def _log_sub()

in opacus/accountants/analysis/rdp.py [0:0]

• 12 lines of code
• 5 McCabe index (conditional complexity)

def _log_sub(logx: float, logy: float) -> float:
    r"""Subtracts two numbers in the log space.

    Args:
        logx: First term in log space. Expected to be greater than the second term.
        logy: First term in log space. Expected to be less than the first term.

    Returns:
        Difference of numbers in log space.

    Raises:
        ValueError
            If the result is negative.
    """
    if logx < logy:
        raise ValueError("The result of subtraction must be non-negative.")
    if logy == -np.inf:  # subtracting 0
        return logx
    if logx == logy:
        return -np.inf  # 0 is represented as -np.inf in the log space.

    try:
        # Use exp(x) - exp(y) = (exp(x - y) - 1) * exp(y).
        return math.log(math.expm1(logx - logy)) + logy  # expm1(x) = exp(x) - 1
    except OverflowError:
        return logx