/****************************************************************************
 * Core Library Version 1.7, August 2004
 * Copyright (c) 1995-2004 Exact Computation Project
 * All rights reserved.
 *
 * This file is part of CGAL (www.cgal.org).
 *
 * File: Filter.h
 * Synopsis:
 *      This is a simple filtered floating point number,
 *      represented by the main class, FilterFp.
 *      based on the Burnikel-Funke-Schirra (BFS) filter scheme.
 *      We do not use IEEE exception mechanism here.
 *      It is used by the Expr class.
 *
 * Written by
 *       Zilin Du <zilin@cs.nyu.edu>
 *       Chee Yap <yap@cs.nyu.edu>
 *
 * WWW URL: https://cs.nyu.edu/exact/
 * Email: exact@cs.nyu.edu
 *
 * $URL: https://github.com/CGAL/cgal/blob/v6.1/CGAL_Core/include/CGAL/CORE/Filter.h $
 * $Id: include/CGAL/CORE/Filter.h b26b07a1242 $
 * SPDX-License-Identifier: LGPL-3.0-or-later
 ***************************************************************************/

#ifndef _CORE_FILTER_H_
#define _CORE_FILTER_H_

#include <CGAL/config.h>
#include <CGAL/assertions.h>
#include <CGAL/CORE/Real.h>
#include <cmath>
#include <limits>

#define CGAL_CORE_finite(x)        std::isfinite(x)
#define CGAL_CORE_ilogb(x)        ilogb(x)

#if defined(sun) || defined(__sun)
  #include <ieeefp.h>
#endif

namespace CORE {

const int POWTWO_26 = (1 << 26);  ///< constant 2^26

/// \class filteredFp Filter.h
/// \brief filteredFp represents filtered floating point
///        numbers, based on BFS filter
class filteredFp {
  double fpVal;         // approximate double value for some "real value"
  double maxAbs;        // if (|fpVal| > maxAbs * ind * 2^{-53}) then
  int ind;              // sign of value is sign(fpVal).  Else, don't know.
  // REFERENCE: Burnikel, Funke, Schirra (BFS) filter
  // Chee: in isOK(), you used the test "|fpVal| >= maxAbs * ind * 2^{-53}"
  // which seems to be correct (i.e., not |fpVal| > maxAbs * ind * 2^{-53})
public:
  /// \name Constructors
  //@{
  /// constructor
  filteredFp (double val = 0.0)
      : fpVal(val), maxAbs(core_abs(val)), ind(0) {}
  /// constructor
  filteredFp (double val, double m, int i)
      : fpVal(val), maxAbs(m), ind(i) {}

  /// construct a filteredFp from Real v.
  /** if v causes an overflow, fpVal = +/- Infty
      if v causes an underflow, fpVal = ...? */
  filteredFp (const Real & value) : fpVal(0.0), maxAbs(0.0), ind(0) {
    if (value != CORE_REAL_ZERO) {
      ind = 1;
      fpVal = value.doubleValue();
      if (value.MSB() <= -1075)
        maxAbs = 1;
      else
              maxAbs = core_abs(fpVal); // NaN are propagated correctly by core_abs.
    }
  }
  //@}

  /// \name Help Functions
  //@{
  /// return filtered value (for debug)
  double getValue() const {
    return fpVal;
  }
  /// check whether the sign (!) of the filtered value is OK
  bool isOK() const {
    return (get_static_fpFilterFlag()  && // To disable filter
            CGAL_CORE_finite(fpVal) && // Test for infinite and NaNs
            (core_abs(fpVal) >= maxAbs*ind*CORE_EPS));
  }
  /// return the sign of filtered value.
  /** (Note: must call isOK() to check whether the sign is ok
      before call this function.) */
  int sign() const {
#ifdef CORE_DEBUG
    CGAL_assertion(isOK());
#endif
    if (fpVal == 0.0)
      return 0;
    else
      return fpVal > 0.0 ? 1: -1;
  }
  /// lower bound on MSB
  /** defined to be cel(lg(real value));
      ilogb(x) is floor(log_2(|x|)).
      Also, ilogb(0) = -INT_MAX.  ilogb(NaN) = ilogb(+/-Inf) = INT_MAX */
  extLong lMSB() const {
    return extLong(CGAL_CORE_ilogb(core_abs(fpVal)-maxAbs*ind*CORE_EPS));
  }
  /// upper bound on MSB
  extLong uMSB() const {
    return extLong(CGAL_CORE_ilogb(core_abs(fpVal)+maxAbs*ind*CORE_EPS));
  }
  //@}

  /// \name Operators
  //@{
  /// unary minus
  filteredFp operator -() const {
    return filteredFp(-fpVal, maxAbs, ind);
  }
  /// addition
  filteredFp operator+ (const filteredFp& x) const {
    return filteredFp(fpVal+x.fpVal, maxAbs+x.maxAbs, 1+core_max(ind, x.ind));
  }
  /// subtraction
  filteredFp operator- (const filteredFp& x) const {
    return filteredFp(fpVal-x.fpVal, maxAbs+x.maxAbs, 1+core_max(ind, x.ind));
  }
  /// multiplication
  filteredFp operator* (const filteredFp& x) const {
    return filteredFp(fpVal*x.fpVal, maxAbs*x.maxAbs+DBL_MIN, 1+ind+x.ind);
  }
  /// division
  filteredFp operator/ (const filteredFp& x) const {
    CGAL_CORE_warning_msg(x.fpVal != 0.0, "possible zero divisor!");

    double xxx = core_abs(x.fpVal) / x.maxAbs - (x.ind+1)*CORE_EPS + DBL_MIN;
    if (xxx > 0) {
      double val =  fpVal / x.fpVal;
      double maxVal = ( core_abs(val) + maxAbs / x.maxAbs) / xxx + DBL_MIN;
      return filteredFp(val, maxVal, 1 + core_max(ind, x.ind + 1));
    } else
      return filteredFp( std::nan(""), getDoubleInfty(), 1);
  }
  /// square root
  filteredFp sqrt () const {

    CGAL_CORE_warning_msg( !(fpVal < 0.0), "possible negative sqrt!");
    if (fpVal > 0.0) {
      double val = std::sqrt(fpVal);
      return filteredFp(val,  ( maxAbs / fpVal ) * val, 1 + ind);
    } else
      return filteredFp(0.0, std::sqrt(maxAbs) * POWTWO_26, 1 + ind);
  }

  /// dump function
  void dump (std::ostream&os) const {
    os << "Filter=[fpVal=" << fpVal << ",maxAbs=" << maxAbs << ",ind=" << ind << "]";
  }

  /// helper function (to avoid warning under some compilers)
  static double getDoubleInfty() {
    return std::numeric_limits<double>::infinity();
  }
  //@}
}; //filteredFp class

inline std::ostream & operator<< (std::ostream & os, const filteredFp& fp) {
  fp.dump(os);
  return os;
}

} //namespace CORE
#endif // _CORE_FILTER_H_

