source: glpk-cmake/src/amd/amd_aat.c @ 1:c445c931472f

/* ========================================================================= */
/* === AMD_aat ============================================================= */
/* ========================================================================= */

/* ------------------------------------------------------------------------- */
/* AMD, Copyright (c) Timothy A. Davis,                                      */
/* Patrick R. Amestoy, and Iain S. Duff.  See ../README.txt for License.     */
/* email: davis at cise.ufl.edu    CISE Department, Univ. of Florida.        */
/* web: http://www.cise.ufl.edu/research/sparse/amd                          */
/* ------------------------------------------------------------------------- */

/* AMD_aat:  compute the symmetry of the pattern of A, and count the number of
 * nonzeros each column of A+A' (excluding the diagonal).  Assumes the input
 * matrix has no errors, with sorted columns and no duplicates
 * (AMD_valid (n, n, Ap, Ai) must be AMD_OK, but this condition is not
 * checked).
 */

#include "amd_internal.h"

GLOBAL size_t AMD_aat   /* returns nz in A+A' */
(
    Int n,
    const Int Ap [ ],
    const Int Ai [ ],
    Int Len [ ],        /* Len [j]: length of column j of A+A', excl diagonal*/
    Int Tp [ ],         /* workspace of size n */
    double Info [ ]
)
{
    Int p1, p2, p, i, j, pj, pj2, k, nzdiag, nzboth, nz ;
    double sym ;
    size_t nzaat ;

#ifndef NDEBUG
    AMD_debug_init ("AMD AAT") ;
    for (k = 0 ; k < n ; k++) Tp [k] = EMPTY ;
    ASSERT (AMD_valid (n, n, Ap, Ai) == AMD_OK) ;
#endif

    if (Info != (double *) NULL)
    {
        /* clear the Info array, if it exists */
        for (i = 0 ; i < AMD_INFO ; i++)
        {
            Info [i] = EMPTY ;
        }
        Info [AMD_STATUS] = AMD_OK ;
    }

    for (k = 0 ; k < n ; k++)
    {
        Len [k] = 0 ;
    }

    nzdiag = 0 ;
    nzboth = 0 ;
    nz = Ap [n] ;

    for (k = 0 ; k < n ; k++)
    {
        p1 = Ap [k] ;
        p2 = Ap [k+1] ;
        AMD_DEBUG2 (("\nAAT Column: "ID" p1: "ID" p2: "ID"\n", k, p1, p2)) ;

        /* construct A+A' */
        for (p = p1 ; p < p2 ; )
        {
            /* scan the upper triangular part of A */
            j = Ai [p] ;
            if (j < k)
            {
                /* entry A (j,k) is in the strictly upper triangular part,
                 * add both A (j,k) and A (k,j) to the matrix A+A' */
                Len [j]++ ;
                Len [k]++ ;
                AMD_DEBUG3 (("    upper ("ID","ID") ("ID","ID")\n", j,k, k,j));
                p++ ;
            }
            else if (j == k)
            {
                /* skip the diagonal */
                p++ ;
                nzdiag++ ;
                break ;
            }
            else /* j > k */
            {
                /* first entry below the diagonal */
                break ;
            }
            /* scan lower triangular part of A, in column j until reaching
             * row k.  Start where last scan left off. */
            ASSERT (Tp [j] != EMPTY) ;
            ASSERT (Ap [j] <= Tp [j] && Tp [j] <= Ap [j+1]) ;
            pj2 = Ap [j+1] ;
            for (pj = Tp [j] ; pj < pj2 ; )
            {
                i = Ai [pj] ;
                if (i < k)
                {
                    /* A (i,j) is only in the lower part, not in upper.
                     * add both A (i,j) and A (j,i) to the matrix A+A' */
                    Len [i]++ ;
                    Len [j]++ ;
                    AMD_DEBUG3 (("    lower ("ID","ID") ("ID","ID")\n",
                        i,j, j,i)) ;
                    pj++ ;
                }
                else if (i == k)
                {
                    /* entry A (k,j) in lower part and A (j,k) in upper */
                    pj++ ;
                    nzboth++ ;
                    break ;
                }
                else /* i > k */
                {
                    /* consider this entry later, when k advances to i */
                    break ;
                }
            }
            Tp [j] = pj ;
        }
        /* Tp [k] points to the entry just below the diagonal in column k */
        Tp [k] = p ;
    }

    /* clean up, for remaining mismatched entries */
    for (j = 0 ; j < n ; j++)
    {
        for (pj = Tp [j] ; pj < Ap [j+1] ; pj++)
        {
            i = Ai [pj] ;
            /* A (i,j) is only in the lower part, not in upper.
             * add both A (i,j) and A (j,i) to the matrix A+A' */
            Len [i]++ ;
            Len [j]++ ;
            AMD_DEBUG3 (("    lower cleanup ("ID","ID") ("ID","ID")\n",
                i,j, j,i)) ;
        }
    }

    /* --------------------------------------------------------------------- */
    /* compute the symmetry of the nonzero pattern of A */
    /* --------------------------------------------------------------------- */

    /* Given a matrix A, the symmetry of A is:
     *  B = tril (spones (A), -1) + triu (spones (A), 1) ;
     *  sym = nnz (B & B') / nnz (B) ;
     *  or 1 if nnz (B) is zero.
     */

    if (nz == nzdiag)
    {
        sym = 1 ;
    }
    else
    {
        sym = (2 * (double) nzboth) / ((double) (nz - nzdiag)) ;
    }

    nzaat = 0 ;
    for (k = 0 ; k < n ; k++)
    {
        nzaat += Len [k] ;
    }

    AMD_DEBUG1 (("AMD nz in A+A', excluding diagonal (nzaat) = %g\n",
        (double) nzaat)) ;
    AMD_DEBUG1 (("   nzboth: "ID" nz: "ID" nzdiag: "ID" symmetry: %g\n",
                nzboth, nz, nzdiag, sym)) ;

    if (Info != (double *) NULL)
    {
        Info [AMD_STATUS] = AMD_OK ;
        Info [AMD_N] = n ;
        Info [AMD_NZ] = nz ;
        Info [AMD_SYMMETRY] = sym ;         /* symmetry of pattern of A */
        Info [AMD_NZDIAG] = nzdiag ;        /* nonzeros on diagonal of A */
        Info [AMD_NZ_A_PLUS_AT] = nzaat ;   /* nonzeros in A+A' */
    }

    return (nzaat) ;
}