MeasureImplementation.h

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#ifndef SimTK_SimTKCOMMON_MEASURE_IMPLEMENTATION_H_
#define SimTK_SimTKCOMMON_MEASURE_IMPLEMENTATION_H_
 
/* -------------------------------------------------------------------------- *
 *                       Simbody(tm): SimTKcommon                             *
 * -------------------------------------------------------------------------- *
 * This is part of the SimTK biosimulation toolkit originating from           *
 * Simbios, the NIH National Center for Physics-Based Simulation of           *
 * Biological Structures at Stanford, funded under the NIH Roadmap for        *
 * Medical Research, grant U54 GM072970. See https://simtk.org/home/simbody.  *
 *                                                                            *
 * Portions copyright (c) 2008-14 Stanford University and the Authors.        *
 * Authors: Michael Sherman                                                   *
 * Contributors:                                                              *
 *                                                                            *
 * 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.                                             *
 * -------------------------------------------------------------------------- */
 
#include "SimTKcommon/basics.h"
#include "SimTKcommon/Simmatrix.h"
#include "SimTKcommon/internal/State.h"
#include "SimTKcommon/internal/Measure.h"
#include "SimTKcommon/internal/Subsystem.h"
#include "SimTKcommon/internal/System.h"
#include "SimTKcommon/internal/SubsystemGuts.h"
 
#include <cmath>
 
 
namespace SimTK {
 
 
//==============================================================================
//                    ABSTRACT MEASURE :: IMPLEMENTATION
//==============================================================================
 
class SimTK_SimTKCOMMON_EXPORT AbstractMeasure::Implementation {
protected:
    Implementation() : copyNumber(0), mySubsystem(0), refCount(0) {}
 
    Implementation(const Implementation& src)
    :   copyNumber(src.copyNumber+1), mySubsystem(0), refCount(0) {}
    
    Implementation& operator=(const Implementation& src) {
        if (&src != this)
        {   copyNumber=src.copyNumber+1;
            refCount=0; mySubsystem=0; }
        return *this; 
    }
 
    // destructor is virtual; below
 
    // Increment the reference count and return its new value.
    int incrRefCount() const {return ++refCount;}
 
    // Decrement the reference count and return its new value.
    int decrRefCount() const {return --refCount;}
 
    // Get the current value of the reference counter.
    int getRefCount() const {return refCount;}
 
    int           getCopyNumber()  const {return copyNumber;}
 
    Implementation* clone() const {return cloneVirtual();}
 
    // realizeTopology() is pure virtual below for Measure_<T> to supply.
    void realizeModel       (State& s)       const {realizeMeasureModelVirtual(s);}
    void realizeInstance    (const State& s) const {realizeMeasureInstanceVirtual(s);}
    void realizeTime        (const State& s) const {realizeMeasureTimeVirtual(s);}
    void realizePosition    (const State& s) const {realizeMeasurePositionVirtual(s);}
    void realizeVelocity    (const State& s) const {realizeMeasureVelocityVirtual(s);}
    void realizeDynamics    (const State& s) const {realizeMeasureDynamicsVirtual(s);}
    void realizeAcceleration(const State& s) const {realizeMeasureAccelerationVirtual(s);}
    void realizeReport      (const State& s) const {realizeMeasureReportVirtual(s);}
 
    void initialize(State& s) const {initializeVirtual(s);}
 
    int getNumTimeDerivatives() const {return getNumTimeDerivativesVirtual();}
 
    Stage getDependsOnStage(int derivOrder) const {
        SimTK_ERRCHK2(0 <= derivOrder && derivOrder <= getNumTimeDerivatives(),
            "Measure::getDependsOnStage()",
            "derivOrder %d was out of range; this Measure allows 0-%d.",
            derivOrder, getNumTimeDerivatives()); 
        return getDependsOnStageVirtual(derivOrder); 
    }
 
 
    void setSubsystem(Subsystem& sub, MeasureIndex mx) 
    {   assert(!mySubsystem && mx.isValid()); 
        mySubsystem = &sub; myIndex = mx; }
 
    bool             isInSubsystem() const {return mySubsystem != 0;}
    const Subsystem& getSubsystem() const {assert(mySubsystem); return *mySubsystem;}
    Subsystem&       updSubsystem() {assert(mySubsystem); return *mySubsystem;}
    MeasureIndex     getSubsystemMeasureIndex() const {assert(mySubsystem); return myIndex;}
    SubsystemIndex   getSubsystemIndex() const
    {   return getSubsystem().getMySubsystemIndex(); }
 
    void invalidateTopologyCache() const
    {   if (isInSubsystem()) getSubsystem().invalidateSubsystemTopologyCache(); }
 
    Stage getStage(const State& s) const {return getSubsystem().getStage(s);}
 
    // VIRTUALS //
 
    virtual ~Implementation() {}
    virtual Implementation* cloneVirtual() const = 0;
 
    virtual void realizeTopology(State&)const = 0;
 
    virtual void realizeMeasureModelVirtual(State&) const {}
    virtual void realizeMeasureInstanceVirtual(const State&) const {}
    virtual void realizeMeasureTimeVirtual(const State&) const {}
    virtual void realizeMeasurePositionVirtual(const State&) const {}
    virtual void realizeMeasureVelocityVirtual(const State&) const {}
    virtual void realizeMeasureDynamicsVirtual(const State&) const {}
    virtual void realizeMeasureAccelerationVirtual(const State&) const {}
    virtual void realizeMeasureReportVirtual(const State&) const {}
 
    virtual void  initializeVirtual(State&) const {}
    virtual int   getNumTimeDerivativesVirtual() const {return 0;}
    virtual Stage getDependsOnStageVirtual(int order) const = 0;
 
private:
    int             copyNumber; // bumped each time we do a deep copy
 
    // These are set when this Measure is adopted by a Subsystem.
    Subsystem*      mySubsystem;
    MeasureIndex    myIndex;
 
    // Measures have shallow copy semantics so they share the Implementation 
    // objects, which are only deleted when the refCount goes to zero.
    mutable int     refCount;
 
friend class AbstractMeasure;
friend class Subsystem::Guts;
};
 
//==============================================================================
//                      ABSTRACT MEASURE DEFINITIONS
//==============================================================================
// These had to wait for AbstractMeasure::Implementation to be defined.
 
inline AbstractMeasure::
AbstractMeasure(Implementation* g) 
:   impl(g)
{   if (impl) impl->incrRefCount(); }
 
inline AbstractMeasure::
AbstractMeasure(Subsystem& sub, Implementation* g, const SetHandle&) 
:   impl(g) {
    SimTK_ERRCHK(hasImpl(), "AbstractMeasure::AbstractMeasure()",
        "An empty Measure handle can't be put in a Subsystem.");
    impl->incrRefCount();
    sub.adoptMeasure(*this);
}
 
// Shallow copy constructor.
inline AbstractMeasure::AbstractMeasure(const AbstractMeasure& src) 
:   impl(0) {
    if (src.impl) {
        impl = src.impl;
        impl->incrRefCount();
    }
}
 
// Shallow assignment.
inline AbstractMeasure& AbstractMeasure::
shallowAssign(const AbstractMeasure& src) {
    if (impl != src.impl) {
        if (impl && impl->decrRefCount()==0) delete impl;
        impl = src.impl;
        impl->incrRefCount();
    }
    return *this;
}
 
// Note that even if the source and destination are currently pointing
// to the same Implementation, we still have to make a new copy so that
// afterwards the destination has its own, refcount==1 copy.
inline AbstractMeasure& AbstractMeasure::
deepAssign(const AbstractMeasure& src) {
    if (&src != this) {
        if (impl && impl->decrRefCount()==0) delete impl;
        if (src.impl) {
            impl = src.impl->clone();
            impl->incrRefCount();
        } else
            impl = 0;
    }
    return *this;
}
 
inline AbstractMeasure::
~AbstractMeasure()
{   if (impl && impl->decrRefCount()==0) delete impl;}
 
inline bool AbstractMeasure::
isInSubsystem() const
{   return hasImpl() && getImpl().isInSubsystem(); }
 
inline const Subsystem& AbstractMeasure::
getSubsystem() const
{   return getImpl().getSubsystem(); }
 
inline bool AbstractMeasure::
isSameSubsystem(const Subsystem& other) const
{   return getSubsystem().isSameSubsystem(other); }
 
inline MeasureIndex AbstractMeasure::
getSubsystemMeasureIndex() const
{   return getImpl().getSubsystemMeasureIndex();}
 
inline int AbstractMeasure::
getNumTimeDerivatives() const
{   return getImpl().getNumTimeDerivatives(); }
 
inline Stage AbstractMeasure::
getDependsOnStage(int derivOrder) const
{   return getImpl().getDependsOnStage(derivOrder); }
 
inline int AbstractMeasure::
getRefCount() const
{   return getImpl().getRefCount(); }
 
 // Hide from Doxygen.
// This is a helper class that makes it possible to treat Real, Vec, and 
// Vector objects uniformly.
template <class T> class Measure_Num {
};
 
template <> class Measure_Num<float> {
public:
    typedef float Element;
    static int size(const float&) {return 1;}
    static const float& get(const float& v, int i) {assert(i==0); return v;}
    static float& upd(float& v, int i) {assert(i==0); return v;}
    static void makeNaNLike(const float&, float& nanValue) 
    {   nanValue = CNT<float>::getNaN();}
    static void makeZeroLike(const float&, float& zeroValue) {zeroValue=0.f;}
};
 
template <> class Measure_Num<double> {
public:
    typedef double Element;
    static int size(const double&) {return 1;}
    static const double& get(const double& v, int i) {assert(i==0); return v;}
    static double& upd(double& v, int i) {assert(i==0); return v;}
    static void makeNaNLike(const double&, double& nanValue) 
    {  nanValue = CNT<double>::getNaN(); }
    static void makeZeroLike(const double&, double& zeroValue) {zeroValue=0.;}
};
 
