Function test_short_halfway_cases

Modules/_decimal/tests/randfloat.py:11–52

()

def test_short_halfway_cases():
    # exact halfway cases with a small number of significant digits
    for k in 0, 5, 10, 15, 20:
        # upper = smallest integer >= 2**54/5**k
        upper = -(-2**54//5**k)
        # lower = smallest odd number >= 2**53/5**k
        lower = -(-2**53//5**k)
        if lower % 2 == 0:
            lower += 1
        for i in range(10 * TEST_SIZE):
            # Select a random odd n in [2**53/5**k,
            # 2**54/5**k). Then n * 10**k gives a halfway case
            # with small number of significant digits.
            n, e = random.randrange(lower, upper, 2), k

            # Remove any additional powers of 5.
            while n % 5 == 0:
                n, e = n // 5, e + 1
            assert n % 10 in (1, 3, 7, 9)

            # Try numbers of the form n * 2**p2 * 10**e, p2 >= 0,
            # until n * 2**p2 has more than 20 significant digits.
            digits, exponent = n, e
            while digits < 10**20:
                s = '{}e{}'.format(digits, exponent)
                yield s
                # Same again, but with extra trailing zeros.
                s = '{}e{}'.format(digits * 10**40, exponent - 40)
                yield s
                digits *= 2

            # Try numbers of the form n * 5**p2 * 10**(e - p5), p5
            # >= 0, with n * 5**p5 < 10**20.
            digits, exponent = n, e
            while digits < 10**20:
                s = '{}e{}'.format(digits, exponent)
                yield s
                # Same again, but with extra trailing zeros.
                s = '{}e{}'.format(digits * 10**40, exponent - 40)
                yield s
                digits *= 5
                exponent -= 1

def test_halfway_cases():
    # test halfway cases for the round-half-to-even rule