<?php /* -*- mode: php; indent-tabs-mode: nil; -*- */

require_once($inc_dir . 'math.inc');

    $this->time_starts = array();

    $this->time_ends = array();

  /**

   * \brief

   *   Add a potential end time to the pool of end times.

   */

  public function time_end_pool_add($time_end)

  {

    $this->time_ends[] = $time_end;

  }

    if (count($this->time_ends))

      {

        $times = filter_outliers($this->time_ends);

        $this->time_end = max($times);

      }

  /**

   * \brief

   *   Add a potential semester start time to the pool of potential

   *   start times.

   *

   * The idea is that there might be erroneous entries in a school's

   * database (

   * http://www.facebook.com/CalvinRegistrar/posts/299438720070457 )

   * which would skew the detected start time. Use statistics to

   * detect and kill outliers by using a pool of endtimes :-D.

   */

  public function time_start_pool_add($time_start)

  {

    $this->time_starts[] = $time_start;

  }

    if (count($this->time_starts))

      {

        $times = filter_outliers($this->time_starts);

        $this->time_end = min($times);

      }

    /*

     * Make sure that time_end is set to the proper end time before

     * clearing out the pool in the time_ends array.

     */

    $this->time_end_get();

    $this->time_ends = array();

    $this->time_start_get();

    $this->time_starts = array();

<?php /* -*- mode: php; indent-tabs-mode: nil; -*- */

/*

 * Copyright 2011 Nathan Phillip Brink <ohnobinki@ohnopublishing.net>

 *

 * This file is a part of slate_permutate.

 *

 * slate_permutate is free software: you can redistribute it and/or modify

 * it under the terms of the GNU Affero General Public License as published by

 * the Free Software Foundation, either version 3 of the License, or

 * (at your option) any later version.

 *

 * slate_permutate 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 Affero General Public License for more details.

 *

 * You should have received a copy of the GNU Affero General Public License

 * along with slate_permutate.  If not, see <http://www.gnu.org/licenses/>.

 */

if (!function_exists('mean'))

  {

    /**

     * \brief

     *   Calculate the mean of a set of numerical values without

     *   overflowing stuff.

     */

    function mean(array $values)

    {

      /*

       * As the influence of each element reduces with each iteration

       * in the used algorithm, shuffling the array should give a

       * better idea of what the actual mean is for larger arrays.

       */

      shuffle($values);

      $val = 0;

      $i = 0;

      foreach ($values as $value)

        {

          $val = $val * $i / ($i + 1)

            + $value / ($i + 1);

          $i ++;

        }

      return $val;

    }

  }

if (!function_exists('stddev'))

  {

    function stddev(array $values)

    {

      $mean = mean($values);

      $squares = 0;

      $i = 0;

      foreach ($values as $value)

        $squares += pow($mean - $value, 2);

      return sqrt($squares / (count($values) - 1));

    }

  }

/**

 * \brief

 *   Return the four quartile points of an array of sorted values with

 *   normal integral indexes.

 */

function sp_iqr(array $values)

{

  $count = count($values);

  if (!$count)

    return array(0, 0, 0, 0);

  return array(

               $values[0],

               $values[(int)($count / 4)],

               $values[(int)($count / 2)],

               $values[(int)(3 * $count / 4)],

               $values[$count - 1],

               );

}

/**

 * \brief

 *   Remove any `outliers' from an array of values.

 *

 * An outlier is defined as any value that falls further than 1.5

 * standard deviations outside of some sort of inter-quartile range.

 */

function filter_outliers(array $values)

{

  sort($values, SORT_NUMERIC);

  $values = array_values($values);

  $stddev = stddev($values);

  list(, $iqr_min, $iqr_max, ) = sp_iqr($values);

  $min = $iqr_min - 1.5 * $stddev;

  $max = $iqr_max + 1.5 * $stddev;

  $count = count($values);

  for ($i = 0; $i < $count; $i ++)

    if ($values[$i] < $min

        || $values[$i] > $max)

      unset($values[$i]);

  return array_values($values);

}

	      $semester->time_start_pool_add($date_start_time);

	      $semester->time_end_pool_add($date_end_time);