codac2_pow.h

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#pragma once
 
#include "codac2_Interval.h"
#include "codac2_AnalyticType.h"
#include "codac2_AnalyticExprWrapper.h"
#include "codac2_sqr.h"
 
namespace codac2
{
  struct PowOp
  {
    template<typename X1,typename X2>
    static std::string str(const X1& x1, const X2& x2)
    {
      return x1->str(!x1->is_str_leaf()) + "^" + x2->str(!x2->is_str_leaf());
    }
 
    template<typename X1, typename X2>
    static std::pair<Index,Index> output_shape([[maybe_unused]] const X1& s1, [[maybe_unused]] const X2& s2)
    {
      return {1,1};
    }
    
    static Interval fwd(const Interval& x1, const Interval& x2);
    static ScalarType fwd_natural(const ScalarType& x1, const ScalarType& x2);
    static ScalarType fwd_centered(const ScalarType& x1, const ScalarType& x2);
    static void bwd(const Interval& y, Interval& x1, int x2);
    static void bwd(const Interval& y, Interval& x1, Interval& x2);
  };
 
  // Analytic operator
  // The following function can be used to build analytic expressions.
 
  inline ScalarExpr
  pow(const ScalarExpr& x1, const ScalarExpr& x2)
  {
    auto x2_as_const_value = std::dynamic_pointer_cast<ConstValueExpr<ScalarType>>(x2->copy());
    if(x2_as_const_value && x2_as_const_value->value() == 2.)
    {
      // SqrOp may be better than the generic PowOp with a simple power of 2
      return { std::make_shared<AnalyticOperationExpr<SqrOp,ScalarType,ScalarType>>(x1) };
    }
 
    return { std::make_shared<AnalyticOperationExpr<PowOp,ScalarType,ScalarType,ScalarType>>(x1,x2) };
  }
 
  inline ScalarExpr
  operator^(const ScalarExpr& x1, const ScalarExpr& x2)
  {
    return pow(x1,x2);
  }
 
  // Inline functions
 
  inline Interval PowOp::fwd(const Interval& x1, const Interval& x2)
  {
    return pow(x1,x2);
  }
 
  inline ScalarType PowOp::fwd_natural(const ScalarType& x1, const ScalarType& x2)
  {
    bool x2isint = x2.a.is_integer();
    bool x2positive = x2.a.lb()>=0.0;
    return {
      fwd(x1.a, x2.a),
      x1.def_domain && x2.def_domain 
        && (x2isint || x1.a.lb()>=0.0)
        && (x2positive || !x1.a.contains(0.0))
    };
  }
 
  inline ScalarType PowOp::fwd_centered(const ScalarType& x1, const ScalarType& x2)
  {
    if(centered_form_not_available_for_args(x1,x2))
      return fwd_natural(x1,x2);
 
    bool x2isint = x2.a.is_integer();
    bool x2positive = x2.a.lb()>=0.0;
    IntervalMatrix d(1,x1.da.size());
    
    if (x2.a.is_degenerated())
    { 
      // To avoid calling log(x1.a), as it would return emptyset if x1.a<=0
      for(Index i = 0 ; i < d.size() ; i++)
        d(0,i) = x2.a*x1.da(0,i)*pow(x1.a,x2.a-1.);
    }
 
    else
    {
      for(Index i = 0 ; i < d.size() ; i++)
      {
        d(0,i) = x2.a*x1.da(0,i)*pow(x1.a,x2.a-1.) 
                + x2.da(0,i)*log(x1.a)*pow(x1.a,x2.a); 
              // Not good when x1 close to 0
      }
    }
 
    return {
      fwd(x1.m, x2.m),
      fwd(x1.a, x2.a),
      d,
      x1.def_domain && x2.def_domain 
        && (x2isint || x1.a.lb()>=0.0)
        && (x2positive || !x1.a.contains(0.0))
    };
  }
 
  inline void PowOp::bwd(const Interval& y, Interval& x1, int x2)
  {
    // The content of this function comes from the IBEX library.
    // See ibex::Interval (IBEX lib, main author: Gilles Chabert)
    //   https://ibex-lib.readthedocs.io
 
    if(x2 % 2 == 0)
    {
      Interval proj = root(y, x2);
      Interval pos_proj = proj & x1;
      Interval neg_proj = (-proj) & x1;
      x1 = pos_proj | neg_proj;
    }
 
    else
      x1 &= root(y, x2);
  }
 
  inline void PowOp::bwd(const Interval& y, Interval& x1, Interval& x2)
  {
    assert_release(x2.is_degenerated() && "PowOp::bwd(y,x1,x2) (with x1 and x2 intervals) not implemented yet with Gaol");
    assert_release(x2 == (int)(x2.mid()) && "PowOp::bwd(y,x1,x2) (x2 not integer) not implemented yet with Gaol");
    PowOp::bwd(y, x1, x2.mid());
  }
}