text: /*
   * Copyright 2014 Facebook, Inc.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  #ifndef FOLLY_GEN_BASE_H
  #error This file may only be included from folly/gen/Base.h
  #endif
  
  // Ignore shadowing warnings within this file, so includers can use -Wshadow.
  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wshadow"
  
  namespace folly { namespace gen {
  
  /**
   * ArgumentReference - For determining ideal argument type to receive a value.
   */
  template <class T>
  struct ArgumentReference
      : public std::conditional<
            std::is_reference<T>::value,
            T, // T& -> T&, T&& -> T&&, const T& -> const T&
            typename std::conditional<std::is_const<T>::value,
                                      T&, // const int -> const int&
                                      T&& // int -> int&&
                                      >::type> {};
  
  namespace detail {
  
  /*
   * ReferencedSource - Generate values from an STL-like container using
   * iterators from .begin() until .end(). Value type defaults to the type of
   * *container->begin(). For std::vector<int>, this would be int&. Note that the
   * value here is a reference, so the values in the vector will be passed by
   * reference to downstream operators.
   *
   * This type is primarily used through the 'from' helper method, like:
   *
   *   string& longestName = from(names)
   *                       | maxBy([](string& s) { return s.size() });
   */
  template<class Container,
           class Value>
  class ReferencedSource :
      public GenImpl<Value, ReferencedSource<Container, Value>> {
    Container* container_;
  public:
    explicit ReferencedSource(Container* container)
      : container_(container) {}
  
    template<class Body>
    void foreach(Body&& body) const {
      for (auto& value : *container_) {
        body(std::forward<Value>(value));
      }
    }
  
    template<class Handler>
    bool apply(Handler&& handler) const {
      for (auto& value : *container_) {
        if (!handler(std::forward<Value>(value))) {
          return false;
        }
      }
      return true;
    }
  };
  
  /**
   * CopiedSource - For producing values from eagerly from a sequence of values
   * whose storage is owned by this class. Useful for preparing a generator for
   * use after a source collection will no longer be available, or for when the
   * values are specified literally with an initializer list.
   *
   * This type is primarily used through the 'fromCopy' function, like:
   *
   *   auto sourceCopy = fromCopy(makeAVector());
   *   auto sum = sourceCopy | sum;
   *   auto max = sourceCopy | max;
   *
   * Though it is also used for the initializer_list specialization of from().
   */
  template<class StorageType,
           class Container>
  class CopiedSource :
    public GenImpl<const StorageType&,
                   CopiedSource<StorageType, Container>> {
    static_assert(
      !std::is_reference<StorageType>::value, "StorageType must be decayed");
   public:
    // Generator objects are often copied during normal construction as they are
    // encapsulated by downstream generators. It would be bad if this caused
    // a copy of the entire container each time, and since we're only exposing a
    // const reference to the value, it's safe to share it between multiple
    // generators.
    static_assert(
      !std::is_reference<Container>::value,
      "Can't copy into a reference");
    std::shared_ptr<const Container> copy_;
  public:
    typedef Container ContainerType;
  
    template<class SourceContainer>
    explicit CopiedSource(const SourceContainer& container)
      : copy_(new Container(begin(container), end(container))) {}
  
    explicit CopiedSource(Container&& container) :
      copy_(new Container(std::move(container))) {}
  
    // To enable re-use of cached results.
    CopiedSource(const CopiedSource<StorageType, Container>& source)
      : copy_(source.copy_) {}
  
    template<class Body>
    void foreach(Body&& body) const {
      for (const auto& value : *copy_) {
        body(value);
      }
    }
  
    template<class Handler>
    bool apply(Handler&& handler) const {
      // The collection may be reused by others, we can't allow it to be changed.
      for (const auto& value : *copy_) {
        if (!handler(value)) {
          return false;
        }
      }
      return true;
    }
  };
  
  /**
   * RangeSource - For producing values from a folly::Range. Useful for referring
   * to a slice of some container.
   *
   * This type is primarily used through the 'from' function, like:
   *
   *   auto rangeSource = from(folly::range(v.begin(), v.end()));
   *   auto sum = rangeSource | sum;
   *
   * Reminder: Be careful not to invalidate iterators when using ranges like this.
   */
  template<class Iterator>
  class RangeSource : public GenImpl<typename Range<Iterator>::reference,
                                     RangeSource<Iterator>> {
    Range<Iterator> range_;
   public:
    RangeSource() {}
    explicit RangeSource(Range<Iterator> range)
      : range_(std::move(range))
    {}
  
    template<class Handler>
    bool apply(Handler&& handler) const {
      for (auto& value : range_) {
        if (!handler(value)) {
          return false;
        }
      }
      return true;
    }
  
    template<class Body>
    void foreach(Body&& body) const {
      for (auto& value : range_) {
        body(value);
      }
    }
  };
  
  /**
   * Sequence - For generating values from beginning value, incremented along the
   * way with the ++ and += operators. Iteration may continue indefinitely.
   * Value type specified explicitly.
   *
   * This type is primarily used through the 'seq' and 'range' function, like:
   *
   *   int total = seq(1, 10) | sum;
   *   auto indexes = range(0, 10);
   *   auto endless = seq(0); // 0, 1, 2, 3, ...
   */
  template<class Value, class SequenceImpl>
  class Sequence : public GenImpl<const Value&, Sequence<Value, SequenceImpl>> {
    static_assert(!std::is_reference<Value>::value &&
                  !std::is_const<Value>::value, "Value mustn't be const or ref.");
    Value start_;
    SequenceImpl impl_;
  public:
    explicit Sequence(Value start, SequenceImpl impl)
        : start_(std::move(start)), impl_(std::move(impl)) { }
  
    template<class Handler>
    bool apply(Handler&& handler) const {
      for (Value current = start_; impl_.test(current); impl_.step(current)) {
        if (!handler(current)) {
          return false;
        }
      }
      return true;
    }
  
    template<class Body>
    void foreach(Body&& body) const {
      for (Value current = start_; impl_.test(current); impl_.step(current)) {
        body(current);
      }
    }
  };
  
