dcnst0 Subroutine

subroutine dcnst0(neq, y, icnstr, iret)
!! This routine checks for constraint violations in the initial approximate solution `y`.

   use daskr_kinds, only: rk, zero
   implicit none

   integer, intent(in) :: neq
      !! Problem size.
   real(rk), intent(in) :: y(neq)
      !! Initial approximate solution.
   integer, intent(in) :: icnstr(neq)
      !! Array indicating which variables are constrained and how. For any element of
      !! `icnstr`, the meaning is as follows. 
      !!  `2`: `y(i) > 0`.
      !!  `1`: `y(i) >= 0`.
      !! `-1`: `y(i) <= 0`.
      !! `-2`: `y(i) < 0`.
      !!  `0`: `y(i)` is unconstrained.
   integer, intent(out) :: iret
      !! Output flag.
      !!     `0`: All constraints satisfied.
      !! `i > 0`: Constraint `i` violated.

   integer :: i

   iret = 0
   do i = 1, neq
      if (icnstr(i) .eq. 2) then
         if (y(i) .le. zero) then
            iret = i
            exit
         end if
      elseif (icnstr(i) .eq. 1) then
         if (y(i) .lt. zero) then
            iret = i
            exit
         end if
      elseif (icnstr(i) .eq. -1) then
         if (y(i) .gt. zero) then
            iret = i
            exit
         end if
      elseif (icnstr(i) .eq. -2) then
         if (y(i) .ge. zero) then
            iret = i
            exit
         end if
      end if
   end do

end subroutine dcnst0