Method evaluate

datafusion-examples/examples/udf/simple_udwf.rs:207–227

This function is called once per input row. `range`specifies which indexes of `values` should be considered for the calculation. Note this is the SLOWEST, but simplest, way to evaluate a window function. It is much faster to implement evaluate_all or evaluate_all_with_rank, if possible

(
        &mut self,
        values: &[ArrayRef],
        range: &std::ops::Range<usize>,
    )

    /// window function. It is much faster to implement
    /// evaluate_all or evaluate_all_with_rank, if possible
    fn evaluate(
        &mut self,
        values: &[ArrayRef],
        range: &std::ops::Range<usize>,
    ) -> Result<ScalarValue> {
        // Again, the input argument is an array of floating
        // point numbers to calculate a moving average
        let arr: &Float64Array = values[0].as_ref().as_primitive::<Float64Type>();

        let range_len = range.end - range.start;

        // our smoothing function will average all the values in the
        let output = if range_len > 0 {
            let sum: f64 = arr.values().iter().skip(range.start).take(range_len).sum();
            Some(sum / range_len as f64)
        } else {
            None
        };

        Ok(ScalarValue::Float64(output))
    }
}