  /**
   * Sequence implementations (range, sequence, infinite, with/without step)
   **/
  template<class Value>
  class RangeImpl {
    Value end_;
   public:
    explicit RangeImpl(Value end) : end_(std::move(end)) { }
    bool test(const Value& current) const { return current < end_; }
    void step(Value& current) const { ++current; }
  };
  
  template<class Value, class Distance>
  class RangeWithStepImpl {
    Value end_;
    Distance step_;
   public:
    explicit RangeWithStepImpl(Value end, Distance step)
      : end_(std::move(end)), step_(std::move(step)) { }
    bool test(const Value& current) const { return current < end_; }
    void step(Value& current) const { current += step_; }
  };
  
  template<class Value>
  class SeqImpl {
    Value end_;
   public:
    explicit SeqImpl(Value end) : end_(std::move(end)) { }
    bool test(const Value& current) const { return current <= end_; }
    void step(Value& current) const { ++current; }
  };
  
  template<class Value, class Distance>
  class SeqWithStepImpl {
    Value end_;
    Distance step_;
   public:
    explicit SeqWithStepImpl(Value end, Distance step)
      : end_(std::move(end)), step_(std::move(step)) { }
    bool test(const Value& current) const { return current <= end_; }
    void step(Value& current) const { current += step_; }
  };
  
  template<class Value>
  class InfiniteImpl {
   public:
    bool test(const Value& current) const { return true; }
    void step(Value& current) const { ++current; }
  };
  
  /**
   * GenratorBuilder - Helper for GENERTATOR macro.
   **/
  template<class Value>
  struct GeneratorBuilder {
    template<class Source,
             class Yield = detail::Yield<Value, Source>>
    Yield operator+(Source&& source) {
      return Yield(std::forward<Source>(source));
    }
  };
  
  /**
   * Yield - For producing values from a user-defined generator by way of a
   * 'yield' function.
   **/
  template<class Value, class Source>
  class Yield : public GenImpl<Value, Yield<Value, Source>> {
    Source source_;
   public:
    explicit Yield(Source source)
      : source_(std::move(source)) {
    }
  
    template<class Handler>
    bool apply(Handler&& handler) const {
      struct Break {};
      auto body = [&](Value value) {
        if (!handler(std::forward<Value>(value))) {
          throw Break();
        }
      };
      try {
        source_(body);
        return true;
      } catch (Break&) {
        return false;
      }
    }
  
    template<class Body>
    void foreach(Body&& body) const {
      source_(std::forward<Body>(body));
    }
  };
  
  template<class Value>
  class Empty : public GenImpl<Value, Empty<Value>> {
   public:
    template <class Handler>
    bool apply(Handler&&) const {
      return true;
    }
  
    template <class Body>
    void foreach(Body&&) const {}
  };
  
  template<class Value>
  class Just : public GenImpl<const Value&, Just<Value>> {
    static_assert(!std::is_reference<Value>::value,
                  "Just requires non-ref types");
    const Value value_;
   public:
    Just(Value value) : value_(std::forward<Value>(value)) {}
  
    template <class Handler>
    bool apply(Handler&& handler) const {
      return handler(value_);
    }
  
    template <class Body>
    void foreach(Body&& body) const {
      body(value_);
    }
  };
  
  /*
   * Operators
   */
  
  /**
   * Map - For producing a sequence of values by passing each value from a source
   * collection through a predicate.
   *
   * This type is usually used through the 'map' or 'mapped' helper function:
   *
   *   auto squares = seq(1, 10) | map(square) | asVector;
   */
  template<class Predicate>
  class Map : public Operator<Map<Predicate>> {
    Predicate pred_;
   public:
    Map() {}
  
    explicit Map(Predicate pred)
      : pred_(std::move(pred))
    { }
  
    template<class Value,
             class Source,
             class Result = typename ArgumentReference<
                              typename std::result_of<Predicate(Value)>::type
                            >::type>
    class Generator :
        public GenImpl<Result, Generator<Value, Source, Result>> {
      Source source_;
      Predicate pred_;
    public:
      explicit Generator(Source source, const Predicate& pred)
        : source_(std::move(source)), pred_(pred) {}
  
      template<class Body>
      void foreach(Body&& body) const {
        source_.foreach([&](Value value) {
          body(pred_(std::forward<Value>(value)));
        });
      }
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        return source_.apply([&](Value value) {
          return handler(pred_(std::forward<Value>(value)));
        });
      }
  
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), pred_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), pred_);
    }
  };
  
  
  /**
   * Filter - For filtering values from a source sequence by a predicate.
   *
   * This type is usually used through the 'filter' helper function, like:
   *
   *   auto nonEmpty = from(strings)
   *                 | filter([](const string& str) -> bool {
   *                     return !str.empty();
   *                   });
   */
  template<class Predicate>
  class Filter : public Operator<Filter<Predicate>> {
    Predicate pred_;
   public:
    Filter() {}
    explicit Filter(Predicate pred)
      : pred_(std::move(pred))
    { }
  
    template<class Value,
             class Source>
    class Generator : public GenImpl<Value, Generator<Value, Source>> {
      Source source_;
      Predicate pred_;
    public:
      explicit Generator(Source source, const Predicate& pred)
        : source_(std::move(source)), pred_(pred) {}
  
      template<class Body>
      void foreach(Body&& body) const {
        source_.foreach([&](Value value) {
          if (pred_(std::forward<Value>(value))) {
            body(std::forward<Value>(value));
          }
        });
      }
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        return source_.apply([&](Value value) -> bool {
          if (pred_(std::forward<Value>(value))) {
            return handler(std::forward<Value>(value));
          }
          return true;
        });
      }
  
