GSparseSymbolic.cpp

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/***************************************************************************
 *       GSparseSymbolic.cpp - Sparse matrix symbolic analysis class       *
 * ----------------------------------------------------------------------- *
 *  copyright (C) 2006-2021 by Juergen Knoedlseder                         *
 * ----------------------------------------------------------------------- *
 *                                                                         *
 *  This program is free software: you can redistribute it and/or modify   *
 *  it under the terms of the GNU General Public License as published by   *
 *  the Free Software Foundation, either version 3 of the License, or      *
 *  (at your option) any later version.                                    *
 *                                                                         *
 *  This program is distributed in the hope that it will be useful,        *
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of         *
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the          *
 *  GNU General Public License for more details.                           *
 *                                                                         *
 *  You should have received a copy of the GNU General Public License      *
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.  *
 *                                                                         *
 ***************************************************************************/
/**
 * @file GSparseSymbolic.cpp
 * @brief Sparse matrix symbolic analysis class implementation
 * @author Juergen Knoedlseder
 */
 
/* __ Includes ___________________________________________________________ */
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
#include "GException.hpp"
#include "GTools.hpp"
#include "GMatrixSparse.hpp"
#include "GSparseSymbolic.hpp"
 
/* __ Method name definitions ____________________________________________ */
#define G_CHOLESKY        "GSparseSymbolic::cholesky_symbolic_analysis(int, "\
                                                            "GMatrixSparse&)"
#define G_CS_AMD               "GSparseSymbolic::cs_amd(int, GMatrixSparse*)"
 
/* __ Macros _____________________________________________________________ */
#define CS_MIN(a,b) (((a) < (b)) ? (a) : (b))
#define CS_MAX(a,b) (((a) > (b)) ? (a) : (b))
#define CS_FLIP(i) (-(i)-2)
#define HEAD(k,j) (ata ? head [k] : j)
#define NEXT(J)   (ata ? next [J] : -1)
 
/* __ Definitions ________________________________________________________ */
//#define G_DEBUG_SPARSE_SYM_AMD_ORDERING               // Debug AMD ordering
//#define G_DEBUG_SPARSE_CHOLESKY           // Analyse Cholesky decomposition
 
 
/*==========================================================================
 =                                                                         =
 =                         Constructors/destructors                        =
 =                                                                         =
 ==========================================================================*/
 
/***********************************************************************//**
 * @brief Void constructor
 ***************************************************************************/
GSparseSymbolic::GSparseSymbolic(void)
{
    // Initialise private members
    m_n_pinv     = 0;
    m_n_q        = 0;
    m_n_parent   = 0;
    m_n_cp       = 0;
    m_n_leftmost = 0;
    m_pinv       = NULL;
    m_q          = NULL;
    m_parent     = NULL;
    m_cp         = NULL;
    m_leftmost   = NULL;
    m_m2         = 0;
    m_lnz        = 0.0;
    m_unz        = 0.0;
 
    // Return
    return;
}
 
 
/***********************************************************************//**
 * @brief Destructor
 ***************************************************************************/
GSparseSymbolic::~GSparseSymbolic(void)
{
    // De-allocate only if memory has indeed been allocated
    if (m_pinv     != NULL) delete [] m_pinv;
    if (m_q        != NULL) delete [] m_q;
    if (m_parent   != NULL) delete [] m_parent;
    if (m_cp       != NULL) delete [] m_cp;
    if (m_leftmost != NULL) delete [] m_leftmost;
 
    // Return
    return;
}
 
 
/*==========================================================================
 =                                                                         =
 =                                  Operators                              =
 =                                                                         =
 ==========================================================================*/
 
/***************************************************************************
 *                    GSparseSymbolic assignment operator                  *
 ***************************************************************************/
GSparseSymbolic& GSparseSymbolic::operator= (const GSparseSymbolic& s)
{ 
    // Execute only if object is not identical
    if (this != &s) {
 
        // De-allocate only if memory has indeed been allocated
        if (m_pinv     != NULL) delete [] m_pinv;
        if (m_q        != NULL) delete [] m_q;
        if (m_parent   != NULL) delete [] m_parent;
        if (m_cp       != NULL) delete [] m_cp;
        if (m_leftmost != NULL) delete [] m_leftmost;
 
        // Initialise private members for clean destruction
        m_n_pinv     = 0;
        m_n_q        = 0;
        m_n_parent   = 0;
        m_n_cp       = 0;
        m_n_leftmost = 0;
        m_pinv       = NULL;
        m_q          = NULL;
        m_parent     = NULL;
        m_cp         = NULL;
        m_leftmost   = NULL;
        m_m2         = 0;
        m_lnz        = 0.0;
        m_unz        = 0.0;
 
        // Copy data members
        m_m2  = s.m_m2;
        m_lnz = s.m_lnz;
        m_unz = s.m_unz;
    
        // Copy m_pinv array if it exists
        if (s.m_pinv != NULL && s.m_n_pinv > 0) {
            m_pinv = new int[s.m_n_pinv];
            for (int i = 0; i < s.m_n_pinv; ++i) {
                m_pinv[i] = s.m_pinv[i];
            }
            m_n_pinv = s.m_n_pinv;
        }
 
        // Copy m_q array if it exists
        if (s.m_q != NULL && s.m_n_q > 0) {
            m_q = new int[s.m_n_q];
            for (int i = 0; i < s.m_n_q; ++i) {
                m_q[i] = s.m_q[i];
                }
            m_n_q = s.m_n_q;
        }
 
        // Copy m_parent array if it exists
        if (s.m_parent != NULL && s.m_n_parent > 0) {
            m_parent = new int[s.m_n_parent];
            for (int i = 0; i < s.m_n_parent; ++i) {
                m_parent[i] = s.m_parent[i];
                }
            m_n_parent = s.m_n_parent;
        }
 
        // Copy m_cp array if it exists
        if (s.m_cp != NULL && s.m_n_cp > 0) {
            m_cp = new int[s.m_n_cp];
            for (int i = 0; i < s.m_n_cp; ++i) {
                m_cp[i] = s.m_cp[i];
            }
            m_n_cp = s.m_n_cp;
        }
 
        // Copy m_leftmost array if it exists
        if (s.m_leftmost != NULL && s.m_n_leftmost > 0) {
            m_leftmost = new int[s.m_n_leftmost];
            for (int i = 0; i < s.m_n_leftmost; ++i) {
                m_leftmost[i] = s.m_leftmost[i];
            }
            m_n_leftmost = s.m_n_leftmost;
        }
 
