subroutine bc1(kappa,l,jdim,jmax,ndim,n,x,t,io,task,u)

c General information:

c     Subroutine bc1 enforces a periodic boundary condition at the
c     left end of a bar.  It must be combined with a periodic
c     boundary condition bc2 at the other end.

c     Copyright 1996 Leon van Dommelen
c     Version 1.0 Leon van Dommelen 12/16/96

c Usage information:

c     Subroutine bc1 can be called in three different ways depending on
c     the chosen value of parameter task:

c     1. task = 0:
c        This is the standard call. Subroutine bc1 sets the value
c     of u(0,n) equal to u(jmax-1,n).  Note that this means that
c     u(jmax-1,n) must already have a value.

c     2. task = -1:
c        This is the inquiry call. Subroutine bc1 changes task to 2 to
c     indicate it is a periodic boundary condition.

c     3. Any other value:
c        If task is positive, subroutine bc1 merely initializes itself
c     and exits, otherwise it does nothing at all.

c Arguments:

c     Avoid typos
      implicit none

c     Input: conduction constant:
      double precision kappa

c     Input: length of the bar:
      double precision l

c     Input: declared maximum mesh point index in arrays x and u:
      integer jdim

c     Input: actual maximum mesh point index:
      integer jmax

c     Input: declared maximum time level index in arrays t and u:
      integer ndim

c     Input: time level to set in array u:
      integer n

c     Input: array of the x-positions of the mesh points.
      double precision x(0:jdim)

c     Input: array of the mesh times t:
      double precision t(0:ndim)

c     Input: I/O unit of an already open output file or zero:
      integer io
c        Subroutine bc1 only writes to this unit if io is positive.

c     Input/Output: task to perform:
      integer task
c        See the usage information above for more information on task.

c     Input/Output: temperature values:
      double precision u(0:jdim,0:ndim)
c        Provided task equals zero, bc1 sets u(0,n) equal to
c     u(jmax-1,n), which must be defined.
c        If task is nonzero, u is ignored.

c External variables and info for compiling or changing subroutine bc1:

c     The following utility routines from ../../../lib/util.f were used:

c        cwrite(module,text,io,show) writes line "text" to I/O unit
c     "io" if nonzero, and to the screen if "show" is nonzero.
      external cwrite

c        fatal(module,text1,text2,text3) kills the program after a
c     fatal error, printing the lines "text1", "text2" and "text3".
      external fatal

c Local variables:

c     An integer to keep track of whether bc1 has been initialized:
      integer init

c     Integer code for a periodic boundary conditions:
      integer period
      parameter (period=2)

c     Data statements:
      data init/0/

c Executable statements:

c For task = -1, change task to indicate a periodic boundary condition:

      if(task.lt.0)then
         task=period
         return
      endif

c Ignore other negative tasks:

      if(task.lt.0)return

c Initialization during the first time that bc1 is called:

      if(init.eq.0)then

c        Show which subroutine is being initialized:
         call cwrite('bc1',
     &   'Initialization of left boundary condition bc1:',io,0)
         call cwrite('bc1','- periodic boundary '//
     &   'condition enforcement at the left end;',io,0)
         call cwrite('bc1',
     &   '- period/bc_1.f Copyright 1996 Leon van Dommelen.',io,0)

c        All done with the initialization:
         init=1

      endif

c Further ignore nonzero tasks:

      if(task.ne.0)return

c Check the sanity of some of the arguments:

      if(jdim.le.0 .or. ndim.lt.0 .or.
     & jmax.le.0 .or. n.lt.0 .or.
     & jmax.gt.jdim .or. n.gt.ndim)call fatal('bc1',
     & 'Program error: invalid arguments issued to subroutine bc1.',
     & 'Fix the program code.',' ')

      if(jmax.lt.2)call fatal('bc1','Periodic '//
     &'boundary conditions require at least 2 mesh intervals.',' ',' ')

c Set the boundary point:

      u(0,n)=u(jmax-1,n)
      goto 900

c Exit:

c     Jump here when done:
 900  return
      end