Function round_decimal

datafusion/functions/src/math/round.rs:645–706

(
    value: V,
    input_scale: i8,
    output_scale: i8,
    decimal_places: i32,
)

}

fn round_decimal<V: ArrowNativeTypeOp>(
    value: V,
    input_scale: i8,
    output_scale: i8,
    decimal_places: i32,
) -> Result<V, ArrowError> {
    let diff = i64::from(input_scale) - i64::from(decimal_places);
    if diff <= 0 {
        return Ok(value);
    }

    debug_assert!(diff <= i64::from(u32::MAX));
    let diff = diff as u32;

    let one = V::ONE;
    let two = V::from_usize(2).ok_or_else(|| {
        ArrowError::ComputeError("Internal error: could not create constant 2".into())
    })?;
    let ten = V::from_usize(10).ok_or_else(|| {
        ArrowError::ComputeError("Internal error: could not create constant 10".into())
    })?;

    let factor = ten.pow_checked(diff).map_err(|_| {
        ArrowError::ComputeError(format!(
            "Overflow while rounding decimal with scale {input_scale} and decimal places {decimal_places}"
        ))
    })?;

    let mut quotient = value.div_wrapping(factor);
    let remainder = value.mod_wrapping(factor);

    // `factor` is an even number (10^n, n > 0), so `factor / 2` is the tie threshold
    let threshold = factor.div_wrapping(two);
    if remainder >= threshold {
        quotient = quotient.add_checked(one).map_err(|_| {
            ArrowError::ComputeError("Overflow while rounding decimal".into())
        })?;
    } else if remainder <= threshold.neg_wrapping() {
        quotient = quotient.sub_checked(one).map_err(|_| {
            ArrowError::ComputeError("Overflow while rounding decimal".into())
        })?;
    }

    // `quotient` is the rounded value at scale `decimal_places`. Rescale to the desired
    // `output_scale` (which is always >= `decimal_places` in cases where diff > 0).
    let scale_shift = i64::from(output_scale) - i64::from(decimal_places);
    if scale_shift == 0 {
        return Ok(quotient);
    }

    debug_assert!(scale_shift > 0);
    debug_assert!(scale_shift <= i64::from(u32::MAX));
    let scale_shift = scale_shift as u32;
    let shift_factor = ten.pow_checked(scale_shift).map_err(|_| {
        ArrowError::ComputeError(format!(
            "Overflow while rounding decimal with scale {input_scale} and decimal places {decimal_places}"
        ))
    })?;