    } // endif: object was not identical
 
    // Return
    return *this;
}
 
 
/*==========================================================================
 =                                                                         =
 =                     GSparseSymbolic member functions                    =
 =                                                                         =
 ==========================================================================*/
 
/***************************************************************************
 * @brief Symbolc Cholesky analysis
 *
 * @param[in] order Ordering type (0: natural, 1: Cholesky decomposition).
 * @param[in] m Spare matrix.
 *
 * Ordering and symbolic analysis for a Cholesky factorization. This method
 * allocates the memory to hold the information.
 ***************************************************************************/
void GSparseSymbolic::cholesky_symbolic_analysis(int                  order,
                                                 const GMatrixSparse& m)
{
    // Debug
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << endl << "GSparseSymbolic::cholesky_symbolic_analysis entered" << std::endl;
    std::cout << " order ......................: " << order << std::endl;
    std::cout << " number of rows .............: " << m.m_rows << std::endl;
    std::cout << " number of columns ..........: " << m.m_cols << std::endl;
    std::cout << " number of non-zero elements : " << m.m_elements << std::endl;
    #endif
 
    // De-allocate memory that has indeed been previously allocated
    if (m_pinv     != NULL) delete [] m_pinv;
    if (m_q        != NULL) delete [] m_q;
    if (m_parent   != NULL) delete [] m_parent;
    if (m_cp       != NULL) delete [] m_cp;
    if (m_leftmost != NULL) delete [] m_leftmost;
  
    // Initialise members
    m_n_pinv     = 0;
    m_n_q        = 0;
    m_n_parent   = 0;
    m_n_cp       = 0;
    m_n_leftmost = 0;
    m_pinv       = NULL;
    m_q          = NULL;
    m_parent     = NULL;
    m_cp         = NULL;
    m_leftmost   = NULL;
    m_m2         = 0;
    m_lnz        = 0.0;
    m_unz        = 0.0;
 
    // Check if order type is valid
    if (order < 0 || order > 1) {
        std::string msg = "Invalid ordering type "+gammalib::str(order)+
                          "specified. The ordering type must be comprised in "
                          "[0,1]. Please specify a valid ordering type.";
        throw GException::invalid_argument(G_CHOLESKY, msg);
    }
 
    // Assign input matrix attributes
    int n = m.m_cols;
 
    // P = amd(A+A'), or natural
    int* P = cs_amd(order, &m);
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << " AMD ordering permutation (" << P << ")" << std::endl;
    if (P != NULL) {
        for (int i = 0; i < n; ++i) {
            std::cout << " " << P[i];
        }
        std::cout << std::endl;
    }
    #endif
  
    // Find inverse permutation and store it in class member 'm_pinv'.
    // Note that if P = NULL or n < 1 this function returns NULL.
    m_pinv   = cs_pinv(P, n);
    m_n_pinv = n;
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << " Inverse permutation (" << m_pinv << ")" << std::endl;
    if (m_pinv != NULL) {
        for (int i = 0; i < n; ++i) {
            std::cout << " " << m_pinv[i];
        }
        std::cout << std::endl;
    }
    #endif
  
    // Delete workspace
    if (P != NULL) {
        delete [] P;
    }
  
    // C = spones(triu(A(P,P)))
    GMatrixSparse C = cs_symperm(m, m_pinv);
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << " C = spones(triu(A(P,P))) " << C << std::endl;
    #endif
  
    // Find etree of C and store it in class member 'm_parent'
    m_parent   = cs_etree(&C, 0);
    m_n_parent = n;
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << " Elimination tree (" << m_parent << ")" << std::endl;
    if (m_parent != NULL) {
        for (int i = 0; i < n; ++i) {
            std::cout << " " << m_parent[i];
        }
        std::cout << std::endl;
    }
    #endif
 
    // Post order the etree
    // Note that if m_parent = NULL or n < 1 this function returns NULL.
    int* post = cs_post(m_parent, n);
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << " Post ordered elimination tree (" << post << ")" << std::endl;
    if (post != NULL) {
        for (int i = 0; i < n; ++i) {
            std::cout << " " << post[i];
        }
        std::cout << std::endl;
    }
    #endif
  
    // Find column counts of chol(C)
    // Note that if m_parent = NULL or post = NULL this function returns NULL.
    int* c = cs_counts(&C, m_parent, post, 0);
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << " Column counts (" << c << ")" << std::endl;
    if (c != NULL) {
        for (int i = 0; i < n; ++i) {
            std::cout << " " << c[i];
        }
        std::cout << std::endl;
    }
    #endif
 
    // Delete workspace
    if (post != NULL) {
        delete [] post;
    }
  
    // Allocate column pointers for Cholesky decomposition
    m_n_cp = n+1;
    m_cp   = new int[m_n_cp];
 
    // Find column pointers for L
    m_unz = m_lnz = cs_cumsum(m_cp, c, n);
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << " Column pointers for L (" << m_cp << ")" << std::endl;
    if (m_cp != NULL) {
        for (int i = 0; i < m_n_cp; ++i) {
            std::cout << " " << m_cp[i];
        }
        std::cout << std::endl;
    }
    std::cout << " Number of non-zero elements in L: " << m_lnz << std::endl;
    #endif
 
    // Delete workspace
    if (c != NULL) {
        delete [] c;
    }
  
    // Delete symbolic analysis if it is not valid
    if (m_lnz < 0) {
 
        // De-allocate memory that has indeed been previously allocated
        if (m_pinv     != NULL) delete [] m_pinv;
        if (m_q        != NULL) delete [] m_q;
        if (m_parent   != NULL) delete [] m_parent;
        if (m_cp       != NULL) delete [] m_cp;
        if (m_leftmost != NULL) delete [] m_leftmost;
  
        // Initialise members
        m_n_pinv     = 0;
        m_n_q        = 0;
        m_n_parent   = 0;
        m_n_cp       = 0;
        m_n_leftmost = 0;
        m_pinv       = NULL;
        m_q          = NULL;
        m_parent     = NULL;
        m_cp         = NULL;
        m_leftmost   = NULL;
        m_m2         = 0;
        m_lnz        = 0.0;
        m_unz        = 0.0;
    }
 
    // Debug
    #if defined(G_DEBUG_SPARSE_CHOLESKY)
    std::cout << "GSparseSymbolic::cholesky_symbolic_analysis finished" << std::endl;
    #endif
  
