fix: handle negative-scale Decimals in increment, decrement and truncate_decimal

pyiceberg/expressions/literals.py

@@ -514,12 +514,14 @@ def __init__(self, value: Decimal) -> None:
         super().__init__(value, Decimal)
 
     def increment(self) -> Literal[Decimal]:
-        original_scale = abs(int(self.value.as_tuple().exponent))
+        # A Decimal carries the negated scale as its exponent, so a value with a
+        # non-negative exponent (1E+2) has a negative scale and abs() would read it wrong
+        original_scale = -int(self.value.as_tuple().exponent)
         unscaled = decimal_to_unscaled(self.value)
         return DecimalLiteral(unscaled_to_decimal(unscaled + 1, original_scale))
 
     def decrement(self) -> Literal[Decimal]:
-        original_scale = abs(int(self.value.as_tuple().exponent))
+        original_scale = -int(self.value.as_tuple().exponent)
         unscaled = decimal_to_unscaled(self.value)
         return DecimalLiteral(unscaled_to_decimal(unscaled - 1, original_scale))
 

pyiceberg/utils/decimal.py

@@ -112,7 +112,9 @@ def truncate_decimal(value: Decimal, width: int) -> Decimal:
     """
     unscaled_value = decimal_to_unscaled(value)
     applied_value = unscaled_value - (((unscaled_value % width) + width) % width)
-    return unscaled_to_decimal(applied_value, abs(int(value.as_tuple().exponent)))
+    # A Decimal carries the negated scale as its exponent, so a value with a
+    # non-negative exponent (1E+2) has a negative scale and abs() would read it wrong
+    return unscaled_to_decimal(applied_value, -int(value.as_tuple().exponent))
 
 
 MAX_PRECISION = tuple(math.floor(math.log10(math.fabs(math.pow(2, 8 * pos - 1) - 1))) for pos in range(24))

tests/expressions/test_literals.py

@@ -1000,6 +1000,24 @@ def test_decimal_literal_decrement() -> None:
     assert dec.decrement().value.as_tuple() == Decimal("10.122").as_tuple()
 
 
+def test_decimal_literal_increment_negative_scale() -> None:
+    dec = DecimalLiteral(Decimal("1E+2"))
+    # Twice to check that we don't mutate the value
+    assert dec.increment() == DecimalLiteral(Decimal("2E+2"))
+    assert dec.increment() == DecimalLiteral(Decimal("2E+2"))
+    # To check that the scale is still the same (a non-negative exponent means a negative scale)
+    assert dec.increment().value.as_tuple() == Decimal("2E+2").as_tuple()
+
+
+def test_decimal_literal_decrement_negative_scale() -> None:
+    dec = DecimalLiteral(Decimal("1E+2"))
+    # Twice to check that we don't mutate the value
+    assert dec.decrement() == DecimalLiteral(Decimal("0E+2"))
+    assert dec.decrement() == DecimalLiteral(Decimal("0E+2"))
+    # To check that the scale is still the same, also when stepping below zero
+    assert dec.decrement().decrement() == DecimalLiteral(Decimal("-1E+2"))
+
+
 def test_uuid_literal_initialization() -> None:
     test_uuid = uuid.UUID("f79c3e09-677c-4bbd-a479-3f349cb785e7")
     uuid_literal = literal(test_uuid)

tests/utils/test_decimal.py

@@ -18,7 +18,7 @@
 
 import pytest
 
-from pyiceberg.utils.decimal import bytes_required, decimal_required_bytes, decimal_to_bytes
+from pyiceberg.utils.decimal import bytes_required, decimal_required_bytes, decimal_to_bytes, truncate_decimal
 
 
 def test_decimal_required_bytes() -> None:
@@ -65,3 +65,16 @@ def test_decimal_to_bytes() -> None:
     # Unscaled values -128 and -32768 require no sign padding.
     assert decimal_to_bytes(Decimal("-1.28")) == b"\x80"
     assert decimal_to_bytes(Decimal("-327.68")) == b"\x80\x00"
+
+
+def test_truncate_decimal() -> None:
+    assert truncate_decimal(Decimal("12.34"), 10) == Decimal("12.30")
+    # Negative values truncate toward negative infinity
+    assert truncate_decimal(Decimal("-12.34"), 10) == Decimal("-12.40")
+
+
+def test_truncate_decimal_negative_scale() -> None:
+    # A Decimal like 1.5E+3 carries a negative scale; reading its exponent as a
+    # positive scale would return a value on the wrong order of magnitude
+    assert truncate_decimal(Decimal("1.5E+3"), 4) == Decimal("1.2E+3")
+    assert truncate_decimal(Decimal("1.5E+3"), 4).as_tuple() == Decimal("1.2E+3").as_tuple()