// We only support stride 1 (densely packed) Vec types.
template <int M, class E>
class Measure_Num< Vec<M,E,1> > {
    typedef Vec<M,E,1> T;
public:
    typedef E Element;
    static int size(const T&) {return M;}
    static const E& get(const T& v, int i) {return v[i];}
    static E& upd(T& v, int i) {return v[i];}
    static void makeNaNLike (const T&, T& nanValue)  {nanValue.setToNaN();}
    static void makeZeroLike(const T&, T& zeroValue) {zeroValue.setToZero();}
};
 
// We only support column major (densely packed) Mat types.
template <int M, int N, class E>
class Measure_Num< Mat<M,N,E> > {
    typedef Mat<M,N,E> T;
public:
    typedef E Element;
    static int size(const T&) {return N;} // number of columns
    static const typename T::TCol& get(const T& m, int j) {return m.col(j);}
    static typename T::TCol& upd(T& m, int j) {return m.col(j);}
    static void makeNaNLike (const T&, T& nanValue)  {nanValue.setToNaN();}
    static void makeZeroLike(const T&, T& zeroValue) {zeroValue.setToZero();}
};
 
 
template <class E>
class Measure_Num< Vector_<E> > {
    typedef Vector_<E> T;
public:
    typedef E Element;
    static int size(const T& v) {return v.size();}
    static const E& get(const T& v, int i) {return v[i];}
    static E& upd(T& v, int i) {return v[i];}
    static void makeNaNLike(const T& v, T& nanValue) 
    {   nanValue.resize(v.size()); nanValue.setToNaN(); }
    static void makeZeroLike(const T& v, T& zeroValue)
    {   zeroValue.resize(v.size()); zeroValue.setToZero(); }
};
 
 
template <class E>
class Measure_Num< Rotation_<E> > {
    typedef Rotation_<E> T;
public:
    typedef T Element;
    static int size(const T&) {return 1;}
    static const T& get(const T& v, int i) {assert(i==0); return v;}
    static T& upd(T& v, int i) {assert(i==0); return v;}
    static void makeNaNLike(const T&, T& nanValue) 
    {   nanValue.setRotationToNaN(); }
    static void makeZeroLike(const T&, T& zeroValue) 
    {   zeroValue.setRotationToIdentityMatrix(); }
};
 
template <class E>
class Measure_Num< Transform_<E> > {
    typedef Transform_<E> T;
public:
    typedef T Element;
    static int size(const T&) {return 1;}
    static const T& get(const T& v, int i) {assert(i==0); return v;}
    static T& upd(T& v, int i) {assert(i==0); return v;}
    static void makeNaNLike(const T&, T& nanValue) 
    {   nanValue.setToNaN(); }
    static void makeZeroLike(const T&, T& zeroValue) 
    {   zeroValue.setToZero(); }
};
 
//==============================================================================
//                       MEASURE_<T> :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Implementation : public AbstractMeasure::Implementation {
public:
    const T& getValue(const State& s, int derivOrder) const {
        SimTK_ERRCHK2(0 <= derivOrder && derivOrder <= getNumTimeDerivatives(),
            "Measure_<T>::getValue()",
            "derivOrder %d was out of range; this Measure allows 0-%d.",
            derivOrder, getNumTimeDerivatives()); 
 
        // We require the stage to have been advanced to at least the one
        // before this measure's depends-on stage since this will get called
        // towards the end of the depends-on stage realization.
        if (getDependsOnStage(derivOrder) != Stage::Empty) {
#ifndef NDEBUG
            Stage prevStage = getDependsOnStage(derivOrder).prev();
#endif
 
            SimTK_ERRCHK2
                (   ( isInSubsystem() && getStage(s)>=prevStage)
                 || (!isInSubsystem() && s.getSystemStage()>=prevStage),
                "Measure_<T>::getValue()",
                "Expected State to have been realized to at least stage "
                "%s but stage was %s.", 
                prevStage.getName().c_str(), 
                (isInSubsystem() ? getStage(s) : s.getSystemStage())
                    .getName().c_str());
        }
 
        if (derivOrder < getNumCacheEntries()) {
            if (!isCacheValueRealized(s,derivOrder)) {
                T& value = updCacheEntry(s,derivOrder);
                calcCachedValueVirtual(s, derivOrder, value);
                markCacheValueRealized(s,derivOrder);
                return value;
            }
            return getCacheEntry(s,derivOrder);
        }
 
        // We can't handle it here -- punt to the concrete Measure
        // for higher order derivatives.
        return getUncachedValueVirtual(s,derivOrder); 
    }
 
    void setDefaultValue(const T& defaultValue) {
        this->defaultValue = defaultValue;
        Measure_Num<T>::makeZeroLike(defaultValue, zeroValue);
        this->invalidateTopologyCache(); 
    }
 
    const T& getDefaultValue() const {return defaultValue;}
 
    void setIsPresumedValidAtDependsOnStage(bool presume) 
    {   presumeValidAtDependsOnStage = presume;
        this->invalidateTopologyCache(); }
 
    bool getIsPresumedValidAtDependsOnStage() const 
    {   return presumeValidAtDependsOnStage; }
 
protected:
    explicit Implementation(const T& defaultValue, int numCacheEntries=1)
    :   presumeValidAtDependsOnStage(false),
        defaultValue(defaultValue),
        derivIx(numCacheEntries) 
    {
        Measure_Num<T>::makeZeroLike(defaultValue, zeroValue);
    }
 
    explicit Implementation(int numCacheEntries=1) 
    :   presumeValidAtDependsOnStage(false),
        defaultValue(),
        derivIx(numCacheEntries) 
    {
        Measure_Num<T>::makeZeroLike(defaultValue, zeroValue);
    }
 
    Implementation(const Implementation& source) 
    :   presumeValidAtDependsOnStage(source.presumeValidAtDependsOnStage),
        defaultValue(source.defaultValue),
        derivIx(source.derivIx.size()) 
    {
        Measure_Num<T>::makeZeroLike(defaultValue, zeroValue);
    }
 
 
    int size() const {return Measure_Num<T>::size(defaultValue);}
 
    int getNumCacheEntries() const {return (int)derivIx.size();}
 
    const T& getCacheEntry(const State& s, int derivOrder) const {
        SimTK_ERRCHK2(0 <= derivOrder && derivOrder < getNumCacheEntries(),
            "Measure_<T>::Implementation::getCacheEntry()",
            "Derivative order %d is out of range; only %d cache entries"
            " were allocated.", derivOrder, getNumCacheEntries());
 
        return Value<T>::downcast(
            this->getSubsystem().getCacheEntry(s, derivIx[derivOrder]));
    }
 
    T& updCacheEntry(const State& s, int derivOrder) const {
        SimTK_ERRCHK2(0 <= derivOrder && derivOrder < getNumCacheEntries(),
            "Measure_<T>::Implementation::updCacheEntry()",
            "Derivative order %d is out of range; only %d cache entries"
            " were allocated.", derivOrder, getNumCacheEntries());
 
        return Value<T>::updDowncast(
            this->getSubsystem().updCacheEntry(s, derivIx[derivOrder]));
    }
 
    bool isCacheValueRealized(const State& s, int derivOrder) const {
        SimTK_ERRCHK2(0 <= derivOrder && derivOrder < getNumCacheEntries(),
            "Measure_<T>::Implementation::isCacheValueRealized()",
            "Derivative order %d is out of range; only %d cache entries"
            " were allocated.", derivOrder, getNumCacheEntries());
 
        return this->getSubsystem().isCacheValueRealized(s, derivIx[derivOrder]);
    }
 
    void markCacheValueRealized(const State& s, int derivOrder) const {
        SimTK_ERRCHK2(0 <= derivOrder && derivOrder < getNumCacheEntries(),
            "Measure_<T>::Implementation::markCacheValueRealized()",
            "Derivative order %d is out of range; only %d cache entries"
            " were allocated.", derivOrder, getNumCacheEntries());
 
        this->getSubsystem().markCacheValueRealized(s, derivIx[derivOrder]);
    }
 
    void markCacheValueNotRealized(const State& s, int derivOrder) const {
        SimTK_ERRCHK2(0 <= derivOrder && derivOrder < getNumCacheEntries(),
            "Measure_<T>::Implementation::markCacheValueNotRealized()",
            "Derivative order %d is out of range; only %d cache entries"
            " were allocated.", derivOrder, getNumCacheEntries());
 
        this->getSubsystem().markCacheValueNotRealized(s, derivIx[derivOrder]);
    }
 
    // VIRTUALS //
 
    virtual void realizeMeasureTopologyVirtual(State&) const {}
 
    virtual void 
    calcCachedValueVirtual(const State&, int derivOrder, T& value) const
    {   SimTK_ERRCHK1_ALWAYS(!"implemented", 
        "Measure_<T>::Implementation::calcCachedValueVirtual()",
        "This method should have been overridden by the derived"
        " Measure but was not. It is needed to calculate the"
        " cached value for derivOrder=%d.", derivOrder); }
 
