1: !
  2: !
  3: !    Fortran kernel for sparse triangular solve in the AIJ matrix format
  4: ! This ONLY works for factorizations in the NATURAL ORDERING, i.e.
  5: ! with MatSolve_SeqAIJ_NaturalOrdering()
  6: !
  7: #include <petsc/finclude/petscsys.h>
  8: !
  9: pure subroutine FortranSolveAIJ(n,x,ai,aj,adiag,aa,b)
 10:   use, intrinsic :: ISO_C_binding
 11:   implicit none (type, external)
 12:   PetscScalar, intent(in) :: aa(0:*),b(0:*)
 13:   PetscInt, intent(in) :: n,ai(0:*), aj(0:*),adiag(0:*)
 14:   PetscScalar, intent(inout) :: x(0:*)

 16:   PetscInt i,jstart,jend
 17:   !
 18:   ! Forward Solve
 19:   !
 20:   x(0) = b(0)
 21:   do i=1,n-1
 22:     jstart = ai(i)
 23:     jend   = adiag(i) - 1
 24:     x(i) = b(i) - sum(aa(jstart:jend)*x(aj(jstart:jend)))
 25:   end do
 26:   !
 27:   ! Backward solve the upper triangular
 28:   !
 29:   do i=n-1,0,-1
 30:     jstart = adiag(i) + 1
 31:     jend   = ai(i+1) - 1
 32:     x(i) = x(i) - sum(aa(jstart:jend)*x(aj(jstart:jend))) * aa(adiag(i))
 33:   end do
 34: end subroutine FortranSolveAIJ