var geometryArea = geometry;

turf_area = function(_) {
    if (_.type === 'FeatureCollection') {
        for (var i = 0, sum = 0; i < _.features.length; i++) {
            if (_.features[i].geometry) {
                sum += geometryArea(_.features[i].geometry);
            }
        }
        return sum;
    } else if (_.type === 'Feature') {
        return geometryArea(_.geometry);
    } else {
        return geometryArea(_);
    }
};

RADIUS = 6378137;
FLATTENING = 1/298.257223563;
POLAR_RADIUS = 6356752.3142;

ring = ringArea;

function geometry(_) {
    var area = 0, i;
    switch (_.type) {
        case 'Polygon':
            return polygonArea(_.coordinates);
        case 'MultiPolygon':
            for (i = 0; i < _.coordinates.length; i++) {
                area += polygonArea(_.coordinates[i]);
            }
            return area;
        case 'Point':
        case 'MultiPoint':
        case 'LineString':
        case 'MultiLineString':
            return 0;
        case 'GeometryCollection':
            for (i = 0; i < _.geometries.length; i++) {
                area += geometry(_.geometries[i]);
            }
            return area;
    }
}

function polygonArea(coords) {
    var area = 0;
    if (coords && coords.length > 0) {
        area += Math.abs(ringArea(coords[0]));
        for (var i = 1; i < coords.length; i++) {
            area -= Math.abs(ringArea(coords[i]));
        }
    }
    return area;
}

function ringArea(coords) {
    var p1, p2, p3, lowerIndex, middleIndex, upperIndex,
        area = 0,
        coordsLength = coords.length;

    if (coordsLength > 2) {
        for (i = 0; i < coordsLength; i++) {
            if (i === coordsLength - 2) {// i = N-2
                lowerIndex = coordsLength - 2;
                middleIndex = coordsLength -1;
                upperIndex = 0;
            } else if (i === coordsLength - 1) {// i = N-1
                lowerIndex = coordsLength - 1;
                middleIndex = 0;
                upperIndex = 1;
            } else { // i = 0 to N-3
                lowerIndex = i;
                middleIndex = i+1;
                upperIndex = i+2;
            }
            p1 = coords[lowerIndex];
            p2 = coords[middleIndex];
            p3 = coords[upperIndex];
            area += ( rad(p3[0]) - rad(p1[0]) ) * Math.sin( rad(p2[1]));
        }

        area = area * RADIUS * RADIUS / 2;
    }

    return area;
}

function rad(_) {
    return _ * Math.PI / 180;
}