SGSmplstat.cxx

// This may look like C code, but it is really -*- C++ -*-
/* 
Copyright (C) 1988 Free Software Foundation
    written by Dirk Grunwald (grunwald@cs.uiuc.edu)

This file is part of the GNU C++ Library.  This library is free
software; you can redistribute it and/or modify it under the terms of
the GNU Library General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your
option) any later version.  This library 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 Library General Public License for more details.
You should have received a copy of the GNU Library General Public
License along with this library; if not, write to the Free Software
Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
#ifdef __GNUG__
#pragma implementation
#endif

#include <math.h>

#ifndef HUGE_VAL
#ifdef HUGE
#define HUGE_VAL HUGE
#else
#include <float.h>
#define HUGE_VAL DBL_MAX
#endif
#endif


#include <iostream>
#include <fstream>
#include <simgear/debug/logstream.hxx>
#include "SGSmplstat.hxx"


void SampleStatistic::error (const char *msg)
{
  SG_LOG(SG_GENERAL, SG_ALERT,  msg);
}

// t-distribution: given p-value and degrees of freedom, return t-value
// adapted from Peizer & Pratt JASA, vol63, p1416

double tval (double p, int df)
{
  double t;
  int positive = p >= 0.5;
  p = (positive) ? 1.0 - p : p;
  if (p <= 0.0 || df <= 0)
    t = HUGE_VAL;
  else if (p == 0.5)
    t = 0.0;
  else if (df == 1)
    t = 1.0 / tan ((p + p) * 1.57079633);
  else if (df == 2)
    t = sqrt (1.0 / ((p + p) * (1.0 - p)) - 2.0);
  else
    {
      double ddf = df;
      double a = sqrt (log (1.0 / (p * p)));
      double aa = a * a;
      a = a - ((2.515517 + (0.802853 * a) + (0.010328 * aa)) /
               (1.0 + (1.432788 * a) + (0.189269 * aa) +
                (0.001308 * aa * a)));
      t = ddf - 0.666666667 + 1.0 / (10.0 * ddf);
      t = sqrt (ddf * (exp (a * a * (ddf - 0.833333333) / (t * t)) - 1.0));
    }
  return (positive) ? t : -t;
}

void SampleStatistic::reset ()
{
  n = 0;
  x = x2 = 0.0;
  maxValue = -HUGE_VAL;
  minValue = HUGE_VAL;
}

void SampleStatistic::operator += (double value)
{
  n += 1;
  x += value;
  x2 += (value * value);
  if (minValue > value)
    minValue = value;
  if (maxValue < value)
    maxValue = value;
}

double SampleStatistic::mean () const
{
  if (n > 0)
    {
      return (x / n);
    }
  else
    {
      return (0.0);
    }
}

double SampleStatistic::var () const
{
  if (n > 1)
    {
      return ((x2 - ((x * x) / n)) / (n - 1));
    }
  else
    {
      return (0.0);
    }
}

double SampleStatistic::stdDev () const
{
  if (n <= 0 || this->var () <= 0)
    {
      return (0);
    }
  else
    {
      return ((double) sqrt (var ()));
    }
}

double SampleStatistic::confidence (int interval) const
{
  int df = n - 1;
  if (df <= 0)
    return HUGE_VAL;
  double t = tval (double (100 + interval) * 0.005, df);
  if (t == HUGE_VAL)
    return t;
  else
    return (t * stdDev ()) / sqrt (double (n));
}

double SampleStatistic::confidence (double p_value) const
{
  int df = n - 1;
  if (df <= 0)
    return HUGE_VAL;
  double t = tval ((1.0 + p_value) * 0.5, df);
  if (t == HUGE_VAL)
    return t;
  else
    return (t * stdDev ()) / sqrt (double (n));
}

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