replace NearPoint with a more generic context object

The NearPoint is actually common to all SearchDescriptions
and there is other context data as well. like viewbox, that
needs to be available to the search object but is common.

lib/SearchContext.php

@@ -0,0 +1,73 @@
+<?php
+
+namespace Nominatim;
+
+require_once(CONST_BasePath.'/lib/lib.php');
+
+
+/**
+ * Collects search constraints that are independent of the
+ * actual interpretation of the search query.
+ *
+ * The search context is shared between all SearchDescriptions. This
+ * object mainly serves as context provider for the database queries.
+ * Therefore most data is directly cached as SQL statements.
+ */
+class SearchContext
+{
+    private $fNearRadius = false;
+
+    // cached SQL
+
+    public $sqlNear = '';
+
+    public function hasNearPoint()
+    {
+        return $this->fNearRadius !== false;
+    }
+
+    public function nearRadius()
+    {
+        return $this->fNearRadius;
+    }
+
+    public function setNearPoint($fLat, $fLon, $fRadius = 0.1)
+    {
+        $this->fNearRadius = $fRadius;
+        $this->sqlNear = 'ST_SetSRID(ST_Point('.$fLon.','.$fLat.'),4326)';
+    }
+
+    /**
+     * Extract a coordinate point from a query string.
+     *
+     * @param string $sQuery Query to scan.
+     *
+     * @return The remaining query string.
+     */
+    public function setNearPointFromQuery($sQuery)
+    {
+        $aResult = parseLatLon($sQuery);
+
+        if ($aResult !== false
+            && $aResult[1] <= 90.1
+            && $aResult[1] >= -90.1
+            && $aResult[2] <= 180.1
+            && $aResult[2] >= -180.1
+        ) {
+            $this->setNearPoint($aResult[1], $aResult[2]);
+            $sQuery = trim(str_replace($aResult[0], ' ', $sQuery));
+        }
+
+        return $sQuery;
+    }
+
+    public function distanceSQL($sObj)
+    {
+        return 'ST_Distance('.$this->sqlNear.", $sObj)";
+    }
+
+    public function withinSQL($sObj)
+    {
+        return sprintf('ST_DWithin(%s, %s, %F)', $sObj, $this->sqlNear, $this->fNearRadius);
+    }
+}