[poincare] Add a method isPositive on expressions

Change-Id: I3eb0064f8d64678326e74216517e0104eaa007fe

poincare/include/poincare/absolute_value.h

@@ -11,6 +11,7 @@ class AbsoluteValue : public StaticHierarchy<1> {
 public:
   Type type() const override;
   Expression * clone() const override;
+  bool isPositive() const override { return true; }
 private:
   template<typename T> static Complex<T> computeOnComplex(const Complex<T> c, AngleUnit angleUnit);
   virtual Evaluation<float> * privateEvaluate(SinglePrecision p, Context& context, AngleUnit angleUnit) const override {

poincare/include/poincare/expression.h

@@ -95,6 +95,9 @@ public:
   static Expression * parse(char const * string);
   virtual ~Expression() = default;
   virtual Expression * clone() const = 0;
+  /* If isPositive is false, the expression has no sign (it does have to be
+   * negative) */
+  virtual bool isPositive() const { return false; }
 
   /* Poor man's RTTI */
   virtual Type type() const = 0;

poincare/include/poincare/power.h

@@ -13,6 +13,7 @@ class Power : public StaticHierarchy<2> {
 public:
   Type type() const override;
   Expression * clone() const override;
+  bool isPositive() const override;
   template<typename T> static Complex<T> compute(const Complex<T> c, const Complex<T> d);
 private:
   constexpr static float k_maxNumberOfSteps = 10000.0f;

poincare/include/poincare/rational.h

@@ -24,6 +24,7 @@ public:
   // Expression subclassing
   Type type() const override;
   Expression * clone() const override;
+  bool isPositive() const override { return !isNegative(); }
 
   // Basic test
   bool isZero() const { return m_numerator.isZero(); }

poincare/src/power.cpp

@@ -23,6 +23,16 @@ Expression * Power::clone() const {
   return new Power(m_operands, true);
 }
 
+bool Power::isPositive() const {
+  if (operand(1)->type() == Type::Rational) {
+    const Rational * r = static_cast<const Rational *>(operand(1));
+    if (r->denominator().isOne() && Integer::Division(r->numerator(), Integer(2)).remainder.isZero()) {
+      return true;
+    }
+  }
+  return false;
+}
+
 template<typename T>
 Complex<T> Power::compute(const Complex<T> c, const Complex<T> d) {
   if (d.b() != 0) {