    // Copyright (C) 2009  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    // This code was adapted from code from the JAMA part of NIST's TNT library.
    //    See: http://math.nist.gov/tnt/
    #ifndef DLIB_MATRIX_EIGENVALUE_DECOMPOSITION_H
    #define DLIB_MATRIX_EIGENVALUE_DECOMPOSITION_H

    #include "matrix.h"
    #include "matrix_utilities.h"
    #include "matrix_subexp.h"
    #include <algorithm>
    #include <complex>
    #include <cmath>

    #ifdef DLIB_USE_LAPACK
    #include "lapack/geev.h"
    #include "lapack/syev.h"
    #include "lapack/syevr.h"
    #endif

    #define DLIB_LAPACK_EIGENVALUE_DECOMP_SIZE_THRESH 4

    namespace dlib
    {

        template <
            typename matrix_exp_type
            >
        class eigenvalue_decomposition
        {

        public:

            const static long NR = matrix_exp_type::NR;
            const static long NC = matrix_exp_type::NC;
            typedef typename matrix_exp_type::type type;
            typedef typename matrix_exp_type::mem_manager_type mem_manager_type;
            typedef typename matrix_exp_type::layout_type layout_type;

            typedef typename matrix_exp_type::matrix_type matrix_type;
            typedef matrix<type,NR,1,mem_manager_type,layout_type> column_vector_type;

            typedef matrix<std::complex<type>,0,0,mem_manager_type,layout_type> complex_matrix_type;
            typedef matrix<std::complex<type>,NR,1,mem_manager_type,layout_type> complex_column_vector_type;

            // You have supplied an invalid type of matrix_exp_type.  You have
            // to use this object with matrices that contain float or double type data.
            COMPILE_TIME_ASSERT((is_same_type<float, type>::value ||
                                 is_same_type<double, type>::value ));

            template <typename EXP>
            eigenvalue_decomposition(
                const matrix_exp<EXP>& A
            );

            template <typename EXP>
            eigenvalue_decomposition(
                const matrix_op<op_make_symmetric<EXP> >& A
            );

            long dim (
            ) const;

            const complex_column_vector_type get_eigenvalues (
            ) const;

            const column_vector_type& get_real_eigenvalues (
            ) const;

            const column_vector_type& get_imag_eigenvalues (
            ) const;

            const complex_matrix_type get_v (
            ) const;

            const complex_matrix_type get_d (
            ) const;

            const matrix_type& get_pseudo_v (
            ) const;

            const matrix_type get_pseudo_d (
            ) const;

        private:

            /** Row and column dimension (square matrix).  */
            long n;

            bool issymmetric;

            /** Arrays for internal storage of eigenvalues. */

            column_vector_type d;         /* real part */
            column_vector_type e;         /* img part */

            /** Array for internal storage of eigenvectors. */
            matrix_type V;

            /** Array for internal storage of nonsymmetric Hessenberg form.
            @serial internal storage of nonsymmetric Hessenberg form.
            */
            matrix_type H;

            /** Working storage for nonsymmetric algorithm.
            @serial working storage for nonsymmetric algorithm.
            */
            column_vector_type ort;

            // Symmetric Householder reduction to tridiagonal form.
            void tred2();

            // Symmetric tridiagonal QL algorithm.
            void tql2 ();

            // Nonsymmetric reduction to Hessenberg form.
            void orthes ();

            // Complex scalar division.
            type cdivr, cdivi;
            void cdiv_(type xr, type xi, type yr, type yi);

            // Nonsymmetric reduction from Hessenberg to real Schur form.
            void hqr2 ();
        };

    // ----------------------------------------------------------------------------------------
    // ----------------------------------------------------------------------------------------
    //                        Public member functions
    // ----------------------------------------------------------------------------------------
    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        template <typename EXP>
        eigenvalue_decomposition<matrix_exp_type>::
        eigenvalue_decomposition(
            const matrix_exp<EXP>& A_
        )
        {
            COMPILE_TIME_ASSERT((is_same_type<type, typename EXP::type>::value));

            const_temp_matrix<EXP> A(A_);