    // Return
    return;
}
 
 
/*==========================================================================
 =                                                                         =
 =                     GSparseSymbolic private functions                   =
 =                                                                         =
 ==========================================================================*/
 
/***************************************************************************
 * @brief cs_amd
 *
 * @param[in] order Ordering type.
 * @param[in] A Spare Matrix.
 * @return Integer array of n+1 elements
 *
 * Applies an approximate minimum degree ordering algorithm to derive the
 * permutations that minimise the fill-in.
 * p = amd(A+A') if symmetric is true, or amd(A'A) otherwise
 *
 * The following ordering types are supported:
 *      0: natural
 *      1: Cholesky decomposition
 *      2: LU decomposition
 *      3: QR decomposition
 ***************************************************************************/
int* GSparseSymbolic::cs_amd(int order, const GMatrixSparse* A)
{
    // Throw exception if order is invalid
    if (order < 0 || order > 3) {
        std::string msg = "Invalid ordering type "+gammalib::str(order)+
                          "specified. The ordering type must be comprised in "
                          "[0,3]. Please specify a valid ordering type.";
        throw GException::invalid_argument(G_CS_AMD, msg);
    }
  
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << std::endl << "GSparseSymbolic::cs_amd entered" << std::endl;
    std::cout << " order ......................: " << order << std::endl;
    std::cout << " number of rows .............: " << A->m_rows << std::endl;
    std::cout << " number of columns ..........: " << A->m_cols << std::endl;
    std::cout << " number of non-zero elements : " << A->m_elements << std::endl;
    #endif
 
    // Declare
    int dense;
    int  d, dk, dext, lemax = 0, e, elenk, eln, i, j, k, k1,
         k2, k3, jlast, ln, mindeg = 0, nvi, nvj, nvk, mark, wnvi,
         ok, nel = 0, p, p1, p2, p3, p4, pj, pk, pk1, pk2, pn, q;
    unsigned int h;
 
    // Assign input matrix attributes
    int m = A->m_rows;
    int n = A->m_cols;
 
 
    // ======================================================================
    // Step 1: Construct matrix C
    // ======================================================================
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Step 1: Construct matrix C" << std::endl;
    #endif
 
    // Initialise matrix C. We do not care at this point about the actual
    // dimension since we attribute the matrix later anyways. It's just
    // that there is no empty constructor of a matrix, so we have to put
    // something by default ...
    GMatrixSparse C(m,n);
 
    // Get (logical) transpose of A: AT = A'
    // NOTE: WE ONLY NEED THE LOGICAL TRANSPOSE HERE. HOWEVER, WE HAVE NO
    // LOGICAL ADDITION SO FAR ...
    //  GMatrixSparse AT = cs_transpose(*A, 0);
    GMatrixSparse AT = cs_transpose(*A, 1);
 
    // Find dense threshold
    dense = (int)CS_MAX(16, 10 * sqrt((double)n));
    dense = CS_MIN(n-2, dense);
  
    // Case A: Cholesky decomposition of a symmetric matrix: C = A + A'
    if (order == 1 && n == m) {
        //  NOTE: HERE WE ONLY NEED LOGICAL ADDITION
        //  C = cs_add(A, AT, 0, 0);
        C = *A + AT;
    }
  
    // Case B: LU decomposition: C = A' * A with no dense rows
    else if (order == 2) {
  
        // Assign A' matrix attributes
        int* ATp = AT.m_colstart;
        int* ATi = AT.m_rowinx;
    
        // Drop dense columns from AT
        for (p2 = 0, j = 0; j < m; j++) {
            p      = ATp[j];                       // column j of AT starts here
            ATp[j] = p2;                           // new column j starts here
            if (ATp[j+1] - p > dense) {
                continue;                          // skip dense col j
            }
            for ( ; p < ATp[j+1]; p++) {
                ATi[p2++] = ATi[p] ;
            }
        }
    
        // Finalise AT
        ATp[m] = p2;
    
        // Get (logical) transpose of AT: A2 = AT'
        // NOTE: WE ONLY NEED THE LOGICAL TRANSPOSE HERE. HOWEVER, WE HAVE NO
        // LOGICAL MULTIPLICATION SO FAR ...
        //    GMatrixSparse A2 = cs_transpose(AT, 0);
        GMatrixSparse A2 = cs_transpose(AT, 1);
    
        // C = A' * A with no dense rows
        //  NOTE: cs_multiply NOT YET IMPLEMENTED
        //  C = A2 ? cs_multiply(AT, A2) : NULL;
        C = AT * A2;
    }
  
    // Case C: QR decomposition: C = A' * A
    else {
        //  NOTE: cs_multiply NOT YET IMPLEMENTED
        //  C = cs_multiply(AT, A);
        C = AT * *A;
    }
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Matrix C " << C << std::endl;
    #endif
  
    // Drop diagonal entries from C
    cs_fkeep(&C, &cs_diag, NULL);
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Dropped diagonal entries from matrix C " << C << std::endl;
    #endif
  
    // Assign C matrix attributes
    int* Cp  = C.m_colstart;
    int  cnz = Cp[n];
 
    // Allocate result array
    int* P = new int[n+1];
 
    // Allocate workspace
    int  wrk_size = 8*(n+1);
    int* wrk_int  = new int[wrk_size];
 
    // Add elbow room to C
    int elbow_room = cnz/5 + 2*n;           // Request additional # of elements
    C.alloc_elements(cnz, elbow_room);      // Appand elements
    int nzmax = C.m_elements;               // Save the maximum # of entries for garbage coll.
    C.m_elements -= elbow_room;             // Reverse element increase of alloc_elements
 
    // (Re-)assign C matrix attributes
    Cp      = C.m_colstart;
    int* Ci = C.m_rowinx;
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Added elbow room to matrix C " << C << std::endl;
    #endif
 
    // Set workspace pointers
    int* len    = wrk_int;
    int* nv     = wrk_int +   (n+1);
    int* next   = wrk_int + 2*(n+1);
    int* head   = wrk_int + 3*(n+1);
    int* elen   = wrk_int + 4*(n+1);
    int* degree = wrk_int + 5*(n+1);
    int* w      = wrk_int + 6*(n+1);
    int* hhead  = wrk_int + 7*(n+1);
    int* last   = P;                        // use P as workspace for last
 
