First attempt at data generator support

The support is to be considered experimental, that is, the interfaces,
the first party generators and helper functions can change or be removed
at any point in time.

Related to #850

include/internal/catch_generators.cpp

@@ -0,0 +1,45 @@
+/*
+ *  Created by Phil Nash on 15/6/2018.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "catch_generators.hpp"
+#include "catch_random_number_generator.h"
+#include "catch_interfaces_capture.h"
+
+#include <set>
+
+namespace Catch {
+
+IGeneratorTracker::~IGeneratorTracker() {}
+
+namespace Generators {
+
+    GeneratorBase::~GeneratorBase() {}
+
+    std::vector<size_t> randomiseIndices( size_t selectionSize, size_t sourceSize ) {
+
+        assert( selectionSize <= sourceSize );
+        std::vector<size_t> indices;
+        indices.reserve( selectionSize );
+        std::uniform_int_distribution<size_t> uid( 0, sourceSize-1 );
+
+        std::set<size_t> seen;
+        // !TBD: improve this algorithm
+        while( indices.size() < selectionSize ) {
+            auto index = uid( rng() );
+            if( seen.insert( index ).second )
+                indices.push_back( index );
+        }
+        return indices;
+    }
+
+    auto acquireGeneratorTracker( SourceLineInfo const& lineInfo ) -> IGeneratorTracker& {
+        return getResultCapture().acquireGeneratorTracker( lineInfo );
+    }
+
+
+} // namespace Generators
+} // namespace Catch