fix: decimal round_dp on zero with too large dp (#575)

Co-authored-by: jon-chuang <jon-chuang@users.noreply.github.com>

src/decimal.rs

@@ -1274,6 +1274,14 @@ impl Decimal {
     /// ```
     #[must_use]
     pub fn round_dp_with_strategy(&self, dp: u32, strategy: RoundingStrategy) -> Decimal {
+        let old_scale = self.scale();
+
+        // return early if decimal has a smaller number of fractional places than dp
+        // e.g. 2.51 rounded to 3 decimal places is 2.51
+        if old_scale <= dp {
+            return *self;
+        }
+
         // Short circuit for zero
         if self.is_zero() {
             return Decimal {
@@ -1284,14 +1292,6 @@ impl Decimal {
             };
         }
 
-        let old_scale = self.scale();
-
-        // return early if decimal has a smaller number of fractional places than dp
-        // e.g. 2.51 rounded to 3 decimal places is 2.51
-        if old_scale <= dp {
-            return *self;
-        }
-
         let mut value = [self.lo, self.mid, self.hi];
         let mut value_scale = self.scale();
         let negative = self.is_sign_negative();

tests/decimal_tests.rs

@@ -2471,6 +2471,15 @@ fn it_can_round_complex_numbers() {
     assert_eq!("0.16", part.to_string());
 }
 
+#[test]
+fn it_does_not_round_decimals_to_too_many_dp() {
+    // Issue 574
+    let zero = Decimal::new(0, 28);
+    let rounded = zero.round_dp(32);
+    assert_eq!(rounded.scale(), 28); // If dp > old_scale, we retain the old scale.
+    rounded.to_string();
+}
+
 #[test]
 fn it_can_round_down() {
     let tests = &[