Stop maintaining the domain size for IntervalIntVarImpl. (#1050)

For such variables, the solver used to track the lower-bound, the
upper-bound and the domain size to provide quick access to the
domain size. This decision had however a significant impact in
terms of memory usage. It requires to have an additional StoredInt
per variable and for every bound change, 2 values where saved into
the stack.

In this patch, the domain size is no longer stored but computed on
demand. In terms of performance, getDomainSize() performs now 2
 get() and one subtraction while it used to perform one get(). On
 the other side it saves one StoredInt, and only one value is saved
into the stack when a bound is updated.

After some internal experiments on problems having around 100,000
bounded variables and 50,000 enum variables, this reduces the
memory consumption of my JVM by about 30% with no performance
degradation.

solver/src/main/java/org/chocosolver/solver/variables/impl/IntervalIntVarImpl.java

@@ -50,10 +50,6 @@ public final class IntervalIntVarImpl extends AbstractVariable implements IntVar
      * Upper bound of the current domain
      */
     private final IStateInt UB;
-    /**
-     * Current size of domain
-     */
-    private final IStateInt SIZE;
     /**
      * To iterate over removed values
      */
@@ -92,7 +88,6 @@ public IntervalIntVarImpl(String name, int min, int max, Model model) {
         IEnvironment env = model.getEnvironment();
         this.LB = env.makeInt(min);
         this.UB = env.makeInt(max);
-        this.SIZE = env.makeInt(max - min + 1);
     }
 
     ////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
@@ -220,7 +215,6 @@ public boolean instantiateTo(int value, ICause cause) throws ContradictionExcept
             }
             this.LB.set(value);
             this.UB.set(value);
-            this.SIZE.set(1);
             this.notifyPropagators(e, cause);
             return true;
         }
@@ -258,7 +252,6 @@ public boolean updateLowerBound(int value, ICause cause) throws ContradictionExc
                 if (reactOnRemoval) {
                     delta.add(old, value - 1, cause);
                 }
-                SIZE.add(old - value);
                 LB.set(value);
                 if (isInstantiated()) {
                     e = IntEventType.INSTANTIATE;
@@ -301,7 +294,6 @@ public boolean updateUpperBound(int value, ICause cause) throws ContradictionExc
                 if (reactOnRemoval) {
                     delta.add(value + 1, old, cause);
                 }
-                SIZE.add(value - old);
                 UB.set(value);
 
                 if (isInstantiated()) {
@@ -348,7 +340,6 @@ public boolean updateBounds(int lb, int ub, ICause cause) throws ContradictionEx
                 d += ub - oub;
                 UB.set(ub);
             }
-            SIZE.add(d);
             if (isInstantiated()) {
                 e = IntEventType.INSTANTIATE;
             }
@@ -360,7 +351,7 @@ public boolean updateBounds(int lb, int ub, ICause cause) throws ContradictionEx
 
     @Override
     public boolean isInstantiated() {
-        return SIZE.get() == 1;
+        return LB.get() == UB.get();
     }
 
     @Override
@@ -404,7 +395,7 @@ public int getUB() {
 
     @Override
     public int getDomainSize() {
-        return SIZE.get();
+        return UB.get() - LB.get() + 1;
     }
 
     @Override
@@ -468,7 +459,7 @@ public IIntervalDelta getDelta() {
 
     @Override
     public String toString() {
-        if (SIZE.get() == 1) {
+        if (LB.get() == UB.get()) {
             return String.format("%s = %d", name, getLB());
         }
         return String.format("%s = [%d,%d]", name, getLB(), getUB());