Add generic generator modifiers

This means mutiple generic generators and some inference helper
* take(n, generator)
* filter(predicate, generator)
* map(func, generator)
* repeat(generator, repeats)

include/internal/catch_generators.cpp

@@ -16,6 +16,10 @@ namespace Catch {
 
 IGeneratorTracker::~IGeneratorTracker() {}
 
+const char* GeneratorException::what() const noexcept {
+    return m_msg;
+}
+
 namespace Generators {
 
     GeneratorUntypedBase::~GeneratorUntypedBase() {}

include/internal/catch_generators.hpp

@@ -16,8 +16,21 @@
 #include <cassert>
 
 #include <utility>
+#include <exception>
 
 namespace Catch {
+
+class GeneratorException : public std::exception {
+    const char* const m_msg = "";
+
+public:
+    GeneratorException(const char* msg):
+        m_msg(msg)
+    {}
+
+    const char* what() const noexcept override final;
+};
+
 namespace Generators {
 
     // !TBD move this into its own location?
@@ -166,6 +179,175 @@ namespace Generators {
         return makeGenerators( value( T( std::forward<U>( val ) ) ), std::forward<Gs>( moreGenerators )... );
     }
 
+    template <typename T>
+    class TakeGenerator : public IGenerator<T> {
+        GeneratorWrapper<T> m_generator;
+        size_t m_returned = 0;
+        size_t m_target;
+    public:
+        TakeGenerator(size_t target, GeneratorWrapper<T>&& generator):
+            m_generator(std::move(generator)),
+            m_target(target)
+        {
+            assert(target != 0 && "Empty generators are not allowed");
+        }
+        T const& get() const override {
+            return m_generator.get();
+        }
+        bool next() override {
+            ++m_returned;
+            if (m_returned >= m_target) {
+                return false;
+            }
+
+            const auto success = m_generator.next();
+            // If the underlying generator does not contain enough values
+            // then we cut short as well
+            if (!success) {
+                m_returned = m_target;
+            }
+            return success;
+        }
+    };
+
+    template <typename T>
+    GeneratorWrapper<T> take(size_t target, GeneratorWrapper<T>&& generator) {
+        return GeneratorWrapper<T>(pf::make_unique<TakeGenerator<T>>(target, std::move(generator)));
+    }
+
+
+    template <typename T, typename Predicate>
+    class FilterGenerator : public IGenerator<T> {
+        GeneratorWrapper<T> m_generator;
+        Predicate m_predicate;
+    public:
+        template <typename P = Predicate>
+        FilterGenerator(P&& pred, GeneratorWrapper<T>&& generator):
+            m_generator(std::move(generator)),
+            m_predicate(std::forward<P>(pred))
+        {
+            if (!m_predicate(m_generator.get())) {
+                // It might happen that there are no values that pass the
+                // filter. In that case we throw an exception.
+                auto has_initial_value = next();
+                if (!has_initial_value) {
+                    Catch::throw_exception(GeneratorException("No valid value found in filtered generator"));
+                }
+            }
+        }
+
+        T const& get() const override {
+            return m_generator.get();
+        }
+
+        bool next() override {
+            bool success = m_generator.next();
+            if (!success) {
+                return false;
+            }
+            while (!m_predicate(m_generator.get()) && (success = m_generator.next()) == true);
+            return success;
+        }
+    };
+
+
+    template <typename T, typename Predicate>
+    GeneratorWrapper<T> filter(Predicate&& pred, GeneratorWrapper<T>&& generator) {
+        return GeneratorWrapper<T>(std::unique_ptr<IGenerator<T>>(pf::make_unique<FilterGenerator<T, Predicate>>(std::forward<Predicate>(pred), std::move(generator))));
+    }
+
+    template <typename T>
+    class RepeatGenerator : public IGenerator<T> {
+        GeneratorWrapper<T> m_generator;
+        mutable std::vector<T> m_returned;
+        size_t m_target_repeats;
+        size_t m_current_repeat = 0;
+        size_t m_repeat_index = 0;
+    public:
+        RepeatGenerator(size_t repeats, GeneratorWrapper<T>&& generator):
+            m_generator(std::move(generator)),
+            m_target_repeats(repeats)
+        {
+            assert(m_target_repeats > 0 && "Repeat generator must repeat at least once");
+        }
+
+        T const& get() const override {
+            if (m_current_repeat == 0) {
+                m_returned.push_back(m_generator.get());
+                return m_returned.back();
+            }
+            return m_returned[m_repeat_index];
+        }
+
+        bool next() override {
+            // There are 2 basic cases:
+            // 1) We are still reading the generator
+            // 2) We are reading our own cache
+
+            // In the first case, we need to poke the underlying generator.
+            // If it happily moves, we are left in that state, otherwise it is time to start reading from our cache
+            if (m_current_repeat == 0) {
+                const auto success = m_generator.next();
+                if (!success) {
+                    ++m_current_repeat;
+                }
+                return m_current_repeat < m_target_repeats;
+            }
+
+            // In the second case, we need to move indices forward and check that we haven't run up against the end
+            ++m_repeat_index;
+            if (m_repeat_index == m_returned.size()) {
+                m_repeat_index = 0;
+                ++m_current_repeat;
+            }
+            return m_current_repeat < m_target_repeats;
+        }
+    };
+
+    template <typename T>
+    GeneratorWrapper<T> repeat(size_t repeats, GeneratorWrapper<T>&& generator) {
+        return GeneratorWrapper<T>(pf::make_unique<RepeatGenerator<T>>(repeats, std::move(generator)));
+    }
+
+    template <typename T, typename U, typename Func>
+    class MapGenerator : public IGenerator<T> {
+        // TBD: provide static assert for mapping function, for friendly error message
+        GeneratorWrapper<U> m_generator;
+        Func m_function;
+        // To avoid returning dangling reference, we have to save the values
+        T m_cache;
+    public:
+        template <typename F2 = Func>
+        MapGenerator(F2&& function, GeneratorWrapper<U>&& generator) :
+            m_generator(std::move(generator)),
+            m_function(std::forward<F2>(function)),
+            m_cache(m_function(m_generator.get()))
+        {}
+
+        T const& get() const override {
+            return m_cache;
+        }
+        bool next() override {
+            const auto success = m_generator.next();
+            if (success) {
+                m_cache = m_function(m_generator.get());
+            }
+            return success;
+        }
+    };
+
+    template <typename T, typename U, typename Func>
+    GeneratorWrapper<T> map(Func&& function, GeneratorWrapper<U>&& generator) {
+        return GeneratorWrapper<T>(
+            pf::make_unique<MapGenerator<T, U, Func>>(std::forward<Func>(function), std::move(generator))
+        );
+    }
+    template <typename T, typename Func>
+    GeneratorWrapper<T> map(Func&& function, GeneratorWrapper<T>&& generator) {
+        return GeneratorWrapper<T>(
+            pf::make_unique<MapGenerator<T, T, Func>>(std::forward<Func>(function), std::move(generator))
+        );
+    }
 
     auto acquireGeneratorTracker( SourceLineInfo const& lineInfo ) -> IGeneratorTracker&;