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), pred_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), pred_);
    }
  };
  
  /**
   * Until - For producing values from a source until a predicate is satisfied.
   *
   * This type is usually used through the 'until' helper function, like:
   *
   *   auto best = from(sortedItems)
   *             | until([](Item& item) { return item.score > 100; })
   *             | asVector;
   */
  template<class Predicate>
  class Until : public Operator<Until<Predicate>> {
    Predicate pred_;
   public:
    Until() {}
    explicit Until(Predicate pred)
      : pred_(std::move(pred))
    {}
  
    template<class Value,
             class Source>
    class Generator : public GenImpl<Value, Generator<Value, Source>> {
      Source source_;
      Predicate pred_;
     public:
      explicit Generator(Source source, const Predicate& pred)
        : source_(std::move(source)), pred_(pred) {}
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        bool cancelled = false;
        source_.apply([&](Value value) -> bool {
          if (pred_(value)) { // un-forwarded to disable move
            return false;
          }
          if (!handler(std::forward<Value>(value))) {
            cancelled = true;
            return false;
          }
          return true;
        });
        return !cancelled;
      }
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), pred_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), pred_);
    }
  
    // Theoretically an 'until' might stop an infinite
    static constexpr bool infinite = false;
  };
  
  /**
   * Take - For producing up to N values from a source.
   *
   * This type is usually used through the 'take' helper function, like:
   *
   *   auto best = from(docs)
   *             | orderByDescending(scoreDoc)
   *             | take(10);
   */
  class Take : public Operator<Take> {
    size_t count_;
   public:
    explicit Take(size_t count)
      : count_(count) {}
  
    template<class Value,
             class Source>
    class Generator :
        public GenImpl<Value, Generator<Value, Source>> {
      Source source_;
      size_t count_;
    public:
      explicit Generator(Source source, size_t count)
        : source_(std::move(source)) , count_(count) {}
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        if (count_ == 0) { return false; }
        size_t n = count_;
        bool cancelled = false;
        source_.apply([&](Value value) -> bool {
          if (!handler(std::forward<Value>(value))) {
            cancelled = true;
            return false;
          }
          return --n;
        });
        return !cancelled;
      }
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), count_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), count_);
    }
  };
  
  /**
   * Sample - For taking a random sample of N elements from a sequence
   * (without replacement).
   */
  template<class Random>
  class Sample : public Operator<Sample<Random>> {
    size_t count_;
    Random rng_;
   public:
    explicit Sample(size_t count, Random rng)
      : count_(count), rng_(std::move(rng)) {}
  
    template<class Value,
             class Source,
             class Rand,
             class StorageType = typename std::decay<Value>::type>
    class Generator :
            public GenImpl<StorageType&&,
                           Generator<Value, Source, Rand, StorageType>> {
      static_assert(!Source::infinite, "Cannot sample infinite source!");
      // It's too easy to bite ourselves if random generator is only 16-bit
      static_assert(Random::max() >= std::numeric_limits<int32_t>::max() - 1,
                    "Random number generator must support big values");
      Source source_;
      size_t count_;
      mutable Rand rng_;
    public:
      explicit Generator(Source source, size_t count, Random rng)
        : source_(std::move(source)) , count_(count), rng_(std::move(rng)) {}
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        if (count_ == 0) { return false; }
        std::vector<StorageType> v;
        v.reserve(count_);
        // use reservoir sampling to give each source value an equal chance
        // of appearing in our output.
        size_t n = 1;
        source_.foreach([&](Value value) -> void {
            if (v.size() < count_) {
              v.push_back(std::forward<Value>(value));
            } else {
              // alternatively, we could create a std::uniform_int_distribution
              // instead of using modulus, but benchmarks show this has
              // substantial overhead.
              size_t index = rng_() % n;
              if (index < v.size()) {
                v[index] = std::forward<Value>(value);
              }
            }
            ++n;
          });
  
        // output is unsorted!
        for (auto& val: v) {
          if (!handler(std::move(val))) {
            return false;
          }
        }
        return true;
      }
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source, Random>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), count_, rng_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source, Random>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), count_, rng_);
    }
  };
  
  /**
   * Skip - For skipping N items from the beginning of a source generator.
   *
   * This type is usually used through the 'skip' helper function, like:
   *
   *   auto page = from(results)
   *             | skip(pageSize * startPage)
   *             | take(10);
   */
  class Skip : public Operator<Skip> {
    size_t count_;
   public:
    explicit Skip(size_t count)
      : count_(count) {}
  
    template<class Value,
             class Source>
    class Generator :
        public GenImpl<Value, Generator<Value, Source>> {
      Source source_;
      size_t count_;
     public:
      explicit Generator(Source source, size_t count)
        : source_(std::move(source)) , count_(count) {}
  
      template<class Body>
      void foreach(Body&& body) const {
        if (count_ == 0) {
          source_.foreach(body);
          return;
        }
        size_t n = 0;
        source_.foreach([&](Value value) {
            if (n < count_) {
              ++n;
            } else {
              body(std::forward<Value>(value));
            }
          });
      }
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        if (count_ == 0) {
          return source_.apply(std::forward<Handler>(handler));
        }
        size_t n = 0;
        return source_.apply([&](Value value) -> bool {
            if (n < count_) {
              ++n;
              return true;
            }
            return handler(std::forward<Value>(value));
          });
      }
  
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), count_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), count_);
    }
  };
  
