from typing import Sequence


def moving_average(source: Sequence[float], window_size: int):
    """
    Compute a simple moving average from a source list.

    Arguments:
        source:       The source list of data from which to compute the moving average.
        window_size:  The size of the moving average to compute.

    Returns:
        list(float):  The computed moving average. The returned list will contain 0.0 for the first 'window_size'
                      elements.
    """

    result = [0.0] * len(source)    

    for source_index in range(window_size, len(source)):
        window_sum = sum(source[source_index - window_size:source_index])
        result[source_index] = window_sum / window_size

    return result