Circular Array

is_circular.py

def is_circular(arr, start):
    if not isinstance(arr, list): raise TypeError('Expected arr to be list')
    if not isinstance(start, int): raise TypeError('Expected start to be int')
    
    size = len(arr)
    n = size
    pos = start

    # key running count to track that each index is hit
    # use the index as the count, total_exp will be the
    # sum of all the index positions
    total = 0
    total_exp = 0

    while n > 0:
        next = arr[pos]
        
        # use modulus to normalize negatives and numbers that loop
        # around the array
        pos = (next + pos) % size
                
        # add the index found to the accumulator
        total += pos

        n -= 1
        
        # add to expected indexes hit
        total_exp += n


    # We should be at the initial value of the array again
    if pos != start: return False

    # verify that all indexes were hit
    if total != total_exp: return False

    return True

if __name__ == '__main__':
    import unittest

    class Test(unittest.TestCase):
        def setUp(self):
            self.tests = [
                ([5, 2, -1], True),
                ([1, 2, 3], False),
                ([0, 2, 1], False),
                ([1, 2, 1], False),
                ([2, 2, 2], True),
                ([1, 2, 2, -1], True),
                ([-1, 2, -1], True),
                ([-2, 3, 3, 3, -2], True)
            ]

        def test_types(self):
            with self.assertRaises(TypeError):
                is_circular(1, 2)

            with self.assertRaises(TypeError):
                is_circular([1, -1], {})

        def test_value(self):
            for (vals, boolean) in self.tests:
                # test circularness works from any starting point
                for i in xrange(len(vals)): 
                    self.assertEqual(
                        is_circular(vals, i), 
                        boolean, 
                        'Vals %s should be %s with rand %d' % (vals, boolean, i)
                    )

    unittest.main()