            // make sure requires clause is not broken
            DLIB_ASSERT(A.nr() == A.nc() && A.size() > 0,
                "\teigenvalue_decomposition::eigenvalue_decomposition(A)"
                << "\n\tYou can only use this on square matrices"
                << "\n\tA.nr():   " << A.nr()
                << "\n\tA.nc():   " << A.nc()
                << "\n\tA.size(): " << A.size()
                << "\n\tthis:     " << this
                );

            n = A.nc();
            V.set_size(n,n);
            d.set_size(n);
            e.set_size(n);

            issymmetric = true;
            for (long j = 0; (j < n) && issymmetric; j++)
            {
                for (long i = 0; (i < n) && issymmetric; i++)
                {
                    issymmetric = (A(i,j) == A(j,i));
                }
            }

            if (issymmetric)
            {
                V = A;

    #ifdef DLIB_USE_LAPACK
                if (A.nr() > DLIB_LAPACK_EIGENVALUE_DECOMP_SIZE_THRESH)
                {
                    e = 0;

                    // We could compute the result using syev()
                    //lapack::syev('V', 'L', V,  d);

                    // Instead, we use syevr because its faster and maybe more stable.
                    matrix_type tempA(A);
                    matrix<lapack::integer,0,0,mem_manager_type,layout_type> isupz;

                    lapack::integer temp;
                    lapack::syevr('V','A','L',tempA,0,0,0,0,-1,temp,d,V,isupz);
                    return;
                }
    #endif
                // Tridiagonalize.
                tred2();

                // Diagonalize.
                tql2();

            }
            else
            {

    #ifdef DLIB_USE_LAPACK
                if (A.nr() > DLIB_LAPACK_EIGENVALUE_DECOMP_SIZE_THRESH)
                {
                    matrix<type,0,0,mem_manager_type, column_major_layout> temp, vl, vr;
                    temp = A;
                    lapack::geev('N', 'V', temp, d, e, vl, vr);
                    V = vr;
                    return;
                }
    #endif
                H = A;

                ort.set_size(n);

                // Reduce to Hessenberg form.
                orthes();

                // Reduce Hessenberg to real Schur form.
                hqr2();
            }
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        template <typename EXP>
        eigenvalue_decomposition<matrix_exp_type>::
        eigenvalue_decomposition(
            const matrix_op<op_make_symmetric<EXP> >& A
        )
        {
            COMPILE_TIME_ASSERT((is_same_type<type, typename EXP::type>::value));

            // make sure requires clause is not broken
            DLIB_ASSERT(A.nr() == A.nc() && A.size() > 0,
                "\teigenvalue_decomposition::eigenvalue_decomposition(A)"
                << "\n\tYou can only use this on square matrices"
                << "\n\tA.nr():   " << A.nr()
                << "\n\tA.nc():   " << A.nc()
                << "\n\tA.size(): " << A.size()
                << "\n\tthis:     " << this
                );

            n = A.nc();
            V.set_size(n,n);
            d.set_size(n);
            e.set_size(n);

            V = A;

    #ifdef DLIB_USE_LAPACK
            if (A.nr() > DLIB_LAPACK_EIGENVALUE_DECOMP_SIZE_THRESH)
            {
                e = 0;

                // We could compute the result using syev()
                //lapack::syev('V', 'L', V,  d);

                // Instead, we use syevr because its faster and maybe more stable.
                matrix_type tempA(A);
                matrix<lapack::integer,0,0,mem_manager_type,layout_type> isupz;

                lapack::integer temp;
                lapack::syevr('V','A','L',tempA,0,0,0,0,-1,temp,d,V,isupz);
                return;
            }
    #endif
            // Tridiagonalize.
            tred2();

            // Diagonalize.
            tql2();

        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        const typename eigenvalue_decomposition<matrix_exp_type>::matrix_type& eigenvalue_decomposition<matrix_exp_type>::
        get_pseudo_v (
        ) const
        {
            return V;
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        long eigenvalue_decomposition<matrix_exp_type>::
        dim (
        ) const
        {
            return V.nr();
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        const typename eigenvalue_decomposition<matrix_exp_type>::complex_column_vector_type eigenvalue_decomposition<matrix_exp_type>::
        get_eigenvalues (
        ) const
        {
            return complex_matrix(get_real_eigenvalues(), get_imag_eigenvalues());
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        const typename eigenvalue_decomposition<matrix_exp_type>::column_vector_type& eigenvalue_decomposition<matrix_exp_type>::
        get_real_eigenvalues (
        ) const
        {
            return d;
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        const typename eigenvalue_decomposition<matrix_exp_type>::column_vector_type& eigenvalue_decomposition<matrix_exp_type>::
        get_imag_eigenvalues (
        ) const
        {
            return e;
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        const typename eigenvalue_decomposition<matrix_exp_type>::complex_matrix_type eigenvalue_decomposition<matrix_exp_type>::
        get_d (
        ) const
        {
            return diagm(complex_matrix(get_real_eigenvalues(), get_imag_eigenvalues()));
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        const typename eigenvalue_decomposition<matrix_exp_type>::complex_matrix_type eigenvalue_decomposition<matrix_exp_type>::
        get_v (
        ) const
        {
            complex_matrix_type CV(n,n);

