def interval_overlap_length(i1, i2):
    """Compute the length of overlap of two intervals.

    Parameters
    ----------
    i1, i2 : pairs of two floats
        The two intervals.

    Returns
    -------
    l : float
        The length of the overlap between the two intervals.

    """
    a, b = i1
    c, d = i2
    if a < c:
        if b < c:
            return 0.0
        elif b < d:
            return b - c
        else:
            return d - c
    elif a < d:
        if b < d:
            return b - a
        else:
            return d - a
    else:
        return 0
