#39 #40 git add for last commit

math/random/RandomIterator.h

@@ -0,0 +1,118 @@
+#ifndef RANDOMITERATOR_H
+#define RANDOMITERATOR_H
+
+#include <vector>
+#include <random>
+#include "../../Assertions.h"
+
+namespace Random {
+
+	template <typename Element> class RandomIterator {
+
+	private:
+
+		/** the user's data-vector to randomly iterate */
+		const std::vector<Element>& vec;
+
+		/** the number of random indices */
+		int cnt;
+
+		/** the random number generator */
+		std::minstd_rand gen;
+		bool isRandomized = false;
+
+		/** X random indices */
+		std::vector<int> indices;
+
+
+	public:
+
+		/** ctor */
+		RandomIterator(const std::vector<Element>& vec, const int cnt) : vec(vec), cnt(cnt) {
+
+			//const uint64_t ts = std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::system_clock::now().time_since_epoch()).count();
+			static int seed = 0; ++seed;
+			gen.seed(seed);
+
+			// sanity check
+			if ((int)vec.size() < cnt)	{throw Exception("number of elements in list is smaller than the requested number to draw");}
+			if (cnt == 0)				{throw Exception("number of elements in list must be at least 1");}
+			if (vec.empty())			{throw Exception("empty input vector given");}
+
+			indices.resize(cnt);
+
+		}
+
+		/** create random samples (vector-indicies) that are hereafter available for iteration */
+		void randomize() {
+
+			// random-number generator between [0:size-1]
+			std::uniform_int_distribution<int> dist(0, (int) vec.size()-1);
+
+			// ensure we use each vector-index only ONCE
+			bool used[vec.size()] = {false};
+
+			// draw X random samples
+			for (int i = 0; i < cnt; ) {
+				const int rnd = dist(gen);
+				if (used[rnd]) {continue;}		// already used? try again!
+				used[rnd] = true;				// mark as used
+				indices[i] = rnd;				// add to the index
+				++i;
+			}
+
+			// the vector is setup correctly
+			isRandomized = true;
+
+		}
+
+		/** the iterator state */
+		struct Iterator {
+
+			/** the current position within "indicies" */
+			int pos;
+
+			/** the vector with the user-data to randomly iterate */
+			const std::vector<Element>& vec;
+
+			/** the vector containing the random indices */
+			const std::vector<int>& indices;
+
+			/** ctor */
+			Iterator(const int pos, const std::vector<Element>& vec, const std::vector<int>& indices) : pos(pos), vec(vec), indices(indices) {;}
+
+			/** end-of-iteration? */
+			bool operator != (const Iterator& o) const {return pos != o.pos;}
+
+			/** next value */
+			Iterator& operator ++ () {++pos; return *this;}
+
+			/** get the user-data */
+			Element operator * () {return vec[indices[pos]];}
+
+		};
+
+		//const Element& operator [] (const int idx) const {return vec[indices[idx]]; }
+
+		/** number of available random entries */
+		size_t size() const {return cnt;}
+
+
+		/** for-each access */
+		Iterator begin() const	{ ensureRandomized(); return Iterator(0, vec, indices); }
+
+		/** for-each access */
+		Iterator end() const	{ ensureRandomized(); return Iterator(cnt, vec, indices); }
+
+private:
+
+		/** ensure the coder called randomize() before using the iterator */
+		void ensureRandomized() const {
+			Assert::isTrue(isRandomized, "call randomize() before using the iterator!");
+		}
+
+	};
+
+}
+
+#endif // RANDOMITERATOR_H