Function getPrecisionForBBox

pkg/geo/encode.go:239–300

getPrecisionForBBox is a function imitating PostGIS's ability to go from a world bounding box and truncating a GeoHash to fit the given bounding box. The algorithm halves the world bounding box until it intersects with the feature bounding box to get a precision that will encompass the entire boundi

(bbox *geopb.BoundingBox)

// feature bounding box to get a precision that will encompass the entire
// bounding box.
func getPrecisionForBBox(bbox *geopb.BoundingBox) int {
	bitPrecision := 0

	// This is a point, for points we use the full bitPrecision.
	if bbox.LoX == bbox.HiX && bbox.LoY == bbox.HiY {
		return GeoHashMaxPrecision
	}

	// Starts from a world bounding box:
	lonMin := -180.0
	lonMax := 180.0
	latMin := -90.0
	latMax := 90.0

	// Each iteration shrinks the world bounding box by half in the dimension that
	// does not fit, making adjustments each iteration until it intersects with
	// the object bbox.
	for {
		lonWidth := lonMax - lonMin
		latWidth := latMax - latMin
		latMaxDelta, lonMaxDelta, latMinDelta, lonMinDelta := 0.0, 0.0, 0.0, 0.0

		// Look at whether the longitudes of the bbox are to the left or
		// the right of the world bbox longitudes, shrinks it and makes adjustments
		// for the next iteration.
		if bbox.LoX > lonMin+lonWidth/2.0 {
			lonMinDelta = lonWidth / 2.0
		} else if bbox.HiX < lonMax-lonWidth/2.0 {
			lonMaxDelta = lonWidth / -2.0
		}
		// Look at whether the latitudes of the bbox are to the left or
		// the right of the world bbox latitudes, shrinks it and makes adjustments
		// for the next iteration.
		if bbox.LoY > latMin+latWidth/2.0 {
			latMinDelta = latWidth / 2.0
		} else if bbox.HiY < latMax-latWidth/2.0 {
			latMaxDelta = latWidth / -2.0
		}

		// Every change we make that splits the box up adds precision.
		// If we detect no change, we've intersected a box and so must exit.
		precisionDelta := 0
		if lonMinDelta != 0.0 || lonMaxDelta != 0.0 {
			lonMin += lonMinDelta
			lonMax += lonMaxDelta
			precisionDelta++
		} else {
			break
		}
		if latMinDelta != 0.0 || latMaxDelta != 0.0 {
			latMin += latMinDelta
			latMax += latMaxDelta
			precisionDelta++
		} else {
			break
		}
		bitPrecision += precisionDelta
	}