def _high_pass(data, alpha=0.5):
    """
    Runs a high pass RC filter over the given data.

    Parameters
    ----------
    data : array_like

    alpha : float
            Smoothing factor between 0.0 (exclusive) and 1.0 (inclusive).
            A lower value means more filtering.  A value of 1.0 equals no
            filtering. Defaults is 0.5.

    Returns
    -------
    hpf : array_like
          Filtered data.

    Based on http://en.wikipedia.org/wiki/High-pass_filter

    """
    mean = data.mean()
    data = data - mean
    hpf = data.copy()
    for k in range(1, len(data)):
        hpf[k] = alpha * hpf[k - 1] + alpha * (data[k] - data[k - 1])
    return hpf + mean
