Add a facility for checking/assigning presence of free phases

This is intentionally black-oil specific because we presently do no
know how to handle other cases (e.g., more phases or number of phases
different from number of components).

opm/core/props/BlackoilPhases.hpp

@@ -40,6 +40,40 @@ namespace Opm
         int phase_pos[MaxNumPhases];
     };
 
+    /// Check or assign presence of a formed, free phase.  Limited to
+    /// the 'BlackoilPhases'.
+    ///
+    /// Use a std::vector<PhasePresence> to represent the conditions
+    /// in an entire model.
+    class PhasePresence
+    {
+    public:
+        PhasePresence()
+            : present_(0)
+        {}
+
+        bool hasFreeWater() const { return present(BlackoilPhases::Aqua  ); }
+        bool hasFreeOil  () const { return present(BlackoilPhases::Liquid); }
+        bool hasFreeGas  () const { return present(BlackoilPhases::Vapour); }
+
+        void setFreeWater() { insert(BlackoilPhases::Aqua  ); }
+        void setFreeOil  () { insert(BlackoilPhases::Liquid); }
+        void setFreeGas  () { insert(BlackoilPhases::Vapour); }
+
+    private:
+        unsigned char present_;
+
+        bool present(const BlackoilPhases::PhaseIndex i) const
+        {
+            return present_ & (1 << i);
+        }
+
+        void insert(const BlackoilPhases::PhaseIndex i)
+        {
+            present_ |= (1 << i);
+        }
+    };
+
 } // namespace Opm
 
 #endif // OPM_BLACKOILPHASES_HEADER_INCLUDED