Bug: Take signed bit into account (#677)

```python
>>> (32768).bit_length()
16
```

From https://docs.python.org/3/library/stdtypes.html#int.bit_length

> Return the number of bits necessary to represent an integer in binary, excluding the sign and leading zeros

Fixes #669

Author: Fokko

pyiceberg/utils/decimal.py

@@ -59,9 +59,9 @@ def bytes_required(value: Union[int, Decimal]) -> int:
         int: the minimum number of bytes needed to serialize the value.
     """
     if isinstance(value, int):
-        return (value.bit_length() + 7) // 8
+        return (value.bit_length() + 8) // 8
     elif isinstance(value, Decimal):
-        return (decimal_to_unscaled(value).bit_length() + 7) // 8
+        return (decimal_to_unscaled(value).bit_length() + 8) // 8
 
     raise ValueError(f"Unsupported value: {value}")
 

tests/utils/test_decimal.py

@@ -14,9 +14,11 @@
 # KIND, either express or implied.  See the License for the
 # specific language governing permissions and limitations
 # under the License.
+from decimal import Decimal
+
 import pytest
 
-from pyiceberg.utils.decimal import decimal_required_bytes
+from pyiceberg.utils.decimal import decimal_required_bytes, decimal_to_bytes
 
 
 def test_decimal_required_bytes() -> None:
@@ -38,3 +40,10 @@ def test_decimal_required_bytes() -> None:
     with pytest.raises(ValueError) as exc_info:
         decimal_required_bytes(precision=-1)
     assert "(0, 40]" in str(exc_info.value)
+
+
+def test_decimal_to_bytes() -> None:
+    # Check the boundary between 2 and 3 bytes.
+    # 2 bytes has a minimum of -32,768 and a maximum value of 32,767 (inclusive).
+    assert decimal_to_bytes(Decimal('32767.')) == b'\x7f\xff'
+    assert decimal_to_bytes(Decimal('32768.')) == b'\x00\x80\x00'