    virtual const T& 
    getUncachedValueVirtual(const State&, int derivOrder) const
    {   SimTK_ERRCHK1_ALWAYS(!"implemented", 
            "Measure_<T>::Implementation::getUncachedValueVirtual()",
            "This method should have been overridden by the derived"
            " Measure but was not. It is needed to return the uncached"
            " value at derivOrder=%d.", derivOrder);
        return *reinterpret_cast<T*>(0);
    }
 
    const T& getValueZero() const {return zeroValue;}
 
private:
    // Satisfy the realizeTopology() pure virtual here now that we know the 
    // data type T. Allocate lazy- or auto-validated- cache entries depending 
    // on the setting of presumeValidAtDependsOnStage.
    void realizeTopology(State& s) const override final {
        // Allocate cache entries. Initialize the value cache entry to
        // the given defaultValue; all the derivative cache entries should be
        // initialized to a NaN of the same size.
        if (getNumCacheEntries()) {
            derivIx[0] = presumeValidAtDependsOnStage 
                ? this->getSubsystem().allocateCacheEntry
                        (s, getDependsOnStage(0), new Value<T>(defaultValue))
                : this->getSubsystem().allocateLazyCacheEntry
                        (s, getDependsOnStage(0), new Value<T>(defaultValue));
 
            if (getNumCacheEntries() > 1) {
                T nanValue; Measure_Num<T>::makeNaNLike(defaultValue, nanValue);
                for (int i=1; i < getNumCacheEntries(); ++i) {
                    derivIx[i] = presumeValidAtDependsOnStage 
                        ? this->getSubsystem().allocateCacheEntry
                            (s, getDependsOnStage(i), new Value<T>(nanValue))
                        : this->getSubsystem().allocateLazyCacheEntry
                            (s, getDependsOnStage(i), new Value<T>(nanValue));
                }
            }
        }
 
        // Call the concrete class virtual if any.
        realizeMeasureTopologyVirtual(s);
    }
 
//------------------------------------------------------------------------------
private:
    // TOPOLOGY STATE
    bool    presumeValidAtDependsOnStage;
    T       defaultValue;
    T       zeroValue;
 
    // TOPOLOGY CACHE
    mutable Array_<CacheEntryIndex> derivIx;
};
 
 
 
//==============================================================================
//                       CONSTANT :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Constant::Implementation 
:   public Measure_<T>::Implementation 
{
public:
    // We don't want the base class to allocate *any* cache entries.
    Implementation() : Measure_<T>::Implementation(0) {}
    explicit Implementation(const T& value) 
    :   Measure_<T>::Implementation(value,0) {}
 
    void setValue(const T& v) {this->setDefaultValue(v);}
 
    // Implementations of virtual methods.
    // Measure_<T> virtuals:
    // No cached values.
 
    const T& getUncachedValueVirtual(const State&, int derivOrder) const 
        override
    {   return derivOrder>0 ? this->getValueZero() : this->getDefaultValue(); }
 
    // AbstractMeasure virtuals:
    Implementation* cloneVirtual() const override
    {   return new Implementation(*this); }
    Stage getDependsOnStageVirtual(int derivOrder) const override 
    {   return derivOrder>0 ? Stage::Empty : Stage::Topology; }
    int getNumTimeDerivativesVirtual() const override 
    {   return std::numeric_limits<int>::max(); }
};
 
 
 
//==============================================================================
//                        MEASURE ZERO and ONE
//==============================================================================
// These had to wait for Constant::Implementation to be declared.
 
template <class T> inline
Measure_<T>::Zero::Zero() : Constant(T(0)) {}
template <class T> inline
Measure_<T>::Zero::Zero(Subsystem& sub) : Constant(sub, T(0)) {}
 
inline Measure_< Vector >::Zero::Zero(int size) 
:   Constant(Vector(size, Real(0))) {}
inline Measure_< Vector >::Zero::Zero(Subsystem& sub, int size) 
:  Constant(sub, Vector(size, Real(0))) {}
 
template <class T> inline
Measure_<T>::One::One() : Constant(T(1)) {}
template <class T> inline
Measure_<T>::One::One(Subsystem& sub) : Constant(sub, T(1)) {}
 
inline Measure_< Vector >::One::One(int size) 
:   Constant(Vector(size, Real(1))) {}
inline Measure_< Vector >::One::One(Subsystem& sub, int size) 
:  Constant(sub, Vector(size, Real(1))) {}
 
 
 
//==============================================================================
//                         TIME :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Time::Implementation {};
 
template <>
class Measure_<Real>::Time::Implementation 
:   public Measure_<Real>::Implementation 
{
public:
    // We don't want the base class to allocate *any* cache entries.
    Implementation() : Measure_<Real>::Implementation(0) {}
 
    // Implementations of virtual methods.
    // Measure_<Real> virtuals:
    // No cached values.
 
    const Real& getUncachedValueVirtual(const State& s, int derivOrder) const
        override
    {   return derivOrder==0 ? s.getTime()
            : (derivOrder==1 ? SimTK::One 
                             : SimTK::Zero); } 
 
    // AbstractMeasure virtuals:
    Implementation* cloneVirtual() const override
    {   return new Implementation(*this); }
    Stage getDependsOnStageVirtual(int derivOrder) const override
    {   return derivOrder>0 ? Stage::Empty : Stage::Time; }
 
    // Value is t, 1st derivative is 1, the rest are 0.
    int getNumTimeDerivativesVirtual() const override 
    {   return std::numeric_limits<int>::max(); }
};
 
 
 
//==============================================================================
//                        VARIABLE :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Variable::Implementation 
:   public Measure_<T>::Implementation 
{
public:
    // We don't want the base class to allocate *any* cache entries;
    // we'll use the variable as its own value and zeroes for all
    // the derivatives.
    Implementation() 
    :   Measure_<T>::Implementation(0),
        invalidatedStage(Stage::Empty) {}
 
    Implementation(Stage invalidated, const T& defaultValue) 
    :   Measure_<T>::Implementation(defaultValue, 0),
        invalidatedStage(invalidated) {}
 
    // Copy constructor should not copy the variable.
    Implementation(const Implementation& source)
    :   Measure_<T>::Implementation(source.getDefaultValue(), 0),
        invalidatedStage(source.invalidatedStage) {}
 
    void setInvalidatedStage(Stage invalidates) {
        invalidatedStage = invalidates;
        this->invalidateTopologyCache();
    }
 
    Stage getInvalidatedStage() const {return invalidatedStage;}
 
    void setValue(State& state, const T& value) const 
    {   updVarValue(state) = value; }
 
    // Implementations of virtual methods.
    Implementation* cloneVirtual() const override
    {   return new Implementation(*this); }
 
    int getNumTimeDerivativesVirtual() const override 
    {   return std::numeric_limits<int>::max(); }
 
    // Discrete variable is available after Model stage; but all its 
    // derivatives are zero so are always available.
    Stage getDependsOnStageVirtual(int derivOrder) const override 
    {   return derivOrder>0 ? Stage::Empty : Stage::Model;}
 
    const T& getUncachedValueVirtual(const State& s, int derivOrder) const
        override
    {   return derivOrder>0 ? this->getValueZero() : getVarValue(s); }
 
    // No cached values.
 
    void realizeMeasureTopologyVirtual(State& s) const override {
        discreteVarIndex = this->getSubsystem().allocateDiscreteVariable
            (s, invalidatedStage, new Value<T>(this->getDefaultValue()));
    }
private:
    const T& getVarValue(const State& s) const {
        assert(discreteVarIndex.isValid());
        return Value<T>::downcast(
            this->getSubsystem().getDiscreteVariable(s, discreteVarIndex));
    }
    T& updVarValue(State& s) const {
        assert(discreteVarIndex.isValid());
        return Value<T>::downcast(
            this->getSubsystem().updDiscreteVariable(s, discreteVarIndex));
    }
 
    // TOPOLOGY STATE
    Stage   invalidatedStage; // TODO this shouldn't be needed
 
    // TOPOLOGY CACHE
    mutable DiscreteVariableIndex discreteVarIndex;
};
 
 
 
//==============================================================================
//                         RESULT :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Result::Implementation 
:   public Measure_<T>::Implementation 
{
public:
    // We want the base class to allocate a single cache entry of type T.
    Implementation() 
    :   Measure_<T>::Implementation(1), 
        dependsOnStage(Stage::Topology), invalidatedStage(Stage::Infinity) {}
 
    Implementation(Stage dependsOn, Stage invalidated) 
    :   Measure_<T>::Implementation(1), 
        dependsOnStage(dependsOn==Stage::Empty ? Stage::Topology : dependsOn), 
        invalidatedStage(invalidated)
    {   SimTK_ERRCHK2_ALWAYS(invalidated > dependsOn,"Measure::Result::ctor()",
            "Got invalidated stage %s and dependsOn stage %s which is illegal "
            "because the invalidated stage must be later than dependsOn.",
            invalidated.getName().c_str(), dependsOn.getName().c_str());
    }
 
