Fix clustering density with pixel-based grid calculation

Replace geohash precision mapping with dynamic grid sizing based on
target pixel spacing (60px between cluster centers). Uses Leaflet/OSM
tile math to calculate degrees-per-pixel at each zoom level, adjusted
for Mercator projection at the viewport's center latitude.

At zoom 7, this gives ~52km cells and ~150 clusters statewide,
properly separating Minneapolis from St. Cloud.

Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

wp-content/plugins/mls-by-hansonxyz/includes/class-mls-cluster.php

@@ -18,29 +18,10 @@ class MLS_Cluster {
     const GEOHASH_CHARS = '0123456789bcdefghjkmnpqrstuvwxyz';
 
     /**
-     * Zoom level to geohash precision mapping
-     * Lower precision = larger geographic area = more clustering
+     * Target pixel spacing between cluster centers
+     * With 30px cluster icons, 45px gap gives 75px total spacing
      */
-    const ZOOM_PRECISION = array(
-        3  => 2,  // ~630km cells - continent level
-        4  => 2,
-        5  => 2,
-        6  => 3,  // ~78km cells - state level
-        7  => 3,
-        8  => 3,
-        9  => 4,  // ~20km cells - county level
-        10 => 4,
-        11 => 4,
-        12 => 5,  // ~2.4km cells - city level
-        13 => 5,
-        14 => 5,
-        15 => 6,  // ~610m cells - neighborhood level
-        16 => 6,
-        17 => 6,
-        18 => 7,  // ~76m cells - street level
-        19 => 7,
-        20 => 8,  // Individual points
-    );
+    const CLUSTER_PIXEL_SPACING = 60;
 
     /**
      * Maximum properties to return as individual markers
@@ -156,20 +137,32 @@ class MLS_Cluster {
     }
 
     /**
-     * Get precision for zoom level
+     * Calculate grid cell size in degrees for a given zoom level
+     *
+     * Uses Leaflet/OSM tile math to determine what geographic distance
+     * corresponds to CLUSTER_PIXEL_SPACING pixels at the given zoom.
      *
      * @param int $zoom Map zoom level (1-20)
-     * @return int Geohash precision
+     * @param float $lat Center latitude (affects Mercator projection)
+     * @return array [lat_step, lng_step] in degrees
      */
-    public function get_precision_for_zoom($zoom) {
+    public function get_grid_step_for_zoom($zoom, $lat = 45.0) {
         $zoom = max(3, min(20, (int) $zoom));
 
-        if (isset(self::ZOOM_PRECISION[$zoom])) {
-            return self::ZOOM_PRECISION[$zoom];
-        }
+        // Degrees per pixel at zoom level (longitude)
+        // 360 degrees / (256 pixels * 2^zoom tiles)
+        $degrees_per_pixel_lng = 360.0 / (256 * pow(2, $zoom));
 
-        // Default to precision 5 for unknown zoom levels
-        return 5;
+        // Latitude degrees per pixel (adjusted for Mercator at given latitude)
+        // At the equator it's the same, at poles it's compressed
+        $lat_rad = deg2rad(abs($lat));
+        $degrees_per_pixel_lat = $degrees_per_pixel_lng * cos($lat_rad);
+
+        // Calculate step size to achieve target pixel spacing
+        $lng_step = self::CLUSTER_PIXEL_SPACING * $degrees_per_pixel_lng;
+        $lat_step = self::CLUSTER_PIXEL_SPACING * $degrees_per_pixel_lat;
+
+        return array($lat_step, $lng_step);
     }
 
     /**
@@ -258,14 +251,18 @@ class MLS_Cluster {
         }
 
         // If low count or high zoom, return individual markers
-        $precision = $this->get_precision_for_zoom($args['zoom']);
-
-        if ($total <= self::MAX_INDIVIDUAL_MARKERS || $args['zoom'] >= 18) {
+        if ($total <= self::MAX_INDIVIDUAL_MARKERS || $args['zoom'] >= 16) {
             return $this->get_individual_markers($where_sql, $values, $total);
         }
 
+        // Calculate center latitude for Mercator adjustment
+        $center_lat = 45.0; // Default Minnesota
+        if ($args['bounds'] && count($args['bounds']) === 4) {
+            $center_lat = ($args['bounds'][0] + $args['bounds'][2]) / 2;
+        }
+
         // Return clusters
-        return $this->get_cluster_data($where_sql, $values, $precision, $total);
+        return $this->get_cluster_data($where_sql, $values, $args['zoom'], $center_lat, $total);
     }
 
     /**
@@ -334,27 +331,22 @@ class MLS_Cluster {
     }
 
     /**
-     * Get clustered data using geohash grouping
+     * Get clustered data using grid-based grouping
      *
      * @param string $where_sql WHERE clause
      * @param array $values Prepared values
-     * @param int $precision Geohash precision
+     * @param int $zoom Map zoom level
+     * @param float $center_lat Center latitude for Mercator adjustment
      * @param int $total Total count
      * @return array
      */
-    private function get_cluster_data($where_sql, $values, $precision, $total) {
+    private function get_cluster_data($where_sql, $values, $zoom, $center_lat, $total) {
         global $wpdb;
 
         $table = $this->db->properties_table();
 
-        // Use MySQL SUBSTR to extract geohash prefix for grouping
-        // We compute geohash on-the-fly using latitude/longitude
-        // For better performance with 30k+ records, we use grid-based clustering
-
-        // Calculate grid cell size based on precision
-        // Approximate degrees per geohash precision level
-        $lat_step = 180 / pow(2, (int)($precision * 2.5));
-        $lng_step = 360 / pow(2, (int)($precision * 2.5));
+        // Calculate grid cell size based on zoom level and target pixel spacing
+        list($lat_step, $lng_step) = $this->get_grid_step_for_zoom($zoom, $center_lat);
 
         // Use FLOOR to group coordinates into cells
         $sql = "SELECT
@@ -388,7 +380,8 @@ class MLS_Cluster {
             'type' => 'clusters',
             'total' => $total,
             'cluster_count' => count($clusters),
-            'precision' => $precision,
+            'zoom' => $zoom,
+            'grid_size_deg' => array('lat' => $lat_step, 'lng' => $lng_step),
             'clusters' => $clusters,
         );
     }