fix: Fix casting decimal to decimal for high precision (#16049)

pola-rs · May 5, 2024 · aa2e77b · aa2e77b
1 parent db77896
commit aa2e77b
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Showing 3 changed files with 22 additions and 14 deletions.
diff --git a/crates/polars-arrow/src/compute/cast/decimal_to.rs b/crates/polars-arrow/src/compute/cast/decimal_to.rs
@@ -12,15 +12,13 @@ fn decimal_to_decimal_impl<F: Fn(i128) -> Option<i128>>(
     to_precision: usize,
     to_scale: usize,
 ) -> PrimitiveArray<i128> {
-    let min_for_precision = 9_i128
-        .saturating_pow(1 + to_precision as u32)
-        .saturating_neg();
-    let max_for_precision = 9_i128.saturating_pow(1 + to_precision as u32);
+    let upper_bound_for_precision = 10_i128.saturating_pow(to_precision as u32);
+    let lower_bound_for_precision = upper_bound_for_precision.saturating_neg();
 
     let values = from.iter().map(|x| {
         x.and_then(|x| {
             op(*x).and_then(|x| {
-                if x > max_for_precision || x < min_for_precision {
+                if x >= upper_bound_for_precision || x <= lower_bound_for_precision {
                     None
                 } else {
                     Some(x)

diff --git a/py-polars/tests/unit/operations/test_cast.py b/py-polars/tests/unit/operations/test_cast.py
@@ -640,3 +640,15 @@ def test_cast_array_to_different_width() -> None:
         pl.InvalidOperationError, match="cannot cast Array to a different width"
     ):
         s.cast(pl.Array(pl.Int16, 3))
+
+
+def test_cast_decimal_to_decimal_high_precision() -> None:
+    precision = 22
+    values = [Decimal("9" * precision)]
+    s = pl.Series(values, dtype=pl.Decimal(None, 0))
+
+    target_dtype = pl.Decimal(precision, 0)
+    result = s.cast(target_dtype)
+
+    assert result.dtype == target_dtype
+    assert result.to_list() == values
diff --git a/py-polars/tests/unit/sql/test_numeric.py b/py-polars/tests/unit/sql/test_numeric.py
@@ -50,7 +50,7 @@ def test_modulo() -> None:
     ("value", "sqltype", "prec_scale", "expected_value", "expected_dtype"),
     [
         (64.5, "numeric", "(3,1)", D("64.5"), pl.Decimal(3, 1)),
-        (512.5, "decimal", "(3,1)", D("512.5"), pl.Decimal(3, 1)),
+        (512.5, "decimal", "(4,1)", D("512.5"), pl.Decimal(4, 1)),
         (512.5, "numeric", "(4,0)", D("512"), pl.Decimal(4, 0)),
         (-1024.75, "decimal", "(10,0)", D("-1024"), pl.Decimal(10, 0)),
         (-1024.75, "numeric", "(10)", D("-1024"), pl.Decimal(10, 0)),
@@ -67,18 +67,16 @@ def test_numeric_decimal_type(
     with pl.Config(activate_decimals=True):
         df = pl.DataFrame({"n": [value]})
         with pl.SQLContext(df=df) as ctx:
-            out = ctx.execute(
+            result = ctx.execute(
                 f"""
                 SELECT n::{sqltype}{prec_scale} AS "dec" FROM df
                 """
             )
-            assert_frame_equal(
-                out.collect(),
-                pl.DataFrame(
-                    data={"dec": [expected_value]},
-                    schema={"dec": expected_dtype},
-                ),
-            )
+        expected = pl.LazyFrame(
+            data={"dec": [expected_value]},
+            schema={"dec": expected_dtype},
+        )
+        assert_frame_equal(result, expected)
 
 
 @pytest.mark.parametrize(