            for (long i = 0; i < n; i++)
            {
                if (e(i) > 0)
                {
                    set_colm(CV,i) = complex_matrix(colm(V,i), colm(V,i+1));
                }
                else if (e(i) < 0)
                {
                    set_colm(CV,i) = complex_matrix(colm(V,i), colm(V,i-1));
                }
                else
                {
                    set_colm(CV,i) = complex_matrix(colm(V,i), uniform_matrix<type>(n,1,0));
                }
            }

            return CV;
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        const typename eigenvalue_decomposition<matrix_exp_type>::matrix_type eigenvalue_decomposition<matrix_exp_type>::
        get_pseudo_d (
        ) const
        {
            matrix_type D(n,n);

            for (long i = 0; i < n; i++)
            {
                for (long j = 0; j < n; j++)
                {
                    D(i,j) = 0.0;
                }
                D(i,i) = d(i);
                if (e(i) > 0)
                {
                    D(i,i+1) = e(i);
                }
                else if (e(i) < 0)
                {
                    D(i,i-1) = e(i);
                }
            }

            return D;
        }

    // ----------------------------------------------------------------------------------------
    // ----------------------------------------------------------------------------------------
    //                        Private member functions
    // ----------------------------------------------------------------------------------------
    // ----------------------------------------------------------------------------------------

    // Symmetric Householder reduction to tridiagonal form.
        template <typename matrix_exp_type>
        void eigenvalue_decomposition<matrix_exp_type>::
        tred2()
        {
            using std::abs;
            using std::sqrt;

            //  This is derived from the Algol procedures tred2 by
            //  Bowdler, Martin, Reinsch, and Wilkinson, Handbook for
            //  Auto. Comp., Vol.ii-Linear Algebra, and the corresponding
            //  Fortran subroutine in EISPACK.

            for (long j = 0; j < n; j++)
            {
                d(j) = V(n-1,j);
            }

            // Householder reduction to tridiagonal form.

            for (long i = n-1; i > 0; i--)
            {

                // Scale to avoid under/overflow.

                type scale = 0.0;
                type h = 0.0;
                for (long k = 0; k < i; k++)
                {
                    scale = scale + abs(d(k));
                }
                if (scale == 0.0)
                {
                    e(i) = d(i-1);
                    for (long j = 0; j < i; j++)
                    {
                        d(j) = V(i-1,j);
                        V(i,j) = 0.0;
                        V(j,i) = 0.0;
                    }
                }
                else
                {

                    // Generate Householder vector.

                    for (long k = 0; k < i; k++)
                    {
                        d(k) /= scale;
                        h += d(k) * d(k);
                    }
                    type f = d(i-1);
                    type g = sqrt(h);
                    if (f > 0)
                    {
                        g = -g;
                    }
                    e(i) = scale * g;
                    h = h - f * g;
                    d(i-1) = f - g;
                    for (long j = 0; j < i; j++)
                    {
                        e(j) = 0.0;
                    }

                    // Apply similarity transformation to remaining columns.

                    for (long j = 0; j < i; j++)
                    {
                        f = d(j);
                        V(j,i) = f;
                        g = e(j) + V(j,j) * f;
                        for (long k = j+1; k <= i-1; k++)
                        {
                            g += V(k,j) * d(k);
                            e(k) += V(k,j) * f;
                        }
                        e(j) = g;
                    }
                    f = 0.0;
                    for (long j = 0; j < i; j++)
                    {
                        e(j) /= h;
                        f += e(j) * d(j);
                    }
                    type hh = f / (h + h);
                    for (long j = 0; j < i; j++)
                    {
                        e(j) -= hh * d(j);
                    }
                    for (long j = 0; j < i; j++)
                    {
                        f = d(j);
                        g = e(j);
                        for (long k = j; k <= i-1; k++)
                        {
                            V(k,j) -= (f * e(k) + g * d(k));
                        }
                        d(j) = V(i-1,j);
                        V(i,j) = 0.0;
                    }
                }
                d(i) = h;
            }

