// SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-or-later
// SPDX-FileCopyrightText: Bradley M. Bell <bradbell@seanet.com>
// SPDX-FileContributor: 2003-24 Bradley M. Bell
// ----------------------------------------------------------------------------
/*
{xrst_begin atomic_four_lin_ode_reverse.cpp}
{xrst_spell
   cccc
}

Atomic Linear ODE Reverse Mode: Example and Test
################################################

Purpose
*******
This example demonstrates using reverse mode with
the :ref:`atomic_four_lin_ode-name` class.

f(u)
****
For this example, the function :math:`f(u) = z(r, u)` where
:math:`z(t, u)` solves the following ODE

.. math::

   z_t (t, u) =
   \left( \begin{array}{cccc}
   0   & 0  & 0    & 0   \\
   u_4 & 0  & 0    & 0   \\
   0   & u_5 & 0   & 0   \\
   0   & 0   & u_6 & 0   \\
   \end{array} \right)
   z(t, u)
   \W{,}
   z(0, u) =
   \left( \begin{array}{c}
   u_0 \\
   u_1 \\
   u_2 \\
   u_3 \\
   \end{array} \right)

Solution
********
The actual solution to this ODE is

.. math::

   z(t, u) =
   \left( \begin{array}{l}
   u_0  \\
   u_1 + u_4 u_0 t \\
   u_2 + u_5 u_1 t + u_5 u_4 u_0 t^2 / 2  \\
   u_3 + u_6 u_2 t + u_6 u_5 u_1 t^2 / 2 + u_6 u_5 u_4 u_0 t^3 / 6
   \end{array} \right)

g(u)
****

.. math::

   z_2 (t, u) = u_2 + u_5 u_1 t + u_5 u_4 u_0 t^2 / 2

Fix :math:`r` and define :math:`g(u) = [ \partial_u z(r, u) ]^\R{T}`.
It follows that

.. math::

   g(u)
   =
   \left( \begin{array}{c}
   u_5 u_4 r^2 / 2 \\
   u_5 r \\
   1 \\
   0 \\
   u_5 u_0 r^2 / 2 \\
   u_t r + u_4 u_0 r^2 / 2 \\
   0
   \end{array} \right)

Source
******
{xrst_literal
   // BEGIN C++
   // END C++
}

{xrst_end atomic_four_lin_ode_reverse.cpp}
*/
// BEGIN C++
# include <cppad/cppad.hpp>
# include <cppad/example/atomic_four/lin_ode/lin_ode.hpp>

namespace { // BEGIN_EMPTY_NAMESPACE

template <class Scalar, class Vector>
Vector Z(Scalar t, const Vector& u)
{  size_t nz = 4;
   Vector z(nz);
   //
   z[0]  = u[0];
   z[1]  = u[1] + u[4]*u[0]*t;
   z[2]  = u[2] + u[5]*u[1]*t + u[5]*u[4]*u[0]*t*t/2.0;
   z[3]  = u[3] + u[6]*u[2]*t + u[6]*u[5]*u[1]*t*t/2.0
          + u[6]*u[5]*u[4]*u[0]*t*t*t/6.0;
   //
   return z;
}

template <class Scalar, class Vector>
Vector G(Scalar t, const Vector& u)
{  size_t nu = 7;
   Vector g(nu);
   //
   g[0]  = u[5]*u[4]*t*t/2.0;
   g[1]  = u[5]*t;
   g[2]  = Scalar(1.0);
   g[3]  = Scalar(0.0);
   g[4]  = u[5]*u[0]*t*t/2.0;
   g[5]  = u[1]*t + u[4]*u[0]*t*t/2.0;
   g[6]  = Scalar(0.0);
   //
   return g;
}

