Track.h

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/*!********************************************************************
 
  Audacity: A Digital Audio Editor
 
  @file Track.h
  @brief declares abstract base class Track, TrackList, and iterators over TrackList
 
  Dominic Mazzoni
 
**********************************************************************/
#ifndef __AUDACITY_TRACK__
#define __AUDACITY_TRACK__
 
#include <atomic>
#include <utility>
#include <vector>
#include <list>
#include <functional>
#include <wx/longlong.h>
 
#include "ClientData.h"
#include "Observer.h"
// TrackAttachment needs to be a complete type for the Windows build, though
// not the others, so there is a nested include here:
#include "TrackAttachment.h"
#include "TypeEnumerator.h"
#include "XMLTagHandler.h"
 
#ifdef __WXMSW__
#pragma warning(disable:4284)
#endif
 
class wxTextFile;
class Track;
class ProjectSettings;
class AudacityProject;
 
using TrackArray = std::vector< Track* >;
 
class TrackList;
struct UndoStackElem;
 
using ListOfTracks = std::list< std::shared_ptr< Track > >;
 
 
using TrackNodePointer =
std::pair< ListOfTracks::iterator, ListOfTracks* >;
 
inline bool operator == (const TrackNodePointer &a, const TrackNodePointer &b)
{ return a.second == b.second && a.first == b.first; }
 
inline bool operator != (const TrackNodePointer &a, const TrackNodePointer &b)
{ return !(a == b); }
 
BEGIN_TYPE_ENUMERATION(TrackTypeTag)
 
 
 
#define ENUMERATE_TRACK_TYPE(T) ENUMERATE_TYPE(TrackTypeTag, T)
 
// forward declarations, so we can make them friends
template<typename T>
   std::enable_if_t< std::is_pointer_v<T>, T >
      track_cast(Track *track);
 
template<typename T>
   std::enable_if_t<
      std::is_pointer_v<T> &&
         std::is_const_v< std::remove_pointer_t< T > >,
      T
   >
      track_cast(const Track *track);
 
 
class TrackId
{
public:
   TrackId() : mValue(-1) {}
   explicit TrackId (long value) : mValue(value) {}
 
   bool operator == (const TrackId &other) const
   { return mValue == other.mValue; }
 
   bool operator != (const TrackId &other) const
   { return mValue != other.mValue; }
 
   // Define this in case you want to key a std::map on TrackId
   // The operator does not mean anything else
   bool operator <  (const TrackId &other) const
   { return mValue <  other.mValue; }
 
private:
   long mValue;
};
 
struct TRACK_API TrackIntervalData {
   virtual ~TrackIntervalData();
};
 
 
class ConstTrackInterval {
public:
 
   ConstTrackInterval( double start, double end,
      std::unique_ptr<TrackIntervalData> pExtra = {} )
   : start{ start }, end{ end }, pExtra{ std::move( pExtra ) }
   {
      wxASSERT( start <= end );
   }
 
   ConstTrackInterval( ConstTrackInterval&& ) = default;
   ConstTrackInterval &operator=( ConstTrackInterval&& ) = default;
 
   double Start() const { return start; }
   double End() const { return end; }
   const TrackIntervalData *Extra() const { return pExtra.get(); }
 
private:
   double start, end;
protected:
   // TODO C++17: use std::any instead
   std::unique_ptr< TrackIntervalData > pExtra;
};
 
 
class TrackInterval : public ConstTrackInterval {
public:
   using ConstTrackInterval::ConstTrackInterval;
 
   TrackInterval(TrackInterval&&) = default;
   TrackInterval &operator= (TrackInterval&&) = default;
 
   TrackIntervalData *Extra() const { return pExtra.get(); }
};
 
using AttachedTrackObjects = ClientData::Site<
   Track, TrackAttachment, ClientData::ShallowCopying, std::shared_ptr
>;
 
class TRACK_API Track /* not final */
   : public XMLTagHandler
   , public AttachedTrackObjects
   , public std::enable_shared_from_this<Track> // see SharedPointer()
{
protected:
 
   struct ProtectedCreationArg{};
public:
 
   enum class LinkType : int {
       None = 0, //< No linkage
       Group = 2, //< Tracks are grouped together
       Aligned, //< Tracks are grouped and changes should be synchronized
   };
 
   struct ChannelGroupData;
 
   using ChannelGroupAttachments = ClientData::Site<
      ChannelGroupData, ClientData::Cloneable<>, ClientData::DeepCopying
   >;
 
   // Structure describing data common to channels of a group of tracks
   // Should be deep-copyable (think twice before adding shared pointers!)
   struct TRACK_API ChannelGroupData : ChannelGroupAttachments {
      wxString mName;
      LinkType mLinkType{ LinkType::None };
      bool mSelected{ false };
   };
 
private:
 
   friend class TrackList;
 
 private:
   TrackId mId; 
 
   std::unique_ptr<ChannelGroupData> mpGroupData;
 
 protected:
   std::weak_ptr<TrackList> mList; 
 
   TrackNodePointer mNode{};
   int            mIndex; 
 
 public:
 
   using AttachedObjects = ::AttachedTrackObjects;
 
   TrackId GetId() const { return mId; }
 private:
   void SetId( TrackId id ) { mId = id; }
 public:
 
