Add a gradient helper class

libs/geom/include/psemek/geom/easing.hpp

@@ -0,0 +1,83 @@
+#pragma once
+
+#include <psemek/geom/vector.hpp>
+#include <psemek/util/assert.hpp>
+
+#include <vector>
+
+namespace psemek::geom
+{
+
+	enum class easing
+	{
+		nearest,
+		constant_left,
+		constant_right,
+		linear,
+		quadratic_in,
+		quadratic_out,
+		cubic,
+	};
+
+	template <typename T>
+	struct gradient
+	{
+		template <typename ... Args>
+		gradient(Args const & ...  args)
+		{
+			init(args...);
+		}
+
+		gradient(std::vector<geom::vector<T, 2>> points, std::vector<easing> segments)
+		{
+			assert(points.size() == segments.size() + 1);
+			points_ = std::move(points);
+			segments_ = std::move(segments);
+		}
+
+		T operator()(T const & t) const
+		{
+			auto it = std::upper_bound(points_.begin(), points_.end(), t, [](T const & t, auto const & p){ return t < p[0]; });
+			if (it == points_.begin())
+				return points_.front()[1];
+			if (it == points_.end())
+				return points_.back()[1];
+
+			std::size_t i = (it - points_.begin()) - 1;
+
+			T s = (t - points_[i][0]) * (points_[i + 1][1] - points_[i][1]);
+
+			switch (segments_[i]) {
+				case easing::nearest: s = (2 * s < 1) ? 0 : 1; break;
+				case easing::constant_left: s = 0; break;
+				case easing::constant_right: s = 1; break;
+				case easing::linear: break;
+				case easing::quadratic_in: s = s * s; break;
+				case easing::quadratic_out: s = s * (2 - s); break;
+				case easing::cubic: s = s * s * (3 - 2 * s); break;
+				default:
+					assert(false);
+			}
+
+			return points_[i][1] + s * (points_[i + 1][1] - points_[i][1]);
+		}
+
+	private:
+		std::vector<geom::vector<T, 2>> points_;
+		std::vector<easing> segments_;
+
+		void init(geom::vector<T, 2> const & p)
+		{
+			points_.push_back(p);
+		}
+
+		template <typename ... Args>
+		void init(geom::vector<T, 2> const & p, easing t, Args const & ... args)
+		{
+			points_.push_back(p);
+			segments_.push_back(t);
+			init(args...);
+		}
+	};
+
+}