            // Accumulate transformations.

            for (long i = 0; i < n-1; i++)
            {
                V(n-1,i) = V(i,i);
                V(i,i) = 1.0;
                type h = d(i+1);
                if (h != 0.0)
                {
                    for (long k = 0; k <= i; k++)
                    {
                        d(k) = V(k,i+1) / h;
                    }
                    for (long j = 0; j <= i; j++)
                    {
                        type g = 0.0;
                        for (long k = 0; k <= i; k++)
                        {
                            g += V(k,i+1) * V(k,j);
                        }
                        for (long k = 0; k <= i; k++)
                        {
                            V(k,j) -= g * d(k);
                        }
                    }
                }
                for (long k = 0; k <= i; k++)
                {
                    V(k,i+1) = 0.0;
                }
            }
            for (long j = 0; j < n; j++)
            {
                d(j) = V(n-1,j);
                V(n-1,j) = 0.0;
            }
            V(n-1,n-1) = 1.0;
            e(0) = 0.0;
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        void eigenvalue_decomposition<matrix_exp_type>::
        tql2 ()
        {
            using std::pow;
            using std::min;
            using std::max;
            using std::abs;

            //  This is derived from the Algol procedures tql2, by
            //  Bowdler, Martin, Reinsch, and Wilkinson, Handbook for
            //  Auto. Comp., Vol.ii-Linear Algebra, and the corresponding
            //  Fortran subroutine in EISPACK.

            for (long i = 1; i < n; i++)
            {
                e(i-1) = e(i);
            }
            e(n-1) = 0.0;

            type f = 0.0;
            type tst1 = 0.0;
            const type eps = std::numeric_limits<type>::epsilon();
            for (long l = 0; l < n; l++)
            {

                // Find small subdiagonal element

                tst1 = max(tst1,abs(d(l)) + abs(e(l)));
                long m = l;

                // Original while-loop from Java code
                while (m < n)
                {
                    if (abs(e(m)) <= eps*tst1)
                    {
                        break;
                    }
                    m++;
                }
                if (m == n)
                    --m;

                // If m == l, d(l) is an eigenvalue,
                // otherwise, iterate.

                if (m > l)
                {
                    long iter = 0;
                    do
                    {
                        iter = iter + 1;  // (Could check iteration count here.)

                        // Compute implicit shift

                        type g = d(l);
                        type p = (d(l+1) - g) / (2.0 * e(l));
                        type r = hypot(p,(type)1.0);
                        if (p < 0)
                        {
                            r = -r;
                        }
                        d(l) = e(l) / (p + r);
                        d(l+1) = e(l) * (p + r);
                        type dl1 = d(l+1);
                        type h = g - d(l);
                        for (long i = l+2; i < n; i++)
                        {
                            d(i) -= h;
                        }
                        f = f + h;

                        // Implicit QL transformation.

                        p = d(m);
                        type c = 1.0;
                        type c2 = c;
                        type c3 = c;
                        type el1 = e(l+1);
                        type s = 0.0;
                        type s2 = 0.0;
                        for (long i = m-1; i >= l; i--)
                        {
                            c3 = c2;
                            c2 = c;
                            s2 = s;
                            g = c * e(i);
                            h = c * p;
                            r = hypot(p,e(i));
                            e(i+1) = s * r;
                            s = e(i) / r;
                            c = p / r;
                            p = c * d(i) - s * g;
                            d(i+1) = h + s * (c * g + s * d(i));

                            // Accumulate transformation.

                            for (long k = 0; k < n; k++)
                            {
                                h = V(k,i+1);
                                V(k,i+1) = s * V(k,i) + c * h;
                                V(k,i) = c * V(k,i) - s * h;
                            }
                        }
                        p = -s * s2 * c3 * el1 * e(l) / dl1;
                        e(l) = s * p;
                        d(l) = c * p;

                        // Check for convergence.

