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Fix TiesToAway rounding below the smallest-subnormal midpoint #68

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Merged
awf merged 2 commits into from
Oct 18, 2026
Merged

Fix TiesToAway rounding below the smallest-subnormal midpoint #68

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Copilot Oct 18, 2026
76cdf5c
Fix TiesToAway rounding just below subnormal midpoint
Copilot Oct 18, 2026
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2 changes: 1 addition & 1 deletion src/gfloat/round.py
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Original file line number Diff line number Diff line change
Expand Up @@ -137,7 +137,7 @@ def round_float(
case RoundMode.TowardNegative:
should_round_away = sign and delta > 0
case RoundMode.TiesToAway:
should_round_away = delta + 0.5 >= 1.0
should_round_away = delta >= 0.5
case RoundMode.TiesToEven:
should_round_away = delta > 0.5 or (delta == 0.5 and code_is_odd)
case RoundMode.ToOdd:
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21 changes: 19 additions & 2 deletions test/test_round_underflow.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2,12 +2,14 @@

from typing import Callable

import math

import numpy as np
import pytest

from gfloat import FormatInfo, RoundMode, decode_float, encode_float
from gfloat import FormatInfo, RoundMode, Signedness, decode_float, encode_float
from gfloat import round_float, round_ndarray
from gfloat.formats import Domain, format_info_ocp_e8m0
from gfloat.formats import Domain, format_info_ocp_e8m0, format_info_p3109


def scalar_round(
Expand Down Expand Up @@ -41,6 +43,21 @@ def no_subnormal_format(precision: int, signed: bool = False) -> FormatInfo:
)


@pytest.mark.parametrize("rounder", [scalar_round, array_round])
@pytest.mark.parametrize("sign", [1, -1])
@pytest.mark.parametrize("direction", [0.0, 0.5, 1.0])
def test_round_ties_to_away_subnormal_midpoint(
rounder: Callable, sign: int, direction: float
) -> None:
fi = format_info_p3109(8, 3, Signedness.Signed, Domain.Extended)
midpoint = fi.smallest / 2
value = math.nextafter(midpoint, direction * fi.smallest)
expected = 0.0 if direction == 0.0 else sign * fi.smallest
actual = rounder(fi, sign * value, RoundMode.TiesToAway, False, 0)
assert actual == expected
assert np.signbit(actual) == np.signbit(expected)


@pytest.mark.parametrize("rounder", [scalar_round, array_round])
@pytest.mark.parametrize("rnd", RoundMode)
@pytest.mark.parametrize("sat", [False, True])
Expand Down
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