 
    // ======================================================================
    // Step 2: Initialize quotient graph
    // ======================================================================
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Step 2: Initialize quotient graph" << std::endl;
    #endif
 
    // Setup array that contains for each column the number of non-zero elements
    for (k = 0 ; k < n ; k++) {
        len[k] = Cp[k+1] - Cp[k];
    }
    len[n] = 0;
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Non-zero elements per column: ";
    for (k = 0 ; k <= n ; k++) {
        std::cout << " " << len[k];
    }
    std::cout << std::endl;
    #endif
    
    // Loop over all nodes (i.e. matrix columns)
    for (i = 0 ; i <= n ; i++) {
        head[i]   = -1;                       // degree list i is empty
        last[i]   = -1;
        next[i]   = -1;
        hhead[i]  = -1;                       // hash list i is empty
        nv[i]     = 1;                        // node i is just one node
        w[i]      = 1;                        // node i is alive
        elen[i]   = 0;                        // Ek of node i is empty
        degree[i] = len[i];                   // degree of node i
    }
  
    // Clear w
    mark = cs_wclear(0, 0, w, n);
  
    // End-point initialisations
    elen[n] = -2;                           // n is a dead element
    Cp[n]   = -1;                           // n is a root of assembly tree
    w[n]    = 0;                            // n is a dead element
  
  
    // ======================================================================
    // Step 3: Initialize degree lists
    // ======================================================================
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Step 3: Initialize degree lists" << std::endl;
    #endif
 
    // Loop over all nodes
    for (i = 0 ; i < n ; i++) {
  
        // Get degree of node
        d = degree[i];
    
        // Case A: Node i is empty
        if (d == 0) {
            elen[i] = -2;                       // element i is dead
            nel++ ;
            Cp[i]   = -1;                       // i is a root of assembly tree
            w[i]    = 0;                        // i is a dead element
            }
    
        // Case B: Node is dense
        else if (d > dense) {
            nv[i]   = 0;                        // absorb i into element n
            elen[i] = -1;                       // node i is dead
            nel++;
            Cp[i] = CS_FLIP(n);
            nv[n]++;
        }
    
        // Case C: Node is sparse
        else {
            if (head[d] != -1) last[head[d]] = i;
            next[i] = head[d];                  // put node i in degree list d
            head[d] = i;
            }
 
    } // endfor: looped over all columns
 
 
    // ======================================================================
    // Step 4: Build permutations
    // ======================================================================
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Step 4: Build permutations" << std::endl;
    #endif
  
    // While (selecting pivots) do
    while (nel < n) {
  
        // Select node of minimum approximate degree
        for (k = -1; mindeg < n && (k = head [mindeg]) == -1; mindeg++) ;
        if (next[k] != -1) last [next[k]] = -1;
        head[mindeg]  = next[k];              // remove k from degree list
        elenk         = elen[k];              // elenk = |Ek|
        nvk           = nv[k];                // # of nodes k represents
        nel          += nvk;                  // nv[k] nodes of A eliminated
    
        // Garbage collection. This frees the elbow rooms in matrix C
        if (elenk > 0 && cnz + mindeg >= nzmax) {
    
            // Loop over all columns
            for (j = 0; j < n; j++) {
                // If j is a live node or element, then ...
                if ((p = Cp[j]) >= 0) {
                    Cp[j] = Ci[p];            // save first entry of object
                    Ci[p] = CS_FLIP(j);       // first entry is now CS_FLIP(j)
                }
            }
      
            // Scan all memory
            for (q = 0, p = 0; p < cnz; ) {
                // If we found object j, then ...
                if ((j = CS_FLIP(Ci[p++])) >= 0) {
                    Ci[q] = Cp[j];            // restore first entry of object
                    Cp[j] = q++;              // new pointer to object j
                    for (k3 = 0; k3 < len[j]-1; k3++) {
                        Ci[q++] = Ci[p++];
                    }
                }
            }
            cnz = q;                          // Ci[cnz...nzmax-1] now free
      
        } // endif: garbage collection done
    
        // Construct new element
        dk    = 0;
        nv[k] = -nvk;                         // flag k as in Lk
        p     = Cp[k];
        pk1   = (elenk == 0) ? p : cnz;       // do in place if elen[k] == 0
        pk2   = pk1;
        for (k1 = 1; k1 <= elenk + 1; k1++) {
    
            // ...
            if (k1 > elenk) {
                e  = k;                       // search the nodes in k
                pj = p;                       // list of nodes starts at Ci[pj]
                ln = len[k] - elenk;          // length of list of nodes in k
            }
            else {
                e  = Ci[p++];                 // search the nodes in e
                pj = Cp[e];
                ln = len[e];                  // length of list of nodes in e
            }
      
            // ...
            for (k2 = 1 ; k2 <= ln ; k2++) {
                i = Ci[pj++];
                if ((nvi = nv [i]) <= 0) {    // Skip of node i is dead or seen
                    continue;
                }
                dk        += nvi;             // degree[Lk] += size of node i
                nv[i]      = -nvi;            // negate nv[i] to denote i in Lk
                Ci[pk2++]  = i;               // place i in Lk
                if (next[i] != -1) {
                    last[next[i]] = last[i];
                }
        
                // remove i from degree list
                if (last[i] != -1) {
                    next[last[i]] = next[i];
                }
                else {
                    head[degree[i]] = next[i];
                }
 
            } // endfor: k2
      
            // ...
            if (e != k) {
                Cp[e] = CS_FLIP(k);           // absorb e into k
                w[e]  = 0;                    // e is now a dead element
            }
 
        } // endfor: k1
    
        if (elenk != 0) {
            cnz = pk2;                        // Ci [cnz...nzmax] is free
        }
        degree[k] = dk;                       // external degree of k - |Lk\i|
        Cp[k]     = pk1;                      // element k is in Ci[pk1..pk2-1]
        len[k]    = pk2 - pk1;
        elen[k]   = -2;                       // k is now an element
    
        // Clear w if necessary
        mark = cs_wclear(mark, lemax, w, n);
    
        // Scan 1: Find set differences |Le\Lk|
        for (pk = pk1 ; pk < pk2 ; pk++) {
 
            i = Ci [pk];
            if ((eln = elen[i]) <= 0) {       // skip if elen[i] empty
                continue;
            }
            nvi  = -nv[i];                    // nv [i] was negated
            wnvi = mark - nvi;
      
