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fix(bitvec): return 0 for concrete ADDMOD/MULMOD with a zero modulus #598

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fix(bitvec): return 0 for concrete ADDMOD/MULMOD with a zero modulus #598

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091724a
fix(bitvec): return 0 for concrete ADDMOD/MULMOD with a zero modulus
mayuriphad Oct 21, 2026
3a5f187
style: format new bitvec tests
mayuriphad Oct 21, 2026
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6 changes: 6 additions & 0 deletions src/halmos/bitvec.py
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Original file line number Diff line number Diff line change
Expand Up @@ -734,6 +734,9 @@ def addmod(
assert size == modulus.size

if self.is_concrete and other.is_concrete and modulus.is_concrete:
# mod by zero is zero
if modulus.value == 0:
return HalmosBitVec(0, size=size)
return HalmosBitVec((self.value + other.value) % modulus.value, size=size)

# to avoid add overflow; and to be a multiple of 8-bit
Expand Down Expand Up @@ -762,6 +765,9 @@ def mulmod(
assert size == modulus.size

if self.is_concrete and other.is_concrete and modulus.is_concrete:
# mod by zero is zero
if modulus.value == 0:
return HalmosBitVec(0, size=size)
return HalmosBitVec((self.value * other.value) % modulus.value, size=size)

# to avoid mul overflow
Expand Down
14 changes: 14 additions & 0 deletions tests/test_bitvec.py
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Original file line number Diff line number Diff line change
Expand Up @@ -305,3 +305,17 @@ def test_bool_hashable():
assert y not in dict
assert TRUE not in dict
assert FALSE in dict


@pytest.mark.parametrize(
"a, b, n, expected", [(2, 3, 0, 0), (2, 3, 4, 1), (7, 5, 1, 0)]
)
def test_addmod_concrete(a, b, n, expected):
assert int(BV(a).addmod(BV(b), BV(n))) == expected


@pytest.mark.parametrize(
"a, b, n, expected", [(2, 3, 0, 0), (2, 3, 4, 2), (7, 5, 1, 0)]
)
def test_mulmod_concrete(a, b, n, expected):
assert int(BV(a).mulmod(BV(b), BV(n))) == expected