#! /usr/bin/env python3
#
def cellular_automaton ( step_num, cell_num ):

  import matplotlib.pyplot as plt
  import numpy as np
#
#  Compute the entire X array.
#
  x = np.zeros ( [ step_num + 1, cell_num ], dtype = int  )
#
#  Set the initial condition in row 1.
#
  mid = ( cell_num - 1 ) // 2
  x[0,mid] = 1
#
#  Compute rows 0 through step_num by applying rule 30.
#
  for i in range ( 1, step_num + 1 ):
#
#  Compute columns 1 through cell_num-2 by applying rule 30.
#  (Leave first and last values at 0)
#
    for j in range ( 1, cell_num - 1 ):

      if ( ( x[i-1,j-1] == 0 and x[i-1,j] == 0 and x[i-1,j+1] == 1 ) or \
           ( x[i-1,j-1] == 0 and x[i-1,j] == 1 and x[i-1,j+1] == 0 ) or \
           ( x[i-1,j-1] == 0 and x[i-1,j] == 1 and x[i-1,j+1] == 1 ) or \
           ( x[i-1,j-1] == 1 and x[i-1,j] == 0 and x[i-1,j+1] == 0 ) ):
        x[i,j] = 1
#
#  Make a plot of red and white squares, with an outline.
#
  plt.clf ( )

  for i in range ( 0, step_num + 1 ):
    for j in range ( 0, cell_num ):
      if ( x[i,j] == 1 ):
        k = 'r'
      else:
        k = 'w'
      i2 = step_num - i
      plt.fill ( [ j, j+1, j+1, j    ], [ i2, i2, i2+1, i2+1    ], color = k )
      plt.plot ( [ j, j+1, j+1, j, j ], [ i2, i2, i2+1, i2+1, i2 ], 'k-' )

  plt.axis ( 'off' )
  plt.axis ( 'equal' )

  filename = 'cellular_automaton.png'
  plt.savefig ( filename )
  print ( '  Graphics saved as "' + filename + '"' )
  plt.close ( )

  return

if ( __name__ == "__main__" ):

  step_num = 25
  cell_num = 51
  cellular_automaton ( step_num, cell_num )