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import numpy as np
from math import ceil


def gaussian_kernel(sigma, cutoff=4, force_odd_size=False):
    """
    Generates a Gaussian convolution kernel.

    :param sigma: Standard Deviation of the Gaussian curve.
    :param cutoff: Parameter tuning the truncation of the Gaussian.
        The higher cutoff, the biggest the array will be (and the closest to
        a "true" Gaussian function).
    :param force_odd_size: when set to True, the resulting array will always
        have an odd size, regardless of the values of "sigma" and "cutoff".
    :return: a numpy.ndarray containing the truncated Gaussian function.
        The array size is 2*c*s+1 where c=cutoff, s=sigma.

    Nota: due to the quick decay of the Gaussian function, small values of the
        "cutoff" parameter are usually fine. The energy difference between a
        Gaussian truncated to [-c, c] and a "true" one is
            erfc(c/(sqrt(2)*s))
        so choosing cutoff=4*sigma keeps the truncation error below 1e-4.
    """
    size = int(ceil(2 * cutoff * sigma + 1))
    if force_odd_size and size % 2 == 0:
        size += 1
    x = np.arange(size) - (size - 1.0) / 2.0
    g = np.exp(-((x / sigma) ** 2) / 2.0)
    g /= g.sum()
    return g