} // END_EMPTY_NAMESPACE

bool reverse(void)
{  // ok
   bool ok = true;
   //
   // AD, NearEqual, eps99
   using CppAD::AD;
   using CppAD::NearEqual;
   double eps99 = std::numeric_limits<double>::epsilon() * 99.0;
   // -----------------------------------------------------------------------
   // Record f
   // -----------------------------------------------------------------------
   //
   // afun
   CppAD::atomic_lin_ode<double> afun("atomic_lin_ode");
   //
   // m, r
   size_t m      = 4;
   double r      = 2.0;
   double step   = 2.0;
   //
   // pattern, transpose
   size_t nr  = m;
   size_t nc  = m;
   size_t nnz = 3;
   CppAD::sparse_rc< CppAD::vector<size_t> > pattern(nr, nc, nnz);
   for(size_t k = 0; k < nnz; ++k)
   {  size_t i = k + 1;
      size_t j = k;
      pattern.set(k, i, j);
   }
   bool transpose = false;
   //
   // ny, ay
   size_t ny = m;
   CPPAD_TESTVECTOR( AD<double> ) ay(ny);
   //
   // nu, au
   size_t nu = nnz + m;
   CPPAD_TESTVECTOR( AD<double> ) au(nu);
   for(size_t j = 0; j < nu; ++j)
      au[j] = AD<double>(j + 1);
   CppAD::Independent(au);
   //
   // ax
   CPPAD_TESTVECTOR( AD<double> ) ax(nnz + m);
   for(size_t k = 0; k < nnz; ++k)
      ax[k] = au[m + k];
   for(size_t i = 0; i < m; ++i)
      ax[nnz + i] = au[i];
   //
   // ay
   size_t call_id = afun.set(r, step, pattern, transpose);
   afun(call_id, ax, ay);
   //
   // f
   CppAD::ADFun<double> f(au, ay);
   // -----------------------------------------------------------------------
   // ar, check_f
   CppAD::Independent(au);
   AD<double> ar = r;
   ay = Z(ar, au);
   CppAD::ADFun<double> check_f(au, ay);
   // -----------------------------------------------------------------------
   // reverse mode on f
   // -----------------------------------------------------------------------
   //
   // u
   CPPAD_TESTVECTOR(double) u(nu);
   for(size_t j = 0; j < nu; ++j)
      u[j] = double( j + 2 );
   //
   // y
   // zero order forward mode computation of f(u)
   CPPAD_TESTVECTOR(double) y(ny);
   y = f.Forward(0, u);
   //
   // ok
   CPPAD_TESTVECTOR(double) check_y = check_f.Forward(0, u);
   for(size_t i = 0; i < ny; ++i)
      ok &= NearEqual(y[i], check_y[i], eps99, eps99);
   //
   // w, ok
   CPPAD_TESTVECTOR(double) w(ny), dw(nu), check_dw(nu);
   for(size_t i = 0; i < ny; ++i)
      w[i] = 0.0;
   for(size_t i = 0; i < ny; ++i)
   {  w[i] = 1.0;
      dw        = f.Reverse(1, w);
      check_dw  = check_f.Reverse(1, w);
      for(size_t j = 0; j < nu; ++j)
         ok &= NearEqual(dw[j], check_dw[j], eps99, eps99);
      w[i] = 0.0;
   }
   // -----------------------------------------------------------------------
   // Record g
   // -----------------------------------------------------------------------
   //
   // af
   CppAD::ADFun< AD<double>, double> af = f.base2ad();
   //
   // au
   CppAD::Independent(au);
   CPPAD_TESTVECTOR( AD<double> ) aw(ny), adw(nu);
   af.Forward(0, au);
   for(size_t i = 0; i < ny; ++i)
      aw[i] = 0.0;
   aw[2] = 1.0;
   adw = af.Reverse(1, aw);
   // g
   CppAD::ADFun<double> g(au, adw);
   // -----------------------------------------------------------------------
   // check_g
   CppAD::Independent(au);
   ay = G(ar, au);
   CppAD::ADFun<double> check_g(au, ay);
   // -----------------------------------------------------------------------
   //
   // v
   // zero order forward mode computation of g(u)
   CPPAD_TESTVECTOR(double) v(nu);
   v = g.Forward(0, u);
   //
   // ok
   CPPAD_TESTVECTOR(double) check_v = check_g.Forward(0, u);
   for(size_t i = 0; i < nu; ++i)
      ok &= NearEqual(v[i], check_v[i], eps99, eps99);
   //
   // w, ok
   w.resize(nu);
   for(size_t i = 0; i < nu; ++i)
      w[i] = 0.0;
   for(size_t i = 0; i < nu; ++i)
   {  w[i] = 1.0;
      dw        = g.Reverse(1, w);
      check_dw  = check_g.Reverse(1, w);
      for(size_t j = 0; j < nu; ++j)
         ok &= NearEqual(dw[j], check_dw[j], eps99, eps99);
      w[i] = 0.0;
   }
   // -----------------------------------------------------------------------
   return ok;
}
// END C++
