/* -*- c++ -*- (enables emacs c++ mode) */
/*===========================================================================

 Copyright (C) 2003-2017 Yves Renard

 This file is a part of GetFEM++

 GetFEM++  is  free software;  you  can  redistribute  it  and/or modify it
 under  the  terms  of the  GNU  Lesser General Public License as published
 by  the  Free Software Foundation;  either version 3 of the License,  or
 (at your option) any later version along with the GCC Runtime Library
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 This program  is  distributed  in  the  hope  that it will be useful,  but
 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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 License and GCC Runtime Library Exception for more details.
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 As a special exception, you  may use  this file  as it is a part of a free
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===========================================================================*/

/**@file gmm_opt.h
   @author  Yves Renard <Yves.Renard@insa-lyon.fr>
   @date July 9, 2003.
   @brief Optimization for some small cases (inversion of 2x2 matrices etc.)
*/
#ifndef GMM_OPT_H__
#define GMM_OPT_H__

namespace gmm {

  /* ********************************************************************* */
  /*    Optimized determinant and inverse for small matrices (2x2 and 3x3) */
  /*    with dense_matrix<T>.                                              */
  /* ********************************************************************* */

  template <typename T>  T lu_det(const dense_matrix<T> &A) {
    size_type n(mat_nrows(A));
    if (n) {
      const T *p = &(A(0,0));
      switch (n) {
      case 1 : return (*p);
      case 2 : return (*p) * (*(p+3)) - (*(p+1)) * (*(p+2));
// Not stable for nearly singular matrices
//       case 3 : return (*p) * ((*(p+4)) * (*(p+8)) - (*(p+5)) * (*(p+7)))
// 		 - (*(p+1)) * ((*(p+3)) * (*(p+8)) - (*(p+5)) * (*(p+6)))
// 		 + (*(p+2)) * ((*(p+3)) * (*(p+7)) - (*(p+4)) * (*(p+6)));
      default :
	{
	  dense_matrix<T> B(mat_nrows(A), mat_ncols(A));
	  std::vector<size_type> ipvt(mat_nrows(A));
	  gmm::copy(A, B);
	  lu_factor(B, ipvt);
	  return lu_det(B, ipvt);	
	}
      }
    }
    return T(1);
  }


  template <typename T> T lu_inverse(const dense_matrix<T> &A_, bool doassert = true) {
    dense_matrix<T>& A = const_cast<dense_matrix<T> &>(A_);
    size_type N = mat_nrows(A);
    T det(1);
    if (N) {
      T *p = &(A(0,0));
      if (N <= 2) {
	switch (N) {
	  case 1 : {
	    det = *p;
	    if (doassert) GMM_ASSERT1(det!=T(0), "non invertible matrix");
            if (det == T(0)) break;
	    *p = T(1) / det; 
	  } break;
	  case 2 : {
	    det = (*p) * (*(p+3)) - (*(p+1)) * (*(p+2));
	    if (doassert) GMM_ASSERT1(det!=T(0), "non invertible matrix");
            if (det == T(0)) break;
	    std::swap(*p, *(p+3));
	    *p++ /= det; *p++ /= -det; *p++ /= -det; *p++ /= det; 
	  } break;
// 	  case 3 : { // not stable for nearly singular matrices
// 	    T a, b, c, d, e, f, g, h, i;
// 	    a =   (*(p+4)) * (*(p+8)) - (*(p+5)) * (*(p+7));
// 	    b = - (*(p+1)) * (*(p+8)) + (*(p+2)) * (*(p+7));
// 	    c =   (*(p+1)) * (*(p+5)) - (*(p+2)) * (*(p+4));
// 	    d = - (*(p+3)) * (*(p+8)) + (*(p+5)) * (*(p+6));
// 	    e =   (*(p+0)) * (*(p+8)) - (*(p+2)) * (*(p+6));
// 	    f = - (*(p+0)) * (*(p+5)) + (*(p+2)) * (*(p+3));
// 	    g =   (*(p+3)) * (*(p+7)) - (*(p+4)) * (*(p+6));
// 	    h = - (*(p+0)) * (*(p+7)) + (*(p+1)) * (*(p+6));
// 	    i =   (*(p+0)) * (*(p+4)) - (*(p+1)) * (*(p+3));
// 	    det = (*p) * a + (*(p+1)) * d + (*(p+2)) * g;
// 	    GMM_ASSERT1(det!=T(0), "non invertible matrix");
// 	    *p++ = a / det; *p++ = b / det; *p++ = c / det; 
// 	    *p++ = d / det; *p++ = e / det; *p++ = f / det; 
// 	    *p++ = g / det; *p++ = h / det; *p++ = i / det; 
// 	  } break;
	}
      }
      else {
	dense_matrix<T> B(mat_nrows(A), mat_ncols(A));
	std::vector<int> ipvt(mat_nrows(A));
	gmm::copy(A, B);
	size_type info = lu_factor(B, ipvt);
	GMM_ASSERT1(!info, "non invertible matrix");
	lu_inverse(B, ipvt, A);
	return lu_det(B, ipvt);
      }
    }
    return det;
  }

  
}

#endif //  GMM_OPT_H__