Calculate the zoom level of bounds

MapUtils.swift

private let LN2 = 0.6931471805599453
private let WORLD_PX_HEIGHT = 256
private let WORLD_PX_WIDTH = 256

func getBoundsZoomLevel(bounds: GMSCoordinateBounds, mapWidthPx: Int, mapHeightPx: Int) -> Double {
        let ne = bounds.northEast
        let sw = bounds.southWest

        let latFraction = (getLatitudeRadius(ne.latitude) - getLatitudeRadius(sw.latitude)) / M_PI

        let lngDiff = ne.longitude - sw.longitude
        let lngFraction = ((lngDiff < 0) ? (lngDiff + 360) : lngDiff) / 360

        let latZoom = getZoomWithFraction(mapHeightPx, worldPx: WORLD_PX_HEIGHT, fraction: latFraction)
        let lngZoom = getZoomWithFraction(mapWidthPx, worldPx: WORLD_PX_WIDTH, fraction: lngFraction)

        return min(latZoom, lngZoom)
    }

    func getLatitudeRadius(lat: Double) -> Double {
        let sinLat = sin(lat * M_PI / 180)
        let radX2 = log((1 + sinLat) / (1 - sinLat)) / 2
        return max(min(radX2, M_PI), -M_PI) / 2
    }

    func getZoomWithFraction(mapPx: Int, worldPx: Int, fraction: Double) -> Double {
        return floor(log(Double(mapPx) / Double(worldPx) / fraction) / LN2)
    }