   // Given a bare pointer, find a shared_ptr.  Undefined results if the track
   // is not yet managed by a shared_ptr.  Undefined results if the track is
   // not really of the subclass.  (That is, trusts the caller and uses static
   // not dynamic casting.)
   template<typename Subclass = Track>
   inline std::shared_ptr<Subclass> SharedPointer()
   {
      // shared_from_this is injected into class scope by base class
      // std::enable_shared_from_this<Track>
      return std::static_pointer_cast<Subclass>( shared_from_this() );
   }
 
   template<typename Subclass = const Track>
   inline auto SharedPointer() const ->
      std::enable_if_t<
         std::is_const_v<Subclass>, std::shared_ptr<Subclass>
      >
   {
      // shared_from_this is injected into class scope by base class
      // std::enable_shared_from_this<Track>
      return std::static_pointer_cast<Subclass>( shared_from_this() );
   }
 
   // Static overloads of SharedPointer for when the pointer may be null
   template<typename Subclass = Track>
   static inline std::shared_ptr<Subclass> SharedPointer( Track *pTrack )
   { return pTrack ? pTrack->SharedPointer<Subclass>() : nullptr; }
 
   template<typename Subclass = const Track>
   static inline std::shared_ptr<Subclass> SharedPointer( const Track *pTrack )
   { return pTrack ? pTrack->SharedPointer<Subclass>() : nullptr; }
 
   // Find anything registered with TrackList::RegisterPendingChangedTrack and
   // not yet cleared or applied; if no such exists, return this track
   std::shared_ptr<Track> SubstitutePendingChangedTrack();
   std::shared_ptr<const Track> SubstitutePendingChangedTrack() const;
 
   // If this track is a pending changed track, return the corresponding
   // original; else return this track
   std::shared_ptr<const Track> SubstituteOriginalTrack() const;
 
   using IntervalData = TrackIntervalData;
   using Interval = TrackInterval;
   using Intervals = std::vector< Interval >;
   using ConstInterval = ConstTrackInterval;
   using ConstIntervals = std::vector< ConstInterval >;
 
 
   struct TypeNames {
      wxString info; 
      wxString property; 
      TranslatableString name; 
   };
   struct TypeInfo {
      TypeNames names;
      bool concrete = false;
      const TypeInfo *pBaseInfo = nullptr;
 
      bool IsBaseOf(const TypeInfo &other) const
      {
         for (auto pInfo = &other;
              pInfo; pInfo = pInfo->pBaseInfo)
            if (this == pInfo)
               return true;
         return false;
      }
   };
   virtual const TypeInfo &GetTypeInfo() const = 0;
   static const TypeInfo &ClassTypeInfo();
   virtual const TypeNames &GetTypeNames() const
   { return GetTypeInfo().names; }
 
   virtual bool SupportsBasicEditing() const;
 
   using Holder = std::shared_ptr<Track>;
 
 
   virtual Holder PasteInto( AudacityProject & ) const = 0;
 
 
   virtual ConstIntervals GetIntervals() const;
 
   virtual Intervals GetIntervals();
 
 public:
   mutable std::pair<int, int> vrulerSize;
 
public:
   static void FinishCopy (const Track *n, Track *dest);
 
 
   virtual bool LinkConsistencyFix(bool doFix = true, bool completeList = true);
 
   bool LinkConsistencyCheck(bool completeList)
   { return const_cast<Track*>(this)->LinkConsistencyFix(false, completeList); }
 
   bool HasOwner() const { return static_cast<bool>(GetOwner());}
 
   std::shared_ptr<TrackList> GetOwner() const { return mList.lock(); }
 
   LinkType GetLinkType() const noexcept;
   bool IsAlignedWithLeader() const;
 
   ChannelGroupData &GetGroupData();
   const ChannelGroupData &GetGroupData() const;
 
protected:
   
   void SetLinkType(LinkType linkType, bool completeList = true);
 
private:
   int GetIndex() const;
   void SetIndex(int index);
 
   ChannelGroupData &MakeGroupData();
   void DoSetLinkType(LinkType linkType, bool completeList = true);
 
   Track* GetLinkedTrack() const;
   bool HasLinkedTrack() const noexcept;
   
   TrackNodePointer GetNode() const;
   void SetOwner
      (const std::weak_ptr<TrackList> &list, TrackNodePointer node);
 
 // Keep in Track
 
 protected:
   double              mOffset;
 
 public:
 
   Track();
   Track(const Track &orig, ProtectedCreationArg&&);
   Track& operator =(const Track &orig) = delete;
 
   virtual ~ Track();
 
   void Init(const Track &orig);
 
   // public nonvirtual duplication function that invokes Clone():
   virtual Holder Duplicate() const;
 
   const wxString &GetName() const;
   void SetName( const wxString &n );
 
   bool GetSelected() const;
   virtual void SetSelected(bool s);
 
   // The argument tells whether the last undo history state should be
   // updated for the appearance change
   void EnsureVisible( bool modifyState = false );
 
public:
 
   virtual double GetOffset() const = 0;
 
   void Offset(double t) { SetOffset(GetOffset() + t); }
   virtual void SetOffset (double o) { mOffset = o; }
 
   // Create a NEW track and modify this track
   // Return non-NULL or else throw
   // May assume precondition: t0 <= t1
   virtual Holder Cut(double WXUNUSED(t0), double WXUNUSED(t1)) = 0;
 
   // Create a NEW track and don't modify this track
   // Return non-NULL or else throw
   // Note that subclasses may want to distinguish tracks stored in a clipboard
   // from those stored in a project
   // May assume precondition: t0 <= t1
   // Should invoke Track::Init
   virtual Holder Copy
      (double WXUNUSED(t0), double WXUNUSED(t1), bool forClipboard = true) const = 0;
 
   // May assume precondition: t0 <= t1
   virtual void Clear(double WXUNUSED(t0), double WXUNUSED(t1)) = 0;
 
   virtual void Paste(double WXUNUSED(t), const Track * WXUNUSED(src)) = 0;
 
