Draws the smallest possible convex polygon that surrounds a set of successful geocaching finds/geohashpoint expeditions. See: https://danq.me/geo-limits

geocaching-extent.php

<?php
/**
 * Given an array of points, returns the convex hull over those points.
 */
function convex_hull_over( array $points ): array {
  $cross = function($o, $a, $b) {
    return ($a[0] - $o[0]) * ($b[1] - $o[1]) - ($a[1] - $o[1]) * ($b[0] - $o[0]);
  };

  $pointCount = count($points);
  sort($points);
  if ($pointCount > 1) {
    $n = $pointCount;
    $k = 0;
    $h = array();

    // Build lower portion of hull
    for ($i = 0; $i < $n; ++$i) {
      while ($k >= 2 && $cross($h[$k - 2], $h[$k - 1], $points[$i]) <= 0)
        $k--;
      $h[$k++] = $points[$i];
    }

    // Build upper portion of hull
    for ($i = $n - 2, $t = $k + 1; $i >= 0; $i--) {
      while ($k >= $t && $cross($h[$k - 2], $h[$k - 1], $points[$i]) <= 0)
        $k--;
      $h[$k++] = $points[$i];
    }

    // Remove all vertices after k as they are inside of the hull
    if ($k > 1) {
      // ensure self-closing -
      $h = array_splice($h, 0, $k); 
    }
    return $h;
  } else if ($pointCount <= 1) {
    return $points;
  } else {
    return null;
  }
}

/**
 * Returns an array of arrays of the lon/lat coordinates of all successful geocaching finds/geohashing expeditions.
 */
function q23_geo_convex_hull() {
  $coords_sql = <<<END_OF_SQL
    SELECT DISTINCT coord_lon.meta_value lon, coord_lat.meta_value lat
    FROM wp_posts
    LEFT JOIN wp_postmeta expedition_result ON wp_posts.ID = expedition_result.post_id AND expedition_result.meta_key = 'checkin_type'
    LEFT JOIN wp_postmeta coord_lat ON wp_posts.ID = coord_lat.post_id AND coord_lat.meta_key = 'checkin_latitude'
    LEFT JOIN wp_postmeta coord_lon ON wp_posts.ID = coord_lon.post_id AND coord_lon.meta_key = 'checkin_longitude'
    LEFT JOIN wp_term_relationships ON wp_posts.ID = wp_term_relationships.object_id
    LEFT JOIN wp_term_taxonomy ON wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_taxonomy_id
    LEFT JOIN wp_terms ON wp_term_taxonomy.term_id = wp_terms.term_id
    WHERE wp_posts.post_type = 'post' AND wp_posts.post_status = 'publish'
    AND wp_term_taxonomy.taxonomy = 'kind'
    AND wp_terms.slug = 'checkin'
    AND expedition_result.meta_value IN ('Found it', 'found', 'coordinates reached', 'Attended');
END_OF_SQL;
  $coords = $GLOBALS['wpdb']->get_results($coords_sql, ARRAY_N);
  $hull = convex_hull_over( $coords );
  return $hull;
}

/**
 * Generates, caches, and renders a WebP map showing the convex hull of all successful geocaching finds/geohashing expeditions.
 */
function q23_hull_generator(){
  define('GEOAPIFY_API_KEY', 'INSERT A VALID API KEY HERE!');
  define('CACHE_DURATION', 14 * 24 * 3600); // two weeks
  define('MAP_SCHEMA_VERSION', 1); // Update this every time you change fixed map params e.g. marker color, to bust the cache
  define('MARKER_COLOR', '062d4d');
  define('CACHE_DIR', get_stylesheet_directory() . '/cache/maps/' . MAP_SCHEMA_VERSION);
  define('WIDTH', 640);
  define('HEIGHT', 480);
  define('ZOOM', 1);

  if(!is_dir(CACHE_DIR)) mkdir(CACHE_DIR, 0777, true);
  $tmp_file = CACHE_DIR . "/hull" . WIDTH . "x" . HEIGHT . "z" . ZOOM . ".tmp.jpg";
  $cache_file = CACHE_DIR . "/hull" . WIDTH . "x" . HEIGHT . "z" . ZOOM . ".webp";
  header('Content-type: image/webp');

  header('Expires: ' . gmdate("D, d M Y H:i:s", time() + CACHE_DURATION) . " GMT");
  header('Pragma: cache');
  header('Cache-control: max-age=' . CACHE_DURATION);

  if(!file_exists($cache_file) || (time()-filemtime($cache_file) > CACHE_DURATION)) {
    // nonexistant or out of date; redownload
    $hull = q23_geo_convex_hull();
    $lons = array_map(function($coord){ return $coord[0]; }, $hull);
    $lats = array_map(function($coord){ return $coord[1]; }, $hull);
    $mean_lon = array_sum($lons) / count($lons);
    $mean_lat = array_sum($lats) / count($lats);
    $hull_points = implode( ',', array_merge(...$hull) );

    $url = "https://maps.geoapify.com/v1/staticmap?style=osm-bright-smooth&width=" . WIDTH . "&height=" . HEIGHT . "&center=lonlat:$mean_lon,$mean_lat&zoom=" . ZOOM . "&apiKey=" . GEOAPIFY_API_KEY . "&geometry=polygon:$hull_points;linecolor:%23" . MARKER_COLOR . ";linewidth:2;lineopacity:0.5;fillcolor:%23" . MARKER_COLOR . ";fillopacity:0.2";
    file_put_contents($tmp_file, file_get_contents($url));
    system("convert \"$tmp_file\" \"$cache_file\"");
  }
  readfile($cache_file);
  die();
}

/**
 * Requests for /_q23-map/hull return my convex hull map:
 */
function maybe_run_q23_hull_generator(){
  if(preg_match('/^\/_q23-map\/hull$/', $_SERVER['REQUEST_URI'], $matches)) {
    q23_hull_generator();
  }
}
add_action( 'init', 'maybe_run_q23_hull_generator' );
?>