declare namespace xsd = "http://www.w3.org/2001/XMLSchema";
declare namespace math = "http://www.w3.org/2005/xpath-functions/math";

declare function local:haversine($lat1 as xsd:float, $lon1 as xsd:float, $lat2 as xsd:float, $lon2 as xsd:float)
    as xsd:float
{
    let $dlat  := ($lat2 - $lat1) * math:pi() div 180
    let $dlon  := ($lon2 - $lon1) * math:pi() div 180
    let $rlat1 := $lat1 * math:pi() div 180
    let $rlat2 := $lat2 * math:pi() div 180
    let $a     := math:sin($dlat div 2) * math:sin($dlat div 2) + math:sin($dlon div 2) * math:sin($dlon div 2) * math:cos($rlat1) * math:cos($rlat2)
    let $c     := 2 * math:atan2(math:sqrt($a), math:sqrt(1-$a))
    return xsd:float($c * 6371.0)
};

local:haversine(36.12, -86.67, 33.94, -118.4)