!>
!> @file dtblok.f90
!>
!> @brief
!>
!> @copyright
!> Copyright (©) 2021 EPFL (Ecole Polytechnique Fédérale de Lausanne)
!> SPC (Swiss Plasma Center)
!>
!> SPClibs 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.
!>
!> SPClibs is distributed in the hope that it will be useful, but WITHOUT ANY
!> WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
!> FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
!>
!> You should have received a copy of the GNU Lesser General Public License
!> along with this program. If not, see <https://www.gnu.org/licenses/>.
!>
!> @authors
!> (in alphabetical order)
!> @author Trach-Minh Tran <trach-minh.tran@epfl.ch>
!>
subroutine dtblok ( bloks, integs, nbloks, ipivot, iflag, detsgn, detlog )

!*************************************************************************
!
!! DTBLOK gets the determinant of an almost block diagonal matrix.
!
!  Discussion:
!
!    The matrix's PLU factorization must have been obtained
!    previously by FCBLOK.
!
!    The logarithm of the determinant is computed instead of the
!    determinant itself to avoid the danger of overflow or underflow
!    inherent in this calculation.
!
!  Reference:
!
!    Carl DeBoor,
!    A Practical Guide to Splines,
!    Springer Verlag.
!
!  Parameters:
!
!  bloks, integs, nbloks, ipivot, iflag  are as on return from fcblok.
!            in particular, iflag=(-1)**(number of interchanges dur-
!            ing factorization) if successful, otherwise iflag=0.
!
!  detsgn  on output, contains the sign of the determinant.
!
!  detlog  on output, contains the natural logarithm of the determi-
!            nant if determinant is not zero. otherwise contains 0.
!
  implicit none

  integer nbloks

  real ( kind = 8 ) bloks(1)
  real ( kind = 8 ) detlog
  real ( kind = 8 ) detsgn
  integer i
  integer iflag
  integer index
  integer indexp
  integer integs(3,nbloks)
  integer ip
  integer ipivot(1)
  integer k
  integer last
  integer nrow

  detsgn = iflag
  detlog = 0.0D+00

  if ( iflag == 0 ) then
    return
  end if

  index = 0
  indexp = 0

  do i = 1, nbloks

    nrow = integs(1,i)
    last = integs(3,i)

    do k = 1, last
      ip = index + nrow * (k-1) + ipivot(indexp+k)
      detlog = detlog + log ( abs ( bloks(ip) ) )
      detsgn = detsgn * sign ( 1.0D+00, bloks(ip) )
    end do

    index = nrow*integs(2,i)+index
    indexp = indexp+nrow

  end do

  return
end