<?php
/**
 * libgpx.php
 *
 * Copyright 2018 Andy Street <andy@street.me.uk>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301, USA.
 *
 *
 */

namespace libgpx;

use \DateTimeZone;
use \RuntimeException;

/**
 * Constants and miscellaneous functions for the libgpx library.
 *
 * @static
 * @author Andy Street <andy@street.me.uk>
 */
final class libGPX
{

  /**
   * The XML namespace for GPX 1.0.
   */
  const NAMESPACE_GPX_1_0 = 'http://www.topografix.com/GPX/1/0';

  /**
   * The XML namespace for GPX 1.1.
   */
  const NAMESPACE_GPX_1_1 = 'http://www.topografix.com/GPX/1/1';

  /**
   * The XML namespace for XML Schema.
   */
  const NAMESPACE_XMLSCHEMA = 'http://www.w3.org/2001/XMLSchema-instance';

  /**
   * The URL of the schema document for GPX 1.0.
   */
  const SCHEMA_GPX_1_0 = 'http://www.topografix.com/GPX/1/0/gpx.xsd';

  /**
   * The URL of the schema document for GPX 1.1.
   */
  const SCHEMA_GPX_1_1 = 'http://www.topografix.com/GPX/1/1/gpx.xsd';

  /**
   * If libgpx::init() has been completed successfully.
   *
   * @var boolean
   */
  private static $initialized = false;

  /**
   * Empty constructor - class is static.
   */
  private function __construct() {}

  /**
   * Check if the system has the required dependencies.
   *
   * @param boolean $throw If true, throw an exception when dependencies are unmet.
   * @return boolean If all dependencies are met.
   * @throws RuntimeException
   */
  public static function checkDependencies($throw = false)
  {
    // Define dependencies
    $php_version = '7.0.0';
    $modules = ['xmlreader', 'xmlwriter'];

    // Check for PHP
    if (version_compare(PHP_VERSION, $php_version) < 0) {
      if ($throw)
        throw new RuntimeException(
          sprintf('PHP version %s required.', $php_version)
        );
      else
        return false;
    }

    // Check for modules
    foreach ($modules as $module) {
      if (!extension_loaded($module)) {
        if ($throw)
          throw new RuntimeException(sprintf(
            'PHP module "%s" required but not installed.',
            $module
          ));
        else
          return false;
      }
    }

    return true;
  }

  /**
   * Ensure that dependencies are met and an autoloader is registered.
   *
   * @return void
   * @throws RuntimeException If the dependencies are not met.
   */
  public static function init()
  {
    if (self::$initialized) return;
    self::checkDependencies(true);
    spl_autoload_register(
      function ($class) {
        if (strncmp($class, 'libgpx\\', 7) !== 0) return;
        $class = strtolower(substr($class, 7));
        $filename = __DIR__ . DIRECTORY_SEPARATOR . $class . '.php';
        if (is_readable($filename)) {
          include $filename;
        }
      }
    );
    self::$initialized = true;
  }

  /**
   * Measure the distance between two points.
   *
   * Haversine formula, great circle distance.
   *
   * @param float $lat1 The latitude of the first point.
   * @param float $lon1 The longitude of the first point.
   * @param float $lat2 The latitude of the second point.
   * @param float $lon2 The longitude of the second point.
   * @return float The distance in meters.
   */
  public static function distance(
    float $lat1,
    float $lon1,
    float $lat2,
    float $lon2
  ) {
    $deg2rad = pi() / 180;
    $r = 6371000; // Radius of the Earth in meters
    $dlat = ($lat2 - $lat1) * $deg2rad;
    $dlon = ($lon2 - $lon1) * $deg2rad;
    $a =
      sin($dlat / 2) * sin($dlat / 2) +
      cos($lat1 * $deg2rad) * cos($lat2 * $deg2rad) *
      sin($dlon / 2) * sin($dlon / 2);
    $c = 2 * atan2(sqrt($a), sqrt(1-$a));
    return $r * $c;
  }

  /**
   * Convert meters in to miles.
   *
   * @see https://en.wikipedia.org/wiki/Mile
   *
   * @param float $meters A distance in meters.
   * @return float A distance in miles.
   */
  public static function meters2miles(float $meters)
  {
    return $meters / 1609.344;
  }

}