    // Copy constructor will not copy the cache entry index.
    Implementation(const Implementation& source)
    :   Measure_<T>::Implementation(source),
        dependsOnStage(source.dependsOnStage), 
        invalidatedStage(source.invalidatedStage) {} 
 
    void setDependsOnStage(Stage dependsOn) {
        if (dependsOn == Stage::Empty) dependsOn = Stage::Topology;
        SimTK_ERRCHK2_ALWAYS(dependsOn < getInvalidatedStage(),
            "Measure::Result::setDependsOnStage()",
            "The provided dependsOn stage %s is illegal because it is not "
            "less than the current invalidated stage %s. Change the "
            "invalidated stage first with setInvalidatedStage().",
            dependsOn.getName().c_str(), 
            getInvalidatedStage().getName().c_str());
 
        dependsOnStage = dependsOn;
        this->invalidateTopologyCache();
    }
 
    void setInvalidatedStage(Stage invalidated) {
        SimTK_ERRCHK2_ALWAYS(invalidated > getDependsOnStage(),
            "Measure::Result::setInvalidatedStage()",
            "The provided invalidated stage %s is illegal because it is not "
            "greater than the current dependsOn stage %s. Change the "
            "dependsOn stage first with setDependsOnStage().",
            invalidated.getName().c_str(),
            getDependsOnStage().getName().c_str());
 
        invalidatedStage = invalidated;
        this->invalidateTopologyCache();
    }
 
 
    Stage getDependsOnStage()   const {return dependsOnStage;}
    Stage getInvalidatedStage() const {return invalidatedStage;}
 
 
    void markAsValid(const State& state) const
    {   const Stage subsystemStage = this->getSubsystem().getStage(state);
        SimTK_ERRCHK3_ALWAYS(subsystemStage >= getDependsOnStage().prev(),
            "Measure::Result::markAsValid()",
            "This Result Measure cannot be marked valid in a State where this "
            "measure's Subsystem has been realized only to stage %s, because "
            "its value was declared to depend on stage %s. To mark it valid, "
            "we require that the State have been realized at least to the "
            "previous stage (%s in this case); that is, you must at least be "
            "*working on* the dependsOn stage in order to claim this result is "
            "available.",
            subsystemStage.getName().c_str(),
            getDependsOnStage().getName().c_str(),
            getDependsOnStage().prev().getName().c_str());
        this->markCacheValueRealized(state, 0); }
 
    bool isValid(const State& state) const
    {   return this->isCacheValueRealized(state, 0); }
    
    void markAsNotValid(const State& state) const
    {   this->markCacheValueNotRealized(state, 0); 
        state.invalidateAllCacheAtOrAbove(invalidatedStage); }
 
    T& updValue(const State& state) const 
    {   markAsNotValid(state); return this->updCacheEntry(state, 0); }
 
 
    // Implementations of virtual methods.
    Implementation* cloneVirtual() const override
    {   return new Implementation(*this); }
 
    int getNumTimeDerivativesVirtual() const override {return 0;} 
 
    Stage getDependsOnStageVirtual(int derivOrder) const override 
    {   return derivOrder>0 ? Stage::Empty : dependsOnStage;}
 
    void calcCachedValueVirtual(const State&, int derivOrder, T& value) const
        override
    {   SimTK_ERRCHK_ALWAYS(!"calcCachedValueVirtual() implemented",
        "Measure_<T>::Result::getValue()",
        "Measure_<T>::Result::getValue() was called when the value was not "
        "yet valid. For most Measure types, this would have initiated "
        "computation of the value, but Result measures must have their values "
        "calculated and set externally, and then marked valid."); }
 
private:
    // TOPOLOGY STATE
    Stage   dependsOnStage;
    Stage   invalidatedStage;
};
 
 
 
//==============================================================================
//                        SINUSOID :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Sinusoid::Implementation
:   public Measure_<T>::Implementation 
{
    static const int NumDerivs = 3;
public:
    Implementation() 
    :   Measure_<T>::Implementation(NumDerivs+1),
        a(CNT<T>::getNaN()), w(CNT<T>::getNaN()), p(CNT<T>::getNaN()) {}
 
    Implementation(const T& amplitude, 
                   const T& frequency, 
                   const T& phase=T(0))
    :   Measure_<T>::Implementation(NumDerivs+1),
        a(amplitude), w(frequency), p(phase) {}
 
    // Default copy constructor is fine.
 
    // Implementations of virtual methods.
    Implementation* cloneVirtual() const override
    {   return new Implementation(*this); }
 
    int getNumTimeDerivativesVirtual() const override {return NumDerivs;}
 
    Stage getDependsOnStageVirtual(int order) const override 
    {   return Stage::Time; }
 
    void calcCachedValueVirtual(const State& s, int derivOrder, T& value) const
        override
    {
        // We need to allow the compiler to select std::sin or SimTK::sin
        // based on the argument type.
        using std::sin; using std::cos;
 
        assert(NumDerivs == 3);
        const Real t = s.getTime();
        const T arg = w*t + p;
 
        switch (derivOrder) {
        case 0: value =        a*sin(arg); break;
        case 1: value =      w*a*cos(arg); break;
        case 2: value =   -w*w*a*sin(arg); break;
        case 3: value = -w*w*w*a*cos(arg); break;
        default: SimTK_ASSERT1_ALWAYS(!"out of range",
                     "Measure::Sinusoid::Implementation::calcCachedValueVirtual():"
                     " derivOrder %d is out of range 0-3.", derivOrder);
        }
    }
 
    // There are no uncached values.
 
private:
    // TOPOLOGY STATE
    T a, w, p;
 
    // TOPOLOGY CACHE
    // nothing
};
 
 
 
//==============================================================================
//                          PLUS :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Plus::Implementation : public Measure_<T>::Implementation {
public:
    // TODO: Currently allocates just one cache entry.
    // left and right will be empty handles.
    Implementation() {}
 
    Implementation(const Measure_<T>& left, 
                   const Measure_<T>& right)
    :   left(left), right(right) {}
 
    // Default copy constructor gives us a new Implementation object,
    // but with references to the *same* operand measures.
 
    // Implementations of virtual methods.
 
    // This uses the default copy constructor.
    Implementation* cloneVirtual() const override 
    {   return new Implementation(*this); }
 
    // TODO: Let this be settable up to the min number of derivatives 
    // provided by the arguments.
    int getNumTimeDerivativesVirtual() const override {return 0;} 
    //{   return std::min(left.getNumTimeDerivatives(), 
    //                    right.getNumTimeDerivatives()); }
 
    Stage getDependsOnStageVirtual(int order) const override 
    {   return Stage(std::max(left.getDependsOnStage(order),
                              right.getDependsOnStage(order))); }
 
 
    void calcCachedValueVirtual(const State& s, int derivOrder, T& value) const
        override
    {
        value = left.getValue(s,derivOrder) + right.getValue(s,derivOrder);
    }
 
    // There are no uncached values.
 
private:
    // TOPOLOGY STATE
    Measure_<T> left;
    Measure_<T> right;
 
    // TOPOLOGY CACHE
    // nothing
};
 
 
 
//==============================================================================
//                          MINUS :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Minus::Implementation : public Measure_<T>::Implementation {
public:
    // TODO: Currently allocates just one cache entry.
    // left and right will be empty handles.
    Implementation() {}
 
    Implementation(const Measure_<T>& left, 
                   const Measure_<T>& right)
    :   left(left), right(right) {}
 
    // Default copy constructor gives us a new Implementation object,
    // but with references to the *same* operand measures.
 
    // Implementations of virtual methods.
 
    // This uses the default copy constructor.
    Implementation* cloneVirtual() const override 
    {   return new Implementation(*this); }
 
    // TODO: Let this be settable up to the min number of derivatives 
    // provided by the arguments.
    int getNumTimeDerivativesVirtual() const override {return 0;} 
    //{   return std::min(left.getNumTimeDerivatives(), 
    //                    right.getNumTimeDerivatives()); }
 
    Stage getDependsOnStageVirtual(int order) const override 
    {   return Stage(std::max(left.getDependsOnStage(order),
                              right.getDependsOnStage(order))); }
 
 
    void calcCachedValueVirtual(const State& s, int derivOrder, T& value) const
        override
    {
        value = left.getValue(s,derivOrder) - right.getValue(s,derivOrder);
    }
 
    // There are no uncached values.
 
private:
    // TOPOLOGY STATE
    Measure_<T> left;
    Measure_<T> right;
 
    // TOPOLOGY CACHE
    // nothing
};
 
 
 
//==============================================================================
//                          SCALE :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Scale::Implementation
:   public Measure_<T>::Implementation 
{
public:
    // TODO: Currently allocates just one cache entry.
    // scale will be uninitialized, operand will be empty handle.
    Implementation() : factor(NaN) {}
 
    Implementation(Real factor, const Measure_<T>& operand)
    :   factor(factor), operand(operand) {}
 
    // Default copy constructor gives us a new Implementation object,
    // but with references to the *same* operand measure.
 
    void setScaleFactor(Real sf) {
        factor = sf;
        this->invalidateTopologyCache();
    }
 
    const Measure_<T>& getOperandMeasure() const
    {
        return operand;
    }
 
    // Implementations of virtual methods.
 
