text: from sympy.combinatorics.graycode import (GrayCode, bin_to_gray,
      random_bitstring, get_subset_from_bitstring, graycode_subsets,
      gray_to_bin)
  from sympy.utilities.pytest import raises
  
  def test_graycode():
      g = GrayCode(2)
      got = []
      for i in g.generate_gray():
          if i.startswith('0'):
              g.skip()
          got.append(i)
      assert got == '00 11 10'.split()
      a = GrayCode(6)
      assert a.current == '0'*6
      assert a.rank == 0
      assert len(list(a.generate_gray())) == 64
      codes = ['011001', '011011', '011010',
      '011110', '011111', '011101', '011100', '010100', '010101', '010111',
      '010110', '010010', '010011', '010001', '010000', '110000', '110001',
      '110011', '110010', '110110', '110111', '110101', '110100', '111100',
      '111101', '111111', '111110', '111010', '111011', '111001', '111000',
      '101000', '101001', '101011', '101010', '101110', '101111', '101101',
      '101100', '100100', '100101', '100111', '100110', '100010', '100011',
      '100001', '100000']
      assert list(a.generate_gray(start='011001')) == codes
      assert list(
          a.generate_gray(rank=GrayCode(6, start='011001').rank)) == codes
      assert a.next().current == '000001'
      assert a.next(2).current == '000011'
      assert a.next(-1).current == '100000'
  
      a = GrayCode(5, start='10010')
      assert a.rank == 28
      a = GrayCode(6, start='101000')
      assert a.rank == 48
  
      assert GrayCode(6, rank=4).current == '000110'
      assert GrayCode(6, rank=4).rank == 4
      assert [GrayCode(4, start=s).rank for s in
              GrayCode(4).generate_gray()] == [0, 1, 2, 3, 4, 5, 6, 7, 8,
                                               9, 10, 11, 12, 13, 14, 15]
      a = GrayCode(15, rank=15)
      assert a.current == '000000000001000'
  
      assert bin_to_gray('111') == '100'
  
      a = random_bitstring(5)
      assert type(a) is str
      assert len(a) == 5
      assert all(i in ['0', '1'] for i in a)
  
      assert get_subset_from_bitstring(
          ['a', 'b', 'c', 'd'], '0011') == ['c', 'd']
      assert get_subset_from_bitstring('abcd', '1001') == ['a', 'd']
      assert list(graycode_subsets(['a', 'b', 'c'])) == \
          [[], ['c'], ['b', 'c'], ['b'], ['a', 'b'], ['a', 'b', 'c'],
      ['a', 'c'], ['a']]
  
      raises(ValueError, lambda: GrayCode(0))
      raises(ValueError, lambda: GrayCode(2.2))
      raises(ValueError, lambda: GrayCode(2, start=[1, 1, 0]))
      raises(ValueError, lambda: GrayCode(2, rank=2.5))
  
  def test_live_issue_117():
      assert bin_to_gray('0100') == '0110'
      assert bin_to_gray('0101') == '0111'
      for bits in ('0100', '0101'):
          assert gray_to_bin(bin_to_gray(bits)) == bits
  