/*
 * Copyright(C) 1999-2020, 2023 National Technology & Engineering Solutions
 * of Sandia, LLC (NTESS).  Under the terms of Contract DE-NA0003525 with
 * NTESS, the U.S. Government retains certain rights in this software.
 *
 * See packages/seacas/LICENSE for details
 */

#include "prototypes.h"
#include <math.h>
#include <stdio.h>

/* Finds first extended eigenpair of system corresponding to
   tridiagonal T using using Rafael's bisection technique. */

void get_extval(double *alpha,   /* j-vector of Lanczos scalars (using elements 1 to j) */
                double *beta,    /* (j+1)-vector of " " (has 0 element but using 1 to j-1) */
                int     j,       /* number of Lanczos iterations taken */
                double  ritzval, /* Ritz value */
                double *s,       /* Ritz vector (length n, re-computed in this routine) */
                double  eigtol,  /* tolerance on eigenpair */
                double  wnorm_g, /* W-norm of n-vector g, the rhs in the extended eig. problem */
                double  sigma,   /* the norm constraint on the extended eigenvector */
                double *extval,  /* the extended eigenvalue this routine computes */
                double *v,       /* the j-vector solving the extended eig problem in T */
                double *work1,   /* j-vector of workspace */
                double *work2    /* j-vector of workspace */
)
{
  extern int DEBUG_EVECS; /* debug flag for eigen computation */
  double     lambda_low;  /* lower bound on extended eval */
  double     lambda_high; /* upper bound on extended eval */
  double     tol;         /* bisection tolerance */
  double     norm_v;      /* norm of the extended T eigenvector v */
  double     lambda;      /* the parameter that iterates to extval */
  int        cnt;         /* debug iteration counter */
  double     diff;        /* distance between lambda limits */

  /* Compute the Ritz vector */
  Tevec(alpha, beta - 1, j, ritzval, s);

  /* Shouldn't happen, but just in case ... */
  if (wnorm_g == 0.0) {
    *extval = ritzval;
    cpvec(v, 1, j, s);
    if (DEBUG_EVECS > 0) {
      printf("Degenerate extended eigenvector problem (g = 0).\n");
    }
    return;
    /* ... not really an extended eigenproblem; just return Ritz pair */
  }

  /* Set up the bisection parameters */
  lambda_low  = ritzval - wnorm_g / sigma;
  lambda_high = ritzval - (wnorm_g / sigma) * s[1];
  lambda      = 0.5 * (lambda_low + lambda_high);
  tol         = eigtol * eigtol * (1 + fabs(lambda_low) + fabs(lambda_high));

  if (DEBUG_EVECS > 2) {
    printf("Computing extended eigenpairs of T\n");
    printf("  target norm_v (= sigma) %g\n", sigma);
    printf("  bisection tolerance %g\n", tol);
  }
  if (DEBUG_EVECS > 3) {
    printf("  lambda iterates to the extended eigenvalue\n");
    printf("         lambda_low           lambda            lambda_high      norm_v\n");
  }

  /* Bisection loop - iterate until norm constraint is satisfied */
  cnt  = 1;
  diff = 2 * tol;
  while (diff > tol) {
    lambda = 0.5 * (lambda_low + lambda_high);
    tri_solve(alpha, beta, j, lambda, v, wnorm_g, work1, work2);
    norm_v = ch_norm(v, 1, j);
    if (DEBUG_EVECS > 3) {
      printf("%2i   %18.16f  %18.16f  %18.16f  %g\n", cnt++, lambda_low, lambda, lambda_high,
             norm_v);
    }
    if (norm_v <= sigma) {
      lambda_low = lambda;
    }
    if (norm_v >= sigma) {
      lambda_high = lambda;
    }
    diff = lambda_high - lambda_low;
  }

  /* Return the extended eigenvalue (eigvec is automatically returned) */
  *extval = lambda;
}