe_access::storage(hist)));
   for (auto&& x : indexed(hist, coverage::all)) {
     auto i = idx.begin();
     auto o = opts.begin();
+    auto ir = irange.begin();
+    bool valid = true;
+
     for (auto j : x.indices()) {
       *i = (j - o->begin);
-      if (*i <= -1)
+      if (*i <= -1) {
         *i = -1;
-      else {
+        if (*i < ir->first) valid = false;
+      } else {
         *i /= o->merge;
         const int end = (o->end - o->begin) / o->merge;
-        if (*i > end) *i = end;
+        if (*i >= end) {
+          *i = end;
+          if (*i >= ir->second) valid = false;
+        }
       }
+
       ++i;
       ++o;
+      ++ir;
     }
-    result.at(idx) += *x;
+
+    if (valid) result.at(idx) += *x;
   }
 
   return result;

test/algorithm_reduce_test.cpp

@@ -224,6 +224,43 @@ void run_tests() {
     BOOST_TEST_EQ(reduce(h, shrink(2, -1.001)).axis(), R(4, 2, -2));
     BOOST_TEST_EQ(reduce(h, shrink(2, -1)).axis(), R(3, 2, -1));
   }
+
+  // reduce on histogram with axis without flow bins, see GitHub issue #257
+  {
+    auto h = make(Tag(), axis::integer<int, use_default, axis::option::underflow_t>(0, 3),
+                  axis::integer<int, use_default, axis::option::overflow_t>(0, 3));
+
+    std::fill(h.begin(), h.end(), 1);
+
+    /*
+    Original histogram:
+                x
+         -1  0  1  2
+       -------------
+      0|  1  1  1  1
+    x 1|  1  1  1  1
+      2|  1  1  1  1
+      3|  1  1  1  1
+
+    Shrunk histogram:
+         -1  0
+       -------
+      0|  2  1
+      1|  4  2
+    */
+
+    auto hr = reduce(h, slice(0, 1, 2), slice(1, 1, 2));
+    BOOST_TEST_EQ(hr.size(), 2 * 2);
+    BOOST_TEST_EQ(hr.axis(0).size(), 1);
+    BOOST_TEST_EQ(hr.axis(1).size(), 1);
+    BOOST_TEST_EQ(hr.axis(0).bin(0), 1);
+    BOOST_TEST_EQ(hr.axis(1).bin(0), 1);
+
+    BOOST_TEST_EQ(hr.at(-1, 0), 2);
+    BOOST_TEST_EQ(hr.at(0, 0), 1);
+    BOOST_TEST_EQ(hr.at(-1, 1), 4);
+    BOOST_TEST_EQ(hr.at(0, 1), 2);
+  }
 }
 
 int main() {