   // This can be used to adjust a sync-lock selected track when the selection
   // is replaced by one of a different length.
   virtual void SyncLockAdjust(double oldT1, double newT1);
 
   // May assume precondition: t0 <= t1
   virtual void Silence(double WXUNUSED(t0), double WXUNUSED(t1)) = 0;
 
   // May assume precondition: t0 <= t1
   virtual void InsertSilence(double WXUNUSED(t), double WXUNUSED(len)) = 0;
 
private:
   // Subclass responsibility implements only a part of Duplicate(), copying
   // the track data proper (not associated data such as for groups and views):
   virtual Holder Clone() const = 0;
 
   template<typename T>
      friend std::enable_if_t< std::is_pointer_v<T>, T >
         track_cast(Track *track);
   template<typename T>
      friend std::enable_if_t<
         std::is_pointer_v<T> &&
            std::is_const_v< std::remove_pointer_t< T > >,
         T
      >
         track_cast(const Track *track);
 
public:
   bool SameKindAs(const Track &track) const
      { return &GetTypeInfo() == &track.GetTypeInfo(); }
 
   template < typename R = void >
   using Continuation = std::function< R() >;
   using Fallthrough = Continuation<>;
   
private:
   template< typename ...Params >
   struct Executor{};
 
 
   struct Dispatcher {
      template< typename Tag, typename R, typename ArgumentType,
                typename Function, typename ...Functions >
      struct inapplicable
      {
         using QualifiedTrackType =
            std::conditional_t< std::is_const_v<ArgumentType>,
               const Track, Track >;
   
         using Tail = Executor< Tag, R, ArgumentType, Functions... >;
         enum : unsigned { SetUsed = Tail::SetUsed << 1 };
 
         R operator ()
            (QualifiedTrackType *pTrack,
             const Function &, const Functions &...functions) const
         { return Tail{}( pTrack, functions... ); }
      };
 
      template< typename R, typename BaseClass, typename ArgumentType,
                typename Function >
      struct applicable1
      {
         using QualifiedTrackType =
            std::conditional_t< std::is_const_v<ArgumentType>,
               const Track, Track >;
         using QualifiedBaseClass =
            std::conditional_t< std::is_const_v<ArgumentType>,
               const BaseClass, BaseClass >;
   
         enum : unsigned { SetUsed = 1u };
 
         R operator ()
            (QualifiedTrackType *pTrack, const Function &function, ...) const
         { return function( static_cast<QualifiedBaseClass *>(pTrack) ); }
      };
 
      template< typename Tag,
                typename R, typename BaseClass, typename ArgumentType,
                typename Function, typename ...Functions >
      struct applicable2
      {
         using QualifiedTrackType =
            std::conditional_t< std::is_const_v<ArgumentType>,
               const Track, Track >;
         using QualifiedBaseClass =
            std::conditional_t< std::is_const_v<ArgumentType>,
               const BaseClass, BaseClass >;
   
         using Tail = Executor< Tag, R, ArgumentType, Functions... >;
         enum : unsigned { SetUsed = (Tail::SetUsed << 1) | 1u };
 
         R operator ()
            (QualifiedTrackType *pTrack, const Function &function,
             const Functions &...functions) const
         {
            auto continuation = Continuation<R>{ [&] {
               return Tail{}( pTrack, functions... );
            } };
            return function( static_cast<QualifiedBaseClass *>(pTrack),
               continuation );
         }
      };
 
      template< typename ... > struct Switch {};
 
 
      template<typename Tag, typename R, typename ArgumentType>
      struct Switch<Tag, R, ArgumentType, TypeList::Nil>
      {
         template< typename Function, typename ...Functions >
            static auto test()
               -> inapplicable< Tag, R, ArgumentType, Function, Functions... >;
      };
 
 
      template<typename Tag, typename R, typename ArgumentType,
         typename BaseClass, typename ...BaseClasses>
      struct Switch<Tag, R, ArgumentType,
         TypeList::List<BaseClass, BaseClasses...>>
      {
         using QualifiedBaseClass =
            std::conditional_t< std::is_const_v<ArgumentType>,
               const BaseClass, BaseClass >;
 
         using Retry =
            Switch<Tag, R, ArgumentType, TypeList::List<BaseClasses...>>;
 
 
         template< typename Function, typename ...Functions >
            static auto test( const void * )
               -> decltype( Retry::template test< Function, Functions... >() );
 
 
         template< typename Function, typename ...Functions >
            static auto test( std::true_type * )
               -> decltype(
                  (void) std::declval<Function>()
                     ( (QualifiedBaseClass*)nullptr ),
                  applicable1< R, BaseClass, ArgumentType, Function>{}
               );
 
 
         template< typename Function, typename ...Functions >
            static auto test( std::true_type * )
               -> decltype(
                  (void) std::declval<Function>()
                     ( (QualifiedBaseClass*)nullptr,
                       std::declval< Continuation<R> >() ),
                  applicable2< Tag, R, BaseClass, ArgumentType,
                               Function, Functions... >{}
               );
 
         static constexpr bool Compatible =
            std::is_base_of_v<BaseClass, ArgumentType>;
         template< typename Function, typename ...Functions >
            static auto test()
               -> decltype(
                  test< Function, Functions... >(
                     (std::integral_constant<bool, Compatible>*)nullptr) );
      };
   };
 
   template< typename Tag, typename R, typename ArgumentType >
   struct Executor< Tag, R, ArgumentType >
   {
      using NominalType = ArgumentType;
      enum : unsigned { SetUsed = 0 };
      R operator () (const void *, ...)
      {
         if constexpr (std::is_void_v<R>)
            return;
         else
            return R{};
      }
   };
 