            // Scan Ei
            for (p = Cp[i]; p <= Cp[i] + eln - 1; p++) {
                e = Ci[p];
                if (w[e] >= mark) {
                    w[e] -= nvi;              // decrement |Le\Lk|
                }
                else if (w[e] != 0) {         // ensure e is a live element
                    w[e] = degree[e] + wnvi;  // 1st time e seen in scan 1
                }
            } // endfor: scanned Ei
 
        } // endfor: scan 1 finished
    
        // Scan 2: Degree update
        for (pk = pk1; pk < pk2; pk++) {
 
            i  = Ci[pk];                      // consider node i in Lk
            p1 = Cp[i];
            p2 = p1 + elen [i] - 1;
            pn = p1;
      
            // Scan Ei
            for (h = 0, d = 0, p = p1 ; p <= p2 ; p++) {
                e = Ci[p];
                // If e is an unabsorbed element, then ...
                if (w[e] != 0) {
                    dext = w[e] - mark;       // dext = |Le\Lk|
                    if (dext > 0) {
                        d += dext;            // sum up the set differences
                        Ci[pn++] = e;         // keep e in Ei
                        h += e;               // compute the hash of node i
                    }
                    else {
                        Cp[e] = CS_FLIP(k);   // aggressive absorb. e->k
                        w[e]  = 0;            // e is a dead element
                    }
                }
            } // endfor: scanned Ei
      
            // elen[i] = |Ei|
            elen[i] = pn - p1 + 1;
            p3 = pn ;
            p4 = p1 + len[i];
      
            // Prune edges in Ai
            for (p = p2 + 1 ; p < p4 ; p++) {
                j = Ci[p];
                if ((nvj = nv[j]) <= 0) {     // node j dead or in Lk
                    continue;
                }
                d += nvj;                     // degree(i) += |j|
                Ci[pn++] = j;                 // place j in node list of i
                h += j;                       // compute hash for node i
            }
      
            // Check for mass elimination
            if (d == 0) {
                Cp[i]   = CS_FLIP(k);         // absorb i into k
                nvi     = -nv[i];
                dk     -= nvi;                // |Lk| -= |i|
                nvk    += nvi;                // |k| += nv[i]
                nel    += nvi;
                nv[i]   = 0;
                elen[i] = -1;                 // node i is dead
            }
            else {
                degree[i] = CS_MIN(degree[i], d);   // update degree(i)
                Ci[pn] = Ci[p3];              // move first node to end
                Ci[p3] = Ci[p1];              // move 1st el. to end of Ei
                Ci[p1] = k;                   // add k as 1st element in of Ei
                len[i] = pn - p1 + 1;         // new len of adj. list of node i
                h %= n;                       // finalize hash of i
                next[i]  = hhead[h];          // place i in hash bucket
                hhead[h] = i;
                last[i]  = h;                 // save hash of i in last[i]
            }
 
        } // endfor: scan 2 is done
    
        // finalize |Lk|
        degree[k] = dk;
        lemax     = CS_MAX(lemax, dk);
    
        // Clear w
        mark = cs_wclear(mark+lemax, lemax, w, n);
    
        // Supernode detection
        for (pk = pk1 ; pk < pk2 ; pk++) {
 
            i = Ci[pk];
            if (nv[i] >= 0) {
                continue;                     // skip if i is dead
            }
            h = last[i];                      // scan hash bucket of node i
            i = hhead[h];
            hhead[h] = -1;                    // hash bucket will be empty
      
            // ...
            for ( ; i != -1 && next[i] != -1 ; i = next[i], mark++) {
                ln  = len[i];
                eln = elen[i];
                for (p = Cp[i]+1 ; p <= Cp[i] + ln-1 ; p++) {
                    w[Ci[p]] = mark;
                }
                jlast = i;
        
                // Compare i with all j
                for (j = next[i]; j != -1; )     {
                    ok = (len[j] == ln) && (elen[j] == eln);
                    for (p = Cp[j] + 1; ok && p <= Cp[j] + ln - 1; p++) {
                        if (w[Ci[p]] != mark) {          // compare i and j
                            ok = 0;
                        }
                    }
          
                    // i and j are identical
                    if (ok) {
                        Cp[j]       = CS_FLIP(i);       // absorb j into i
                        nv[i]      += nv[j];
                        nv[j]       = 0;
                        elen[j]     = -1;               // node j is dead
                        j           = next[j];          // delete j from hash bucket
                        next[jlast] = j;
                    }
                    // j and i are different
                    else {
                        jlast = j;
                        j     = next[j];
                    }
                } // endfor: comparison of i with all j
 
            } // endfor: ...
 
        } // endfor: supernode detection
    
        // Finalize new element Lk
        for (p = pk1, pk = pk1; pk < pk2; pk++) {
            i = Ci[pk];
            if ((nvi = -nv[i]) <= 0) {             // skip if i is dead
                continue;
            }
            nv[i] = nvi;                          // restore nv[i]
            d = degree[i] + dk - nvi;             // compute external degree(i)
            d = CS_MIN(d, n - nel - nvi);
            if (head[d] != -1) {
                last[head[d]] = i;
            }
            next[i]   = head[d];                  // put i back in degree list
            last[i]   = -1;
            head[d]   = i;
            mindeg    = CS_MIN(mindeg, d);          // find new minimum degree
            degree[i] = d;
            Ci[p++]   = i;                        // place i in Lk
        }
    
        // Number of nodes absorbed into k
        nv[k] = nvk;
    
        // ...
        if ((len [k] = p-pk1) == 0) {           // length of adj list of element k
            Cp[k] = -1;                       // k is a root of the tree
            w[k]  = 0;                         // k is now a dead element
        }
        if (elenk != 0) {
            cnz = p;               // free unused space in Lk
        }
 
    } // endwhile
 
 
    // ======================================================================
    // Step 5: Postordering
    // ======================================================================
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Step 5: Postordering" << std::endl;
    #endif
    
    // Fix assembly tree
    for (i = 0; i < n; i++) {
        Cp[i] = CS_FLIP(Cp[i]);
    }
    for (j = 0 ; j <= n ; j++) {
        head[j] = -1;
    }
    
    // Place unordered nodes in lists
    for (j = n ; j >= 0 ; j--) {
        if (nv[j] > 0) {
            continue;                // skip if j is an element
        }
        next[j]     = head [Cp[j]];                 // place j in list of its parent
        head[Cp[j]] = j;
    }
    
