house.c

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/*  house.c    CCMATH mathematics library source code.
 *
 *  Copyright (C)  2000   Daniel A. Atkinson    All rights reserved.
 *  This code may be redistributed under the terms of the GNU library
 *  public license (LGPL). ( See the lgpl.license file for details.)
 * ------------------------------------------------------------------------
 */
#include <stdlib.h>
#include "ccmath.h"
void house(double *a, double *d, double *dp, int n)
{
    double sc, x, y, h;

    int i, j, k, m, e;

    double *qw, *qs, *pc, *p;

    qs = (double *)calloc(2 * n, sizeof(double));
    for (j = 0, qw = qs + n, pc = a; j < n; pc += n + 1)
        qw[j++] = *pc;
    for (j = 0, pc = a; j < n - 2; ++j, pc += n + 1) {
        m = n - j - 1;
        for (i = 1, sc = 0.; i <= m; ++i)
            sc += pc[i] * pc[i];
        if (sc > 0.) {
            sc = sqrt(sc);
            if ((x = *(pc + 1)) < 0.) {
                y = x - sc;
                h = 1. / sqrt(-2. * sc * y);
            }
            else {
                y = x + sc;
                h = 1. / sqrt(2. * sc * y);
                sc = -sc;
            }
            for (i = 0, qw = pc + 1; i < m; ++i) {
                qs[i] = 0.;
                if (i)
                    qw[i] *= h;
                else
                    qw[i] = y * h;
            }
            for (i = 0, e = j + 2, p = pc + n + 1, h = 0.; i < m;
                 ++i, p += e++) {
                qs[i] += (y = qw[i]) * *p++;
                for (k = i + 1; k < m; ++k) {
                    qs[i] += qw[k] * *p;
                    qs[k] += y * *p++;
                }
                h += y * qs[i];
            }
            for (i = 0; i < m; ++i) {
                qs[i] -= h * qw[i];
                qs[i] += qs[i];
            }
            for (i = 0, e = j + 2, p = pc + n + 1; i < m; ++i, p += e++) {
                for (k = i; k < m; ++k)
                    *p++ -= qw[i] * qs[k] + qs[i] * qw[k];
            }
        }
        d[j] = *pc;
        dp[j] = sc;
    }
    d[j] = *pc;
    dp[j] = *(pc + 1);
    d[j + 1] = *(pc + n + 1);
    for (j = 0, pc = a, qw = qs + n; j < n; ++j, pc += n + 1) {
        *pc = qw[j];
        for (i = 1, p = pc + n; i < n - j; p += n)
            pc[i++] = *p;
    }
    free(qs);
}