  /**
   * Order - For ordering a sequence of values from a source by key.
   * The key is extracted by the given selector functor, and this key is then
   * compared using the specified comparator.
   *
   * This type is usually used through the 'order' helper function, like:
   *
   *   auto closest = from(places)
   *                | orderBy([](Place& p) {
   *                    return -distance(p.location, here);
   *                  })
   *                | take(10);
   */
  template<class Selector, class Comparer>
  class Order : public Operator<Order<Selector, Comparer>> {
    Selector selector_;
    Comparer comparer_;
   public:
    Order() {}
  
    explicit Order(Selector selector)
      : selector_(std::move(selector))
    {}
  
    Order(Selector selector,
          Comparer comparer)
      : selector_(std::move(selector))
      , comparer_(std::move(comparer))
    {}
  
    template<class Value,
             class Source,
             class StorageType = typename std::decay<Value>::type,
             class Result = typename std::result_of<Selector(Value)>::type>
    class Generator :
      public GenImpl<StorageType&&,
                     Generator<Value, Source, StorageType, Result>> {
      static_assert(!Source::infinite, "Cannot sort infinite source!");
      Source source_;
      Selector selector_;
      Comparer comparer_;
  
      typedef std::vector<StorageType> VectorType;
  
      VectorType asVector() const {
        auto comparer = [&](const StorageType& a, const StorageType& b) {
          return comparer_(selector_(a), selector_(b));
        };
        auto vals = source_ | as<VectorType>();
        std::sort(vals.begin(), vals.end(), comparer);
        return std::move(vals);
      }
     public:
      Generator(Source source,
                Selector selector,
                Comparer comparer)
        : source_(std::move(source)),
          selector_(std::move(selector)),
          comparer_(std::move(comparer)) {}
  
      VectorType operator|(const Collect<VectorType>&) const {
        return asVector();
      }
  
      VectorType operator|(const CollectTemplate<std::vector>&) const {
        return asVector();
      }
  
      template<class Body>
      void foreach(Body&& body) const {
        for (auto& value : asVector()) {
          body(std::move(value));
        }
      }
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        auto comparer = [&](const StorageType& a, const StorageType& b) {
          // swapped for minHeap
          return comparer_(selector_(b), selector_(a));
        };
        auto heap = source_ | as<VectorType>();
        std::make_heap(heap.begin(), heap.end(), comparer);
        while (!heap.empty()) {
          std::pop_heap(heap.begin(), heap.end(), comparer);
          if (!handler(std::move(heap.back()))) {
            return false;
          }
          heap.pop_back();
        }
        return true;
      }
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), selector_, comparer_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), selector_, comparer_);
    }
  };
  
  /*
   * TypeAssertion - For verifying the exact type of the value produced by a
   * generator. Useful for testing and debugging, and acts as a no-op at runtime.
   * Pass-through at runtime. Used through the 'assert_type<>()' factory method
   * like so:
   *
   *   auto c =  from(vector) | assert_type<int&>() | sum;
   *
   */
  template<class Expected>
  class TypeAssertion : public Operator<TypeAssertion<Expected>> {
   public:
    template<class Source, class Value>
    const Source& compose(const GenImpl<Value, Source>& source) const {
      static_assert(std::is_same<Expected, Value>::value,
                    "assert_type() check failed");
      return source.self();
    }
  
    template<class Source, class Value>
    Source&& compose(GenImpl<Value, Source>&& source) const {
      static_assert(std::is_same<Expected, Value>::value,
                    "assert_type() check failed");
      return std::move(source.self());
    }
  };
  
  /**
   * Distinct - For filtering duplicates out of a sequence. A selector may be
   * provided to generate a key to uniquify for each value.
   *
   * This type is usually used through the 'distinct' helper function, like:
   *
   *   auto closest = from(results)
   *                | distinctBy([](Item& i) {
   *                    return i.target;
   *                  })
   *                | take(10);
   */
  template<class Selector>
  class Distinct : public Operator<Distinct<Selector>> {
    Selector selector_;
   public:
    Distinct() {}
  
    explicit Distinct(Selector selector)
      : selector_(std::move(selector))
    {}
  
    template<class Value,
             class Source>
    class Generator : public GenImpl<Value, Generator<Value, Source>> {
      Source source_;
      Selector selector_;
  
      typedef typename std::decay<Value>::type StorageType;
  
      // selector_ cannot be passed an rvalue or it would end up passing the husk
      // of a value to the downstream operators.
      typedef const StorageType& ParamType;
  
      typedef typename std::result_of<Selector(ParamType)>::type KeyType;
      typedef typename std::decay<KeyType>::type KeyStorageType;
  
     public:
      Generator(Source source,
                Selector selector)
        : source_(std::move(source)),
          selector_(std::move(selector)) {}
  
      template<class Body>
      void foreach(Body&& body) const {
        std::unordered_set<KeyStorageType> keysSeen;
        source_.foreach([&](Value value) {
          if (keysSeen.insert(selector_(ParamType(value))).second) {
            body(std::forward<Value>(value));
          }
        });
      }
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        std::unordered_set<KeyStorageType> keysSeen;
        return source_.apply([&](Value value) -> bool {
          if (keysSeen.insert(selector_(ParamType(value))).second) {
            return handler(std::forward<Value>(value));
          }
          return true;
        });
      }
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), selector_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), selector_);
    }
  };
  
  /**
   * Composer - Helper class for adapting pipelines into functors. Primarily used
   * for 'mapOp'.
   */
  template<class Operators>
  class Composer {
    Operators op_;
   public:
    explicit Composer(Operators op)
      : op_(std::move(op)) {}
  
    template<class Source,
             class Ret = decltype(std::declval<Operators>()
                                    .compose(std::declval<Source>()))>
    Ret operator()(Source&& source) const {
      return op_.compose(std::forward<Source>(source));
    }
  };
  