    // Place elements in lists
    for (e = n ; e >= 0 ; e--) {
        if (nv[e] <= 0) {
            continue;               // skip unless e is an element
        }
        if (Cp[e] != -1) {
        next[e]     = head[Cp[e]];                // place e in list of its parent
        head[Cp[e]] = e;
        }
    }
 
 
    // ======================================================================
    // Step 6: Postorder the assembly tree
    // ======================================================================
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Step 6: Postorder the assembly tree" << std::endl;
    #endif
  
    // Postorder the assembly tree
    for (k = 0, i = 0 ; i <= n ; i++) {
        if (Cp [i] == -1) {
            k = cs_tdfs(i, k, head, next, P, w);
        }
    }
  
    // Free workspace
    delete [] wrk_int;
 
    // Debug
    #if defined(G_DEBUG_SPARSE_SYM_AMD_ORDERING)
    std::cout << " Permutation:" << std::endl;
    for (i = 0; i < n; ++i) {
        std::cout << " " << P[i];
    }
    std::cout << endl;
    std::cout << "GSparseSymbolic::cs_amd finished" << std::endl;
    #endif
  
    // Return result
    return P;
}
 
 
/***************************************************************************
 * @brief cs_counts
 *
 * @param[in] A Spare matrix.
 * @param[in] parent Parent array.
 * @param[in] post Post ordering array.
 * @param[in] ata 0: count LL'=A, 1: count LL'=A'A
 * @return Integer array of n+1 elements
 *
 * Column counts of LL'=A or LL'=A'A, given parent & post ordering
 ***************************************************************************/
int* GSparseSymbolic::cs_counts(const GMatrixSparse* A,
                                const int*           parent,
                                const int*           post,
                                int                  ata)
{
    // Declare loop variables
    int i, j, k, J, p, q, jleaf;
 
    // Return NULL pointer if input arrays are NULL or sparse matrix is
    // empty
    if (!parent || !post) {
        return NULL;
    }
 
    // Assign input matrix attributes
    int m = A->m_rows;
    int n = A->m_cols;
  
    // Allocate result
    int* colcount = new int[n];
    int* delta = colcount;
 
    // Allocate workspace
    int  wrk_size = 4*n + (ata ? (n+m+1) : 0);
    int* wrk_int  = new int[wrk_size];
 
    // Get (logical) transpose of A: AT = A'
    GMatrixSparse AT = cs_transpose(*A, 0);
 
    // Set-up pointers to workspace
    int* ancestor = wrk_int;
    int* maxfirst = wrk_int + n;
    int* prevleaf = wrk_int + 2*n;
    int* first    = wrk_int + 3*n;
 
    // Clear workspace
    for (k = 0; k < wrk_size; ++k) {
        wrk_int[k] = -1;
    }
 
    // Find first j
    for (k = 0; k < n; k++) {
        j        = post[k];
        delta[j] = (first[j] == -1) ? 1 : 0;    // delta[j]=1 if j is a leaf
        for ( ; j != -1 && first[j] == -1; j = parent[j]) {
            first[j] = k;
        }
    }
 
    // Assign output matrix attributes
    int* ATp = AT.m_colstart;
    int* ATi = AT.m_rowinx;
 
    // Initialise. Note that init_ata wires the working array into the *head
    // and *next pointers, so both arrays are just aliases of the wrk_int
    // array.
    int* head = NULL;
    int* next = NULL;
    if (ata) {
        init_ata(&AT, post, wrk_int, &head, &next);
    }
 
    // Each node in its own set
    for (i = 0; i < n; i++) {
        ancestor[i] = i;
    }
  
    // Loop over all columns of matrix A (or rows of matrix AT)
    for (k = 0; k < n; k++) {
        j = post[k];        // j is the kth node in postordered etree
        if (parent[j] != -1) {
            delta[parent[j]]--;        // j is not a root
        }
        for (J = HEAD(k,j); J != -1; J = NEXT(J)) {     // J=j for LL'=A case
    
            // Loop over all non-zero elements in column J
            for (p = ATp[J]; p < ATp[J+1]; p++) {
                i = ATi[p];
                q = cs_leaf(i, j, first, maxfirst, prevleaf, ancestor, &jleaf);
                if (jleaf >= 1) {
                    delta[j]++;         // A(i,j) is in skeleton
                }
                if (jleaf == 2) {
                    delta[q]--;         // account for overlap in q
                }
            }
        }
        if (parent[j] != -1) {
            ancestor[j] = parent[j];
        }
    }
  
    // Sum up delta's of each child
    for (j = 0; j < n; j++) {
        if (parent[j] != -1) {
            colcount[parent[j]] += colcount[j];
        }
    }
  
    // Free workspace
    delete [] wrk_int;
  
    // Return result
    return colcount;
} 
 
 
/***************************************************************************
 * @brief cs_etree
 *
 * @param[in] A Sparse matrix
 * @param[in] ata Flag.
 * @return Evaluation tree of matrix @p A.
 *
 * Compute the etree of A using triu(A), or A'A without forming A'A
 ***************************************************************************/
int* GSparseSymbolic::cs_etree(const GMatrixSparse* A, int ata)
{
    // Declare loop variables
    int i, k, p;
 
    // Assign matrix attributes
    int m = A->m_rows;
    int n = A->m_cols;
 
    // Allocate result array
    int* parent = new int[n];
 
    // Allocate workspace
    int  wrk_size = n + (ata ? m : 0);
    int* wrk_int  = new int[wrk_size];
 
    // Set-up pointers to workspace. If 'ata=1' then we use also a prev array
    int* ancestor = wrk_int;
    int* prev     = wrk_int + n;
  
    // If 'prev' is requested, initialise array with -1
    if (ata) {
        for (i = 0; i < m ; ++i) {
            prev[i] = -1;
        }
    }
 
    // Loop over all nodes (columns)
    for (k = 0; k < n; ++k) {
  
        // Initialise node k to having no parent and no ancestor
        parent[k]   = -1;           // node k has no parent yet
        ancestor[k] = -1;           // nor does k have an ancestor
    
        // Loop over all elements in column k
        for (p = A->m_colstart[k]; p < A->m_colstart[k+1]; ++p) {
    
            // Get element index
            i = ata ? (prev[A->m_rowinx[p]]) : (A->m_rowinx[p]);
      
            // Traverse from i to k
            int inext;
            for ( ; i != -1 && i < k; i = inext) {
                inext       = ancestor[i];        // inext = ancestor of i
                ancestor[i] = k;                  // path compression
                if (inext == -1) {
                    parent[i] = k;   // no ancestor, parent is k
                }
            }
            if (ata) {
                prev[A->m_rowinx[p]] = k;
            }
            
