ENH: add bitSet::begin(pos) to provide alternative start for indexing

applications/test/bitSet2/Test-bitSet2.C

@@ -412,6 +412,30 @@ int main(int argc, char *argv[])
     );
 
 
+    // Test begin vs find_first etc
+    {
+        // Clear some values
+        list4.unset(labelRange(0, 10));
+
+        Info<< nl
+            << "Test first vs begin" << nl
+            << "    values:" << flatOutput(list4.toc()) << nl
+            << "    first:" << list4.find_first() << nl
+            << "    begin:" << *(list4.begin()) << nl
+            << "    begin(0):" << *(list4.begin(0)) << nl;
+
+        // With offset
+
+        Info<< nl
+            << "Test first vs begin" << nl
+            << "    next(35):" << list4.find_next(35)
+            << "    begin(35):" << *(list4.begin(35)) << nl
+            << "    next(40):" << list4.find_next(40)
+            << "    begin(40):" << *(list4.begin(40)) << nl
+            << nl;
+    }
+
+
     // Construct from labelUList, labelUIndList
     {
         DynamicList<label> indices({10, 50, 300});

src/OpenFOAM/containers/Bits/bitSet/bitSet.H

@@ -478,6 +478,9 @@ public:
             //- Construct begin iterator
             inline const_iterator(const bitSet* bitset);
 
+            //- Construct iterator, starting at or beyond the given position
+            inline const_iterator(const bitSet* bitset, label pos);
+
         public:
 
             //- Return the current \a on position
@@ -497,6 +500,14 @@ public:
         //- Iterator set to the position of the first \a on bit
         inline const_iterator cbegin() const;
 
+        //- Iterator set to the position of the first \a on bit that occurs
+        //- at or beyond the given position
+        inline const_iterator begin(label pos) const;
+
+        //- Iterator set to the position of the first \a on bit that occurs
+        //- at or beyond the given position
+        inline const_iterator cbegin(label pos) const;
+
         //- Iterator beyond the end of the bitSet
         inline const_iterator end() const noexcept;
 

src/OpenFOAM/containers/Bits/bitSet/bitSetI.H

@@ -230,6 +230,17 @@ inline Foam::bitSet::const_iterator::const_iterator(const bitSet* parent)
 {}
 
 
+inline Foam::bitSet::const_iterator::const_iterator
+(
+    const bitSet* parent,
+    label pos
+)
+:
+    set_(parent),
+    pos_(set_->find_next(pos-1))
+{}
+
+
 inline Foam::label Foam::bitSet::const_iterator::operator*() const noexcept
 {
     return pos_;
@@ -273,6 +284,18 @@ inline Foam::bitSet::const_iterator Foam::bitSet::cbegin() const
 }
 
 
+inline Foam::bitSet::const_iterator Foam::bitSet::begin(label pos) const
+{
+    return const_iterator(this, pos);
+}
+
+
+inline Foam::bitSet::const_iterator Foam::bitSet::cbegin(label pos) const
+{
+    return const_iterator(this, pos);
+}
+
+
 inline Foam::bitSet::const_iterator Foam::bitSet::end() const noexcept
 {
     return const_iterator();