_struct.py 文件源码

def float_pack(x, size):
    """Convert a Python float x into a 64-bit unsigned integer
    with the same byte representation."""

    if size == 8:
        MIN_EXP = -1021  # = sys.float_info.min_exp
        MAX_EXP = 1024   # = sys.float_info.max_exp
        MANT_DIG = 53    # = sys.float_info.mant_dig
        BITS = 64
    elif size == 4:
        MIN_EXP = -125   # C's FLT_MIN_EXP
        MAX_EXP = 128    # FLT_MAX_EXP
        MANT_DIG = 24    # FLT_MANT_DIG
        BITS = 32
    else:
        raise ValueError("invalid size value")

    sign = math.copysign(1.0, x) < 0.0
    if math.isinf(x):
        mant = 0
        exp = MAX_EXP - MIN_EXP + 2
    elif math.isnan(x):
        mant = 1 << (MANT_DIG-2) # other values possible
        exp = MAX_EXP - MIN_EXP + 2
    elif x == 0.0:
        mant = 0
        exp = 0
    else:
        m, e = math.frexp(abs(x))  # abs(x) == m * 2**e
        exp = e - (MIN_EXP - 1)
        if exp > 0:
            # Normal case.
            mant = round_to_nearest(m * (1 << MANT_DIG))
            mant -= 1 << MANT_DIG - 1
        else:
            # Subnormal case.
            if exp + MANT_DIG - 1 >= 0:
                mant = round_to_nearest(m * (1 << exp + MANT_DIG - 1))
            else:
                mant = 0
            exp = 0

        # Special case: rounding produced a MANT_DIG-bit mantissa.
        assert 0 <= mant <= 1 << MANT_DIG - 1
        if mant == 1 << MANT_DIG - 1:
            mant = 0
            exp += 1

        # Raise on overflow (in some circumstances, may want to return
        # infinity instead).
        if exp >= MAX_EXP - MIN_EXP + 2:
             raise OverflowError("float too large to pack in this format")

    # check constraints
    assert 0 <= mant < 1 << MANT_DIG - 1
    assert 0 <= exp <= MAX_EXP - MIN_EXP + 2
    assert 0 <= sign <= 1
    return ((sign << BITS - 1) | (exp << MANT_DIG - 1)) | mant