fix(rust): parse sign for decimal properly (#11302)

crates/polars-arrow/src/compute/decimal.rs

@@ -28,12 +28,20 @@ pub fn infer_scale(bytes: &[u8]) -> Option<u8> {
 /// Deserializes bytes to a single i128 representing a decimal
 /// The decimal precision and scale are not checked.
 #[inline]
-pub(super) fn deserialize_decimal(bytes: &[u8], precision: Option<u8>, scale: u8) -> Option<i128> {
+pub(super) fn deserialize_decimal(
+    mut bytes: &[u8],
+    precision: Option<u8>,
+    scale: u8,
+) -> Option<i128> {
+    let negative = bytes.first() == Some(&b'-');
+    if negative {
+        bytes = &bytes[1..];
+    };
     let (lhs, rhs) = split_decimal_bytes(bytes);
     let precision = precision.unwrap_or(u8::MAX);
 
     let lhs_b = lhs?;
-    parse_integer_checked(lhs_b).and_then(|x| {
+    let abs = parse_integer_checked(lhs_b).and_then(|x| {
         match rhs {
             Some(rhs) => {
                 parse_integer_checked(rhs)
@@ -77,9 +85,7 @@ pub(super) fn deserialize_decimal(bytes: &[u8], precision: Option<u8>, scale: u8
                             Some((lhs, rhs))
                         }
                     })
-                    .map(|(lhs, rhs)| {
-                        lhs * 10i128.pow(scale as u32) + (if lhs < 0 { -rhs } else { rhs })
-                    })
+                    .map(|(lhs, rhs)| lhs * 10i128.pow(scale as u32) + rhs)
             },
             None => {
                 if lhs_b.len() > precision as usize || scale != 0 {
@@ -88,7 +94,12 @@ pub(super) fn deserialize_decimal(bytes: &[u8], precision: Option<u8>, scale: u8
                 parse_integer_checked(lhs_b)
             },
         }
-    })
+    });
+    if negative {
+        Some(-abs?)
+    } else {
+        abs
+    }
 }
 
 #[cfg(test)]
@@ -117,6 +128,12 @@ mod test {
             Some(14390)
         );
 
+        let val = "-0.5";
+        assert_eq!(
+            deserialize_decimal(val.as_bytes(), precision, scale),
+            Some(-50)
+        );
+
         let val = "-1.5";
         assert_eq!(
             deserialize_decimal(val.as_bytes(), precision, scale),