4.3.4 Changing requirements

In the previous section, we concentrated on the general requirements of a Gray code. It is possible to specify these without specifying the actual code. It is easy to see that there are several codes that satisfy these requirements. In good XP style, we only tested the requirements and nothing more.

It may be that more control is needed. For example, the requirement may be for a particular code, instead of compliance with general properties. As an illustration, we will show how to test for the original Gray code, which is one specific instance that satisfies the requirements of the previous section. In this particular case, this test will actually be easier than the previous one.

L1 = ['0', '1']
L2 = ['00', '01', '11', '10']
L3 = ['000', '001', '011', '010', '110', '111', '101', 100']

Python is well-known for its elegant algorithmic descriptions, and this is a good example. We can write the algorithm in Python as follows:

def nextLn(Ln):
    """ Return Gray code Ln+1, given Ln. """
    Ln0 = ['0' + codeword for codeword in Ln]
    Ln1 = ['1' + codeword for codeword in Ln]
    Ln1.reverse()
    return Ln0 + Ln1

The code "['0' + codeword for ...]" is called a list comprehension. It is a concise way to describe lists built by short computations in a for loop.

The requirement is now that the output code matches the expected code Ln. We use the nextLn function to compute the expected result. The new test case code is as follows:

class TestOriginalGrayCode(TestCase):

    def testOriginalGrayCode(self):
        """ Check that the code is an original Gray code """

        Rn = []
        
        def stimulus(B, G, n):
            for i in range(2**n):
                B.next = intbv(i)
                yield delay(10)
                Rn.append(bin(G, width=n))
        
        Ln = ['0', '1'] # n == 1
        for n in range(2, MAX_WIDTH):
            Ln = nextLn(Ln)
            del Rn[:]
            B = Signal(intbv(-1))
            G = Signal(intbv(0))
            dut = bin2gray(B, G, n)
            stim = stimulus(B, G, n)
            sim = Simulation(dut, stim)
            sim.run(quiet=1)
            self.assertEqual(Ln, Rn)

% python test_gray.py -v TestOriginalGrayCode
Check that the code is an original Gray code ... ok

----------------------------------------------------------------------
Ran 1 tests in 3.091s

OK