    // This uses the default copy constructor.
    Implementation* cloneVirtual() const override 
    {   return new Implementation(*this); }
 
    // TODO: Let this be settable up to the min number of derivatives 
    // provided by the arguments.
    int getNumTimeDerivativesVirtual() const override {return 0;} 
    //{   return std::min(left.getNumTimeDerivatives(), 
    //                    right.getNumTimeDerivatives()); }
 
    Stage getDependsOnStageVirtual(int order) const override
    {   return operand.getDependsOnStage(order); }
 
 
    void calcCachedValueVirtual(const State& s, int derivOrder, T& value) const
        override
    {
        value = factor * operand.getValue(s,derivOrder);
    }
 
    // There are no uncached values.
 
private:
    // TOPOLOGY STATE
    Real        factor;
    Measure_<T> operand;
 
    // TOPOLOGY CACHE
    // nothing
};
 
 
 
//==============================================================================
//                        INTEGRATE :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Integrate::Implementation 
:   public Measure_<T>::Implementation {
public:
    Implementation() : Measure_<T>::Implementation(1) {}
 
    Implementation(const Measure_<T>& deriv, const Measure_<T>& ic,
                   const T& defaultValue)
    :   Measure_<T>::Implementation(defaultValue, 1), 
        derivMeasure(deriv), icMeasure(ic) {}
 
    Implementation(const Implementation& source)
    :   Measure_<T>::Implementation(source.getDefaultValue(), 1), 
        derivMeasure(source.derivMeasure), icMeasure(source.icMeasure) {}
 
    void setValue(State& s, const T& value) const
    {   assert(zIndex >= 0);
        for (int i=0; i < this->size(); ++i)
            this->getSubsystem().updZ(s)[zIndex+i] = 
                Measure_Num<T>::get(value, i); }
    
    const Measure_<T>& getDerivativeMeasure() const
    {   SimTK_ERRCHK(!derivMeasure.isEmptyHandle(), 
            "Measure_<T>::Integrate::getDerivativeMeasure()",
            "No derivative measure is available for this integrated measure."); 
        return derivMeasure; }
 
    const Measure_<T>& getInitialConditionMeasure() const
    {   SimTK_ERRCHK(!icMeasure.isEmptyHandle(), 
            "Measure_<T>::Integrate::getInitialConditionMeasure()",
            "No initial condition measure is available for this "
            "integrated measure."); 
        return icMeasure; }
 
    void setDerivativeMeasure(const Measure_<T>& d)
    {   derivMeasure = d; this->invalidateTopologyCache(); }
    void setInitialConditionMeasure(const Measure_<T>& ic)
    {   icMeasure = ic; this->invalidateTopologyCache(); }
 
    // Implementations of virtuals.
 
    // This uses the copy constructor defined above.
    Implementation* cloneVirtual() const override 
    {   return new Implementation(*this); }
 
    int getNumTimeDerivativesVirtual() const override 
    {   int integralDerivs = getDerivativeMeasure().getNumTimeDerivatives();
        // Careful - can't add 1 to max int and stay an int.
        if (integralDerivs < std::numeric_limits<int>::max())
            ++integralDerivs;        
        return integralDerivs; }
 
    void calcCachedValueVirtual(const State& s, int derivOrder, T& value) const
        override
    {   assert(derivOrder == 0); // only one cache entry
        assert(Measure_Num<T>::size(value) == this->size());
        assert(zIndex.isValid());
        const Vector& allZ = this->getSubsystem().getZ(s);
        for (int i=0; i < this->size(); ++i)
            Measure_Num<T>::upd(value,i) = allZ[zIndex+i];
    }
 
    const T& getUncachedValueVirtual(const State& s, int derivOrder) const
        override
    {   assert(derivOrder > 0); // 0th entry is cached
        return getDerivativeMeasure().getValue(s, derivOrder-1);
    }
 
    Stage getDependsOnStageVirtual(int derivOrder) const override 
    {   return derivOrder>0 
            ? getDerivativeMeasure().getDependsOnStage(derivOrder-1)
            : Stage::Time; }
 
    void initializeVirtual(State& s) const override {
        assert(zIndex.isValid());
        Vector& allZ = this->getSubsystem().updZ(s);
        if (!icMeasure.isEmptyHandle()) {
            this->getSubsystem().getSystem()
                .realize(s, icMeasure.getDependsOnStage());
            const T& ic = icMeasure.getValue(s);
            for (int i=0; i < this->size(); ++i)
                allZ[zIndex+i] = Measure_Num<T>::get(ic,i);
        } else {
            for (int i=0; i < this->size(); ++i)
                allZ[zIndex+i] = Measure_Num<T>::get(this->getDefaultValue(),i);
        }
    }
 
    void realizeMeasureTopologyVirtual(State& s) const override {
        Vector init(this->size());
        for (int i=0; i < this->size(); ++i) 
            init[i] = Measure_Num<T>::get(this->getDefaultValue(),i);
        zIndex = this->getSubsystem().allocateZ(s, init);
    }
 
    void realizeMeasureAccelerationVirtual(const State& s) const override {
        assert(zIndex.isValid());
        Vector& allZDot = this->getSubsystem().updZDot(s);
        if (!derivMeasure.isEmptyHandle()) {
            const T& deriv = derivMeasure.getValue(s);
             for (int i=0; i < this->size(); ++i)
                 allZDot[zIndex+i] = Measure_Num<T>::get(deriv,i);
        } else {
            allZDot(zIndex,this->size()) = 0; // derivative is zero
        }
    }
 
private:
    // TOPOLOGY STATE
    Measure_<T> derivMeasure; // just handles
    Measure_<T> icMeasure;
 
    // TOPOLOGY CACHE
    mutable ZIndex zIndex;  // This is the first index if more than one z.
};
 
 
 
//==============================================================================
//                      DIFFERENTIATE :: IMPLEMENTATION
//==============================================================================
 // Hide from Doxygen.
// This helper class is the contents of the discrete state variable and 
// corresponding cache entry maintained by this measure. The variable is 
// auto-update, meaning the value of the cache entry replaces the state 
// variable at the start of each step.
// TODO: This was a local class in Measure_<T>::Differentiate::Implementation
// but VC++ 8 (2005) failed to properly instantiate the templatized operator<<()
// in that case; doing it this way is a workaround.
template <class T>
class Measure_Differentiate_Result {
public:
    Measure_Differentiate_Result() : derivIsGood(false) {}
    T       operand;    // previous value of operand
    T       operandDot; // previous value of derivative
    bool    derivIsGood; // do we think the deriv is a good one?
};
template <class T>
class Measure_<T>::Differentiate::Implementation
:   public Measure_<T>::Implementation 
{
    typedef Measure_Differentiate_Result<T> Result;
public:
    // Don't allocate any cache entries in the base class.
    Implementation() : Measure_<T>::Implementation(0) {}
 
    Implementation(const Measure_<T>& operand)
    :   Measure_<T>::Implementation(0),
        operand(operand), forceUseApprox(false), isApproxInUse(false) {}
 
    // Default copy constructor gives us a new Implementation object,
    // but with reference to the *same* operand measure.
 
    void setForceUseApproximation(bool mustApproximate) {
        forceUseApprox = mustApproximate;
        this->invalidateTopologyCache();
    }
 
    void setOperandMeasure(const Measure_<T>& operand) {
        this->operand = operand;
        this->invalidateTopologyCache();
    }
 
    bool getForceUseApproximation() const {return forceUseApprox;}
    bool isUsingApproximation() const {return isApproxInUse;}
    const Measure_<T>& getOperandMeasure() const {return operand;}
 
    // Implementations of virtual methods.
 
    // This uses the default copy constructor.
    Implementation* cloneVirtual() const override
    {   return new Implementation(*this); }
 
    // This has one fewer than the operand.
    int getNumTimeDerivativesVirtual() const override
    {   if (!isApproxInUse) return operand.getNumTimeDerivatives()-1;
        else return 0; }
 
    Stage getDependsOnStageVirtual(int order) const override 
    {   if (!isApproxInUse) return operand.getDependsOnStage(order+1);
        else return operand.getDependsOnStage(order); }
 
 
    // We're not using the Measure_<T> base class cache services, but
    // we do have one of our own. It looks uncached from the base class
    // point of view which is why we're implementing it here.
    const T& getUncachedValueVirtual(const State& s, int derivOrder) const
        override
    {   if (!isApproxInUse) 
            return operand.getValue(s, derivOrder+1);
 
        ensureDerivativeIsRealized(s);
        const Subsystem& subsys = this->getSubsystem();
        const Result& result = Value<Result>::downcast
                                (subsys.getDiscreteVarUpdateValue(s,resultIx));
        return result.operandDot; // has a value but might not be a good one
    }
 
    void initializeVirtual(State& s) const override {
        if (!isApproxInUse) return;
 
        assert(resultIx.isValid());
        const Subsystem& subsys = this->getSubsystem();
        Result& result = Value<Result>::updDowncast
                            (subsys.updDiscreteVariable(s,resultIx));
        this->getSubsystem().getSystem().realize(s,operand.getDependsOnStage());
        result.operand = operand.getValue(s);
        result.operandDot = this->getValueZero();
        result.derivIsGood = false;
    }
 
    void realizeMeasureTopologyVirtual(State& s) const override {
        isApproxInUse = (forceUseApprox || operand.getNumTimeDerivatives()==0);
        if (!isApproxInUse)
            return;
 
        resultIx = this->getSubsystem()
            .allocateAutoUpdateDiscreteVariable(s, operand.getDependsOnStage(0),
                new Value<Result>(), operand.getDependsOnStage(0));
    }
 
    void realizeMeasureAccelerationVirtual(const State& s) const override {
        ensureDerivativeIsRealized(s);
    }
 
    void ensureDerivativeIsRealized(const State& s) const {
        assert(resultIx.isValid());
        const Subsystem& subsys = this->getSubsystem();
        if (subsys.isDiscreteVarUpdateValueRealized(s,resultIx))
            return;
 
        const Real t0 = subsys.getDiscreteVarLastUpdateTime(s,resultIx);
        const Result& prevResult = Value<Result>::downcast
           (subsys.getDiscreteVariable(s,resultIx));
        const T&   f0         = prevResult.operand;
        const T&   fdot0      = prevResult.operandDot;   // may be invalid
        const bool good0      = prevResult.derivIsGood;
 
        const Real t  = s.getTime();
        Result& result = Value<Result>::updDowncast
           (subsys.updDiscreteVarUpdateValue(s,resultIx));
        T&         f          = result.operand;          // renaming
        T&         fdot       = result.operandDot;
        bool&      good       = result.derivIsGood;
 
        f = operand.getValue(s);
        good = false;
        if (!isFinite(t0))
            fdot = this->getValueZero(); 
        else if (t == t0) {
            fdot = fdot0;
            good = good0;
        } else {
            fdot = (f-f0)/(t-t0); // 1st order
            if (good0)
                fdot = Real(2)*fdot - fdot0; // now 2nd order
            good = true; // either 1st or 2nd order estimate
        }
        subsys.markDiscreteVarUpdateValueRealized(s,resultIx);
    }
private:
    // TOPOLOGY STATE
    Measure_<T>     operand;
    bool            forceUseApprox;
 