                    } while (abs(e(l)) > eps*tst1);
                }
                d(l) = d(l) + f;
                e(l) = 0.0;
            }

            /*
                The code to sort the eigenvalues and eigenvectors
                has been removed from here since, in the non-symmetric case,
                we can't sort the eigenvalues in a meaningful way.  If we left this
                code in here then the user might supply what they thought was a symmetric
                matrix but was actually slightly non-symmetric due to rounding error
                and then they would end up in the non-symmetric eigenvalue solver
                where the eigenvalues don't end up getting sorted.  So to avoid
                any possible user confusion I'm just removing this.
            */
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        void eigenvalue_decomposition<matrix_exp_type>::
        orthes ()
        {
            using std::abs;
            using std::sqrt;

            //  This is derived from the Algol procedures orthes and ortran,
            //  by Martin and Wilkinson, Handbook for Auto. Comp.,
            //  Vol.ii-Linear Algebra, and the corresponding
            //  Fortran subroutines in EISPACK.

            long low = 0;
            long high = n-1;

            for (long m = low+1; m <= high-1; m++)
            {

                // Scale column.

                type scale = 0.0;
                for (long i = m; i <= high; i++)
                {
                    scale = scale + abs(H(i,m-1));
                }
                if (scale != 0.0)
                {

                    // Compute Householder transformation.

                    type h = 0.0;
                    for (long i = high; i >= m; i--)
                    {
                        ort(i) = H(i,m-1)/scale;
                        h += ort(i) * ort(i);
                    }
                    type g = sqrt(h);
                    if (ort(m) > 0)
                    {
                        g = -g;
                    }
                    h = h - ort(m) * g;
                    ort(m) = ort(m) - g;

                    // Apply Householder similarity transformation
                    // H = (I-u*u'/h)*H*(I-u*u')/h)

                    for (long j = m; j < n; j++)
                    {
                        type f = 0.0;
                        for (long i = high; i >= m; i--)
                        {
                            f += ort(i)*H(i,j);
                        }
                        f = f/h;
                        for (long i = m; i <= high; i++)
                        {
                            H(i,j) -= f*ort(i);
                        }
                    }

                    for (long i = 0; i <= high; i++)
                    {
                        type f = 0.0;
                        for (long j = high; j >= m; j--)
                        {
                            f += ort(j)*H(i,j);
                        }
                        f = f/h;
                        for (long j = m; j <= high; j++)
                        {
                            H(i,j) -= f*ort(j);
                        }
                    }
                    ort(m) = scale*ort(m);
                    H(m,m-1) = scale*g;
                }
            }

            // Accumulate transformations (Algol's ortran).

            for (long i = 0; i < n; i++)
            {
                for (long j = 0; j < n; j++)
                {
                    V(i,j) = (i == j ? 1.0 : 0.0);
                }
            }

            for (long m = high-1; m >= low+1; m--)
            {
                if (H(m,m-1) != 0.0)
                {
                    for (long i = m+1; i <= high; i++)
                    {
                        ort(i) = H(i,m-1);
                    }
                    for (long j = m; j <= high; j++)
                    {
                        type g = 0.0;
                        for (long i = m; i <= high; i++)
                        {
                            g += ort(i) * V(i,j);
                        }
                        // Double division avoids possible underflow
                        g = (g / ort(m)) / H(m,m-1);
                        for (long i = m; i <= high; i++)
                        {
                            V(i,j) += g * ort(i);
                        }
                    }
                }
            }
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        void eigenvalue_decomposition<matrix_exp_type>::
        cdiv_(type xr, type xi, type yr, type yi)
        {
            using std::abs;
            type r,d;
            if (abs(yr) > abs(yi))
            {
                r = yi/yr;
                d = yr + r*yi;
                cdivr = (xr + r*xi)/d;
                cdivi = (xi - r*xr)/d;
            }
            else
            {
                r = yr/yi;
                d = yi + r*yr;
                cdivr = (r*xr + xi)/d;
                cdivi = (r*xi - xr)/d;
            }
        }

    // ----------------------------------------------------------------------------------------

        template <typename matrix_exp_type>
        void eigenvalue_decomposition<matrix_exp_type>::
        hqr2 ()
        {
            using std::pow;
            using std::min;
            using std::max;
            using std::abs;
            using std::sqrt;

            //  This is derived from the Algol procedure hqr2,
            //  by Martin and Wilkinson, Handbook for Auto. Comp.,
            //  Vol.ii-Linear Algebra, and the corresponding
            //  Fortran subroutine in EISPACK.

