From 077a95e78b835dbdd39c37efb13533200d64143d Mon Sep 17 00:00:00 2001
From: lisyarus <lisyarus@gmail.com>
Date: Thu, 10 Dec 2020 10:39:04 +0300
Subject: [PATCH] Add hemispherical random distributions

---
 .../psemek/random/uniform_hemiball.hpp        | 60 ++++++++++++++++++
 .../psemek/random/uniform_hemisphere.hpp      | 61 +++++++++++++++++++
 2 files changed, 121 insertions(+)
 create mode 100644 libs/random/include/psemek/random/uniform_hemiball.hpp
 create mode 100644 libs/random/include/psemek/random/uniform_hemisphere.hpp

diff --git a/libs/random/include/psemek/random/uniform_hemiball.hpp b/libs/random/include/psemek/random/uniform_hemiball.hpp
new file mode 100644
index 00000000..25baadd7
--- /dev/null
+++ b/libs/random/include/psemek/random/uniform_hemiball.hpp
@@ -0,0 +1,60 @@
+#pragma once
+
+#include <psemek/random/uniform_hemisphere.hpp>
+
+namespace psemek::random
+{
+
+	template <typename T, std::size_t N>
+	struct uniform_hemiball_vector_distribution
+	{
+		using result_type = geom::vector<T, N>;
+
+		uniform_hemiball_vector_distribution(geom::vector<T, N> const & n, T r = T{1})
+			: sphere_d_{n, r}
+		{}
+
+		uniform_hemiball_vector_distribution(uniform_hemiball_vector_distribution const &) = default;
+
+		template <typename RNG>
+		auto operator()(RNG && rng)
+		{
+			result_type v = sphere_d_(rng);
+			auto r = uniform_real_distribution<T>{}(rng);
+			return v * std::pow(r, T{1} / N);
+		}
+
+		T radius() const { return sphere_d_.radius(); }
+
+	private:
+		uniform_hemisphere_vector_distribution<T, N> sphere_d_;
+	};
+
+	template <typename T, std::size_t N>
+	struct uniform_hemiball_point_distribution
+	{
+		using result_type = geom::point<T, N>;
+
+		uniform_hemiball_point_distribution(result_type origin, geom::vector<T, N> const & n, T r = T{1})
+			: origin_{origin}
+			, vector_d_{n, r}
+		{}
+
+		uniform_hemiball_point_distribution(uniform_hemiball_point_distribution const &) = default;
+
+		template <typename RNG>
+		auto operator()(RNG && rng)
+		{
+			return origin_ + vector_d_(rng);
+		}
+
+		result_type origin() const { return origin_; }
+
+		T radius() const { return vector_d_.radius(); }
+
+	private:
+		result_type origin_;
+		uniform_hemiball_vector_distribution<T, N> vector_d_;
+	};
+
+}
diff --git a/libs/random/include/psemek/random/uniform_hemisphere.hpp b/libs/random/include/psemek/random/uniform_hemisphere.hpp
new file mode 100644
index 00000000..83963917
--- /dev/null
+++ b/libs/random/include/psemek/random/uniform_hemisphere.hpp
@@ -0,0 +1,61 @@
+#pragma once
+
+#include <psemek/random/uniform_sphere.hpp>
+
+namespace psemek::random
+{
+
+	template <typename T, std::size_t N>
+	struct uniform_hemisphere_vector_distribution
+	{
+		using result_type = geom::vector<T, N>;
+
+		uniform_hemisphere_vector_distribution(geom::vector<T, N> const & n, T r = T{1})
+			: d_{r}
+			, n_{n}
+		{}
+
+		uniform_hemisphere_vector_distribution(uniform_hemisphere_vector_distribution const &) = default;
+
+		template <typename RNG>
+		auto operator()(RNG && rng)
+		{
+			result_type v = d_(rng);
+			if (dot(v, n_) < 0)
+				v = -v;
+			return v;
+		}
+
+		T radius() const { return d_.radius(); }
+
+	private:
+		uniform_sphere_vector_distribution<T, N> d_;
+		geom::vector<T, N> n_;
+	};
+
+	template <typename T, std::size_t N>
+	struct uniform_hemisphere_point_distribution
+	{
+		using result_type = geom::point<T, N>;
+
+		uniform_hemisphere_point_distribution(result_type origin, geom::vector<T, N> const & n, T r = T{1})
+			: origin_{origin}
+			, vector_d_{n, r}
+		{}
+
+		template <typename RNG>
+		auto operator()(RNG && rng)
+		{
+			return origin_ + vector_d_(rng);
+		}
+
+		result_type origin() const { return origin_; }
+
+		T radius() const { return vector_d_.radius(); }
+
+	private:
+		result_type origin_;
+		uniform_hemisphere_vector_distribution<T, N> vector_d_;
+	};
+
+}