#' Obtain delta forecast by the Persistence method
#'
#' @inheritParams getForecast
#' @inheritParams getZeroDeltaForecast
getPersistenceDeltaForecast = function(data, today, memory, horizon, shape_params, ...)
{
	#return gap between end of similar day curve and first day of tomorrow (in the past)
	first_day = max(1, today-memory)
	index = today-7
	repeat
	{
		{
			last_similar_serie_end = tail( data$getCenteredSerie(index), 1)
			last_similar_tomorrow_begin = data$getCenteredSerie(index+1)[1]
			index = index - 7
		};
		if (!is.na(last_similar_serie_end) && !is.na(last_similar_tomorrow_begin))
			return (last_similar_tomorrow_begin - last_similar_serie_end);
		if (index < first_day)
			return (NA)
	}
}