From a4c264d8db2f02e7d7bc433e22f571b65450aba1 Mon Sep 17 00:00:00 2001
From: Roland Conybeare <rconybeare@gmail.com>
Date: Tue, 17 Oct 2023 14:54:27 -0400
Subject: [PATCH] randomgen: distribution + related concepts

---
 include/xo/randomgen/distribution_concept.hpp | 35 +++++++++++++
 include/xo/randomgen/generator.hpp            | 49 +++++++++++++++++++
 include/xo/randomgen/normalgen.hpp            | 14 ++++++
 3 files changed, 98 insertions(+)
 create mode 100644 include/xo/randomgen/distribution_concept.hpp
 create mode 100644 include/xo/randomgen/generator.hpp
 create mode 100644 include/xo/randomgen/normalgen.hpp

diff --git a/include/xo/randomgen/distribution_concept.hpp b/include/xo/randomgen/distribution_concept.hpp
new file mode 100644
index 00000000..61c764e0
--- /dev/null
+++ b/include/xo/randomgen/distribution_concept.hpp
@@ -0,0 +1,35 @@
+/* @file distribution_concept.hpp */
+
+#pragma once
+
+#include <concepts>
+
+namespace xo {
+    namespace rng {
+        template <class Distribution, class Engine>
+        concept distribution_concept = requires(Distribution dist, typename Distribution::param_type p) {
+            typename Distribution::result_type;
+            typename Distribution::param_type;
+            { Distribution() };
+            { Distribution(p) };
+            { dist.reset() };
+            { dist.param() };
+            { dist.param(p) };
+            // { dist(g) };   // generator g satisfying engine_concept
+            // { dist(g, p) };
+            { dist.min() };
+            { dist.max() };
+            { dist == dist };
+            { dist != dist };
+            // os << dist
+            // is >> dist
+
+        } && std::copyable<Distribution>
+        && std::copyable<typename Distribution::param_type>
+        && std::equality_comparable<typename Distribution::param_type>;
+    } /*namespace rng*/
+
+} /*namespace xo*/
+
+
+/* end distribution_concept.hpp */
diff --git a/include/xo/randomgen/generator.hpp b/include/xo/randomgen/generator.hpp
new file mode 100644
index 00000000..f35c0994
--- /dev/null
+++ b/include/xo/randomgen/generator.hpp
@@ -0,0 +1,49 @@
+/* @file generator.hpp */
+
+#pragma once
+
+#include "engine_concept.hpp"
+#include "distribution_concept.hpp"
+#include <utility>
+
+namespace xo {
+    namespace rng {
+        /* Engine:        uniform integer random number generator,  e.g. xoshiro256ss
+         * Distribution:  random number distribution,  e.g. std::normal_distribution
+         */
+        template <class Engine, class Distribution> requires engine_concept<Engine> && distribution_concept<Distribution, Engine>
+        class generator {
+        public:
+            using result_type = typename Distribution::result_type;
+            using engine_type = Engine;
+
+        public:
+            generator(Engine & e, Distribution const & d)
+                : engine_{e},
+                  distribution_{d} {}
+            generator(Engine && e, Distribution && d)
+                : engine_{std::move(e)},
+                  distribution_{std::move(d)} {}
+
+            static generator make(Engine e, Distribution d) {
+                return generator(e, d);
+            }
+
+            result_type operator()() { return this->distribution_(this->engine_); }
+
+        private:
+            /* random number generator;  generates uniformly-distributed integers */
+            Engine engine_;
+            /* distribution object */
+            Distribution distribution_;
+        }; /*generator*/
+
+        template <class Engine, class Distribution>
+        generator<Engine, Distribution> make_generator(Engine e, Distribution d) {
+            return generator<Engine, Distribution>::make(std::move(e),
+                                                         std::move(d));
+        } /*make_generator*/
+    } /*namespace rng*/
+} /*namespace xo*/
+
+/* end generator.hpp */
diff --git a/include/xo/randomgen/normalgen.hpp b/include/xo/randomgen/normalgen.hpp
new file mode 100644
index 00000000..dad04328
--- /dev/null
+++ b/include/xo/randomgen/normalgen.hpp
@@ -0,0 +1,14 @@
+/* @file normalgen.hpp */
+
+#pragma once
+
+#include "generator.hpp"
+#include <random>
+
+namespace xo {
+    namespace rng {
+        /* Engine: e.g. xo::rng::xoshiro256 or std::mt19937 */
+        template <class Engine>
+        using normalgen = generator<Engine, std::normal_distribution<double>>;
+    } /*namespace rng*/
+} /*namespace xo*/