#! /usr/bin/env python
#
from dolfin import *

def burgers_time_viscous ( e_num, nu ):

#*****************************************************************************80
#
## burgers_time_viscous, 1D time-dependent viscous Burgers equation.
#
#  Discussion:
#
#    dudt - nu u" + u del u = 0, 
#    -1 < x < 1, 0 < t
#    u(-1,t) = -1, u(1,t) = 1
#    u(x,0) = x
#
#    This equation is nonlinear in U.
#
#  Licensing:
#
#    This code is distributed under the MIT license.
#
#  Modified:
#
#    21 October 2018
#
#  Author:
#
#    John Burkardt
#
#  Parameters:
#
#    Input, integer E_NUM, the number of elements to use.
#
#    Input, real NU, the viscosity, which should be positive.
#    The larger it is, the smoother the solution will be.
#
  import matplotlib.pyplot as plt

  print ( '' )
  print ( '  Number of elements is %d' % ( e_num ) )
  print ( '  Viscosity set to %g' % ( nu ) )
#
#  Create a mesh on the interval [0,+1].
#
  x_left = -1.0
  x_right = +1.0
  mesh = IntervalMesh ( e_num, x_left, x_right )
#
#  Define the function space to be of Lagrange type
#  using piecewise linear basis functions.
#
  V = FunctionSpace ( mesh, "CG", 1 )
#
#  Define the boundary conditions.
#  if X <= XLEFT + eps, then U = U_LEFT
#  if X_RIGHT - eps <= X, then U = U_RIGHT
#
  u_left = -1.0
  def on_left ( x, on_boundary ):
    return ( on_boundary and near ( x[0], x_left ) )
  bc_left = DirichletBC ( V, u_left, on_left )

  u_right = +1.0
  def on_right ( x, on_boundary ):
    return ( on_boundary and near ( x[0], x_right ) )
  bc_right = DirichletBC ( V, u_right, on_right )

  bc = [ bc_left, bc_right ]
#
#  Define the initial condition.
#
  u_init = Expression ( "x[0]", degree = 1 )
#
#  Define the trial functions (u) and test functions (v).
#
  u = Function ( V )
  u_old = Function ( V )
  v = TestFunction ( V )
#
#  Set U and U0 by interpolation.
#
  u.interpolate ( u_init )
  u_old.assign ( u )
#
#  Set the time step.
#  We need a UFL version "DT" for the function F,
#  and a Python version "dt" to do a conditional in the time loop.
#
  DT = Constant ( 0.01 )
  dt = 0.01
#
#  Set the source term.
#
  f = Expression ( "0.0", degree = 0 )
#
#  Write the function to be satisfied.
#
  n = FacetNormal ( mesh )
#
#  Write the function F.
#
  F = \
  ( \
    dot ( u - u_old, v ) / DT \
  + nu * inner ( grad ( u ), grad ( v ) ) \
  + inner ( u * u.dx(0), v ) \
  - dot ( f, v ) \
  ) * dx
#
#  Specify the jacobian.
#
  J = derivative ( F, u )
#
#  Do the time integration.
#
  k = 0
  t = 0.0

  t_plot = 0.0
  t_final = 1.0

  while ( True ):

    if ( k % 10 == 0 ):
      plot ( u, title = ( 'burgers time viscous %g' % ( t ) ) )
      plt.grid ( True )
      filename = ( 'burgers_time_viscous_%d.png' % ( k ) )
      plt.savefig ( filename )
      print ( 'Graphics saved as "%s"' % ( filename ) )
      plt.close ( )
      t_plot = t_plot + 0.1

    if ( t_final <= t ):
      print ( '' )
      print ( 'Reached final time.' )
      break

    k = k + 1
    t = t + dt

    solve ( F == 0, u, bc, J = J )

    u_old.assign ( u )

  return

def burgers_time_viscous_test ( ):

#*****************************************************************************80
#
## burgers_time_viscous_test tests burgers_time_viscous.
#
#  Licensing:
#
#    This code is distributed under the MIT license.
#
#  Modified:
#
#    21 October 2018
#
#  Author:
#
#    John Burkardt
#
  import dolfin
  import platform
  import time

  print ( time.ctime ( time.time() ) )
#
#  Report level = only warnings or higher.
#
  level = 30
  set_log_level ( level )

  print ( '' )
  print ( 'burgers_time_viscous_test:' )
  print ( '  Python version: %s' % ( platform.python_version ( ) ) )
  print ( '  FENICS version %s'% ( dolfin.__version__ ) )
  print ( '  Solve the time-dependent 1d viscous Burgers equation.' )

  e_num = 32
  nu = 0.05
  burgers_time_viscous ( e_num, nu )
#
#  Terminate.
#
  print ( "" )
  print ( "burgers_time_viscous_test:" )
  print ( "  Normal end of execution." )
  print ( '' )
  print ( time.ctime ( time.time() ) )
  return

if ( __name__ == '__main__' ):

  burgers_time_viscous_test ( )