!>
!> @file shiftb.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/>.
!>
!> @author
!> (in alphabetical order)
!> @author Trach-Minh Tran <trach-minh.tran@epfl.ch>
!>
subroutine shiftb ( ai, ipivot, nrowi, ncoli, last, ai1, nrowi1, ncoli1 )

!*************************************************************************
!
!! SHIFTB shifts the rows in current block, ai, not used as pivot
!  rows, if any, i.e., rows ipivot(last+1),...,ipivot(nrowi),
!  onto the first mmax=nrow-last rows of the next block, ai1,
!  with column last+j of ai  going to column j ,
!  for j=1,...,jmax=ncoli-last. the remaining columns of these
!  rows of ai1 are zeroed out.
!
!                             picture
!
!       original situation after         results in a new block i+1
!       last=2 columns have been       created and ready to be
!       done in factrb (assuming no      factored by next factrb call.
!       interchanges of rows)
!                   1
!              x  x 1x  x  x           x  x  x  x  x
!                   1
!              0  x 1x  x  x           0  x  x  x  x
!  block i          1                       ---
!  nrowi=4     0  0 1x  x  x           0  0 1x  x  x  0  01
!  ncoli=5          1                       1             1
!  last=2      0  0 1x  x  x           0  0 1x  x  x  0  01
!              -------------------          1             1   new
!                   1x  x  x  x  x          1x  x  x  x  x1  block
!                   1                       1             1   i+1
!  block i+1        1x  x  x  x  x          1x  x  x  x  x1
!  nrowi1= 5        1                       1             1
!  ncoli1= 5        1x  x  x  x  x          1x  x  x  x  x1
!              -------------------          1-------------1
!                   1
!
!  Reference:
!
!    Carl DeBoor,
!    A Practical Guide to Splines,
!    Springer Verlag.
!
  implicit none

  integer ncoli
  integer ncoli1
  integer nrowi1
  integer nrowi

  real ( kind = 8 ) ai(nrowi,ncoli)
  real ( kind = 8 ) ai1(nrowi1,ncoli1)
  integer ip
  integer ipivot(nrowi)
  integer j
  integer last
  integer m

  if ( nrowi-last < 1 ) then
    return
  end if

  if ( ncoli-last < 1 ) then
    return
  end if
!
!  Put the remainder of block I into AI1.
!
  do m = 1, nrowi-last
    ip = ipivot(last+m)
    do j = 1, ncoli-last
      ai1(m,j) = ai(ip,last+j)
    end do
  end do
!
!  Zero out the upper right corner of ai1.
!
  do j = ncoli+1-last, ncoli1
    do m = 1, nrowi-last
      ai1(m,j) = 0.0D+00
    end do
  end do

  return
end