 
   template< typename Tag, typename R, typename ArgumentType,
             typename Function, typename ...Functions >
   struct Executor< Tag, R, ArgumentType, Function, Functions... >
      : decltype(
         Dispatcher::Switch<Tag, R, ArgumentType,
            // Each track subtype occurs earlier than its base classes in this
            // list of types
            TypeList::Reverse_t<
               typename TypeEnumerator::CollectTypes<TrackTypeTag, Tag>::type>
         >::template test<Function, Functions... >())
   {
      using NominalType = ArgumentType;
   };
 
public:
 
   template<typename TrackType>
   static void checkTrackType()
   {
      static_assert(
         std::is_same_v<Track, TrackType> ||
         std::is_same_v<const Track, TrackType>, "Error" );
   }
   template<typename R, typename TrackType, typename... Functions>
   static R CallExecutor(R*, std::tuple<>*, TrackType&, const Functions&...)
   {
      // This overload is needed so that the other overload of CallExecutor
      // compiles, but it should never be reached at run-time, because an
      // Executor generated for (const) Track should have been the catch-all.
      wxASSERT(false);
      checkTrackType<TrackType>();
      if constexpr (std::is_void_v<R>)
         return;
      else
         return R{};
   }
   template<
      typename R, typename TrackType, typename... Functions,
      typename Executor, typename... Executors>
   static R CallExecutor(
      R*, std::tuple<Executor, Executors...>*, TrackType &track,
      const Functions &...functions)
   {
      checkTrackType<TrackType>();
      const auto &info = Executor::NominalType::ClassTypeInfo();
      // Dynamic type test of track
      // Assumes Executor classes are sequenced with more specific accepted
      // types earlier
      if ( info.IsBaseOf(track.GetTypeInfo()) )
         // Dispatch to an Executor that knows which of functions applies
         return Executor{}(&track, functions...);
      else
         // Recur, with fewer candidate Executors and all of functions
         return CallExecutor( (R*)nullptr,
            (std::tuple<Executors...>*)nullptr, track, functions...);
   }
 
   template<typename ...Executors>
   static constexpr unsigned UsedCases(std::tuple<Executors...>*)
   {
      return (Executors::SetUsed | ...); // My very first fold expression :-)
   }
 
   template<
      typename Tag,
      bool IsConst,
      typename R,
      typename ...TrackTypes,
      typename ...Functions
   >
   static R DoTypeSwitch(
      std::conditional_t<IsConst, const Track, Track> &track,
      TypeList::List<TrackTypes...>,
      const Functions &...functions )
   {
      // Generate Executor classes, for each of TrackTypes,
      // each zero-sized and with an operator () that calls the correct
      // one of functions, assuming the track is of the corresponding type
      using Executors = std::tuple< Executor<
         Tag, R,
         std::conditional_t<IsConst, const TrackTypes, TrackTypes>,
         Functions...
      >... >;
      // Don't even construct the tuple of zero-sized types, just point
      constexpr Executors *executors = nullptr;
 
      // Compile time reachability check of the given functions
      enum { All = sizeof...( functions ) };
      static_assert( (1u << All) - 1u == UsedCases(executors),
         "Uncallable case in Track::TypeSwitch");
 
      // Do dynamic dispatch to one of the Executors
      return CallExecutor((R *)nullptr, executors, track, functions...);
   }
 
 
   template<
      typename R = void,
      typename ...Functions
   >
   R TypeSwitch(const Functions &...functions)
   {
      struct Tag : TrackTypeTag {};
      // Collect all concrete and abstract track types known at compile time
      using TrackTypes = TypeList::Reverse_t<
         typename TypeEnumerator::CollectTypes<TrackTypeTag, Tag>::type>;
      // Generate a function that dispatches dynamically on track type
      return DoTypeSwitch<Tag, false, R>(*this, TrackTypes{}, functions...);
   }
 
   template<
      typename R = void,
      typename ...Functions
   >
   R TypeSwitch(const Functions &...functions) const
   {
      struct Tag : TrackTypeTag {};
      // Collect all concrete and abstract track types known at compile time
      using TrackTypes = TypeList::Reverse_t<
         typename TypeEnumerator::CollectTypes<TrackTypeTag, Tag>::type>;
      // Generate a function that dispatches dynamically on track type
      return DoTypeSwitch<Tag, true, R>(*this, TrackTypes{}, functions...);
   }
 
   // XMLTagHandler callback methods -- NEW virtual for writing
   virtual void WriteXML(XMLWriter &xmlFile) const = 0;
 
   // Returns true if an error was encountered while trying to
   // open the track from XML
   virtual bool GetErrorOpening() { return false; }
 
   virtual double GetStartTime() const = 0;
   virtual double GetEndTime() const = 0;
 
   // Send a notification to subscribers when state of the track changes
   // To do: define values for the argument to distinguish different parts
   // of the state
   void Notify(bool allChannels, int code = -1);
 
   // An always-true predicate useful for defining iterators
   bool Any() const;
 
   // Frequently useful operands for + and -
   bool IsSelected() const;
   bool IsLeader() const;
   bool IsSelectedLeader() const;
 
   // Cause this track and following ones in its TrackList to adjust
   void AdjustPositions();
 
   // Serialize, not with tags of its own, but as attributes within a tag.
   void WriteCommonXMLAttributes(
      XMLWriter &xmlFile, bool includeNameAndSelected = true) const;
 
   // Return true iff the attribute is recognized.
   bool HandleCommonXMLAttribute(const std::string_view& attr, const XMLAttributeValueView& valueView);
};
 