  /**
   * Batch - For producing fixed-size batches of each value from a source.
   *
   * This type is usually used through the 'batch' helper function:
   *
   *   auto batchSums
   *     = seq(1, 10)
   *     | batch(3)
   *     | map([](const std::vector<int>& batch) {
   *         return from(batch) | sum;
   *       })
   *     | as<vector>();
   */
  class Batch : public Operator<Batch> {
    size_t batchSize_;
   public:
    explicit Batch(size_t batchSize)
      : batchSize_(batchSize) {
      if (batchSize_ == 0) {
        throw std::invalid_argument("Batch size must be non-zero!");
      }
    }
  
    template<class Value,
             class Source,
             class StorageType = typename std::decay<Value>::type,
             class VectorType = std::vector<StorageType>>
    class Generator :
        public GenImpl<VectorType&,
                       Generator<Value, Source, StorageType, VectorType>> {
      Source source_;
      size_t batchSize_;
    public:
      explicit Generator(Source source, size_t batchSize)
        : source_(std::move(source))
        , batchSize_(batchSize) {}
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        VectorType batch_;
        batch_.reserve(batchSize_);
        bool shouldContinue = source_.apply([&](Value value) -> bool {
            batch_.push_back(std::forward<Value>(value));
            if (batch_.size() == batchSize_) {
              bool needMore = handler(batch_);
              batch_.clear();
              return needMore;
            }
            // Always need more if the handler is not called.
            return true;
          });
        // Flush everything, if and only if `handler` hasn't returned false.
        if (shouldContinue && !batch_.empty()) {
          shouldContinue = handler(batch_);
          batch_.clear();
        }
        return shouldContinue;
      }
  
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), batchSize_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), batchSize_);
    }
  };
  /*
   * Sinks
   */
  
  /**
   * FoldLeft - Left-associative functional fold. For producing an aggregate value
   * from a seed and a folder function. Useful for custom aggregators on a
   * sequence.
   *
   * This type is primarily used through the 'foldl' helper method, like:
   *
   *   double movingAverage = from(values)
   *                        | foldl(0.0, [](double avg, double sample) {
   *                            return sample * 0.1 + avg * 0.9;
   *                          });
   */
  template<class Seed,
           class Fold>
  class FoldLeft : public Operator<FoldLeft<Seed, Fold>> {
    Seed seed_;
    Fold fold_;
   public:
    FoldLeft() {}
    FoldLeft(Seed seed,
             Fold fold)
      : seed_(std::move(seed))
      , fold_(std::move(fold))
    {}
  
    template<class Source,
             class Value>
    Seed compose(const GenImpl<Value, Source>& source) const {
      static_assert(!Source::infinite, "Cannot foldl infinite source");
      Seed accum = seed_;
      source | [&](Value v) {
        accum = fold_(std::move(accum), std::forward<Value>(v));
      };
      return accum;
    }
  };
  
  /**
   * First - For finding the first value in a sequence.
   *
   * This type is primarily used through the 'first' static value, like:
   *
   *   int firstThreeDigitPrime = seq(100) | filter(isPrime) | first;
   */
  class First : public Operator<First> {
   public:
    First() { }
  
    template<class Source,
             class Value,
             class StorageType = typename std::decay<Value>::type>
    StorageType compose(const GenImpl<Value, Source>& source) const {
      Optional<StorageType> accum;
      source | [&](Value v) -> bool {
        accum = std::forward<Value>(v);
        return false;
      };
      if (!accum.hasValue()) {
        throw EmptySequence();
      }
      return std::move(accum.value());
    }
  };
  
  
  /**
   * Any - For determining whether any values in a sequence satisfy a predicate.
   *
   * This type is primarily used through the 'any' static value, like:
   *
   *   bool any20xPrimes = seq(200, 210) | filter(isPrime) | any;
   *
   * Note that it may also be used like so:
   *
   *   bool any20xPrimes = seq(200, 210) | any(isPrime);
   *
   */
  class Any : public Operator<Any> {
   public:
    Any() { }
  
    template<class Source,
             class Value>
    bool compose(const GenImpl<Value, Source>& source) const {
      bool any = false;
      source | [&](Value v) -> bool {
        any = true;
        return false;
      };
      return any;
    }
  
    /**
     * Convenience function for use like:
     *
     *  bool found = gen | any([](int i) { return i * i > 100; });
     */
    template<class Predicate,
             class Filter = Filter<Predicate>,
             class Composed = Composed<Filter, Any>>
    Composed operator()(Predicate pred) const {
      return Composed(Filter(std::move(pred)), Any());
    }
  };
  
  /**
   * All - For determining whether all values in a sequence satisfy a predicate.
   *
   * This type is primarily used through the 'any' static value, like:
   *
   *   bool valid = from(input) | all(validate);
   *
   * Note: Passing an empty sequence through 'all()' will always return true.
   */
  template<class Predicate>
  class All : public Operator<All<Predicate>> {
    Predicate pred_;
   public:
    All() {}
    explicit All(Predicate pred)
      : pred_(std::move(pred))
    { }
  
    template<class Source,
             class Value>
    bool compose(const GenImpl<Value, Source>& source) const {
      static_assert(!Source::infinite, "Cannot call 'all' on infinite source");
      bool all = true;
      source | [&](Value v) -> bool {
        if (!pred_(std::forward<Value>(v))) {
          all = false;
          return false;
        }
        return true;
      };
      return all;
    }
  };
  
