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+///////////////////////////////////////////////////////////////////////////////
+// mean.hpp
+//
+//  Copyright 2005 Eric Niebler. Distributed under the Boost
+//  Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_ACCUMULATORS_STATISTICS_MEAN_HPP_EAN_28_10_2005
+#define BOOST_ACCUMULATORS_STATISTICS_MEAN_HPP_EAN_28_10_2005
+
+#include <boost/mpl/placeholders.hpp>
+#include <boost/accumulators/framework/accumulator_base.hpp>
+#include <boost/accumulators/framework/extractor.hpp>
+#include <boost/accumulators/numeric/functional.hpp>
+#include <boost/accumulators/framework/parameters/sample.hpp>
+#include <boost/accumulators/framework/depends_on.hpp>
+#include <boost/accumulators/statistics_fwd.hpp>
+#include <boost/accumulators/statistics/count.hpp>
+#include <boost/accumulators/statistics/sum.hpp>
+
+namespace boost { namespace accumulators
+{
+
+namespace impl
+{
+    ///////////////////////////////////////////////////////////////////////////////
+    // mean_impl
+    //      lazy, by default
+    template<typename Sample, typename SumFeature>
+    struct mean_impl
+      : accumulator_base
+    {
+        // for boost::result_of
+        typedef typename numeric::functional::average<Sample, std::size_t>::result_type result_type;
+
+        mean_impl(dont_care) {}
+
+        template<typename Args>
+        result_type result(Args const &args) const
+        {
+            extractor<SumFeature> sum;
+            return numeric::average(sum(args), count(args));
+        }
+    };
+
+    template<typename Sample, typename Tag>
+    struct immediate_mean_impl
+      : accumulator_base
+    {
+        // for boost::result_of
+        typedef typename numeric::functional::average<Sample, std::size_t>::result_type result_type;
+
+        template<typename Args>
+        immediate_mean_impl(Args const &args)
+          : mean(numeric::average(args[sample | Sample()], numeric::one<std::size_t>::value))
+        {
+        }
+
+        template<typename Args>
+        void operator ()(Args const &args)
+        {
+            std::size_t cnt = count(args);
+            this->mean = numeric::average(
+                (this->mean * (cnt - 1)) + args[parameter::keyword<Tag>::get()]
+              , cnt
+            );
+        }
+
+        result_type result(dont_care) const
+        {
+            return this->mean;
+        }
+
+    private:
+        result_type mean;
+    };
+
+} // namespace impl
+
+///////////////////////////////////////////////////////////////////////////////
+// tag::mean
+// tag::immediate_mean
+// tag::mean_of_weights
+// tag::immediate_mean_of_weights
+// tag::mean_of_variates
+// tag::immediate_mean_of_variates
+//
+namespace tag
+{
+    struct mean
+      : depends_on<count, sum>
+    {
+        /// INTERNAL ONLY
+        ///
+        typedef accumulators::impl::mean_impl<mpl::_1, sum> impl;
+    };
+    struct immediate_mean
+      : depends_on<count>
+    {
+        /// INTERNAL ONLY
+        ///
+        typedef accumulators::impl::immediate_mean_impl<mpl::_1, tag::sample> impl;
+    };
+    struct mean_of_weights
+      : depends_on<count, sum_of_weights>
+    {
+        typedef mpl::true_ is_weight_accumulator;
+        /// INTERNAL ONLY
+        ///
+        typedef accumulators::impl::mean_impl<mpl::_2, sum_of_weights> impl;
+    };
+    struct immediate_mean_of_weights
+      : depends_on<count>
+    {
+        typedef mpl::true_ is_weight_accumulator;
+        /// INTERNAL ONLY
+        ///
+        typedef accumulators::impl::immediate_mean_impl<mpl::_2, tag::weight> impl;
+    };
+    template<typename VariateType, typename VariateTag>
+    struct mean_of_variates
+      : depends_on<count, sum_of_variates<VariateType, VariateTag> >
+    {
+        /// INTERNAL ONLY
+        ///
+        typedef mpl::always<accumulators::impl::mean_impl<VariateType, sum_of_variates<VariateType, VariateTag> > > impl;
+    };
+    template<typename VariateType, typename VariateTag>
+    struct immediate_mean_of_variates
+      : depends_on<count>
+    {
+        /// INTERNAL ONLY
+        ///
+        typedef mpl::always<accumulators::impl::immediate_mean_impl<VariateType, VariateTag> > impl;
+    };
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// extract::mean
+// extract::mean_of_weights
+// extract::mean_of_variates
+//
+namespace extract
+{
+    extractor<tag::mean> const mean = {};
+    extractor<tag::mean_of_weights> const mean_of_weights = {};
+    BOOST_ACCUMULATORS_DEFINE_EXTRACTOR(tag, mean_of_variates, (typename)(typename))
+
+    BOOST_ACCUMULATORS_IGNORE_GLOBAL(mean)