            // Initialize

            long nn = this->n;
            long n = nn-1;
            long low = 0;
            long high = nn-1;
            const type eps = std::numeric_limits<type>::epsilon();
            type exshift = 0.0;
            type p=0,q=0,r=0,s=0,z=0,t,w,x,y;

            // Store roots isolated by balanc and compute matrix norm

            type norm = 0.0;
            for (long i = 0; i < nn; i++)
            {
                if ((i < low) || (i > high))
                {
                    d(i) = H(i,i);
                    e(i) = 0.0;
                }
                for (long j = max(i-1,0L); j < nn; j++)
                {
                    norm = norm + abs(H(i,j));
                }
            }

            // Outer loop over eigenvalue index

            long iter = 0;
            while (n >= low)
            {

                // Look for single small sub-diagonal element

                long l = n;
                while (l > low)
                {
                    s = abs(H(l-1,l-1)) + abs(H(l,l));
                    if (s == 0.0)
                    {
                        s = norm;
                    }
                    if (abs(H(l,l-1)) < eps * s)
                    {
                        break;
                    }
                    l--;
                }

                // Check for convergence
                // One root found

                if (l == n)
                {
                    H(n,n) = H(n,n) + exshift;
                    d(n) = H(n,n);
                    e(n) = 0.0;
                    n--;
                    iter = 0;

                    // Two roots found

                }
                else if (l == n-1)
                {
                    w = H(n,n-1) * H(n-1,n);
                    p = (H(n-1,n-1) - H(n,n)) / 2.0;
                    q = p * p + w;
                    z = sqrt(abs(q));
                    H(n,n) = H(n,n) + exshift;
                    H(n-1,n-1) = H(n-1,n-1) + exshift;
                    x = H(n,n);

                    // type pair

                    if (q >= 0)
                    {
                        if (p >= 0)
                        {
                            z = p + z;
                        }
                        else
                        {
                            z = p - z;
                        }
                        d(n-1) = x + z;
                        d(n) = d(n-1);
                        if (z != 0.0)
                        {
                            d(n) = x - w / z;
                        }
                        e(n-1) = 0.0;
                        e(n) = 0.0;
                        x = H(n,n-1);
                        s = abs(x) + abs(z);
                        p = x / s;
                        q = z / s;
                        r = sqrt(p * p+q * q);
                        p = p / r;
                        q = q / r;

                        // Row modification

                        for (long j = n-1; j < nn; j++)
                        {
                            z = H(n-1,j);
                            H(n-1,j) = q * z + p * H(n,j);
                            H(n,j) = q * H(n,j) - p * z;
                        }

                        // Column modification

                        for (long i = 0; i <= n; i++)
                        {
                            z = H(i,n-1);
                            H(i,n-1) = q * z + p * H(i,n);
                            H(i,n) = q * H(i,n) - p * z;
                        }

                        // Accumulate transformations

                        for (long i = low; i <= high; i++)
                        {
                            z = V(i,n-1);
                            V(i,n-1) = q * z + p * V(i,n);
                            V(i,n) = q * V(i,n) - p * z;
                        }

                        // Complex pair

                    }
                    else
                    {
                        d(n-1) = x + p;
                        d(n) = x + p;
                        e(n-1) = z;
                        e(n) = -z;
                    }
                    n = n - 2;
                    iter = 0;

                    // No convergence yet

                }
                else
                {

                    // Form shift

                    x = H(n,n);
                    y = 0.0;
                    w = 0.0;
                    if (l < n)
                    {
                        y = H(n-1,n-1);
                        w = H(n,n-1) * H(n-1,n);
                    }

                    // Wilkinson's original ad hoc shift

                    if (iter == 10)
                    {
                        exshift += x;
                        for (long i = low; i <= n; i++)
                        {
                            H(i,i) -= x;
                        }
                        s = abs(H(n,n-1)) + abs(H(n-1,n-2));
                        x = y = 0.75 * s;
                        w = -0.4375 * s * s;
                    }

                    // MATLAB's new ad hoc shift

                    if (iter == 30)
                    {
                        s = (y - x) / 2.0;
                        s = s * s + w;
                        if (s > 0)
                        {
                            s = sqrt(s);
                            if (y < x)
                            {
                                s = -s;
                            }
                            s = x - w / ((y - x) / 2.0 + s);
                            for (long i = low; i <= n; i++)
                            {
                                H(i,i) -= s;
                            }
                            exshift += s;
                            x = y = w = 0.964;
                        }
                    }

                    iter = iter + 1;   // (Could check iteration count here.)

