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[Type] Mat: Fix {add,sub}Transposed() self-aliasing calls #6276

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[Type] Mat: Fix {add,sub}Transposed() self-aliasing calls #6276

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865d141
demonstrate that addTransposed/subTransposed selfaliasing is wrong
fredroy Aug 31, 2026
2a9edb5
fixes selfaliases version
fredroy Aug 31, 2026
0708781
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fredroy Sep 2, 2026
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49 changes: 43 additions & 6 deletions Sofa/framework/Type/src/sofa/type/Mat.h
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Original file line number Diff line number Diff line change
Expand Up @@ -708,17 +708,54 @@ class Mat
/// Addition of the transposed of m
constexpr void addTransposed(const Mat<C,L,real>& m) noexcept
{
for(Size i=0; i<L; i++)
for(Size j=0; j<C; j++)
(*this)(i,j) += m(j,i);
if (canSelfTranspose(*this, m))
{
// m aliases *this: process each off-diagonal pair once, from values read
// before either of the two positions is written.
for(Size i=0; i<L; i++)
{
(*this)(i,i) += (*this)(i,i);
for(Size j=i+1; j<C; j++)
{
const real sum = (*this)(i,j) + (*this)(j,i);
(*this)(i,j) = sum;
(*this)(j,i) = sum;
}
}
}
else
{
for(Size i=0; i<L; i++)
for(Size j=0; j<C; j++)
(*this)(i,j) += m(j,i);
}
}

/// Subtraction of the transposed of m
constexpr void subTransposed(const Mat<C,L,real>& m) noexcept
{
for(Size i=0; i<L; i++)
for(Size j=0; j<C; j++)
(*this)(i,j) -= m(j,i);
if (canSelfTranspose(*this, m))
{
// m aliases *this: process each off-diagonal pair once, from values read
// before either of the two positions is written.
for(Size i=0; i<L; i++)
{
(*this)(i,i) = real{};
for(Size j=i+1; j<C; j++)
{
const real mij = (*this)(i,j);
const real mji = (*this)(j,i);
(*this)(i,j) = mij - mji;
(*this)(j,i) = mji - mij;
}
}
}
else
{
for(Size i=0; i<L; i++)
for(Size j=0; j<C; j++)
(*this)(i,j) -= m(j,i);
}
}

/// Subtraction assignment operator.
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22 changes: 22 additions & 0 deletions Sofa/framework/Type/test/MatTypes_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -699,6 +699,28 @@ TEST(MatTypesTest, addSubTransposed)
EXPECT_EQ(M, Matrix3::Identity() + A.transposed());
}

TEST(MatTypesTest, addTransposedSelfAliasing)
{
Matrix3 M(Matrix3::Line(1., 2., 3.), Matrix3::Line(4., 5., 6.), Matrix3::Line(7., 8., 9.));
const Matrix3 expected(Matrix3::Line(2., 6., 10.),
Matrix3::Line(6., 10., 14.),
Matrix3::Line(10., 14., 18.));

M.addTransposed(M);
EXPECT_EQ(M, expected);
}

TEST(MatTypesTest, subTransposedSelfAliasing)
{
Matrix3 M(Matrix3::Line(1., 2., 3.), Matrix3::Line(4., 5., 6.), Matrix3::Line(7., 8., 9.));
const Matrix3 expected(Matrix3::Line(0., -2., -4.),
Matrix3::Line(2., 0., -2.),
Matrix3::Line(4., 2., 0.));

M.subTransposed(M);
EXPECT_EQ(M, expected);
}

TEST(MatTypesTest, symmetrize)
{
Matrix3 A(Matrix3::Line(1., 2., 3.), Matrix3::Line(4., 5., 6.), Matrix3::Line(7., 8., 9.));
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