Check overflow when casting floating point value to decimal256

arrow-buffer/src/bigint.rs

@@ -16,7 +16,7 @@
 // under the License.
 
 use num::cast::AsPrimitive;
-use num::BigInt;
+use num::{BigInt, FromPrimitive};
 use std::cmp::Ordering;
 
 /// A signed 256-bit integer
@@ -102,6 +102,19 @@ impl i256 {
         Self::from_parts(v as u128, v >> 127)
     }
 
+    /// Create an optional i256 from the provided `f64`. Returning `None`
+    /// if overflow occurred
+    pub fn from_f64(v: f64) -> Option<Self> {
+        BigInt::from_f64(v).and_then(|i| {
+            let (integer, overflow) = i256::from_bigint_with_overflow(i);
+            if overflow {
+                None
+            } else {
+                Some(integer)
+            }
+        })
+    }
+
     /// Create an i256 from the provided low u128 and high i128
     #[inline]
     pub const fn from_parts(low: u128, high: i128) -> Self {

arrow-cast/src/cast.rs

@@ -387,16 +387,38 @@ fn cast_floating_point_to_decimal256<T: ArrowPrimitiveType>(
     array: &PrimitiveArray<T>,
     precision: u8,
     scale: u8,
+    cast_options: &CastOptions,
 ) -> Result<ArrayRef, ArrowError>
 where
     <T as ArrowPrimitiveType>::Native: AsPrimitive<f64>,
 {
     let mul = 10_f64.powi(scale as i32);
 
-    array
-        .unary::<_, Decimal256Type>(|v| i256::from_i128((v.as_() * mul).round() as i128))
-        .with_precision_and_scale(precision, scale)
-        .map(|a| Arc::new(a) as ArrayRef)
+    if cast_options.safe {
+        let iter = array
+            .iter()
+            .map(|v| v.and_then(|v| i256::from_f64((v.as_() * mul).round())));
+        let casted_array =
+            unsafe { PrimitiveArray::<Decimal256Type>::from_trusted_len_iter(iter) };
+        casted_array
+            .with_precision_and_scale(precision, scale)
+            .map(|a| Arc::new(a) as ArrayRef)
+    } else {
+        array
+            .try_unary::<_, Decimal256Type, _>(|v| {
+                i256::from_f64((v.as_() * mul).round()).ok_or_else(|| {
+                    ArrowError::CastError(format!(
+                        "Cannot cast to {}({}, {}). Overflowing on {:?}",
+                        Decimal256Type::PREFIX,
+                        precision,
+                        scale,
+                        v
+                    ))
+                })
+            })
+            .and_then(|a| a.with_precision_and_scale(precision, scale))
+            .map(|a| Arc::new(a) as ArrayRef)
+    }
 }
 
 /// Cast the primitive array using [`PrimitiveArray::reinterpret_cast`]
@@ -666,11 +688,13 @@ pub fn cast_with_options(
                     as_primitive_array::<Float32Type>(array),
                     *precision,
                     *scale,
+                    cast_options,
                 ),
                 Float64 => cast_floating_point_to_decimal256(
                     as_primitive_array::<Float64Type>(array),
                     *precision,
                     *scale,
+                    cast_options,
                 ),
                 Null => Ok(new_null_array(to_type, array.len())),
                 _ => Err(ArrowError::CastError(format!(
@@ -6166,4 +6190,31 @@ mod tests {
             err
         );
     }
+
+    #[test]
+    fn test_cast_floating_point_to_decimal256_overflow() {
+        let array = Float64Array::from(vec![f64::MAX]);
+        let array = Arc::new(array) as ArrayRef;
+        let casted_array = cast_with_options(
+            &array,
+            &DataType::Decimal256(76, 50),
+            &CastOptions { safe: true },
+        );
+        assert!(casted_array.is_ok());
+        assert!(casted_array.unwrap().is_null(0));
+
+        let casted_array = cast_with_options(
+            &array,
+            &DataType::Decimal256(76, 50),
+            &CastOptions { safe: false },
+        );
+        let err = casted_array.unwrap_err().to_string();
+        let expected_error = "Cast error: Cannot cast to Decimal256(76, 50)";
+        assert!(
+            err.contains(expected_error),
+            "did not find expected error '{}' in actual error '{}'",
+            expected_error,
+            err
+        );
+    }
 }