    // TOPOLOGY CACHE
    mutable bool                    isApproxInUse;
    mutable DiscreteVariableIndex   resultIx;    // auto-update
};
 
 
 
//==============================================================================
//                         EXTREME :: IMPLEMENTATION
//==============================================================================
template <class T>
class Measure_<T>::Extreme::Implementation : public Measure_<T>::Implementation 
{
    typedef typename Measure_<T>::Extreme Extreme;
    typedef typename Extreme::Operation   Operation;
public:
    Implementation() 
    :   Measure_<T>::Implementation(0), operation(Extreme::MaxAbs) {}
 
    Implementation(const Measure_<T>& operand, Operation op)
    :   Measure_<T>::Implementation(0), operand(operand), operation(op) {}
 
    // Default copy constructor gives us a new Implementation object,
    // but with reference to the *same* operand measure.
 
    void setOperandMeasure(const Measure_<T>& operand) {
        this->operand = operand;
        this->invalidateTopologyCache();
    }
 
    void setOperation(Operation op) {
        this->operation = op;
        this->invalidateTopologyCache();
    }
 
    const Measure_<T>& getOperandMeasure() const {return operand;}
 
    Operation getOperation() const {return operation;}
 
    void setValue(State& s, const T& value) const {
        assert(extremeIx.isValid());
        const Subsystem& subsys = this->getSubsystem();
        T& prevMin = Value<T>::updDowncast
                            (subsys.updDiscreteVariable(s,extremeIx));
        prevMin = value;
    }
 
    Real getTimeOfExtremeValue(const State& s) const {
        const Subsystem& subsys = this->getSubsystem();
        const bool hasNewExtreme = ensureExtremeHasBeenUpdated(s);
        Real tUpdate;
        if (hasNewExtreme)
            tUpdate = s.getTime(); // i.e., now
        else
            tUpdate = subsys.getDiscreteVarLastUpdateTime(s,extremeIx);
        return tUpdate;
    }
 
    // Implementations of virtual methods.
 
    // This uses the default copy constructor.
    Implementation* cloneVirtual() const override 
    {   return new Implementation(*this); }
 
    int getNumTimeDerivativesVirtual() const override
    {   return operand.getNumTimeDerivatives(); }
 
    Stage getDependsOnStageVirtual(int order) const override
    {   return operand.getDependsOnStage(order); }
 
 
    const T& getUncachedValueVirtual(const State& s, int derivOrder) const
        override
    {   
        const Subsystem& subsys = this->getSubsystem();
        const bool hasNewExtreme = ensureExtremeHasBeenUpdated(s);
        if (derivOrder > 0) {
            // TODO: should be handled elementwise and zero unless the
            // derivative is acting in the direction that changes the 
            // extreme.
            return hasNewExtreme ? operand.getValue(s, derivOrder)
                                 : this->getValueZero();
        }
        if (hasNewExtreme) {
            const T& newExt = Value<T>::downcast
                                (subsys.getDiscreteVarUpdateValue(s,extremeIx));
            return newExt;
        } else {
            const T& currentExt = Value<T>::downcast
                                (subsys.getDiscreteVariable(s,extremeIx));
            return currentExt;
        }
    }
 
    void initializeVirtual(State& s) const override {
        this->getSubsystem().getSystem().realize(s,operand.getDependsOnStage()); 
        setValue(s, operand.getValue(s));
    }
 
    void realizeMeasureTopologyVirtual(State& s) const override {
        // TODO: this should be NaN once initialization is working properly.
        T initVal = this->getDefaultValue();
        switch(operation) {
        case Minimum: initVal = Infinity; break;
        case Maximum: initVal = -Infinity; break;
        case MinAbs:  initVal = Infinity; break;
        case MaxAbs:  initVal = 0; break;
        };
 
        extremeIx = this->getSubsystem()
            .allocateAutoUpdateDiscreteVariable(s, Stage::Dynamics,
                new Value<T>(initVal), operand.getDependsOnStage(0));
 
        isNewExtremeIx = this->getSubsystem()
            .allocateAutoUpdateDiscreteVariable(s, Stage::Dynamics,              
                new Value<bool>(false), operand.getDependsOnStage(0));
    }
 
    void realizeMeasureAccelerationVirtual(const State& s) const override {
        ensureExtremeHasBeenUpdated(s);
    }
 
    bool ensureExtremeHasBeenUpdated(const State& s) const {
        assert(extremeIx.isValid() && isNewExtremeIx.isValid());
        const Subsystem& subsys = this->getSubsystem();
 
        // We may have already determined whether we're at a new extreme in 
        // which case we don't need to do it again.
        if (subsys.isDiscreteVarUpdateValueRealized(s, isNewExtremeIx))
            return Value<bool>::downcast
               (subsys.getDiscreteVarUpdateValue(s,isNewExtremeIx));
 
        // We're going to have to decide if we're at a new extreme, and if
        // so record the new extreme value in the auto-update cache entry of
        // the extreme value state variable.
 
        // Get the previous extreme value and the current operand value.
        const T& prevExtreme = Value<T>::downcast
                                    (subsys.getDiscreteVariable(s,extremeIx));
        const T& currentVal = operand.getValue(s);
 
        // Search to see if any element has reached a new extreme.
        bool foundNewExt = false;
        for (int i=0; i < this->size() && !foundNewExt; ++i) 
            foundNewExt = isNewExtreme(Measure_Num<T>::get(currentVal,i), 
                                       Measure_Num<T>::get(prevExtreme,i));
 
        // Record the result and mark the auto-update cache entry valid
        // so we won't have to recalculate. When the integrator advances to the
        // next step this cache entry will be swapped with the corresponding 
        // state and marked invalid so we'll be sure to recalculate each step. 
        Value<bool>::updDowncast
           (subsys.updDiscreteVarUpdateValue(s,isNewExtremeIx)) = foundNewExt;
        subsys.markDiscreteVarUpdateValueRealized(s,isNewExtremeIx);
 
        // Don't update the auto-update cache entry if we didn't see a new
        // extreme. That way no auto-update will occur and the state variable
        // will remain unchanged with the existing update time preserved.
        if (!foundNewExt)
            return false;
 
        // We have encountered a new extreme. We'll record the new extreme
        // in the auto-update cache entry which will be used as the current
        // result until the integrator advances to the next step at which time
        // this will be swapped with the state variable to serve as the previous
        // extreme value until a further extreme is encountered.
        T& newExtreme = Value<T>::updDowncast
                                (subsys.updDiscreteVarUpdateValue(s,extremeIx));
 
        for (int i=0; i < this->size(); ++i)
            Measure_Num<T>::upd(newExtreme,i) =
                extremeOf(Measure_Num<T>::get(currentVal,i),
                          Measure_Num<T>::get(prevExtreme,i));
 
        // Marking this valid is what ensures that an auto-update occurs later.
        subsys.markDiscreteVarUpdateValueRealized(s,extremeIx);
        return true;
    }
private:
    // Return true if newVal is "more extreme" than oldExtreme, according
    // to the operation we're performing.
    bool isNewExtreme(const typename Measure_Num<T>::Element& newVal,
                      const typename Measure_Num<T>::Element& oldExtreme) const
    {
        switch (operation) {
        case Extreme::Maximum: return newVal > oldExtreme;
        case Extreme::Minimum: return newVal < oldExtreme;
        case Extreme::MaxAbs: return std::abs(newVal) > std::abs(oldExtreme);
        case Extreme::MinAbs: return std::abs(newVal) < std::abs(oldExtreme);
        };
        SimTK_ASSERT1_ALWAYS(!"recognized", 
            "Measure::Extreme::Implementation::isNewExtreme(): "
            "unrecognized operation %d", (int)operation);
        return false; /*NOTREACHED*/
    }
 
