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/* Ergo, version 3.8.2, a program for linear scaling electronic structure
* calculations.
* Copyright (C) 2023 Elias Rudberg, Emanuel H. Rubensson, Pawel Salek,
* and Anastasia Kruchinina.
*
* 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/>.
*
* Primary academic reference:
* Ergo: An open-source program for linear-scaling electronic structure
* calculations,
* Elias Rudberg, Emanuel H. Rubensson, Pawel Salek, and Anastasia
* Kruchinina,
* SoftwareX 7, 107 (2018),
* <http://dx.doi.org/10.1016/j.softx.2018.03.005>
*
* For further information about Ergo, see <http://www.ergoscf.org>.
*/
/* This file belongs to the template_lapack part of the Ergo source
* code. The source files in the template_lapack directory are modified
* versions of files originally distributed as CLAPACK, see the
* Copyright/license notice in the file template_lapack/COPYING.
*/
#ifndef TEMPLATE_LAPACK_LARRA_HEADER
#define TEMPLATE_LAPACK_LARRA_HEADER
template<class Treal>
int template_lapack_larra(const integer *n, Treal *d__, Treal *e,
Treal *e2, Treal *spltol, Treal *tnrm, integer *nsplit,
integer *isplit, integer *info)
{
/* System generated locals */
integer i__1;
Treal d__1, d__2;
/* Local variables */
integer i__;
Treal tmp1, eabs;
/* -- LAPACK auxiliary routine (version 3.2) -- */
/* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd.. */
/* November 2006 */
/* .. Scalar Arguments .. */
/* .. */
/* .. Array Arguments .. */
/* .. */
/* Purpose */
/* ======= */
/* Compute the splitting points with threshold SPLTOL. */
/* DLARRA sets any "small" off-diagonal elements to zero. */
/* Arguments */
/* ========= */
/* N (input) INTEGER */
/* The order of the matrix. N > 0. */
/* D (input) DOUBLE PRECISION array, dimension (N) */
/* On entry, the N diagonal elements of the tridiagonal */
/* matrix T. */
/* E (input/output) DOUBLE PRECISION array, dimension (N) */
/* On entry, the first (N-1) entries contain the subdiagonal */
/* elements of the tridiagonal matrix T; E(N) need not be set. */
/* On exit, the entries E( ISPLIT( I ) ), 1 <= I <= NSPLIT, */
/* are set to zero, the other entries of E are untouched. */
/* E2 (input/output) DOUBLE PRECISION array, dimension (N) */
/* On entry, the first (N-1) entries contain the SQUARES of the */
/* subdiagonal elements of the tridiagonal matrix T; */
/* E2(N) need not be set. */
/* On exit, the entries E2( ISPLIT( I ) ), */
/* 1 <= I <= NSPLIT, have been set to zero */
/* SPLTOL (input) DOUBLE PRECISION */
/* The threshold for splitting. Two criteria can be used: */
/* SPLTOL<0 : criterion based on absolute off-diagonal value */
/* SPLTOL>0 : criterion that preserves relative accuracy */
/* TNRM (input) DOUBLE PRECISION */
/* The norm of the matrix. */
/* NSPLIT (output) INTEGER */
/* The number of blocks T splits into. 1 <= NSPLIT <= N. */
/* ISPLIT (output) INTEGER array, dimension (N) */
/* The splitting points, at which T breaks up into blocks. */
/* The first block consists of rows/columns 1 to ISPLIT(1), */
/* the second of rows/columns ISPLIT(1)+1 through ISPLIT(2), */
/* etc., and the NSPLIT-th consists of rows/columns */
/* ISPLIT(NSPLIT-1)+1 through ISPLIT(NSPLIT)=N. */
/* INFO (output) INTEGER */
/* = 0: successful exit */
/* Further Details */
/* =============== */
/* Based on contributions by */
/* Beresford Parlett, University of California, Berkeley, USA */
/* Jim Demmel, University of California, Berkeley, USA */
/* Inderjit Dhillon, University of Texas, Austin, USA */
/* Osni Marques, LBNL/NERSC, USA */
/* Christof Voemel, University of California, Berkeley, USA */
/* ===================================================================== */
/* .. Parameters .. */
/* .. */
/* .. Local Scalars .. */
/* .. */
/* .. Intrinsic Functions .. */
/* .. */
/* .. Executable Statements .. */
/* Parameter adjustments */
--isplit;
--e2;
--e;
--d__;
/* Function Body */
*info = 0;
/* Compute splitting points */
*nsplit = 1;
if (*spltol < 0.) {
/* Criterion based on absolute off-diagonal value */
tmp1 = absMACRO(*spltol) * *tnrm;
i__1 = *n - 1;
for (i__ = 1; i__ <= i__1; ++i__) {
eabs = (d__1 = e[i__], absMACRO(d__1));
if (eabs <= tmp1) {
e[i__] = 0.;
e2[i__] = 0.;
isplit[*nsplit] = i__;
++(*nsplit);
}
/* L9: */
}
} else {
/* Criterion that guarantees relative accuracy */
i__1 = *n - 1;
for (i__ = 1; i__ <= i__1; ++i__) {
eabs = (d__1 = e[i__], absMACRO(d__1));
if (eabs <= *spltol * template_blas_sqrt((d__1 = d__[i__], absMACRO(d__1))) * template_blas_sqrt((
d__2 = d__[i__ + 1], absMACRO(d__2)))) {
e[i__] = 0.;
e2[i__] = 0.;
isplit[*nsplit] = i__;
++(*nsplit);
}
/* L10: */
}
}
isplit[*nsplit] = *n;
return 0;
/* End of DLARRA */
} /* dlarra_ */
#endif
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