MRSymMatrix2.h

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#pragma once
 
#include "MRVector2.h"
#include "MRMatrix2.h"
#include <limits>
 
namespace MR
{
 
template <typename T>
struct SymMatrix2
{
    using ValueType = T;
 
    T xx = 0, xy = 0, yy = 0;
 
    constexpr SymMatrix2() noexcept = default;
 
    // Here `T == U` doesn't seem to cause any issues in the C++ code, but we're still disabling it because it somehow gets emitted
    //   when generating the bindings, and results in duplicate functions in C#.
    template <typename U> MR_REQUIRES_IF_SUPPORTED( !std::is_same_v<T, U> )
    constexpr explicit SymMatrix2( const SymMatrix2<U> & m ) : xx( T( m.xx ) ), xy( T( m.xy ) ), yy( T( m.yy ) ) { }
 
    static constexpr SymMatrix2 identity() noexcept { SymMatrix2 res; res.xx = res.yy = 1; return res; }
    static constexpr SymMatrix2 diagonal( T diagVal ) noexcept { SymMatrix2 res; res.xx = res.yy = diagVal; return res; }
 
    constexpr T trace() const noexcept { return xx + yy; }
    constexpr T normSq() const noexcept { return sqr( xx ) + 2 * sqr( xy ) + sqr( yy ); }
    constexpr T det() const noexcept;
    constexpr SymMatrix2<T> inverse() const noexcept { return inverse( det() ); }
    constexpr SymMatrix2<T> inverse( T det ) const noexcept;
 
    SymMatrix2 & operator +=( const SymMatrix2<T> & b ) { xx += b.xx; xy += b.xy; yy += b.yy; return * this; }
    SymMatrix2 & operator -=( const SymMatrix2<T> & b ) { xx -= b.xx; xy -= b.xy; yy -= b.yy; return * this; }
    SymMatrix2 & operator *=( T b ) { xx *= b; xy *= b; yy *= b; return * this; }
    SymMatrix2 & operator /=( T b )
    {
        if constexpr ( std::is_integral_v<T> )
            { xx /= b; xy /= b; yy /= b; return * this; }
        else
            return *this *= ( 1 / b );
    }
 
    Vector2<T> eigens( Matrix2<T> * eigenvectors = nullptr ) const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> );
    Vector2<T> eigenvector( T eigenvalue ) const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> );
    Vector2<T> maxEigenvector() const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> );
 
    SymMatrix2<T> pseudoinverse( T tol = std::numeric_limits<T>::epsilon(), int * rank = nullptr, Vector2<T> * space = nullptr ) const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> );
};
 
 
template <typename T>
inline Vector2<T> operator *( const SymMatrix2<T> & a, const Vector2<T> & b )
{
    return
    {
        a.xx * b.x + a.xy * b.y,
        a.xy * b.x + a.yy * b.y
    };
}
 
template <typename T>
inline SymMatrix2<T> outerSquare( const Vector2<T> & a )
{
    SymMatrix2<T> res;
    res.xx = a.x * a.x;
    res.xy = a.x * a.y;
    res.yy = a.y * a.y;
    return res;
}
 
template <typename T>
inline SymMatrix2<T> outerSquare( T k, const Vector2<T> & a )
{
    const auto ka = k * a;
    SymMatrix2<T> res;
    res.xx = ka.x * a.x;
    res.xy = ka.x * a.y;
    res.yy = ka.y * a.y;
    return res;
}
 
template <typename T>
inline bool operator ==( const SymMatrix2<T> & a, const SymMatrix2<T> & b )
    { return a.xx = b.xx && a.xy = b.xy && a.yy = b.yy; }
 
template <typename T>
inline bool operator !=( const SymMatrix2<T> & a, const SymMatrix2<T> & b )
    { return !( a == b ); }
 
template <typename T>
inline SymMatrix2<T> operator +( const SymMatrix2<T> & a, const SymMatrix2<T> & b )
    { SymMatrix2<T> res{ a }; res += b; return res; }
 