        } // endfor: looped over all elements in column k
 
    } // endfor: looped over all columns
 
    // Free workspace
    delete [] wrk_int;
  
    // Return result
    return parent;
}
 
 
/***********************************************************************//**
 * @brief cs_fkeep
 *
 * @param[in] A Sparse matrix.
 * @param[in] fkeep Screening function.
 * @param[in] other Other function.
 * @return Number of non-zero elements (-1 if not okay)
 *
 * Drop entries for which fkeep(A(i,j)) is false. Return the number of
 * non-zero elements if ok, otherwise return -1.
 ***************************************************************************/
int GSparseSymbolic::cs_fkeep(GMatrixSparse* A, 
                              int(*fkeep)(int, int, double, void*), 
                              void* other)
{
    // Return error if some of the input pointers is invalid
    if (!A || !fkeep) {
        return (-1);
    }
  
    // Initialise counter for non-zero elements 
    int nz = 0;
 
    // Assign matrix attributes
    int     n  = A->m_cols;
    int*    Ap = A->m_colstart;
    int*    Ai = A->m_rowinx; 
    double* Ax = A->m_data;
 
    // Operate only if we have some data
    if (Ap != NULL && Ai != NULL && Ax != NULL) {
 
        // Loop over all columns
        for (int j = 0; j < n; j++) {
            int p = Ap[j];                     // get current location of col j
            Ap[j] = nz;                        // record new location of col j
            for ( ; p < Ap[j+1] ; p++) {
                if (fkeep(Ai[p], j, Ax ? Ax[p] : 1, other)) {
                    if (Ax) {
                        Ax[nz] = Ax[p];    // keep A(i,j)
                    }
                    Ai[nz++] = Ai[p];
                }
            }
        }
  
        // Finalise A
        Ap[n]         = nz;
        A->m_elements = nz;
 
    } // endif: we had data
  
    // Remove extra space from A
    //cs_sprealloc(A, 0);
    A->free_elements(nz, (A->m_elements-nz));
 
    // Return number of non-zero elements
    return nz;
}
 
 
/***************************************************************************
 * @brief cs_leaf
 *
 * @param[in] i Row to consider
 * @param[in] j Column to consider
 * @param[in] first ...
 * @param[in] maxfirst ...
 * @param[in] prevleaf ...
 * @param[in] ancestor ...
 * @param[out] jleaf ...
 * @return q
 *
 * Consider A(i,j), node j in ith row subtree and return lca(jprev,j)
 ***************************************************************************/
int GSparseSymbolic::cs_leaf(int        i,
                             int        j,
                             const int *first,
                             int       *maxfirst,
                             int       *prevleaf,
                             int       *ancestor,
                             int       *jleaf)
{
    // If one of the input pointers is invalid then return -1
    if (!first || !maxfirst || !prevleaf || !ancestor || !jleaf) {
        return (-1);
    }
 
    // Declare loop variables
    int q, s, sparent;
 
    // ...
    *jleaf = 0 ;
  
    // If j is not a leaf then return -1
    if (i <= j || first[j] <= maxfirst[i]) {
        return (-1);
    }
 
    // Update max first[j] seen so far
    maxfirst[i] = first[j];
  
    // jprev = previous leaf of ith subtree
    int jprev = prevleaf[i];
    prevleaf[i] = j;
  
    // j is first or subsequent leaf
    *jleaf = (jprev == -1) ? 1: 2;
  
    // If j is 1st leaf then q is the root of ith subtree
    if (*jleaf == 1) {
        return (i);
    }
  
    // ...
    for (q = jprev; q != ancestor[q]; q = ancestor[q]) ;
  
    // ...
    for (s = jprev; s != q; s = sparent) {
        sparent     = ancestor[s];     // path compression
        ancestor[s] = q;
    }
  
    // Return least common ancester (jprev,j)
    return (q);
}
 
 
/***************************************************************************
 * @brief Inverts the permutation p[0..n-1]
 *
 * @param[in] p Integer array of n elements (NULL denotes ID)
 * @param[in] n Number of elements
 * @return Integer array of n elements pinv[0..n-1] = p[n-1..0]
 *
 * Inverts the permutation p[0..n-1]
 ***************************************************************************/
int* GSparseSymbolic::cs_pinv(int const *p, int n)
{
    // Return NULL pointer if input pointer is NULL or the number of
    // elements is zero. This denotes identity.
    if (!p || n < 1) {
        return NULL;
    }
  
    // Allocate result array
    int* pinv = new int[n];
  
    // Invert the permutation
    for (int k = 0; k < n; ++k) {
        pinv[p[k]] = k;
    }
  
    // Return result
    return pinv;
}
 
 
/***************************************************************************
 * @brief Post order a forest
 *
 * @param[in] parent Integer array of n elements
 * @param[in] n Number of elements
 * @return Integer array of n elements
 *
 * Post order a forest
 ***************************************************************************/
int* GSparseSymbolic::cs_post(const int* parent, int n)
{
    // Return NULL pointer if input pointer is NULL or number of elements
    // is zero
    if (!parent || n < 1) {
        return (NULL);
    }
 
    // Allocate result array
    int* post = new int[n];
 
    // Allocate workspace
    int  wrk_size = 3 * n;
    int* wrk_int  = new int[wrk_size];
 
    // Set-up pointers to workspace
    int* head  = wrk_int;
    int* next  = wrk_int + n;
    int* stack = wrk_int + 2*n;  // Working array for 'cs_tdfs' function
 
    // Declare and initialise loop variables
    int j, k = 0;
  
    // Empty linked list
    for (j = 0 ; j < n ; j++) {
        head[j] = -1;
    }
  
    // Traverse nodes in reverse order to build a linked list 'head'
    for (j = n-1; j >= 0; j--) {
  
        // Skip if j is a root
        if (parent[j] == -1) {
            continue;
        }
    
        // Add j to list of its parent
        next[j]         = head[parent[j]];
        head[parent[j]] = j;
    
    }
  
    // Traverse nodes
    for (j = 0 ; j < n ; j++) {
 
        // Skip if j is not a root
        if (parent[j] != -1) {
            continue;
        }
    
        // Depth-first search and postorder of a tree rooted at node j
        k = cs_tdfs(j, k, head, next, post, stack);
 
