Function half_deviance 

pub fn half_deviance(obs: f64, mean: f64) -> f64

obs·ln(obs/mean) + mean − obs, by C. Loader’s bd0 (“Fast and accurate computation of binomial probabilities”, 2000): a series in v = (obs − mean)/(obs + mean) when |obs − mean| < 0.1 (obs + mean). For obs ≥ 0 and mean > 0; 0 when both are 0, and +∞ when only mean is.