ENUMERATE_TRACK_TYPE(Track);
 
 
template<typename T>
   inline std::enable_if_t< std::is_pointer_v<T>, T >
      track_cast(Track *track)
{
   using BareType = std::remove_pointer_t< T >;
   if (track &&
       BareType::ClassTypeInfo().IsBaseOf(track->GetTypeInfo() ))
      return reinterpret_cast<T>(track);
   else
      return nullptr;
}
 
template<typename T>
   inline std::enable_if_t<
      std::is_pointer_v<T> && std::is_const_v< std::remove_pointer_t< T > >,
      T
   >
      track_cast(const Track *track)
{
   using BareType = std::remove_pointer_t< T >;
   if (track &&
       BareType::ClassTypeInfo().IsBaseOf(track->GetTypeInfo() ))
      return reinterpret_cast<T>(track);
   else
      return nullptr;
}
 
template < typename TrackType > struct TrackIterRange;
 
 
template <
   typename TrackType
> class TrackIter
   : public ValueIterator< TrackType *, std::bidirectional_iterator_tag >
{
public:
   using FunctionType = std::function< bool(
      std::add_pointer_t< std::add_const_t< std::remove_pointer_t<TrackType> > >
   ) >;
 
   TrackIter(
         TrackNodePointer begin, 
         TrackNodePointer iter, 
         TrackNodePointer end, 
         FunctionType pred = {} 
   )
      : mBegin( begin ), mIter( iter ), mEnd( end )
      , mPred( std::move(pred) )
   {
      // Establish the class invariant
      if (this->mIter != this->mEnd && !this->valid())
         this->operator ++ ();
   }
 
 
   template < typename Predicate2 >
   TrackIter Filter( const Predicate2 &pred2 ) const
   {
      return { this->mBegin, this->mIter, this->mEnd, pred2 };
   }
 
 
   template < typename TrackType2 >
      auto Filter() const
         -> std::enable_if_t<
            std::is_base_of_v< TrackType, TrackType2 > &&
               (!std::is_const_v<TrackType> ||
                 std::is_const_v<TrackType2>),
            TrackIter< TrackType2 >
         >
   {
      return { this->mBegin, this->mIter, this->mEnd, this->mPred };
   }
 
   const FunctionType &GetPredicate() const
   { return this->mPred; }
 
 
   TrackIter &operator ++ ()
   {
      // Maintain the class invariant
      if (this->mIter != this->mEnd) do
         ++this->mIter.first;
      while (this->mIter != this->mEnd && !this->valid() );
      return *this;
   }
 
   TrackIter operator ++ (int)
   {
      TrackIter result { *this };
      this-> operator ++ ();
      return result;
   }
 
 
   TrackIter &operator -- ()
   {
      // Maintain the class invariant
      do {
         if (this->mIter == this->mBegin)
            // Go circularly
            this->mIter = this->mEnd;
         else
            --this->mIter.first;
      } while (this->mIter != this->mEnd && !this->valid() );
      return *this;
   }
 
   TrackIter operator -- (int)
   {
      TrackIter result { *this };
      this->operator -- ();
      return result;
   }
 
 
   TrackType *operator * () const
   {
      if (this->mIter == this->mEnd)
         return nullptr;
      else
         // Other methods guarantee that the cast is correct
         // (provided no operations on the TrackList invalidated
         // underlying iterators or replaced the tracks there)
         return static_cast< TrackType * >( &**this->mIter.first );
   }
 
   TrackIter advance(
      long amount 
   ) const
   {
      auto copy = *this;
      std::advance( copy, amount );
      return copy;
   }
 
   friend inline bool operator == (TrackIter a, TrackIter b)
   {
      // Assume the predicate is not stateful.  Just compare the iterators.
      return
         a.mIter == b.mIter
         // Assume this too:
         // && a.mBegin == b.mBegin && a.mEnd == b.mEnd
      ;
   }
 
   friend inline bool operator != (TrackIter a, TrackIter b)
   {
      return !(a == b);
   }
 
private:
   bool valid() const
   {
      // assume mIter != mEnd
      const auto pTrack = track_cast< TrackType * >( &**this->mIter.first );
      if (!pTrack)
         return false;
      return !this->mPred || this->mPred( pTrack );
   }
 
   friend TrackIterRange< TrackType >;
 
   TrackNodePointer
      mBegin, 
      mIter, 
      mEnd; 
   FunctionType mPred; 
};
 
template <
   typename TrackType // Track or a subclass, maybe const-qualified
> struct TrackIterRange
   : public IteratorRange< TrackIter< TrackType > >
{
   TrackIterRange
      ( const TrackIter< TrackType > &begin,
        const TrackIter< TrackType > &end )
         : IteratorRange< TrackIter< TrackType > >
            ( begin, end )
   {}
 
   // Conjoin the filter predicate with another predicate
   // Read + as "and"
   template< typename Predicate2 >
      TrackIterRange operator + ( const Predicate2 &pred2 ) const
   {
      const auto &pred1 = this->first.GetPredicate();
      using Function = typename TrackIter<TrackType>::FunctionType;
      const auto &newPred = pred1
         ? Function{ [=] (typename Function::argument_type track) {
            return pred1(track) && pred2(track);
         } }
         : Function{ pred2 };
      return {
         this->first.Filter( newPred ),
         this->second.Filter( newPred )
      };
   }
 
   // Specify the added conjunct as a pointer to member function
   // Read + as "and"
   template< typename R, typename C >
      TrackIterRange operator + ( R ( C ::* pmf ) () const ) const
   {
      return this->operator + ( std::mem_fn( pmf ) );
   }
 
