/*
 * 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 "defs.h"
#include <math.h>
#include <stdio.h>

/* Finds needed eigenvalues of tridiagonal T using either the QL algorithm
   or Sturm sequence bisection, whichever is predicted to be faster based
   on a simple complexity model. If one fails (which is rare), the other
   is tried. The return value is 0 if one of the routines succeeds. If they
   both fail, the return value is 1, and Lanczos should compute the best
   approximation it can based on previous iterations. */
int get_ritzvals(double *alpha,         /* vector of Lanczos scalars */
                 double *beta,          /* vector of Lanczos scalars */
                 int     j,             /* number of Lanczos iterations taken */
                 double  Anorm,         /* Gershgorin estimate */
                 double *workj,         /* work vector for Sturm sequence */
                 double *ritz,          /* array holding evals */
                 int     d,             /* problem dimension = num. eigenpairs needed */
                 int     left_goodlim,  /* number of ritz pairs checked on left end */
                 int     right_goodlim, /* number of ritz pairs checked on right end */
                 double  eigtol,        /* tolerance on eigenpair */
                 double  bis_safety     /* bisection tolerance function divisor */
)
{
  extern int DEBUG_EVECS;     /* debug flag for eigen computation */
  extern int WARNING_EVECS;   /* warning flag for eigen computation */
  int        nvals_left;      /* numb. evals to find on left end of spectrum */
  int        nvals_right;     /* numb. evals to find on right end of spectrum */
  double     bisection_tol;   /* width of interval bisection should converge to */
  int        pred_steps;      /* predicts # of required bisection steps per eval */
  int        tot_pred_steps;  /* predicts total # of required bisection steps */
  double    *ritz_sav = NULL; /* copy of ritzvals for debugging */
  int        bisect_flag;     /* return status of bisect() */
  int        ql_flag;         /* return status of ql() */
  int        local_debug;     /* whether to check bisection results with ql */
  /* Determine number of ritzvals to find on left and right ends */
  nvals_left  = max(d, left_goodlim);
  nvals_right = min(j - nvals_left, right_goodlim);

  /* Estimate work for bisection vs. ql assuming bisection takes 5j flops per
     step, ql takes 30j^2 flops per call. (Ignore sorts, copies, addressing.) */

  bisection_tol  = eigtol * eigtol / bis_safety;
  pred_steps     = (log10(Anorm / bisection_tol) / log10(2.0)) + 1;
  tot_pred_steps = (nvals_left + nvals_right) * pred_steps;

  bisect_flag = ql_flag = 0;

  if (5 * tot_pred_steps < 30 * j) {
    if (DEBUG_EVECS > 2) {
      printf("  tridiagonal solver: bisection\n");
    }

    /* Set local_debug = TRUE for a table checking bisection against QL. */
    local_debug = FALSE;
    if (local_debug) {
      ritz_sav = mkvec(1, j);
      cpvec(ritz_sav, 1, j, alpha);
      cpvec(workj, 0, j, beta);
      ql_flag = ql(ritz_sav, workj, j);
      if (ql_flag != 0) {
        bail("Aborting debugging procedure in get_ritzvals().\n", 1);
      }
      shell_sort(j, &ritz_sav[1]);
    }

    bisect_flag = bisect(alpha, beta, j, Anorm, workj, ritz, nvals_left, nvals_right, bisection_tol,
                         ritz_sav, pred_steps + 10);

    if (local_debug) {
      frvec(ritz_sav, 1);
    }
  }

  else {
    if (DEBUG_EVECS > 2) {
      printf("  tridiagonal solver: ql\n");
    }
    cpvec(ritz, 1, j, alpha);
    cpvec(workj, 0, j, beta);
    ql_flag = ql(ritz, workj, j);
    shell_sort(j, &ritz[1]);
  }

  if (bisect_flag != 0 && ql_flag == 0) {
    if (DEBUG_EVECS > 0 || WARNING_EVECS > 0) {
      strout("WARNING: Sturm bisection of T failed; switching to QL.\n");
    }
    if (DEBUG_EVECS > 1 || WARNING_EVECS > 1) {
      if (bisect_flag == 1) {
        strout("         - failure detected in sturmcnt().\n");
      }
      if (bisect_flag == 2) {
        strout("         - maximum number of bisection steps reached.\n");
      }
    }
    cpvec(ritz, 1, j, alpha);
    cpvec(workj, 0, j, beta);
    ql_flag = ql(ritz, workj, j);
    shell_sort(j, &ritz[1]);
  }

  if (ql_flag != 0 && bisect_flag == 0) {
    if (DEBUG_EVECS > 0 || WARNING_EVECS > 0) {
      strout("WARNING: QL failed for T; switching to Sturm bisection.\n");
    }
    bisect_flag = bisect(alpha, beta, j, Anorm, workj, ritz, nvals_left, nvals_right, bisection_tol,
                         ritz_sav, pred_steps + 3);
  }

  if (bisect_flag != 0 && ql_flag != 0) {
    if (DEBUG_EVECS > 0 || WARNING_EVECS > 0) {
      return (1); /* can't recover; bail out with error code */
    }
  }

  return (0); /* ... things seem ok. */
}