  /**
   * Reduce - Functional reduce, for recursively combining values from a source
   * using a reducer function until there is only one item left. Useful for
   * combining values when an empty sequence doesn't make sense.
   *
   * This type is primarily used through the 'reduce' helper method, like:
   *
   *   sring longest = from(names)
   *                 | reduce([](string&& best, string& current) {
   *                     return best.size() >= current.size() ? best : current;
   *                   });
   */
  template<class Reducer>
  class Reduce : public Operator<Reduce<Reducer>> {
    Reducer reducer_;
   public:
    Reduce() {}
    explicit Reduce(Reducer reducer)
      : reducer_(std::move(reducer))
    {}
  
    template<class Source,
             class Value,
             class StorageType = typename std::decay<Value>::type>
    StorageType compose(const GenImpl<Value, Source>& source) const {
      Optional<StorageType> accum;
      source | [&](Value v) {
        if (accum.hasValue()) {
          accum = reducer_(std::move(accum.value()), std::forward<Value>(v));
        } else {
          accum = std::forward<Value>(v);
        }
      };
      if (!accum.hasValue()) {
        throw EmptySequence();
      }
      return accum.value();
    }
  };
  
  /**
   * Count - for simply counting the items in a collection.
   *
   * This type is usually used through its singleton, 'count':
   *
   *   auto shortPrimes = seq(1, 100) | filter(isPrime) | count;
   */
  class Count : public Operator<Count> {
   public:
    Count() { }
  
    template<class Source,
             class Value>
    size_t compose(const GenImpl<Value, Source>& source) const {
      static_assert(!Source::infinite, "Cannot count infinite source");
      return foldl(size_t(0),
                   [](size_t accum, Value v) {
                     return accum + 1;
                   }).compose(source);
    }
  };
  
  /**
   * Sum - For simply summing up all the values from a source.
   *
   * This type is usually used through its singleton, 'sum':
   *
   *   auto gaussSum = seq(1, 100) | sum;
   */
  class Sum : public Operator<Sum> {
   public:
    Sum() : Operator<Sum>() {}
  
    template<class Source,
             class Value,
             class StorageType = typename std::decay<Value>::type>
    StorageType compose(const GenImpl<Value, Source>& source) const {
      static_assert(!Source::infinite, "Cannot sum infinite source");
      return foldl(StorageType(0),
                   [](StorageType&& accum, Value v) {
                     return std::move(accum) + std::forward<Value>(v);
                   }).compose(source);
    }
  };
  
  /**
   * Contains - For testing whether a value matching the given value is contained
   * in a sequence.
   *
   * This type should be used through the 'contains' helper method, like:
   *
   *   bool contained = seq(1, 10) | map(square) | contains(49);
   */
  template<class Needle>
  class Contains : public Operator<Contains<Needle>> {
    Needle needle_;
   public:
    explicit Contains(Needle needle)
      : needle_(std::move(needle))
    {}
  
    template<class Source,
             class Value,
             class StorageType = typename std::decay<Value>::type>
    bool compose(const GenImpl<Value, Source>& source) const {
      static_assert(!Source::infinite,
                    "Calling contains on an infinite source might cause "
                    "an infinite loop.");
      return !(source | [this](Value value) {
          return !(needle_ == std::forward<Value>(value));
        });
    }
  };
  
  /**
   * Min - For a value which minimizes a key, where the key is determined by a
   * given selector, and compared by given comparer.
   *
   * This type is usually used through the singletone 'min' or through the helper
   * functions 'minBy' and 'maxBy'.
   *
   *   auto oldest = from(people)
   *               | minBy([](Person& p) {
   *                   return p.dateOfBirth;
   *                 });
   */
  template<class Selector,
           class Comparer>
  class Min : public Operator<Min<Selector, Comparer>> {
    Selector selector_;
    Comparer comparer_;
   public:
    Min() {}
  
    explicit Min(Selector selector)
      : selector_(std::move(selector))
    {}
  
    Min(Selector selector,
          Comparer comparer)
      : selector_(std::move(selector))
      , comparer_(std::move(comparer))
    {}
  
    template<class Value,
             class Source,
             class StorageType = typename std::decay<Value>::type,
             class Key = typename std::decay<
                 typename std::result_of<Selector(Value)>::type
               >::type>
    StorageType compose(const GenImpl<Value, Source>& source) const {
      Optional<StorageType> min;
      Optional<Key> minKey;
      source | [&](Value v) {
        Key key = selector_(std::forward<Value>(v));
        if (!minKey.hasValue() || comparer_(key, minKey.value())) {
          minKey = key;
          min = std::forward<Value>(v);
        }
      };
      if (!min.hasValue()) {
        throw EmptySequence();
      }
      return min.value();
    }
  };
  
  /**
   * Append - For collecting values from a source into a given output container
   * by appending.
   *
   * This type is usually used through the helper function 'appendTo', like:
   *
   *   vector<int64_t> ids;
   *   from(results) | map([](Person& p) { return p.id })
   *                 | appendTo(ids);
   */
  template<class Collection>
  class Append : public Operator<Append<Collection>> {
    Collection* collection_;
   public:
    explicit Append(Collection* collection)
      : collection_(collection)
    {}
  
    template<class Value,
             class Source>
    Collection& compose(const GenImpl<Value, Source>& source) const {
      source | [&](Value v) {
        collection_->insert(collection_->end(), std::forward<Value>(v));
      };
      return *collection_;
    }
  };
  
  /**
   * Collect - For collecting values from a source in a collection of the desired
   * type.
   *
   * This type is usually used through the helper function 'as', like:
   *
   *   std::string upper = from(stringPiece)
   *                     | map(&toupper)
   *                     | as<std::string>();
   */
  template<class Collection>
  class Collect : public Operator<Collect<Collection>> {
   public:
    Collect() { }
  
    template<class Value,
             class Source,
             class StorageType = typename std::decay<Value>::type>
    Collection compose(const GenImpl<Value, Source>& source) const {
      Collection collection;
      source | [&](Value v) {
        collection.insert(collection.end(), std::forward<Value>(v));
      };
      return collection;
    }
  };
  