   // Conjoin the filter predicate with the negation of another predicate
   // Read - as "and not"
   template< typename Predicate2 >
      TrackIterRange operator - ( const Predicate2 &pred2 ) const
   {
      using ArgumentType =
         typename TrackIterRange::iterator::FunctionType::argument_type;
      auto neg = [=] (ArgumentType track) { return !pred2( track ); };
      return this->operator + ( neg );
   }
 
   // Specify the negated conjunct as a pointer to member function
   // Read - as "and not"
   template< typename R, typename C >
      TrackIterRange operator - ( R ( C ::* pmf ) () const ) const
   {
      return this->operator + ( std::not1( std::mem_fn( pmf ) ) );
   }
 
   template< typename TrackType2 >
      TrackIterRange< TrackType2 > Filter() const
   {
      return {
         this-> first.template Filter< TrackType2 >(),
         this->second.template Filter< TrackType2 >()
      };
   }
 
   TrackIterRange StartingWith( const Track *pTrack ) const
   {
      auto newBegin = this->find( pTrack );
      // More careful construction is needed so that the independent
      // increment and decrement of each iterator in the NEW pair
      // has the expected behavior at boundaries of the range
      return {
         { newBegin.mIter, newBegin.mIter,    this->second.mEnd,
           this->first.GetPredicate() },
         { newBegin.mIter, this->second.mIter, this->second.mEnd,
           this->second.GetPredicate() }
      };
   }
 
   TrackIterRange EndingAfter( const Track *pTrack ) const
   {
      const auto newEnd = this->reversal().find( pTrack ).base();
      // More careful construction is needed so that the independent
      // increment and decrement of each iterator in the NEW pair
      // has the expected behavior at boundaries of the range
      return {
         { this->first.mBegin, this->first.mIter, newEnd.mIter,
           this->first.GetPredicate() },
         { this->first.mBegin, newEnd.mIter,      newEnd.mIter,
           this->second.GetPredicate() }
      };
   }
 
   // Exclude one given track
   TrackIterRange Excluding ( const TrackType *pExcluded ) const
   {
      return this->operator - (
         [=](const Track *pTrack){ return pExcluded == pTrack; } );
   }
 
   template< typename ...Functions >
   void Visit(const Functions &...functions)
   {
      for (auto track : *this)
         track->TypeSwitch(functions...);
   }
 
 
   template< typename Flag, typename ...Functions >
   void VisitWhile(Flag &flag, const Functions &...functions)
   {
      if ( flag ) for (auto track : *this) {
         track->TypeSwitch(functions...);
         if (!flag)
            break;
      }
   }
};
 
 
struct TrackListEvent
{
   enum Type {
      SELECTION_CHANGE,
 
      TRACK_DATA_CHANGE,
 
      TRACK_REQUEST_VISIBLE,
 
 
      PERMUTED,
 
      RESIZING,
 
      ADDITION,
 
 
      DELETION,
   };
 
   TrackListEvent( Type type,
      const std::weak_ptr<Track> &pTrack = {}, int extra = -1)
      : mType{ type }
      , mpTrack{ pTrack }
      , mExtra{ extra }
   {}
 
   TrackListEvent( const TrackListEvent& ) = default;
 
   const Type mType;
   const std::weak_ptr<Track> mpTrack;
   const int mExtra;
};
 
class TRACK_API TrackList final
   : public Observer::Publisher<TrackListEvent>
   , public ListOfTracks
   , public std::enable_shared_from_this<TrackList>
   , public ClientData::Base
{
   // privatize this, make you use Add instead:
   using ListOfTracks::push_back;
 
   // privatize this, make you use Swap instead:
   using ListOfTracks::swap;
 
   // Disallow copy
   TrackList(const TrackList &that) = delete;
   TrackList &operator= (const TrackList&) = delete;
 
   // No need for move, disallow it
   TrackList(TrackList &&that) = delete;
   TrackList& operator= (TrackList&&) = delete;
 
   void clear() = delete;
 
 public:
   static TrackList &Get( AudacityProject &project );
   static const TrackList &Get( const AudacityProject &project );
 
   static TrackList *FindUndoTracks(const UndoStackElem &state);
 
   // Create an empty TrackList
   // Don't call directly -- use Create() instead
   explicit TrackList( AudacityProject *pOwner );
 
   // Create an empty TrackList
   static std::shared_ptr<TrackList> Create( AudacityProject *pOwner );
 
   // Move is defined in terms of Swap
   void Swap(TrackList &that);
 
   // Destructor
   virtual ~TrackList();
 
   // Find the owning project, which may be null
   AudacityProject *GetOwner() { return mOwner; }
   const AudacityProject *GetOwner() const { return mOwner; }
 
   wxString MakeUniqueTrackName(const wxString& baseTrackName) const;
 
   // Iteration
 
   // Hide the inherited begin() and end()
   using iterator = TrackIter<Track>;
   using const_iterator = TrackIter<const Track>;
   using value_type = Track *;
   iterator begin() { return Any().begin(); }
   iterator end() { return Any().end(); }
   const_iterator begin() const { return Any().begin(); }
   const_iterator end() const { return Any().end(); }
   const_iterator cbegin() const { return begin(); }
   const_iterator cend() const { return end(); }
 
   template < typename TrackType = Track >
      auto Find(Track *pTrack)
         -> TrackIter< TrackType >
   {
      if (!pTrack || pTrack->GetOwner().get() != this)
         return EndIterator<TrackType>();
      else
         return MakeTrackIterator<TrackType>( pTrack->GetNode() );
   }
 
 
   template < typename TrackType = const Track >
      auto Find(const Track *pTrack) const
         -> std::enable_if_t< std::is_const_v<TrackType>,
            TrackIter< TrackType >
         >
   {
      if (!pTrack || pTrack->GetOwner().get() != this)
         return EndIterator<TrackType>();
      else
         return MakeTrackIterator<TrackType>( pTrack->GetNode() );
   }
 
