added exponential distrbution + tests

new shortest-path walker (still todo)
modified dijkstra-path (new helper methods)

math/distribution/Exponential.h

@@ -1,4 +1,52 @@
 #ifndef EXPONENTIAL_H
 #define EXPONENTIAL_H
 
+#include <cmath>
+#include <random>
+
+
+namespace Distribution {
+
+	/** exponential distribution */
+	template <typename T> class Exponential {
+
+	private:
+
+		const T lambda;
+
+		std::minstd_rand gen;
+		std::exponential_distribution<T> dist;
+
+	public:
+
+		/** ctor */
+		Exponential(const T lambda) : lambda(lambda), dist(lambda) {
+			;
+		}
+
+		/** get probability for the given value */
+		T getProbability(const T val) const {
+			return getProbability(lambda, val);
+		}
+
+		/** get an exponentially distributed random number */
+		T draw() {
+			return dist(gen);
+		}
+
+		/** set the seed to use */
+		void setSeed(const long seed) {
+			gen.seed(seed);
+		}
+
+
+		/** get the probability for the given value */
+		static T getProbability(const T lambda, const T val) {
+			return (val < 0) ? (0) : (lambda * std::exp( -lambda * val ));
+		}
+
+	};
+
+}
+
 #endif // EXPONENTIAL_H