                    // Look for two consecutive small sub-diagonal elements

                    long m = n-2;
                    while (m >= l)
                    {
                        z = H(m,m);
                        r = x - z;
                        s = y - z;
                        p = (r * s - w) / H(m+1,m) + H(m,m+1);
                        q = H(m+1,m+1) - z - r - s;
                        r = H(m+2,m+1);
                        s = abs(p) + abs(q) + abs(r);
                        p = p / s;
                        q = q / s;
                        r = r / s;
                        if (m == l)
                        {
                            break;
                        }
                        if (abs(H(m,m-1)) * (abs(q) + abs(r)) <
                            eps * (abs(p) * (abs(H(m-1,m-1)) + abs(z) +
                                             abs(H(m+1,m+1)))))
                        {
                            break;
                        }
                        m--;
                    }

                    for (long i = m+2; i <= n; i++)
                    {
                        H(i,i-2) = 0.0;
                        if (i > m+2)
                        {
                            H(i,i-3) = 0.0;
                        }
                    }

                    // Double QR step involving rows l:n and columns m:n

                    for (long k = m; k <= n-1; k++)
                    {
                        long notlast = (k != n-1);
                        if (k != m)
                        {
                            p = H(k,k-1);
                            q = H(k+1,k-1);
                            r = (notlast ? H(k+2,k-1) : 0.0);
                            x = abs(p) + abs(q) + abs(r);
                            if (x != 0.0)
                            {
                                p = p / x;
                                q = q / x;
                                r = r / x;
                            }
                        }
                        if (x == 0.0)
                        {
                            break;
                        }
                        s = sqrt(p * p + q * q + r * r);
                        if (p < 0)
                        {
                            s = -s;
                        }
                        if (s != 0)
                        {
                            if (k != m)
                            {
                                H(k,k-1) = -s * x;
                            }
                            else if (l != m)
                            {
                                H(k,k-1) = -H(k,k-1);
                            }
                            p = p + s;
                            x = p / s;
                            y = q / s;
                            z = r / s;
                            q = q / p;
                            r = r / p;

                            // Row modification

                            for (long j = k; j < nn; j++)
                            {
                                p = H(k,j) + q * H(k+1,j);
                                if (notlast)
                                {
                                    p = p + r * H(k+2,j);
                                    H(k+2,j) = H(k+2,j) - p * z;
                                }
                                H(k,j) = H(k,j) - p * x;
                                H(k+1,j) = H(k+1,j) - p * y;
                            }

                            // Column modification

                            for (long i = 0; i <= min(n,k+3); i++)
                            {
                                p = x * H(i,k) + y * H(i,k+1);
                                if (notlast)
                                {
                                    p = p + z * H(i,k+2);
                                    H(i,k+2) = H(i,k+2) - p * r;
                                }
                                H(i,k) = H(i,k) - p;
                                H(i,k+1) = H(i,k+1) - p * q;
                            }

                            // Accumulate transformations

                            for (long i = low; i <= high; i++)
                            {
                                p = x * V(i,k) + y * V(i,k+1);
                                if (notlast)
                                {
                                    p = p + z * V(i,k+2);
                                    V(i,k+2) = V(i,k+2) - p * r;
                                }
                                V(i,k) = V(i,k) - p;
                                V(i,k+1) = V(i,k+1) - p * q;
                            }
                        }  // (s != 0)
                    }  // k loop
                }  // check convergence
            }  // while (n >= low)

            // Backsubstitute to find vectors of upper triangular form

            if (norm == 0.0)
            {
                return;
            }

            for (n = nn-1; n >= 0; n--)
            {
                p = d(n);
                q = e(n);

                // Real vector

                if (q == 0)
                {
                    long l = n;
                    H(n,n) = 1.0;
                    for (long i = n-1; i >= 0; i--)
                    {
                        w = H(i,i) - p;
                        r = 0.0;
                        for (long j = l; j <= n; j++)
                        {
                            r = r + H(i,j) * H(j,n);
                        }
                        if (e(i) < 0.0)
                        {
                            z = w;
                            s = r;
                        }
                        else
                        {
                            l = i;
                            if (e(i) == 0.0)
                            {
                                if (w != 0.0)
                                {
                                    H(i,n) = -r / w;
                                }
                                else
                                {
                                    H(i,n) = -r / (eps * norm);
                                }