   // If the track is not an audio track, or not one of a group of channels,
   // return the track itself; else return the first channel of its group --
   // in either case as an iterator that will only visit other leader tracks.
   // (Generalizing away from the assumption of at most stereo)
   TrackIter< Track > FindLeader( Track *pTrack );
 
   TrackIter< const Track >
      FindLeader( const Track *pTrack ) const
   {
      return const_cast<TrackList*>(this)->
         FindLeader( const_cast<Track*>(pTrack) ).Filter< const Track >();
   }
 
 
   template < typename TrackType = Track >
      auto Any()
         -> TrackIterRange< TrackType >
   {
      return Tracks< TrackType >();
   }
 
   template < typename TrackType = const Track >
      auto Any() const
         -> std::enable_if_t< std::is_const_v<TrackType>,
            TrackIterRange< TrackType >
         >
   {
      return Tracks< TrackType >();
   }
 
   // Abbreviating some frequently used cases
   template < typename TrackType = Track >
      auto Selected()
         -> TrackIterRange< TrackType >
   {
      return Tracks< TrackType >( &Track::IsSelected );
   }
 
   template < typename TrackType = const Track >
      auto Selected() const
         -> std::enable_if_t< std::is_const_v<TrackType>,
            TrackIterRange< TrackType >
         >
   {
      return Tracks< TrackType >( &Track::IsSelected );
   }
 
 
   template < typename TrackType = Track >
      auto Leaders()
         -> TrackIterRange< TrackType >
   {
      return Tracks< TrackType >( &Track::IsLeader );
   }
 
   template < typename TrackType = const Track >
      auto Leaders() const
         -> std::enable_if_t< std::is_const_v<TrackType>,
            TrackIterRange< TrackType >
         >
   {
      return Tracks< TrackType >( &Track::IsLeader );
   }
 
 
   template < typename TrackType = Track >
      auto SelectedLeaders()
         -> TrackIterRange< TrackType >
   {
      return Tracks< TrackType >( &Track::IsSelectedLeader );
   }
 
   template < typename TrackType = const Track >
      auto SelectedLeaders() const
         -> std::enable_if_t< std::is_const_v<TrackType>,
            TrackIterRange< TrackType >
         >
   {
      return Tracks< TrackType >( &Track::IsSelectedLeader );
   }
 
 
   template<typename TrackType>
      static auto SingletonRange( TrackType *pTrack )
         -> TrackIterRange< TrackType >
   {
      return pTrack->GetOwner()->template Any<TrackType>()
         .StartingWith( pTrack ).EndingAfter( pTrack );
   }
 
 
private:
   Track *DoAddToHead(const std::shared_ptr<Track> &t);
   Track *DoAdd(const std::shared_ptr<Track> &t);
 
   template< typename TrackType, typename InTrackType >
      static TrackIterRange< TrackType >
         Channels_( TrackIter< InTrackType > iter1 )
   {
      // Assume iterator filters leader tracks
      if (*iter1) {
         return {
            iter1.Filter( &Track::Any )
               .template Filter<TrackType>(),
            (++iter1).Filter( &Track::Any )
               .template Filter<TrackType>()
         };
      }
      else
         // empty range
         return {
            iter1.template Filter<TrackType>(),
            iter1.template Filter<TrackType>()
         };
   }
 
public:
   // Find an iterator range of channels including the given track.
   template< typename TrackType >
      static auto Channels( TrackType *pTrack )
         -> TrackIterRange< TrackType >
   {
      return Channels_<TrackType>( pTrack->GetOwner()->FindLeader(pTrack) );
   }
 
   static size_t NChannels(const Track &track)
   {
      return Channels(&track).size();
   }
 
 
   static bool SwapChannels(Track &track);
 
   friend class Track;
 
   void Permute(const std::vector<TrackNodePointer> &permutation);
 
   Track *FindById( TrackId id );
 
   template<typename TrackKind>
      TrackKind *AddToHead( const std::shared_ptr< TrackKind > &t )
         { return static_cast< TrackKind* >( DoAddToHead( t ) ); }
 
   template<typename TrackKind>
      TrackKind *Add( const std::shared_ptr< TrackKind > &t )
         { return static_cast< TrackKind* >( DoAdd( t ) ); }
   
   void UnlinkChannels(Track& track);
   bool MakeMultiChannelTrack(Track& first, int nChannels, bool aligned);
 
   ListOfTracks::value_type Replace(
      Track * t, const ListOfTracks::value_type &with);
 
   TrackNodePointer Remove(Track *t);
 
   void Clear(bool sendEvent = true);
 
   bool CanMoveUp(Track * t) const;
   bool CanMoveDown(Track * t) const;
 
   bool MoveUp(Track * t);
   bool MoveDown(Track * t);
   bool Move(Track * t, bool up) { return up ? MoveUp(t) : MoveDown(t); }
 
   bool Contains(const Track * t) const;
 
   // Return non-null only if the weak pointer is not, and the track is
   // owned by this list; constant time.
   template <typename Subclass>
   std::shared_ptr<Subclass> Lock(const std::weak_ptr<Subclass> &wTrack)
   {
      auto pTrack = wTrack.lock();
      if (pTrack) {
         auto pList = pTrack->mList.lock();
         if (pTrack && this == pList.get())
            return pTrack;
      }
      return {};
   }
 
   bool empty() const;
   size_t NChannels() const;
   size_t Size() const { return Leaders().size(); }
 
   double GetStartTime() const;
   double GetEndTime() const;
 
   double GetMinOffset() const;
 
private:
   using ListOfTracks::size;
 