    // Given the value of one element of the operand, and that value's time
    // derivative, determine whether the derivative is moving the element
    // into the "more extreme" direction, according to the operation.
    bool isExtremeDir(const typename Measure_Num<T>::Element& value,
                      const typename Measure_Num<T>::Element& deriv) const 
    {
        const int sv = sign(value), sd = sign(deriv);
        if (sd == 0) return false; // derivative is zero; not changing
        switch (operation) {
        case Extreme::Maximum: return sd ==  1; // getting larger
        case Extreme::Minimum: return sd == -1; // getting smaller
        case Extreme::MaxAbs: return sv==0 || sd==sv; // abs is growing
        case Extreme::MinAbs: return sd == -sv;
        };
        SimTK_ASSERT1_ALWAYS(!"recognized", 
            "Measure::Extreme::Implementation::isExtremeDir(): "
            "unrecognized operation %d", (int)operation);
        return false; /*NOTREACHED*/
    }
 
    typename Measure_Num<T>::Element 
    extremeOf(const typename Measure_Num<T>::Element& newVal,
              const typename Measure_Num<T>::Element& oldExtreme) const
    {
        return isNewExtreme(newVal,oldExtreme) ? newVal : oldExtreme;
    }
 
    // TOPOLOGY STATE
    Measure_<T>                     operand;
    Operation                       operation;
 
    // TOPOLOGY CACHE
    mutable DiscreteVariableIndex   extremeIx; // extreme so far; auto-update
 
    // This auto-update flag records whether the current value is a new
    // extreme. We don't really need to save it as a state variable since you
    // can figure this out from the timestamp, but we need to to get invalidated
    // by the auto-update swap so that we'll figure it out anew each step.
    mutable DiscreteVariableIndex   isNewExtremeIx;
};
 
 
 
//==============================================================================
//                         DELAY :: IMPLEMENTATION
//============================================================================== // Hide from Doxygen.
// This helper class is the contents of the discrete state variable and 
// corresponding cache entry maintained by this measure. The variable is 
// auto-update, meaning the value of the cache entry replaces the state 
// variable at the start of each step.
//
// Circular buffers look like this:
//
//             oldest=0, n=0
//                  v
// Empty buffer:   |    available    |
//
// By convention, oldest=0 whenever the buffer is empty. 
//
//
//           oldest            next=(oldest+n)%capacity
//                 v           v
//    | available | | | | | | | available |
//     ^                n=6                ^
//     0                                   capacity
//     v                                   v
// or | | | | | | available | | | | | | | |    n=12
//               ^           ^
//               next        oldest
//                 = (oldest+n)%capacity
//
// Number of entries = n (called size() below)
// Empty = n==0
// Full = n==capacity()
// Next available = (oldest+n)%capacity()
template <class T>
class Measure_Delay_Buffer {
public:
    explicit Measure_Delay_Buffer() {initDataMembers();}
    void clear() {initDataMembers();}
    int  size() const {return m_size;} // # saved entries, *not* size of arrays
    int  capacity() const {return m_times.size();}
    bool empty() const {return size()==0;}
    bool full()  const {return size()==capacity();}
 
    double getEntryTime(int i) const
    {   assert(i < size()); return m_times[getArrayIndex(i)];}
    const T& getEntryValue(int i) const
    {   assert(i < size()); return m_values[getArrayIndex(i)];}
 
    enum {  
        InitialAllocation  = 8,  // smallest allocation 
        GrowthFactor       = 2,  // how fast to grow (double)
        MaxShrinkProofSize = 16, // won't shrink unless bigger
        TooBigFactor       = 5   // 5X too much->maybe shrink
    };
 
    // Add a new entry to the end of the list, throwing out old entries that
    // aren't needed to answer requests at tEarliest or later.
    void append(double tEarliest, double tNow, const T& valueNow) {
        forgetEntriesMuchOlderThan(tEarliest);
        removeEntriesLaterOrEq(tNow);
        if (full()) 
            makeMoreRoom();
        else if (capacity() > std::max((int)MaxShrinkProofSize, 
                                       (int)TooBigFactor * (size()+1)))
            makeLessRoom(); // less than 1/TooBigFactor full
        const int nextFree = getArrayIndex(m_size++);
        m_times[nextFree] = tNow;
        m_values[nextFree] = valueNow;
        m_maxSize = std::max(m_maxSize, size());
    }
 
    // Prepend an older entry to the beginning of the list. No cleanup is done.
    void prepend(double tNewOldest, const T& value) {
        assert(empty() || tNewOldest < m_times[m_oldest]);
        if (full()) makeMoreRoom();
        m_oldest = empty() ? 0 : getArrayIndex(-1);
        m_times[m_oldest] = tNewOldest;
        m_values[m_oldest] = value;
        ++m_size;
        m_maxSize = std::max(m_maxSize, size());
    }
 
    // This is a specialized copy assignment for copying an old buffer
    // to a new one with updated contents. We are told the earliest time we'll
    // be asked about from now on, and won't copy any entries older than those
    // needed to answer that earliest request. We won't copy anything at or
    // newer than tNow, and finally we'll push (tNow,valueNow) as the newest
    // entry.
    void copyInAndUpdate(const Measure_Delay_Buffer& oldBuf, double tEarliest,
                         double tNow, const T& valueNow) {
        // clear all current entries (no heap activity)
        m_oldest = m_size = 0;
 
        // determine how may old entries we have to keep
        int firstNeeded = oldBuf.countNumUnneededOldEntries(tEarliest);
        int lastNeeded  = oldBuf.findLastEarlier(tNow); // might be -1
        int numOldEntriesToKeep = lastNeeded-firstNeeded+1;
        int newSize = numOldEntriesToKeep+1; // includes the new one
 
        int newSizeRequest = -1;
        if (capacity() < newSize) {
            newSizeRequest = std::max((int)InitialAllocation, 
                                      (int)GrowthFactor * newSize);
            ++m_nGrows;
        } else if (capacity() > std::max((int)MaxShrinkProofSize, 
                                         (int)TooBigFactor * newSize)) {
            newSizeRequest = std::max((int)MaxShrinkProofSize, 
                                      (int)GrowthFactor * newSize);
            ++m_nShrinks;
        }
 
        // Reallocate space if advisable.
        if (newSizeRequest != -1) {
            const double dNaN = NTraits<double>::getNaN();
            m_values.resize(newSizeRequest); 
            if (m_values.capacity() > m_values.size())
                m_values.resize(m_values.capacity()); // don't waste any     
            m_times.resize(m_values.size(), dNaN); 
        }
 
        m_maxCapacity = std::max(m_maxCapacity, capacity());
        
        // Copy the entries we need to keep.
        int nxt = 0;
        for (int i=firstNeeded; i<=lastNeeded; ++i, ++nxt) {
            m_times[nxt]  = oldBuf.getEntryTime(i);
            m_values[nxt] = oldBuf.getEntryValue(i);
        }
        // Now add the newest entry and set the size.
        m_times[nxt]  = tNow;
        m_values[nxt] = valueNow;
        assert(nxt+1==newSize);
        m_size = nxt+1;
        m_maxSize = std::max(m_maxSize, size());
    }
 
    // Given the current time and value and the earlier time at which the
    // value is needed, use the buffer and (if necessary) the current value
    // to estimate the delayed value.
    T calcValueAtTime(double tDelay, double tNow, const T& valueNow) const;
 
    // Given the current time but *not* the current value of the source measure,
    // provide an estimate for the value at tDelay=tNow-delay using only the 
    // buffer contents and linear interpolation or extrapolation.
    void calcValueAtTimeLinearOnly(double tDelay, T& delayedValue) const {
        if (empty()) {
            // Nothing in the buffer?? Shouldn't happen. Return empty Vector
            // or NaN for fixed-size types.
            Measure_Num<T>::makeNaNLike(T(), delayedValue);
            return;
        }
 
        int firstLater = findFirstLaterOrEq(tDelay);
 
        if (firstLater > 0) {
            // Normal case: tDelay is between two buffer entries.
            int firstEarlier = firstLater-1;
            double t0=getEntryTime(firstEarlier), t1=getEntryTime(firstLater);
            const T& v0=getEntryValue(firstEarlier);
            const T& v1=getEntryValue(firstLater);
            Real fraction = Real((tDelay-t0)/(t1-t0));
            delayedValue = T(v0 + fraction*(v1-v0));
            return;
        }
 
        if (firstLater==0) {
            // Startup case: tDelay is at or before the oldest buffer entry.
            // Assume the value was flat before that.
            delayedValue = getEntryValue(firstLater);
            return;
        }
 
        // tDelay is later than the latest entry in the buffer. We are going
        // to have to extrapolate (yuck).
 
        if (size() == 1) {
            // Just one entry; we'll have to assume the value is flat.
            delayedValue = getEntryValue(0);
            return;
        }
 
        // Extrapolate using the last two entries.
        double t0=getEntryTime(size()-2), t1=getEntryTime(size()-1);
        const T& v0=getEntryValue(size()-2);
        const T& v1=getEntryValue(size()-1);
        Real fraction = Real((tDelay-t0)/(t1-t0));  // > 1
        assert(fraction > 1.0);
        delayedValue = T(v0 + fraction*(v1-v0));   // Extrapolate.
    }
 
    // Return the number of times we had to grow the buffer.
    int getNumGrows() const {return m_nGrows;}
    // Return the number of times we decided the buffer was so overallocated
    // that we had to shrink it.
    int getNumShrinks() const {return m_nShrinks;}
    // Return the largest number of values we ever had in the buffer.
    int getMaxSize() const {return m_maxSize;}
    // Return the largest capacity the buffer ever had.
    int getMaxCapacity() const {return m_maxCapacity;}
 
private:
    // Return the i'th oldest entry 
    // (0 -> oldest, size-1 -> newest, size -> first free, -1 -> last free)
    int getArrayIndex(int i) const 
    {   assert(-1<=i && i<=size()); 
        const int rawIndex = m_oldest + i;
        if (rawIndex < 0) return rawIndex + capacity();
        else return rawIndex % capacity(); }
 
