ENH: improve UPtrList sorting

- adjust nullptr checks to discourage flip-flop when confronted with
  multiple null values.

     Old:   (a && b) ? (*a < *b) : bool(a);
     New:   (a && b) ? (*a < *b) : !b;

  comparing (non-null < null) and (null < non-null) behaves
  identically, but comparing (null < null) now tests as true
  (ie, already sorted) whereas before it would have been false
  (ie, needs a swap)

- add UPtrList trimTrailingNull(), which reduces the effective
  (addressable) list size to ignore any trailing null pointers, but
  without reallocation. This is particularly useful when creating a
  UPtrList list view. For example,

     UPtrList<some_iterator> validValues(container.size());

     ...Loop to add valid entries, by some criteria...

     // Shorten list to hide null entries
     validValues.trimTrailingNull();

   This list view now only needs a single allocation, whereas using
   a resize (as was previously necessary) could invoke a second
   allocation, as well as recopying.

src/OpenFOAM/containers/PtrLists/PtrDynList/PtrDynListI.H

@@ -187,7 +187,7 @@ template<class T, int SizeMin>
 inline Foam::label Foam::PtrDynList<T, SizeMin>::squeezeNull()
 {
     const label newLen = UPtrList<T>::squeezeNull();
-    resize(newLen);
+    PtrList<T>::setAddressableSize(newLen);
     return newLen;
 }
 

src/OpenFOAM/containers/PtrLists/UPtrList/UPtrList.C

@@ -65,6 +65,23 @@ Foam::label Foam::UPtrList<T>::squeezeNull()
 }
 
 
+template<class T>
+void Foam::UPtrList<T>::trimTrailingNull()
+{
+    label newLen = this->size();
+
+    for (label i = newLen-1; i >= 0 && !ptrs_[i]; --i)
+    {
+        --newLen;
+    }
+
+    // Or mutable?
+    // const_cast<Detail::PtrListDetail<T>&>(ptrs_).setAddressableSize(newLen);
+
+    ptrs_.setAddressableSize(newLen);
+}
+
+
 template<class T>
 void Foam::UPtrList<T>::reorder(const labelUList& oldToNew, const bool check)
 {
@@ -177,6 +194,22 @@ Foam::Ostream& Foam::operator<<(Ostream& os, const UPtrList<T>& list)
 
 // * * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * //
 
+template<class T>
+void Foam::sort(UPtrList<T>& list)
+{
+    std::stable_sort
+    (
+        list.begin_ptr(),
+        list.end_ptr(),
+        // Compare less, with nullptr protect and sort nullptr to end
+        [](const T* const a, const T* const b) -> bool
+        {
+            return (a && b) ? (*a < *b) : !b;
+        }
+    );
+}
+
+
 template<class T, class Compare>
 void Foam::sort(UPtrList<T>& list, const Compare& comp)
 {
@@ -189,12 +222,4 @@ void Foam::sort(UPtrList<T>& list, const Compare& comp)
 }
 
 
-template<class T>
-void Foam::sort(UPtrList<T>& list)
-{
-    // ie, lessOp<T>() or std::less<T>()
-    Foam::sort(list, [](const T& a, const T& b) { return (a < b); });
-}
-
-
 // ************************************************************************* //

src/OpenFOAM/containers/PtrLists/UPtrList/UPtrList.H

@@ -129,7 +129,7 @@ public:
             //- Compare dereferenced pointers
             bool operator()(const T* const a, const T* const b) const
             {
-                return (a && b) ? comp(*a, *b) : bool(a);
+                return (a && b) ? comp(*a, *b) : !b;
             }
         };
 
@@ -150,7 +150,7 @@ public:
                 const T* const a = values.get(ai);
                 const T* const b = values.get(bi);
 
-                return (a && b) ? (*a < *b) : bool(a);
+                return (a && b) ? (*a < *b) : !b;
             }
         };
 
@@ -171,7 +171,7 @@ public:
                 const T* const a = values.get(ai);
                 const T* const b = values.get(bi);
 
-                return (a && b) ? (*b < *a) : bool(b);
+                return (a && b) ? (*b < *a) : !a;
             }
         };
 
@@ -262,10 +262,15 @@ public:
         //- Alias for resize()
         void setSize(const label n) { this->resize(n); }
 
-        //- Squeeze out intermediate nullptr entries in the list of pointers
+        //- Squeeze out nullptr entries in the list of pointers after which
+        //- any null pointers will be at the end of the list
         //  \return the number of non-null entries
         label squeezeNull();
 
+        //- Reduce addressable list size to ignore any trailing null pointers.
+        //  The reduces the effective list length without reallocation
+        void trimTrailingNull();
+
         //- Append an element to the end of the list
         inline void push_back(T* ptr);
 
@@ -343,7 +348,7 @@ public:
             // Member Functions
 
             //- Return pointer, can be nullptr.
-            inline pointer get() const;
+            pointer get() const { return *ptr_; }
 
             // Member Operators
 
@@ -403,7 +408,7 @@ public:
             // Member Functions
 
             //- Return pointer, can be nullptr.
-            inline pointer get() const;
+            pointer get() const { return *ptr_; }
 
             // Member Operators
 
@@ -507,6 +512,8 @@ public:
 // * * * * * * * * * * * * * * Global Functions  * * * * * * * * * * * * * * //
 
 //- Inplace (stable) sorting of pointer list.
+//  This sort function includes null pointer guards and will also sort
+//  any null pointers to the end (eg, rubbish that can be truncated)
 template<class T>
 void sort(UPtrList<T>& list);
 

src/OpenFOAM/containers/PtrLists/UPtrList/UPtrListI.H

@@ -281,13 +281,6 @@ iterator(T** ptr) noexcept
 {}
 
 
-template<class T>
-inline T* Foam::UPtrList<T>::iterator::get() const
-{
-    return *ptr_;
-}
-
-
 template<class T>
 inline T* Foam::UPtrList<T>::iterator::operator->() const
 {
@@ -455,13 +448,6 @@ const_iterator(const iterator& iter) noexcept
 {}
 
 
-template<class T>
-inline const T* Foam::UPtrList<T>::const_iterator::get() const
-{
-    return *ptr_;
-}
-
-
 template<class T>
 inline const T* Foam::UPtrList<T>::const_iterator::operator->() const
 {