/*! \file
Copyright (c) 2003, The Regents of the University of California, through
Lawrence Berkeley National Laboratory (subject to receipt of any required 
approvals from U.S. Dept. of Energy) 

All rights reserved. 

The source code is distributed under BSD license, see the file License.txt
at the top-level directory.
*/

/*! @file dpivotgrowth.c
 * \brief Computes the reciprocal pivot growth factor
 *
 * <pre>
 * -- SuperLU routine (version 2.0) --
 * Univ. of California Berkeley, Xerox Palo Alto Research Center,
 * and Lawrence Berkeley National Lab.
 * November 15, 1997
 * </pre>
 */
#include <math.h>
#include "slu_ddefs.h"

/*! \brief
 *
 * <pre>
 * Purpose
 * =======
 *
 * Compute the reciprocal pivot growth factor of the leading ncols columns
 * of the matrix, using the formula:
 *     min_j ( max_i(abs(A_ij)) / max_i(abs(U_ij)) )
 *
 * Arguments
 * =========
 *
 * ncols    (input) int
 *          The number of columns of matrices A, L and U.
 *
 * A        (input) SuperMatrix*
 *	    Original matrix A, permuted by columns, of dimension
 *          (A->nrow, A->ncol). The type of A can be:
 *          Stype = NC; Dtype = SLU_D; Mtype = GE.
 *
 * L        (output) SuperMatrix*
 *          The factor L from the factorization Pr*A=L*U; use compressed row 
 *          subscripts storage for supernodes, i.e., L has type: 
 *          Stype = SC; Dtype = SLU_D; Mtype = TRLU.
 *
 * U        (output) SuperMatrix*
 *	    The factor U from the factorization Pr*A*Pc=L*U. Use column-wise
 *          storage scheme, i.e., U has types: Stype = NC;
 *          Dtype = SLU_D; Mtype = TRU.
 * </pre>
 */

double
dPivotGrowth(int ncols, SuperMatrix *A, int *perm_c, 
             SuperMatrix *L, SuperMatrix *U)
{

    NCformat *Astore;
    SCformat *Lstore;
    NCformat *Ustore;
    double  *Aval, *Lval, *Uval;
    int      fsupc, nsupr;
    int_t    luptr, nz_in_U;
    int_t    i, j, k, oldcol;
    int      *inv_perm_c;
    double   rpg, maxaj, maxuj;
    double   smlnum;
    double   *luval;
   
    /* Get machine constants. */
    smlnum = dmach("S");
    rpg = 1. / smlnum;

    Astore = A->Store;
    Lstore = L->Store;
    Ustore = U->Store;
    Aval = Astore->nzval;
    Lval = Lstore->nzval;
    Uval = Ustore->nzval;
    
    inv_perm_c = (int *) SUPERLU_MALLOC(A->ncol*sizeof(int));
    for (j = 0; j < A->ncol; ++j) inv_perm_c[perm_c[j]] = j;

    for (k = 0; k <= Lstore->nsuper; ++k) {
	fsupc = L_FST_SUPC(k);
	nsupr = L_SUB_START(fsupc+1) - L_SUB_START(fsupc);
	luptr = L_NZ_START(fsupc);
	luval = &Lval[luptr];
	nz_in_U = 1;
	
	for (j = fsupc; j < L_FST_SUPC(k+1) && j < ncols; ++j) {
	    maxaj = 0.;
            oldcol = inv_perm_c[j];
	    for (i = Astore->colptr[oldcol]; i < Astore->colptr[oldcol+1]; ++i)
		maxaj = SUPERLU_MAX( maxaj, fabs(Aval[i]) );
	
	    maxuj = 0.;
	    for (i = Ustore->colptr[j]; i < Ustore->colptr[j+1]; i++)
		maxuj = SUPERLU_MAX( maxuj, fabs(Uval[i]) );
	    
	    /* Supernode */
	    for (i = 0; i < nz_in_U; ++i)
		maxuj = SUPERLU_MAX( maxuj, fabs(luval[i]) );

	    ++nz_in_U;
	    luval += nsupr;

	    if ( maxuj == 0. )
		rpg = SUPERLU_MIN( rpg, 1.);
	    else
		rpg = SUPERLU_MIN( rpg, maxaj / maxuj );
	}
	
	if ( j >= ncols ) break;
    }

    SUPERLU_FREE(inv_perm_c);
    return (rpg);
}