  
  /**
   * CollectTemplate - For collecting values from a source in a collection
   * constructed using the specified template type. Given the type of values
   * produced by the given generator, the collection type will be:
   *   Container<Value, Allocator<Value>>
   *
   * The allocator defaults to std::allocator, so this may be used for the STL
   * containers by simply using operators like 'as<set>', 'as<deque>',
   * 'as<vector>'. 'as', here is the helper method which is the usual means of
   * consturcting this operator.
   *
   * Example:
   *
   *   set<string> uniqueNames = from(names) | as<set>();
   */
  template<template<class, class> class Container,
           template<class> class Allocator>
  class CollectTemplate : public Operator<CollectTemplate<Container, Allocator>> {
   public:
    CollectTemplate() { }
  
    template<class Value,
             class Source,
             class StorageType = typename std::decay<Value>::type,
             class Collection = Container<StorageType, Allocator<StorageType>>>
    Collection compose(const GenImpl<Value, Source>& source) const {
      Collection collection;
      source | [&](Value v) {
        collection.insert(collection.end(), std::forward<Value>(v));
      };
      return collection;
    }
  };
  
  /**
   * Concat - For flattening generators of generators.
   *
   * This type is usually used through the 'concat' static value, like:
   *
   *   auto edges =
   *       from(nodes)
   *     | map([](Node& x) {
   *           return from(x.neighbors)
   *                | map([&](Node& y) {
   *                    return Edge(x, y);
   *                  });
   *         })
   *     | concat
   *     | as<std::set>();
   */
  class Concat : public Operator<Concat> {
   public:
    Concat() { }
  
    template<class Inner,
             class Source,
             class InnerValue = typename std::decay<Inner>::type::ValueType>
    class Generator :
        public GenImpl<InnerValue, Generator<Inner, Source, InnerValue>> {
      Source source_;
     public:
      explicit Generator(Source source)
        : source_(std::move(source)) {}
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        return source_.apply([&](Inner inner) -> bool {
            return inner.apply(std::forward<Handler>(handler));
          });
      }
  
      template<class Body>
      void foreach(Body&& body) const {
        source_.foreach([&](Inner inner) {
            inner.foreach(std::forward<Body>(body));
          });
      }
  
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Value,
             class Source,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()));
    }
  
    template<class Value,
             class Source,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self());
    }
  };
  
  /**
   * RangeConcat - For flattening generators of iterables.
   *
   * This type is usually used through the 'rconcat' static value, like:
   *
   *   map<int, vector<int>> adjacency;
   *   auto sinks =
   *       from(adjacency)
   *     | get<1>()
   *     | rconcat()
   *     | as<std::set>();
   */
  class RangeConcat : public Operator<RangeConcat> {
   public:
    RangeConcat() { }
  
    template<class Range,
             class Source,
             class InnerValue = typename ValueTypeOfRange<Range>::RefType>
    class Generator
      : public GenImpl<InnerValue, Generator<Range, Source, InnerValue>> {
      Source source_;
     public:
      explicit Generator(Source source)
        : source_(std::move(source)) {}
  
      template<class Body>
      void foreach(Body&& body) const {
        source_.foreach([&](Range range) {
            for (auto& value : range) {
              body(value);
            }
          });
      }
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        return source_.apply([&](Range range) -> bool {
            for (auto& value : range) {
              if (!handler(value)) {
                return false;
              }
            }
            return true;
          });
      }
    };
  
    template<class Value,
             class Source,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()));
    }
  
    template<class Value,
             class Source,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self());
    }
  };
  
  
  /**
   * GuardImpl - For handling exceptions from downstream computation. Requires the
   * type of exception to catch, and handler function to invoke in the event of
   * the exception. Note that the handler may:
   *   1) return true to continue processing the sequence
   *   2) return false to end the sequence immediately
   *   3) throw, to pass the exception to the next catch
   * The handler must match the signature 'bool(Exception&, Value)'.
   *
   * This type is used through the `guard` helper, like so:
   *
   *  auto indexes
   *    = byLine(STDIN_FILENO)
   *    | guard<std::runtime_error>([](std::runtime_error& e,
   *                                   StringPiece sp) {
   *        LOG(ERROR) << sp << ": " << e.str();
   *        return true; // continue processing subsequent lines
   *      })
   *    | eachTo<int>()
   *    | as<vector>();
   *
   *  TODO(tjackson): Rename this back to Guard.
   **/
  template<class Exception,
           class ErrorHandler>
  class GuardImpl : public Operator<GuardImpl<Exception, ErrorHandler>> {
    ErrorHandler handler_;
   public:
    GuardImpl(ErrorHandler handler)
      : handler_(std::move(handler)) {}
  
    template<class Value,
             class Source>
    class Generator : public GenImpl<Value, Generator<Value, Source>> {
      Source source_;
      ErrorHandler handler_;
    public:
      explicit Generator(Source source,
                         ErrorHandler handler)
        : source_(std::move(source)),
          handler_(std::move(handler)) {}
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        return source_.apply([&](Value value) -> bool {
          try {
            handler(std::forward<Value>(value));
            return true;
          } catch (Exception& e) {
            return handler_(e, std::forward<Value>(value));
          }
        });
      }
  
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Value,
             class Source,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), handler_);
    }
  
    template<class Value,
             class Source,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), handler_);
    }
  };
  
  /**
   * Cycle - For repeating a sequence forever.
   *
   * This type is usually used through the 'cycle' static value, like:
   *
   *   auto tests
   *     = from(samples)
   *     | cycle
   *     | take(100);
   */
  class Cycle : public Operator<Cycle> {
    off_t limit_; // -1 for infinite
   public:
    Cycle()
      : limit_(-1) { }
  
    explicit Cycle(off_t limit)
      : limit_(limit) { }
  
    template<class Value,
             class Source>
    class Generator : public GenImpl<Value, Generator<Value, Source>> {
      Source source_;
      off_t limit_; // -1 for infinite
    public:
      explicit Generator(Source source, off_t limit)
        : source_(std::move(source))
        , limit_(limit) {}
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        bool cont;
        auto handler2 = [&](Value value) {
          cont = handler(std::forward<Value>(value));
          return cont;
        };
        for (off_t count = 0; count != limit_; ++count) {
          cont = false;
          source_.apply(handler2);
          if (!cont) {
            return false;
          }
        }
        return true;
      }
  
