Added HashTbl::shrink() - but it only be useful in particular situations

- for the hashTableTest3, it seemed to slow things down a bit

loop 0 - Erased 100000 elements (size 2900000 capacity 4194304) 0.31 s
loop 1 - Erased 100000 elements (size 2800000 capacity 4194304) 0.01 s
loop 2 - Erased 100000 elements (size 2700000 capacity 4194304) 0 s
loop 3 - Erased 100000 elements (size 2600000 capacity 4194304) 0 s
loop 4 - Erased 100000 elements (size 2500000 capacity 4194304) 0.01 s
loop 5 - Erased 100000 elements (size 2400000 capacity 4194304) 0 s
loop 6 - Erased 100000 elements (size 2300000 capacity 4194304) 0 s
loop 7 - Erased 100000 elements (size 2200000 capacity 4194304) 0 s
loop 8 - Erased 100000 elements (size 2100000 capacity 4194304) 0.01 s
loop 9 - Erased 100000 elements (size 2000000 capacity 4194304) 0.44 s
loop 10 - Erased 100000 elements (size 1900000 capacity 4194304) 0.44 s
loop 11 - Erased 100000 elements (size 1800000 capacity 4194304) 0.39 s
loop 12 - Erased 100000 elements (size 1700000 capacity 4194304) 0.4 s
loop 13 - Erased 100000 elements (size 1600000 capacity 2097152) 0.15 s
loop 14 - Erased 100000 elements (size 1500000 capacity 2097152) 0.01 s
loop 15 - Erased 100000 elements (size 1400000 capacity 2097152) 0 s
loop 16 - Erased 100000 elements (size 1300000 capacity 2097152) 0 s
loop 17 - Erased 100000 elements (size 1200000 capacity 2097152) 0.01 s
loop 18 - Erased 100000 elements (size 1100000 capacity 2097152) 0 s
loop 19 - Erased 100000 elements (size 1000000 capacity 2097152) 0.27 s
loop 20 - Erased 100000 elements (size 900000 capacity 2097152) 0.2 s
loop 21 - Erased 100000 elements (size 800000 capacity 1048576) 0.1 s
loop 22 - Erased 100000 elements (size 700000 capacity 1048576) 0 s
loop 23 - Erased 100000 elements (size 600000 capacity 1048576) 0 s
loop 24 - Erased 100000 elements (size 500000 capacity 1048576) 0.12 s
loop 25 - Erased 100000 elements (size 400000 capacity 524288) 0.04 s
loop 26 - Erased 100000 elements (size 300000 capacity 524288) 0.01 s
loop 27 - Erased 100000 elements (size 200000 capacity 262144) 0.02 s
loop 28 - Erased 100000 elements (size 100000 capacity 131072) 0.02 s
loop 29 - Erased 100000 elements (size 0 capacity 2) 0 s

applications/test/HashTable3/hashTableTest3.C

@@ -74,6 +74,8 @@ int main(int argc, char *argv[])
         {
             map.erase(elemI++);
         }
+
+        map.shrink();
         Info<< "loop " << iLoop << " - Erased " << nBase << " elements"
             << " (size " << map.size() << " capacity " << map.capacity() << ") "
             << timer.cpuTimeIncrement() << " s\n";

src/OpenFOAM/containers/HashTables/HashTbl/HashTbl.C

@@ -542,6 +542,19 @@ void Foam::HashTbl<T, Key, Hash>::clearStorage()
 }
 
 
+template<class T, class Key, class Hash>
+void Foam::HashTbl<T, Key, Hash>::shrink()
+{
+    const label newSize = canonicalSize(nElmts_);
+
+    if (newSize < tableSize_)
+    {
+        // avoid having the table disappear on us
+        resize(newSize ? newSize : 2);
+    }
+}
+
+
 template<class T, class Key, class Hash>
 void Foam::HashTbl<T, Key, Hash>::transfer(HashTbl<T, Key, Hash>& ht)
 {

src/OpenFOAM/containers/HashTables/HashTbl/HashTbl.H

@@ -242,6 +242,9 @@ public:
             //  Equivalent to clear() followed by resize(0)
             void clearStorage();
 
+            //- Shrink the allocated table to approx. twice number of elements
+            void shrink();
+
             //- Transfer the contents of the argument table into this table
             //  and annull the argument table.
             void transfer(HashTbl<T, Key, Hash>&);