[poincare] Add comment on how Units are built

poincare/include/poincare/unit.h

@@ -188,6 +188,13 @@ public:
   typedef UnitNode::Prefix Prefix;
   typedef UnitNode::Representative Representative;
   typedef UnitNode::Dimension Dimension;
+  /* TODO: Prefix, Representative and Dimension defined below must be defined
+   * only once and all units must be constructed from their pointers. This way
+   * we can easily check if two Unit objects are equal by comparing pointers.
+   * This saves us from overloading the == operator on Prefix, Representative
+   * and Dimension and saves time at execution. We should assert at compilation
+   * that only one occurence of each is built by maybe privatizing constructors
+   * on these classes? */
   static constexpr const Prefix
     PicoPrefix  = Prefix("p", -12),
     NanoPrefix  = Prefix("n",  -9),

poincare/src/unit.cpp

@@ -393,15 +393,18 @@ Expression Unit::removeUnit(Expression * unit) {
 }
 
 bool Unit::isSecond() const {
+  // TODO: comparing pointers suffices because all time dimension are built from the same pointers. This should be asserted some way at compile-time?
   return node()->dimension() == TimeDimension && node()->representative() == SecondRepresentative && node()->prefix() == &EmptyPrefix;
 }
 
 bool Unit::isMeter() const {
+  // See comment on isSecond
   return node()->dimension() == DistanceDimension && node()->representative() == MeterRepresentative && node()->prefix() == &EmptyPrefix;
 }
 
 
 bool Unit::isKilogram() const {
+  // See comment on isSecond
   return node()->dimension() == MassDimension && node()->representative() == KilogramRepresentative && node()->prefix() == &KiloPrefix;
 }