pub fn arima_diff(data: &[f64], d: usize) -> Vec<f64>Expand description
ARIMA(p,d,q) differencing step.
Applies first-order differencing d times to produce a stationary series.
Returns the d-th order differenced series.
After one round: result[i] = data[i+1] - data[i], length n-1.
After d rounds: length n-d.