// $Id$
# ifndef CPPAD_CORE_REV_TWO_HPP
# define CPPAD_CORE_REV_TWO_HPP

/* --------------------------------------------------------------------------
CppAD: C++ Algorithmic Differentiation: Copyright (C) 2003-16 Bradley M. Bell

CppAD is distributed under multiple licenses. This distribution is under
the terms of the
                    Eclipse Public License Version 1.0.

A copy of this license is included in the COPYING file of this distribution.
Please visit http://www.coin-or.org/CppAD/ for information on other licenses.
-------------------------------------------------------------------------- */

/*
$begin RevTwo$$
$spell
	ddw
	typename
	Taylor
	const
$$





$section Reverse Mode Second Partial Derivative Driver$$
$mindex order easy$$

$head Syntax$$
$icode%ddw% = %f%.RevTwo(%x%, %i%, %j%)%$$


$head Purpose$$
We use $latex F : B^n \rightarrow B^m$$ to denote the
$cref/AD function/glossary/AD Function/$$ corresponding to $icode f$$.
The syntax above sets
$latex \[
	ddw [ k * p + \ell ]
	=
	\DD{ F_{i[ \ell ]} }{ x_{j[ \ell ]} }{ x_k } (x)
\] $$
for $latex k = 0 , \ldots , n-1$$
and $latex \ell = 0 , \ldots , p$$,
where $latex p$$ is the size of the vectors $icode i$$ and $icode j$$.

$head f$$
The object $icode f$$ has prototype
$codei%
	ADFun<%Base%> %f%
%$$
Note that the $cref ADFun$$ object $icode f$$ is not $code const$$
(see $cref/RevTwo Uses Forward/RevTwo/RevTwo Uses Forward/$$ below).

$head x$$
The argument $icode x$$ has prototype
$codei%
	const %VectorBase% &%x%
%$$
(see $cref/VectorBase/RevTwo/VectorBase/$$ below)
and its size
must be equal to $icode n$$, the dimension of the
$cref/domain/seq_property/Domain/$$ space for $icode f$$.
It specifies
that point at which to evaluate the partial derivatives listed above.

$head i$$
The argument $icode i$$ has prototype
$codei%
	const %VectorSize_t% &%i%
%$$
(see $cref/VectorSize_t/RevTwo/VectorSize_t/$$ below)
We use $icode p$$ to denote the size of the vector $icode i$$.
All of the indices in $icode i$$
must be less than $icode m$$, the dimension of the
$cref/range/seq_property/Range/$$ space for $icode f$$; i.e.,
for $latex \ell = 0 , \ldots , p-1$$, $latex i[ \ell ]  < m$$.

$head j$$
The argument $icode j$$ has prototype
$codei%
	const %VectorSize_t% &%j%
%$$
(see $cref/VectorSize_t/RevTwo/VectorSize_t/$$ below)
and its size must be equal to $icode p$$,
the size of the vector $icode i$$.
All of the indices in $icode j$$
must be less than $icode n$$; i.e.,
for $latex \ell = 0 , \ldots , p-1$$, $latex j[ \ell ]  < n$$.

$head ddw$$
The result $icode ddw$$ has prototype
$codei%
	%VectorBase% %ddw%
%$$
(see $cref/VectorBase/RevTwo/VectorBase/$$ below)
and its size is $latex n * p$$.
It contains the requested partial derivatives; to be specific,
for $latex k = 0 , \ldots , n - 1 $$
and $latex \ell = 0 , \ldots , p - 1$$
$latex \[
	ddw [ k * p + \ell ]
	=
	\DD{ F_{i[ \ell ]} }{ x_{j[ \ell ]} }{ x_k } (x)
\] $$

$head VectorBase$$
The type $icode VectorBase$$ must be a $cref SimpleVector$$ class with
$cref/elements of type Base/SimpleVector/Elements of Specified Type/$$.
The routine $cref CheckSimpleVector$$ will generate an error message
if this is not the case.

$head VectorSize_t$$
The type $icode VectorSize_t$$ must be a $cref SimpleVector$$ class with
$cref/elements of type size_t/SimpleVector/Elements of Specified Type/$$.
The routine $cref CheckSimpleVector$$ will generate an error message
if this is not the case.

$head RevTwo Uses Forward$$
After each call to $cref Forward$$,
the object $icode f$$ contains the corresponding
$cref/Taylor coefficients/glossary/Taylor Coefficient/$$.
After a call to $code RevTwo$$,
the zero order Taylor coefficients correspond to
$icode%f%.Forward(0, %x%)%$$
and the other coefficients are unspecified.

$head Examples$$
$children%
	example/rev_two.cpp
%$$
The routine
$cref/RevTwo/rev_two.cpp/$$ is both an example and test.
It returns $code true$$, if it succeeds and $code false$$ otherwise.

$end
-----------------------------------------------------------------------------
*/

//  BEGIN CppAD namespace
namespace CppAD {

template <typename Base>
template <typename VectorBase, typename VectorSize_t>
VectorBase ADFun<Base>::RevTwo(
	const VectorBase   &x,
	const VectorSize_t &i,
	const VectorSize_t &j)
{	size_t i1;
	size_t j1;
	size_t k;
	size_t l;

	size_t n = Domain();
	size_t m = Range();
	size_t p = i.size();

	// check VectorBase is Simple Vector class with Base elements
	CheckSimpleVector<Base, VectorBase>();

	// check VectorSize_t is Simple Vector class with size_t elements
	CheckSimpleVector<size_t, VectorSize_t>();

	CPPAD_ASSERT_KNOWN(
		x.size() == n,
		"RevTwo: Length of x not equal domain dimension for f."
	);
	CPPAD_ASSERT_KNOWN(
		i.size() == j.size(),
		"RevTwo: Lenght of the i and j vectors are not equal."
	);
	// point at which we are evaluating the second partials
	Forward(0, x);

	// dimension the return value
	VectorBase ddw(n * p);

	// direction vector in argument space
	VectorBase dx(n);
	for(j1 = 0; j1 < n; j1++)
		dx[j1] = Base(0);

	// direction vector in range space
	VectorBase w(m);
	for(i1 = 0; i1 < m; i1++)
		w[i1] = Base(0);

	// place to hold the results of a reverse calculation
	VectorBase r(n * 2);

	// check the indices in i and j
	for(l = 0; l < p; l++)
	{	i1 = i[l];
		j1 = j[l];
		CPPAD_ASSERT_KNOWN(
		i1 < m,
		"RevTwo: an eleemnt of i not less than range dimension for f."
		);
		CPPAD_ASSERT_KNOWN(
		j1 < n,
		"RevTwo: an element of j not less than domain dimension for f."
		);
	}

	// loop over all forward directions
	for(j1 = 0; j1 < n; j1++)
	{	// first order forward mode calculation done
		bool first_done = false;
		for(l = 0; l < p; l++) if( j[l] == j1 )
		{	if( ! first_done )
			{	first_done = true;

				// first order forward mode in j1 direction
				dx[j1] = Base(1);
				Forward(1, dx);
				dx[j1] = Base(0);
			}
			// execute a reverse in this component direction
			i1    = i[l];
			w[i1] = Base(1);
			r     = Reverse(2, w);
			w[i1] = Base(0);

			// place the reverse result in return value
			for(k = 0; k < n; k++)
				ddw[k * p + l] = r[k * 2 + 1];
		}
	}
	return ddw;
}

} // END CppAD namespace

# endif
