#! /usr/bin/env python
#
def ubvec_next_gray ( n, t ):

#*****************************************************************************80
#
#% UBVEC_NEXT_GRAY computes the next UBVEC in the Gray code.
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license.
#
#  Modified:
#
#    22 November 2015
#
#  Author:
#
#    John Burkardt
#
#  Reference:
#
#    Donald Kreher, Douglas Simpson,
#    Combinatorial Algorithms,
#    CRC Press, 1998,
#    ISBN: 0-8493-3988-X,
#    LC: QA164.K73.
#
#  Parameters:
#
#    Input, integer N, the number of digits in each element.
#    N must be positive.
#
#    Input/output, integer T(N).
#    On input, T contains an element of the Gray code, that is,
#    each entry T(I) is either 0 or 1.
#    On output, T contains the successor to the input value this
#    is an element of the Gray code, which differs from the input
#    value in a single position.
#
  import numpy as np

  weight = np.sum ( t )

  if ( ( weight % 2 ) == 0 ):

    if ( t[n-1] == 0 ):
      t[n-1] = 1
    else:
      t[n-1] = 0

    return t

  else:

    for i in range ( n - 1, 0, -1 ):
      if ( t[i] == 1 ):
        if ( t[i-1] == 0 ):
          t[i-1] = 1
        else:
          t[i-1] = 0
        return t
#
#  The final element was input.
#  Return the first element.
#
    for i in range ( 0, n ):
      t[i] = 0

  return t

def ubvec_next_gray_test ( ):

#*****************************************************************************80
#
## UBVEC_NEXT_GRAY_TEST tests UBVEC_NEXT_GRAY.
#
#  Licensing:
#
#    This code is distributed under the GNU LGPL license. 
#
#  Modified:
#
#    22 November 2015
#
#  Author:
#
#    John Burkardt
#
  import numpy as np
  import platform

  n = 4

  print ( '' )
  print ( 'UBVEC_NEXT_GRAY_TEST' )
  print ( '  Python version: %s' % ( platform.python_version ( ) ) )
  print ( '  UBVEC_NEXT_GRAY returns the next UBVEC in the Gray code.' )

  print ( '' )
  print ( '   K  UBVEC' )
  print ( '' )

  k = 0
  g = np.zeros ( n )

  while ( True ):

    print ( '  %2d  ' % ( k ), end = '' )
    for j in range ( 0, n ):
      print ( '%2d' % ( g[j] ), end = '' )
    print ( '' )

    k = k + 1
    g = ubvec_next_gray ( n, g )

    if ( np.sum ( g ) == 0 ):
      break
#
#  Terminate.
#
  print ( '' )
  print ( 'UBVEC_NEXT_GRAY_TEST' )
  print ( '  Normal end of execution.' )
  return

if ( __name__ == '__main__' ):
  from timestamp import timestamp
  timestamp ( )
  ubvec_next_gray_test ( )
  timestamp ( )