    }
  
    // Free workspace
    delete [] wrk_int;
  
    // Return result
    return post;
}
 
 
/***************************************************************************
 * @brief cs_tdfs
 *
 * @param[in] j Node of tree root
 * @param[in] k ...
 * @param[in,out] head Linked list
 * @param[in] next Linked list information
 * @param[in,out] post Array
 * @param[in,OUT] stack Working array
 * @return Update of k
 *
 * Depth-first search and postorder of a tree rooted at node j
 ***************************************************************************/
int GSparseSymbolic::cs_tdfs(int        j,
                             int        k,
                             int*       head,
                             const int* next,
                             int*       post,
                             int*       stack)
{
    // Return -1 if one of the array pointers is NULL
    if (!head || !next || !post || !stack) {
        return (-1);
    }
 
    // Decrare
    int i, p;
 
    // Place j on the first position of the stack
    stack[0] = j;
  
    // Loop while stack is not empty
    int top = 0;
    while (top >= 0) {
        p = stack[top];              // p = top of stack
        i = head[p];                 // i = youngest child of p
        if (i == -1) {
            top--;                   // p has no unordered children left
            post[k++] = p;           // node p is the kth postordered node
        }
        else {
            head[p]      = next[i];  // remove i from children of p
            stack[++top] = i;        // start depth-first search on child node i
        }
    }
  
    // Return result
    return k;
}
 
 
/*==========================================================================
 =                                                                         =
 =                  GSparseSymbolic static member functions                =
 =                                                                         =
 ==========================================================================*/
 
/***************************************************************************
 * @brief Initialise A'A
 *
 * @param[in] AT Sparse matrix.
 * @param[in] post ...
 * @param[in] wrk_int Workspace.
 * @param[out] head Pointer to linked list.
 * @param[out] next Pointer to linked list information.
 *
 * Initialise A'A.
 *
 * Note that the returned pointers @p head and @p info point into the
 * @p wrk_int workspace array.
 ***************************************************************************/
void GSparseSymbolic::init_ata(const GMatrixSparse* AT,
                               const int*           post,
                               int*                 wrk_int,
                               int**                head,
                               int**                next)
{
    // Declare loop variables
    int i, k, p;
 
    // Assign matrix attributes
    int  m   = AT->m_cols;
    int  n   = AT->m_rows;
    int* ATp = AT->m_colstart;
    int* ATi = AT->m_rowinx;
  
    // Set-up pointers to workspace
    *head = wrk_int + 4*n;
    *next = wrk_int + 5*n+1;
 
    // Invert post
    for (k = 0 ; k < n ; k++) {
        wrk_int[post[k]] = k;
    }
    
    // Loop over columns of AT matrix
    for (i = 0 ; i < m ; i++) {
  
        // Loop over elements in column i of AT matrix
        for (k = n, p = ATp[i]; p < ATp[i+1]; p++) {
            k = CS_MIN(k,wrk_int[ATi[p]]);
        }
      
        // Place row i in linked list k
        (*next)[i] = (*head)[k];
        (*head)[k] = i;
 
    } // endfor: looped over columns of matrix
  
    // Return
    return;
}
 
 
/***************************************************************************
 * @brief cs_diag
 *
 * @param[in] i Row index.
 * @param[in] j Column index.
 * @param[in] aij Array element (not used).
 * @param[in] other Not used
 * @return 0: off-diagonal, 1: diagonal element
 *
 * Keep off-diagonal entries and drop diagonal entries.
 ***************************************************************************/
int GSparseSymbolic::cs_diag(int i, int j, double aij, void* other) 
{ 
    return (i != j);
}
 
 
/***************************************************************************
 * @brief cs_wclear
 *
 * @param[in] mark ...
 * @param[in] lemax ...
 * @param[in,out] w Working array of size @p n.
 * @param[in] n Size of the working array
 * @return Update of @p mark
 *
 * Clear w array.
 ***************************************************************************/
int GSparseSymbolic::cs_wclear(int mark, int lemax, int* w, int n)
{
    // ...
    if (mark < 2 || (mark + lemax < 0)) {
        for (int k = 0; k < n; k++) {
            if (w[k] != 0) {
                w[k] = 1;
            }
        }
        mark = 2;
    }
  
    // Return mark. At this point, w[0..n-1] < mark holds
    return mark;
}
 
 
/*==========================================================================
 =                                                                         =
 =                          GSparseSymbolic friends                        =
 =                                                                         =
 ==========================================================================*/
 
/***************************************************************************
 *                             Output operator                             *
 ***************************************************************************/
std::ostream& operator<< (std::ostream& os, const GSparseSymbolic& s)
{
    // Put header is stream
    os << "=== GSparseSymbolic ===";
  
    // Show inverse permutation if it exists
    if (s.m_pinv != NULL) {
        os << std::endl << " Inverse permutation (of ";
        os << s.m_n_pinv << " elements)" << std::endl;
        for (int i = 0; i < s.m_n_pinv; ++i) {
            os << " " << s.m_pinv[i];
        }
    }
 
    // Show fill reducing column permutation if it exists
    if (s.m_q != NULL) {
        os << std::endl << " Fill reducing column permutation (";
        os << s.m_n_cp << " elements)" << std::endl;
        for (int i = 0; i < s.m_n_cp; ++i) {
            os << " " << s.m_q[i];
        }
    }
 
    // Show elimination tree for Cholesky and QR if it exists
    if (s.m_parent != NULL) {
        os << std::endl << " Elimination tree (";
        os << s.m_n_parent << " elements)" << std::endl;
        for (int i = 0; i < s.m_n_parent; ++i) {
            os << " " << s.m_parent[i];
        }
    }
 
    // Show column pointers for Cholesky row counts for QR if it exists
    if (s.m_cp != NULL) {
        os << std::endl << " Cholesky column pointers/QR row counts (";
        os << s.m_n_cp << " elements)" << std::endl;
        for (int i = 0; i < s.m_n_cp; ++i) {
            os << " " << s.m_cp[i];
        }
    }
 
    // Show leftmost[i] = min(find(A(i,:))) if it exists
    if (s.m_leftmost != NULL) {
        os << std::endl << " leftmost[i] = min(find(A(i,:))) (";
        os << s.m_n_leftmost << " elements)" << std::endl;
        for (int i = 0; i < s.m_n_leftmost; ++i) {
            os << " " << s.m_leftmost[i];
        }
    }
  
    // Return output stream
    return os;
}