added support for variable axis

include/boost/histogram/algorithm/reduce.hpp

@@ -39,20 +39,31 @@ struct reduce_option_type {
   operator bool() const noexcept { return merge; }
 };
 
-reduce_option_type shrink(unsigned iaxis, double lower, double upper) {
-  return {iaxis, lower, upper, 1};
-}
-
 reduce_option_type shrink_and_rebin(unsigned iaxis, double lower, double upper,
                                     unsigned merge) {
   return {iaxis, lower, upper, merge};
 }
 
+reduce_option_type shrink(unsigned iaxis, double lower, double upper) {
+  return {iaxis, lower, upper, 1};
+}
+
 reduce_option_type rebin(unsigned iaxis, unsigned merge) {
   return {iaxis, std::numeric_limits<double>::quiet_NaN(),
           std::numeric_limits<double>::quiet_NaN(), merge};
 }
 
+/// Convenience overload for when there is only one axis.
+reduce_option_type shrink_and_rebin(double lower, double upper, unsigned merge) {
+  return shrink_and_rebin(0, lower, upper, merge);
+}
+
+/// Convenience overload for when there is only one axis.
+reduce_option_type shrink(double lower, double upper) { return shrink(0, lower, upper); }
+
+/// Convenience overload for when there is only one axis.
+reduce_option_type rebin(unsigned merge) { return rebin(0, merge); }
+
 template <typename A, typename S, typename C, typename = detail::requires_iterable<C>>
 histogram<A, S> reduce(const histogram<A, S>& h, const C& c) {
   auto options =

include/boost/histogram/axis/variable.hpp

@@ -107,6 +107,25 @@ public:
            allocator_type a = allocator_type())
       : variable(l.begin(), l.end(), std::move(m), o, std::move(a)) {}
 
+  /// Constructor used by algorithm::reduce to shrink and rebin (not for users).
+  variable(const variable& src, unsigned begin, unsigned end, unsigned merge)
+      : base_type((end - begin) / merge, src.metadata(), src.options()), x_(src.x_) {
+    BOOST_ASSERT((end - begin) % merge == 0);
+    using It = const detail::unqual<decltype(*src.x_.first())>*;
+    struct skip_iterator {
+      It it;
+      unsigned skip;
+      skip_iterator operator++(int) {
+        auto tmp = *this;
+        it += skip;
+        return tmp;
+      }
+      decltype(auto) operator*() { return *it; }
+      bool operator==(const skip_iterator& rhs) const { return it == rhs.it; }
+    } iter{src.x_.first() + begin, merge};
+    x_.first() = detail::create_buffer_from_iter(x_.second(), nx(), iter);
+  }
+
   variable() : x_(nullptr) {}
 
   variable(const variable& o) : base_type(o), x_(o.x_) {