                                // Solve real equations

                            }
                            else
                            {
                                x = H(i,i+1);
                                y = H(i+1,i);
                                q = (d(i) - p) * (d(i) - p) + e(i) * e(i);
                                t = (x * s - z * r) / q;
                                H(i,n) = t;
                                if (abs(x) > abs(z))
                                {
                                    H(i+1,n) = (-r - w * t) / x;
                                }
                                else
                                {
                                    H(i+1,n) = (-s - y * t) / z;
                                }
                            }

                            // Overflow control

                            t = abs(H(i,n));
                            if ((eps * t) * t > 1)
                            {
                                for (long j = i; j <= n; j++)
                                {
                                    H(j,n) = H(j,n) / t;
                                }
                            }
                        }
                    }

                    // Complex vector

                }
                else if (q < 0)
                {
                    long l = n-1;

                    // Last vector component imaginary so matrix is triangular

                    if (abs(H(n,n-1)) > abs(H(n-1,n)))
                    {
                        H(n-1,n-1) = q / H(n,n-1);
                        H(n-1,n) = -(H(n,n) - p) / H(n,n-1);
                    }
                    else
                    {
                        cdiv_(0.0,-H(n-1,n),H(n-1,n-1)-p,q);
                        H(n-1,n-1) = cdivr;
                        H(n-1,n) = cdivi;
                    }
                    H(n,n-1) = 0.0;
                    H(n,n) = 1.0;
                    for (long i = n-2; i >= 0; i--)
                    {
                        type ra,sa,vr,vi;
                        ra = 0.0;
                        sa = 0.0;
                        for (long j = l; j <= n; j++)
                        {
                            ra = ra + H(i,j) * H(j,n-1);
                            sa = sa + H(i,j) * H(j,n);
                        }
                        w = H(i,i) - p;

                        if (e(i) < 0.0)
                        {
                            z = w;
                            r = ra;
                            s = sa;
                        }
                        else
                        {
                            l = i;
                            if (e(i) == 0)
                            {
                                cdiv_(-ra,-sa,w,q);
                                H(i,n-1) = cdivr;
                                H(i,n) = cdivi;
                            }
                            else
                            {

                                // Solve complex equations

                                x = H(i,i+1);
                                y = H(i+1,i);
                                vr = (d(i) - p) * (d(i) - p) + e(i) * e(i) - q * q;
                                vi = (d(i) - p) * 2.0 * q;
                                if ((vr == 0.0) && (vi == 0.0))
                                {
                                    vr = eps * norm * (abs(w) + abs(q) +
                                                       abs(x) + abs(y) + abs(z));
                                }
                                cdiv_(x*r-z*ra+q*sa,x*s-z*sa-q*ra,vr,vi);
                                H(i,n-1) = cdivr;
                                H(i,n) = cdivi;
                                if (abs(x) > (abs(z) + abs(q)))
                                {
                                    H(i+1,n-1) = (-ra - w * H(i,n-1) + q * H(i,n)) / x;
                                    H(i+1,n) = (-sa - w * H(i,n) - q * H(i,n-1)) / x;
                                }
                                else
                                {
                                    cdiv_(-r-y*H(i,n-1),-s-y*H(i,n),z,q);
                                    H(i+1,n-1) = cdivr;
                                    H(i+1,n) = cdivi;
                                }
                            }

                            // Overflow control

                            t = max(abs(H(i,n-1)),abs(H(i,n)));
                            if ((eps * t) * t > 1)
                            {
                                for (long j = i; j <= n; j++)
                                {
                                    H(j,n-1) = H(j,n-1) / t;
                                    H(j,n) = H(j,n) / t;
                                }
                            }
                        }
                    }
                }
            }

            // Vectors of isolated roots

            for (long i = 0; i < nn; i++)
            {
                if (i < low || i > high)
                {
                    for (long j = i; j < nn; j++)
                    {
                        V(i,j) = H(i,j);
                    }
                }
            }

            // Back transformation to get eigenvectors of original matrix

            for (long j = nn-1; j >= low; j--)
            {
                for (long i = low; i <= high; i++)
                {
                    z = 0.0;
                    for (long k = low; k <= min(j,high); k++)
                    {
                        z = z + V(i,k) * H(k,j);
                    }
                    V(i,j) = z;
                }
            }
        }

    // ----------------------------------------------------------------------------------------

    }

    #endif // DLIB_MATRIX_EIGENVALUE_DECOMPOSITION_H