+    BOOST_ACCUMULATORS_IGNORE_GLOBAL(mean_of_weights)
+}
+
+using extract::mean;
+using extract::mean_of_weights;
+using extract::mean_of_variates;
+
+// mean(lazy) -> mean
+template<>
+struct as_feature<tag::mean(lazy)>
+{
+    typedef tag::mean type;
+};
+
+// mean(immediate) -> immediate_mean
+template<>
+struct as_feature<tag::mean(immediate)>
+{
+    typedef tag::immediate_mean type;
+};
+
+// mean_of_weights(lazy) -> mean_of_weights
+template<>
+struct as_feature<tag::mean_of_weights(lazy)>
+{
+    typedef tag::mean_of_weights type;
+};
+
+// mean_of_weights(immediate) -> immediate_mean_of_weights
+template<>
+struct as_feature<tag::mean_of_weights(immediate)>
+{
+    typedef tag::immediate_mean_of_weights type;
+};
+
+// mean_of_variates<VariateType, VariateTag>(lazy) -> mean_of_variates<VariateType, VariateTag>
+template<typename VariateType, typename VariateTag>
+struct as_feature<tag::mean_of_variates<VariateType, VariateTag>(lazy)>
+{
+    typedef tag::mean_of_variates<VariateType, VariateTag> type;
+};
+
+// mean_of_variates<VariateType, VariateTag>(immediate) -> immediate_mean_of_variates<VariateType, VariateTag>
+template<typename VariateType, typename VariateTag>
+struct as_feature<tag::mean_of_variates<VariateType, VariateTag>(immediate)>
+{
+    typedef tag::immediate_mean_of_variates<VariateType, VariateTag> type;
+};
+
+// for the purposes of feature-based dependency resolution,
+// immediate_mean provides the same feature as mean
+template<>
+struct feature_of<tag::immediate_mean>
+  : feature_of<tag::mean>
+{
+};
+
+// for the purposes of feature-based dependency resolution,
+// immediate_mean provides the same feature as mean
+template<>
+struct feature_of<tag::immediate_mean_of_weights>
+  : feature_of<tag::mean_of_weights>
+{
+};
+
+// for the purposes of feature-based dependency resolution,
+// immediate_mean provides the same feature as mean
+template<typename VariateType, typename VariateTag>
+struct feature_of<tag::immediate_mean_of_variates<VariateType, VariateTag> >
+  : feature_of<tag::mean_of_variates<VariateType, VariateTag> >
+{
+};
+
+// So that mean can be automatically substituted with
+// weighted_mean when the weight parameter is non-void.
+template<>
+struct as_weighted_feature<tag::mean>
+{
+    typedef tag::weighted_mean type;
+};
+
+template<>
+struct feature_of<tag::weighted_mean>
+  : feature_of<tag::mean>
+{};
+
+// So that immediate_mean can be automatically substituted with
+// immediate_weighted_mean when the weight parameter is non-void.
+template<>
+struct as_weighted_feature<tag::immediate_mean>
+{
+    typedef tag::immediate_weighted_mean type;
+};
+
+template<>
+struct feature_of<tag::immediate_weighted_mean>
+  : feature_of<tag::immediate_mean>
+{};
+
+// So that mean_of_weights<> can be automatically substituted with
+// weighted_mean_of_variates<> when the weight parameter is non-void.
+template<typename VariateType, typename VariateTag>
+struct as_weighted_feature<tag::mean_of_variates<VariateType, VariateTag> >
+{
+    typedef tag::weighted_mean_of_variates<VariateType, VariateTag> type;
+};
+
+template<typename VariateType, typename VariateTag>
+struct feature_of<tag::weighted_mean_of_variates<VariateType, VariateTag> >
+  : feature_of<tag::mean_of_variates<VariateType, VariateTag> >
+{
+};
+
+// So that immediate_mean_of_weights<> can be automatically substituted with
+// immediate_weighted_mean_of_variates<> when the weight parameter is non-void.
+template<typename VariateType, typename VariateTag>
+struct as_weighted_feature<tag::immediate_mean_of_variates<VariateType, VariateTag> >
+{
+    typedef tag::immediate_weighted_mean_of_variates<VariateType, VariateTag> type;
+};
+
+template<typename VariateType, typename VariateTag>
+struct feature_of<tag::immediate_weighted_mean_of_variates<VariateType, VariateTag> >
+  : feature_of<tag::immediate_mean_of_variates<VariateType, VariateTag> >
+{
+};
+
+////////////////////////////////////////////////////////////////////////////
+//// droppable_accumulator<mean_impl>
+////  need to specialize droppable lazy mean to cache the result at the
+////  point the accumulator is dropped.
+///// INTERNAL ONLY
+/////
+//template<typename Sample, typename SumFeature>
+//struct droppable_accumulator<impl::mean_impl<Sample, SumFeature> >
+//  : droppable_accumulator_base<
+//        with_cached_result<impl::mean_impl<Sample, SumFeature> >
+//    >
+//{
+//    template<typename Args>
+//    droppable_accumulator(Args const &args)
+//      : droppable_accumulator::base(args)
+//    {
+//    }
+//};
+
+}} // namespace boost::accumulators
+
+#endif