    // Remove all but two entries older than the given time.
    void forgetEntriesMuchOlderThan(double tEarliest) {
        const int numToRemove = countNumUnneededOldEntries(tEarliest);
        if (numToRemove) {
            m_oldest = getArrayIndex(numToRemove);
            m_size -= numToRemove;
        }
    }
 
    // Count up how many old entries at the beginning of the buffer are so old
    // that they wouldn't be needed to respond to a request at time tEarliest or
    // later. We'll keep no more than two entries earlier than tEarliest.
    int countNumUnneededOldEntries(double tEarliest) const {
        const int firstLater = findFirstLaterOrEq(tEarliest);
        return std::max(0, firstLater-2);
    }
 
    // Given the time now, delete anything at the end of the queue that is
    // at that same time or later.
    void removeEntriesLaterOrEq(double t) {
        int lastEarlier = findLastEarlier(t);
        m_size = lastEarlier+1;
        if (m_size==0) m_oldest=0; // restart at beginning of array
    }
 
    // Return the entry number (0..size-1) of the first entry whose time 
    // is >= the given time, or -1 if there is none such.
    int findFirstLaterOrEq(double tDelay) const {
        for (int i=0; i < size(); ++i)
            if (getEntryTime(i) >= tDelay)
                return i;
        return -1;
    }
 
    // Return the entry number(size-1..0) of the last entry whose time 
    // is < the given time, or -1 if there is none such.
    int findLastEarlier(double t) const {
        for (int i=size()-1; i>=0; --i)
            if (getEntryTime(i) < t)
                return i;
        return -1;
    }
 
    // We don't have enough space. This is either the initial allocation or
    // we need to double the current space.
    void makeMoreRoom() {
        const int newSizeRequest = std::max((int)InitialAllocation, 
                                            (int)GrowthFactor * size());
        resize(newSizeRequest);
        ++m_nGrows;
        m_maxCapacity = std::max(m_maxCapacity, capacity());
    }
 
    // We are wasting a lot of space, reduce the heap allocation to just 
    // double what we're using now.
    void makeLessRoom() {
        const int targetMaxSize = std::max((int)MaxShrinkProofSize, 
                                           (int)GrowthFactor * size());
        if (capacity() > targetMaxSize) {
            resize(targetMaxSize);
            ++m_nShrinks;
        }
    }
 
    // Reallocate memory to get more space or stop wasting space. The new
    // size request must be big enough to hold all the current contents. The
    // amount we actually get may be somewhat larger than the request. On 
    // return, the times and values arrays will have been resized and the 
    // oldest entry will now be entry 0.
    void resize(int newSizeRequest) {
        assert(newSizeRequest >= size());
        const double dNaN = NTraits<double>::getNaN();
        Array_<T,int> newValues(newSizeRequest); 
        if (newValues.capacity() > newValues.size())
            newValues.resize(newValues.capacity()); // don't waste any     
        Array_<double,int> newTimes(newValues.size(), dNaN); 
 
        // Pack existing values into start of new arrays.
        for (int i=0; i < size(); ++i) {
            const int ix = getArrayIndex(i);
            newTimes[i]  = m_times[ix];
            newValues[i] = m_values[ix];
        }
        m_times.swap(newTimes); // switch heap space
        m_values.swap(newValues);
        m_oldest = 0; // starts at the beginning now; size unchanged
    }
 
    // Initialize everything to its default-constructed state.
    void initDataMembers() {
        m_times.clear(); m_values.clear();
        m_oldest=m_size=0;
        m_nGrows=m_nShrinks=m_maxSize=m_maxCapacity=0;
    }
 
    // These are circular buffers of the same size.
    Array_<double,int>  m_times;
    Array_<T,int>       m_values;
    int                 m_oldest; // Array index of oldest (time,value)
    int                 m_size;   // number of entries in use
 
    // Statistics.
    int m_nGrows, m_nShrinks, m_maxSize, m_maxCapacity;
};
template <class T>
class Measure_<T>::Delay::Implementation: public Measure_<T>::Implementation {
    typedef Measure_Delay_Buffer<T> Buffer;
public:
    // Allocate one cache entry in the base class for the value; we allocate
    // a specialized one for the buffer.
    Implementation() 
    :   Measure_<T>::Implementation(1), m_delay(NaN),
        m_canUseCurrentValue(false), m_useLinearInterpolationOnly(false) {}
 
    Implementation(const Measure_<T>& source, Real delay)
    :   Measure_<T>::Implementation(1), m_source(source), m_delay(delay),
        m_canUseCurrentValue(false), m_useLinearInterpolationOnly(false) {}
 
    // Default copy constructor gives us a new Implementation object,
    // but with reference to the *same* source measure.
 
    void setSourceMeasure(const Measure_<T>& source) {
        if (!source.isSameMeasure(this->m_source)) {
            this->m_source = source;
            this->invalidateTopologyCache();
        }
    }
 
    void setDelay(Real delay) {
        if (delay != this->m_delay) {
            this->m_delay = delay;
            this->invalidateTopologyCache();
        }
    }
 
    void setUseLinearInterpolationOnly(bool linearOnly) {
        if (linearOnly != this->m_useLinearInterpolationOnly) {
            this->m_useLinearInterpolationOnly = linearOnly;
            this->invalidateTopologyCache();
        }
    }
 
    void setCanUseCurrentValue(bool canUseCurrentValue) {
        if (canUseCurrentValue != this->m_canUseCurrentValue) {
            this->m_canUseCurrentValue = canUseCurrentValue;
            this->invalidateTopologyCache();
        }
    }
 
    const Measure_<T>& getSourceMeasure() const {return this->m_source;}
    Real getDelay() const {return this->m_delay;}
    bool getUseLinearInterpolationOnly() const
    {   return this->m_useLinearInterpolationOnly; }
    bool getCanUseCurrentValue() const
    {   return this->m_canUseCurrentValue; }
 
 
    // Implementations of virtual methods.
 
    // This uses the default copy constructor.
    Implementation* cloneVirtual() const override
    {   return new Implementation(*this); }
 
    // Currently no derivative supported.
    int getNumTimeDerivativesVirtual() const override
    {   return 0; }
 
    // If we are allowed to use the current value of the source measure to
    // determine the delayed value, the depends-on stage here is the same as
    // for the source; otherwise it is Stage::Time.
    Stage getDependsOnStageVirtual(int order) const override 
    {   return this->m_canUseCurrentValue ? m_source.getDependsOnStage(order)
                                          : Stage::Time; }
 
    // Calculate the delayed value and return it to the Measure base class to
    // be put in a cache entry.
    void calcCachedValueVirtual(const State& s, int derivOrder, T& value) const
        override
    {   const Subsystem& subsys = this->getSubsystem();
        const Buffer& buffer = Value<Buffer>::downcast
                                (subsys.getDiscreteVariable(s,m_bufferIx));
        //TODO: use cubic interpolation if allowed
        buffer.calcValueAtTimeLinearOnly(s.getTime()-m_delay, value);
    }
 
    void initializeVirtual(State& s) const override {
        assert(m_bufferIx.isValid());
        const Subsystem& subsys = this->getSubsystem();
        Buffer& buffer = Value<Buffer>::updDowncast
                            (subsys.updDiscreteVariable(s,m_bufferIx));
        buffer.clear();
        this->getSubsystem().getSystem().realize(s,m_source.getDependsOnStage());
        buffer.append(s.getTime()-m_delay, s.getTime(), m_source.getValue(s));
    }
 
    void realizeMeasureTopologyVirtual(State& s) const override {
        m_bufferIx = this->getSubsystem()
            .allocateAutoUpdateDiscreteVariable(s, Stage::Report,
                new Value<Buffer>(), getDependsOnStageVirtual(0));
    }
 
    void realizeMeasureAccelerationVirtual(const State& s) const override {
        updateBuffer(s);
    }
 
    // This uses the buffer from the state to update the one in the
    // corresponding cache entry. The update adds the current value of the
    // source to the end of the buffer and tosses out unneeded old entries.
    void updateBuffer(const State& s) const {
        assert(m_bufferIx.isValid());
        const Subsystem& subsys = this->getSubsystem();
 
        const Buffer& prevBuffer = Value<Buffer>::downcast
           (subsys.getDiscreteVariable(s,m_bufferIx));
 
        Buffer& nextBuffer = Value<Buffer>::updDowncast
           (subsys.updDiscreteVarUpdateValue(s,m_bufferIx));
 
        const Real t = s.getTime();
        nextBuffer.copyInAndUpdate(prevBuffer, t-m_delay, 
                                   t, m_source.getValue(s));
 
        subsys.markDiscreteVarUpdateValueRealized(s,m_bufferIx);
    }
private:
    // TOPOLOGY STATE
    Measure_<T>     m_source;
    Real            m_delay;
    bool            m_canUseCurrentValue;
    bool            m_useLinearInterpolationOnly;
 
    // TOPOLOGY CACHE
    mutable DiscreteVariableIndex   m_bufferIx;    // auto-update
};
 
 
} // namespace SimTK
 
 
 
 
#endif // SimTK_SimTKCOMMON_MEASURE_IMPLEMENTATION_H_