don't fully cast array in round_sig_figs

crates/polars-ops/src/series/ops/round.rs

@@ -40,21 +40,17 @@ pub trait RoundSeries: SeriesSealed {
 
     fn round_sig_figs(&self, digits: i32) -> PolarsResult<Series> {
         let s = self.as_series();
-        if digits < 1 {
-            polars_bail!(
-                InvalidOperation: "digits must be an integer >= 1"
-            )
-        };
-
+        polars_ensure!(digits >= 1, InvalidOperation: "digits must be an integer >= 1");
+        polars_ensure!(s.dtype().is_numeric(), InvalidOperation: "round_sig_figs can only be used on numeric types" );
         with_match_physical_numeric_polars_type!(s.dtype(), |$T| {
-            let s = s.cast(&DataType::Float64)?;
-            let ca = s.f64()?;
+            let ca: &ChunkedArray<$T> = s.as_ref().as_ref().as_ref();
             let s = ca.apply_values(|value| {
+                let value = value as f64;
                 if value == 0.0 {
-                    return 0.0;
+                    return value as <$T as PolarsNumericType>::Native;
                 }
-                let magnitude = 10.0_f64.powi(digits - 1 - ((value.abs()).log10().floor() as i32));
-                (value * magnitude).round() / magnitude
+                let magnitude = 10.0_f64.powi(digits - 1 - value.abs().log10().floor() as i32);
+                ((value * magnitude).round() / magnitude) as <$T as PolarsNumericType>::Native
             }).into_series();
             return Ok(s);
         });