template <typename T>
inline SymMatrix2<T> operator -( const SymMatrix2<T> & a, const SymMatrix2<T> & b )
    { SymMatrix2<T> res{ a }; res -= b; return res; }
 
template <typename T>
inline SymMatrix2<T> operator *( T a, const SymMatrix2<T> & b )
    { SymMatrix2<T> res{ b }; res *= a; return res; }
 
template <typename T>
inline SymMatrix2<T> operator *( const SymMatrix2<T> & b, T a )
    { SymMatrix2<T> res{ b }; res *= a; return res; }
 
template <typename T>
inline SymMatrix2<T> operator /( SymMatrix2<T> b, T a )
    { b /= a; return b; }
 
template <typename T>
constexpr T SymMatrix2<T>::det() const noexcept
{
    return xx * yy - xy * xy;
}
 
template <typename T>
constexpr SymMatrix2<T> SymMatrix2<T>::inverse( T det ) const noexcept
{
    if ( det == 0 )
        return {};
    SymMatrix2<T> res;
    res.xx =  yy / det;
    res.xy = -xy / det;
    res.yy =  xx / det;
    return res;
}
 
template <typename T>
Vector2<T> SymMatrix2<T>::eigens( Matrix2<T> * eigenvectors ) const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> )
{
    //https://en.wikipedia.org/wiki/Eigenvalue_algorithm#2%C3%972_matrices
    const auto tr = trace();
    const auto q = tr / 2;
    const auto p = std::sqrt( std::max( T(0), sqr( tr ) - 4 * det() ) ) / 2;
    Vector2<T> eig;
    if ( p <= std::abs( q ) * std::numeric_limits<T>::epsilon() )
    {
        // this is proportional to identity matrix
        eig = { q, q };
        if ( eigenvectors )
            *eigenvectors = Matrix2<T>{};
        return eig;
    }
    eig[0] = q - p;
    eig[1] = q + p;
    if ( eigenvectors )
    {
        const auto x = eigenvector( eig[0] ).normalized();
        *eigenvectors = Matrix2<T>::fromRows( x, x.perpendicular() );
    }
    return eig;
}
 
template <typename T>
Vector2<T> SymMatrix2<T>::eigenvector( T eigenvalue ) const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> )
{
    const Vector2<T> row0( xx - eigenvalue, xy );
    const Vector2<T> row1( xy, yy - eigenvalue );
    // not-repeating eigenvalue means that one of two rows is not zero
    const T lsq0 = row0.lengthSq();
    const T lsq1 = row1.lengthSq();
    return lsq0 >= lsq1 ? row0.perpendicular() : row1.perpendicular();
}
 
template <typename T>
Vector2<T> SymMatrix2<T>::maxEigenvector() const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> )
{
    const auto tr = trace();
    const auto q = tr / 2;
    const auto p = std::sqrt( std::max( T(0), sqr( tr ) - 4 * det() ) ) / 2;
    if ( p <= std::abs( q ) * std::numeric_limits<T>::epsilon() )
    {
        // this is proportional to identity matrix
        return Vector2<T>( T(1), T(0) );
    }
    return eigenvector( q + p );
}
 
template <typename T>
SymMatrix2<T> SymMatrix2<T>::pseudoinverse( T tol, int * rank, Vector2<T> * space ) const MR_REQUIRES_IF_SUPPORTED( std::is_floating_point_v<T> )
{
    SymMatrix2<T> res;
    Matrix2<T> eigenvectors;
    const auto eigenvalues = eigens( &eigenvectors );
    const auto threshold = std::max( std::abs( eigenvalues[0] ), std::abs( eigenvalues[1] ) ) * tol;
    int myRank = 0;
    for ( int i = 0; i < 2; ++i )
    {
        if ( std::abs( eigenvalues[i] ) <= threshold )
            continue;
        res += outerSquare( 1 / eigenvalues[i], eigenvectors[i] );
        ++myRank;
        if ( space )
        {
            if ( myRank == 1 )
                *space = eigenvectors[i];
            else
                *space = Vector2<T>{};
        }
    }
    if ( rank )
        *rank = myRank;
    return res;
}
 
 
} // namespace MR