fix: decimal precision/scale for fuzz (#3266)

Signed-off-by: Andrew Duffy <[email protected]>

Author: a10y

vortex-dtype/src/arbitrary.rs

@@ -2,7 +2,10 @@ use std::sync::Arc;
 
 use arbitrary::{Arbitrary, Result, Unstructured};
 
-use crate::{DType, DecimalDType, FieldName, FieldNames, Nullability, PType, StructDType};
+use crate::{
+    DECIMAL256_MAX_PRECISION, DECIMAL256_MAX_SCALE, DType, DecimalDType, FieldName, FieldNames,
+    Nullability, PType, StructDType,
+};
 
 impl<'a> Arbitrary<'a> for DType {
     fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
@@ -65,11 +68,15 @@ impl<'a> Arbitrary<'a> for PType {
 }
 
 impl<'a> Arbitrary<'a> for DecimalDType {
-    #[allow(clippy::unwrap_in_result, clippy::expect_used)]
+    #[allow(
+        clippy::unwrap_in_result,
+        clippy::expect_used,
+        clippy::cast_possible_truncation
+    )]
     fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
         // Get a random integer for the scale
-        let precision = u8::try_from(u.int_in_range(0..=38)?).expect("u8 overflow");
-        let scale = i8::try_from(u.int_in_range(-38..=38)?).expect("i8 overflow");
+        let precision = u.int_in_range(1..=DECIMAL256_MAX_PRECISION)?;
+        let scale = u.int_in_range(-DECIMAL256_MAX_SCALE..=(precision as i8))?;
         Ok(Self::new(precision, scale))
     }
 }

vortex-dtype/src/decimal.rs

@@ -8,6 +8,15 @@ const MAX_SCALE: i8 = 76;
 /// Maximum precision for a Decimal128 type from Arrow
 pub const DECIMAL128_MAX_PRECISION: u8 = 38;
 
+/// Maximum precision for a Decimal256 type from Arrow
+pub const DECIMAL256_MAX_PRECISION: u8 = 76;
+
+/// Maximum sacle for a Decimal128 type from Arrow
+pub const DECIMAL128_MAX_SCALE: i8 = 38;
+
+/// Maximum sacle for a Decimal256 type from Arrow
+pub const DECIMAL256_MAX_SCALE: i8 = 76;
+
 /// Parameters that define the precision and scale of a decimal type.
 ///
 /// Decimal types allow real numbers with a similar precision and scale to be represented exactly.