atan2.hpp

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#ifndef __STAN__AGRAD__REV__ATAN2_HPP__
#define __STAN__AGRAD__REV__ATAN2_HPP__

#include <valarray>
#include <stan/agrad/rev/var.hpp>
#include <stan/agrad/rev/op/vv_vari.hpp>
#include <stan/agrad/rev/op/vd_vari.hpp>
#include <stan/agrad/rev/op/dv_vari.hpp>

namespace stan {
  namespace agrad {
    
    namespace {
      class atan2_vv_vari : public op_vv_vari {
      public:
        atan2_vv_vari(vari* avi, vari* bvi) :
          op_vv_vari(std::atan2(avi->val_,bvi->val_),avi,bvi) {
        }
        void chain() {
          double a_sq_plus_b_sq = (avi_->val_ * avi_->val_) + (bvi_->val_ * bvi_->val_);
          avi_->adj_ += adj_ * bvi_->val_ / a_sq_plus_b_sq;
          bvi_->adj_ -= adj_ * avi_->val_ / a_sq_plus_b_sq;
        }
      };

      class atan2_vd_vari : public op_vd_vari {
      public:
        atan2_vd_vari(vari* avi, double b) :
          op_vd_vari(std::atan2(avi->val_,b),avi,b) {
        }
        void chain() {
          double a_sq_plus_b_sq = (avi_->val_ * avi_->val_) + (bd_ * bd_);
          avi_->adj_ += adj_ * bd_ / a_sq_plus_b_sq;
        }
      };

      class atan2_dv_vari : public op_dv_vari {
      public:
        atan2_dv_vari(double a, vari* bvi) :
          op_dv_vari(std::atan2(a,bvi->val_),a,bvi) {
        }
        void chain() {
          double a_sq_plus_b_sq = (ad_ * ad_) + (bvi_->val_ * bvi_->val_);
          bvi_->adj_ -= adj_ * ad_ / a_sq_plus_b_sq;
        }
      };
    }

    inline var atan2(const var& a, const var& b) {
      return var(new atan2_vv_vari(a.vi_,b.vi_));
    }

    inline var atan2(const var& a, const double b) {
      return var(new atan2_vd_vari(a.vi_,b));
    }

    inline var atan2(const double a, const var& b) {
      return var(new atan2_dv_vari(a,b.vi_));
    }

  }
}
#endif