   // Visit all tracks satisfying a predicate, mutative access
   template <
      typename TrackType = Track,
      typename Pred =
         typename TrackIterRange< TrackType >::iterator::FunctionType
   >
      auto Tracks( const Pred &pred = {} )
         -> TrackIterRange< TrackType >
   {
      auto b = getBegin(), e = getEnd();
      return { { b, b, e, pred }, { b, e, e, pred } };
   }
 
   // Visit all tracks satisfying a predicate, const access
   template <
      typename TrackType = const Track,
      typename Pred =
         typename TrackIterRange< TrackType >::iterator::FunctionType
   >
      auto Tracks( const Pred &pred = {} ) const
         -> std::enable_if_t< std::is_const_v<TrackType>,
            TrackIterRange< TrackType >
         >
   {
      auto b = const_cast<TrackList*>(this)->getBegin();
      auto e = const_cast<TrackList*>(this)->getEnd();
      return { { b, b, e, pred }, { b, e, e, pred } };
   }
 
   Track *GetPrev(Track * t, bool linked = false) const;
   Track *GetNext(Track * t, bool linked = false) const;
   
   template < typename TrackType >
      TrackIter< TrackType >
         MakeTrackIterator( TrackNodePointer iter ) const
   {
      auto b = const_cast<TrackList*>(this)->getBegin();
      auto e = const_cast<TrackList*>(this)->getEnd();
      return { b, iter, e };
   }
 
   template < typename TrackType >
      TrackIter< TrackType >
         EndIterator() const
   {
      auto e = const_cast<TrackList*>(this)->getEnd();
      return { e, e, e };
   }
 
   TrackIterRange< Track > EmptyRange() const;
 
   bool isNull(TrackNodePointer p) const
   { return (p.second == this && p.first == ListOfTracks::end())
      || (p.second == &mPendingUpdates && p.first == mPendingUpdates.end()); }
   TrackNodePointer getEnd() const
   { return { const_cast<TrackList*>(this)->ListOfTracks::end(),
              const_cast<TrackList*>(this)}; }
   TrackNodePointer getBegin() const
   { return { const_cast<TrackList*>(this)->ListOfTracks::begin(),
              const_cast<TrackList*>(this)}; }
 
   TrackNodePointer getNext(TrackNodePointer p) const
   {
      if ( isNull(p) )
         return p;
      auto q = p;
      ++q.first;
      return q;
   }
 
   TrackNodePointer getPrev(TrackNodePointer p) const
   {
      if (p == getBegin())
         return getEnd();
      else {
         auto q = p;
         --q.first;
         return q;
      }
   }
 
   void RecalcPositions(TrackNodePointer node);
   void QueueEvent(TrackListEvent event);
   void SelectionEvent(Track &track);
   void PermutationEvent(TrackNodePointer node);
   void DataEvent(
      const std::shared_ptr<Track> &pTrack, bool allChannels, int code );
   void EnsureVisibleEvent(
      const std::shared_ptr<Track> &pTrack, bool modifyState );
   void DeletionEvent(std::weak_ptr<Track> node, bool duringReplace);
   void AdditionEvent(TrackNodePointer node);
   void ResizingEvent(TrackNodePointer node);
 
   void SwapNodes(TrackNodePointer s1, TrackNodePointer s2);
 
   // Nondecreasing during the session.
   // Nonpersistent.
   // Used to assign ids to added tracks.
   static long sCounter;
 
public:
   using Updater = std::function< void(Track &dest, const Track &src) >;
   // Start a deferred update of the project.
   // The return value is a duplicate of the given track.
   // While ApplyPendingTracks or ClearPendingTracks is not yet called,
   // there may be other direct changes to the project that push undo history.
   // Meanwhile the returned object can accumulate other changes for a deferred
   // push, and temporarily shadow the actual project track for display purposes.
   // The Updater function, if not null, merges state (from the actual project
   // into the pending track) which is not meant to be overridden by the
   // accumulated pending changes.
   // To keep the display consistent, the Y and Height values, minimized state,
   // and Linked state must be copied, and this will be done even if the
   // Updater does not do it.
   // Pending track will have the same TrackId as the actual.
   // Pending changed tracks will not occur in iterations.
   std::shared_ptr<Track> RegisterPendingChangedTrack(
      Updater updater,
      Track *src
   );
 
   // Like the previous, but for a NEW track, not a replacement track.  Caller
   // supplies the track, and there are no updates.
   // Pending track will have an unassigned TrackId.
   // Pending changed tracks WILL occur in iterations, always after actual
   // tracks, and in the sequence that they were added.  They can be
   // distinguished from actual tracks by TrackId.
   void RegisterPendingNewTrack( const std::shared_ptr<Track> &pTrack );
 
   // Invoke the updaters of pending tracks.  Pass any exceptions from the
   // updater functions.
   void UpdatePendingTracks();
 
   // Forget pending track additions and changes;
   // if requested, give back the pending added tracks.
   void ClearPendingTracks( ListOfTracks *pAdded = nullptr );
 
   // Change the state of the project.
   // Strong guarantee for project state in case of exceptions.
   // Will always clear the pending updates.
   // Return true if the state of the track list really did change.
   bool ApplyPendingTracks();
 
   bool HasPendingTracks() const;
 
private:
   AudacityProject *mOwner;
 
 
   ListOfTracks mPendingUpdates;
   std::vector< Updater > mUpdaters;
};
 
#endif