      // not actually infinite, since an empty generator will end the cycles.
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()), limit_);
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self(), limit_);
    }
  
    /**
     * Convenience function for use like:
     *
     *  auto tripled = gen | cycle(3);
     */
    Cycle operator()(off_t limit) const {
      return Cycle(limit);
    }
  };
  
  /**
   * Dereference - For dereferencing a sequence of pointers while filtering out
   * null pointers.
   *
   * This type is usually used through the 'dereference' static value, like:
   *
   *   auto refs = from(ptrs) | dereference;
   */
  class Dereference : public Operator<Dereference> {
   public:
    Dereference() {}
  
    template<class Value,
             class Source,
             class Result = decltype(*std::declval<Value>())>
    class Generator : public GenImpl<Result, Generator<Value, Source, Result>> {
      Source source_;
    public:
      explicit Generator(Source source)
        : source_(std::move(source)) {}
  
      template<class Body>
      void foreach(Body&& body) const {
        source_.foreach([&](Value value) {
          if (value) {
            return body(*value);
          }
        });
      }
  
      template<class Handler>
      bool apply(Handler&& handler) const {
        return source_.apply([&](Value value) -> bool {
          if (value) {
            return handler(*value);
          }
          return true;
        });
      }
  
      // not actually infinite, since an empty generator will end the cycles.
      static constexpr bool infinite = Source::infinite;
    };
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(GenImpl<Value, Source>&& source) const {
      return Gen(std::move(source.self()));
    }
  
    template<class Source,
             class Value,
             class Gen = Generator<Value, Source>>
    Gen compose(const GenImpl<Value, Source>& source) const {
      return Gen(source.self());
    }
  };
  
  } //::detail
  
  /**
   * VirtualGen<T> - For wrapping template types in simple polymorphic wrapper.
   **/
  template<class Value>
  class VirtualGen : public GenImpl<Value, VirtualGen<Value>> {
    class WrapperBase {
     public:
      virtual ~WrapperBase() {}
      virtual bool apply(const std::function<bool(Value)>& handler) const = 0;
      virtual void foreach(const std::function<void(Value)>& body) const = 0;
      virtual std::unique_ptr<const WrapperBase> clone() const = 0;
    };
  
    template<class Wrapped>
    class WrapperImpl : public WrapperBase {
      Wrapped wrapped_;
     public:
      explicit WrapperImpl(Wrapped wrapped)
       : wrapped_(std::move(wrapped)) {
      }
  
      virtual bool apply(const std::function<bool(Value)>& handler) const {
        return wrapped_.apply(handler);
      }
  
      virtual void foreach(const std::function<void(Value)>& body) const {
        wrapped_.foreach(body);
      }
  
      virtual std::unique_ptr<const WrapperBase> clone() const {
        return std::unique_ptr<const WrapperBase>(new WrapperImpl(wrapped_));
      }
    };
  
    std::unique_ptr<const WrapperBase> wrapper_;
  
   public:
    template<class Self>
    /* implicit */ VirtualGen(Self source)
     : wrapper_(new WrapperImpl<Self>(std::move(source)))
    { }
  
    VirtualGen(VirtualGen&& source)
     : wrapper_(std::move(source.wrapper_))
    { }
  
    VirtualGen(const VirtualGen& source)
     : wrapper_(source.wrapper_->clone())
    { }
  
    VirtualGen& operator=(const VirtualGen& source) {
      wrapper_.reset(source.wrapper_->clone());
      return *this;
    }
  
    VirtualGen& operator=(VirtualGen&& source) {
      wrapper_= std::move(source.wrapper_);
      return *this;
    }
  
    bool apply(const std::function<bool(Value)>& handler) const {
      return wrapper_->apply(handler);
    }
  
    void foreach(const std::function<void(Value)>& body) const {
      wrapper_->foreach(body);
    }
  };
  
  /**
   * non-template operators, statically defined to avoid the need for anything but
   * the header.
   */
  static const detail::Sum sum;
  
  static const detail::Count count;
  
  static const detail::First first;
  
  /**
   * Use directly for detecting any values, or as a function to detect values
   * which pass a predicate:
   *
   *  auto nonempty = g | any;
   *  auto evens = g | any(even);
   */
  static const detail::Any any;
  
  static const detail::Min<Identity, Less> min;
  
  static const detail::Min<Identity, Greater> max;
  
  static const detail::Order<Identity> order;
  
  static const detail::Distinct<Identity> distinct;
  
  static const detail::Map<Move> move;
  
  static const detail::Concat concat;
  
  static const detail::RangeConcat rconcat;
  
  /**
   * Use directly for infinite sequences, or as a function to limit cycle count.
   *
   *  auto forever = g | cycle;
   *  auto thrice = g | cycle(3);
   */
  static const detail::Cycle cycle;
  
  static const detail::Dereference dereference;
  
  inline detail::Take take(size_t count) {
    return detail::Take(count);
  }
  
  template<class Random = std::default_random_engine>
  inline detail::Sample<Random> sample(size_t count, Random rng = Random()) {
    return detail::Sample<Random>(count, std::move(rng));
  }
  
  inline detail::Skip skip(size_t count) {
    return detail::Skip(count);
  }
  
  inline detail::Batch batch(size_t batchSize) {
    return detail::Batch(batchSize);
  }
  
  }} //folly::gen
  
  #pragma GCC diagnostic pop
  