geohash.go

package gogeohash

const (
	MIN_LATITUDE = -90.0
	MAX_LATITUDE = 90.0

	MIN_LONGITUDE = -180.0
	MAX_LONGITUDE = 180.0

	STEP = 32

	TEXTREP = "0123456789bcdefghjkmnpqrstuvwxyz"
)

// Interleave 2 uint32's bit representation.
// Interleave lower bits of a and b, so the bits of a are in the even positions
// and bits from y in the odd. Checkout the following page many such bit tweaks
// https://graphics.stanford.edu/~seander/bithacks.html#InterleaveBMN
func interleave64(a, b uint32) uint64 {
	B := []uint64{
		0x5555555555555555,
		0x3333333333333333,
		0x0F0F0F0F0F0F0F0F,
		0x00FF00FF00FF00FF,
		0x0000FFFF0000FFFF,
	}
	S := []uint32{1, 2, 4, 8, 16}

	x := uint64(a)
	y := uint64(b)

	x = (x | (x << S[4])) & B[4]
	y = (y | (y << S[4])) & B[4]

	x = (x | (x << S[3])) & B[3]
	y = (y | (y << S[3])) & B[3]

	x = (x | (x << S[2])) & B[2]
	y = (y | (y << S[2])) & B[2]

	x = (x | (x << S[1])) & B[1]
	y = (y | (y << S[1])) & B[1]

	x = (x | (x << S[0])) & B[0]
	y = (y | (y << S[0])) & B[0]

	return x | (y << 1)
}

// Map each 5 bit to TEXTREP and produce a string representation of the geohash.
func toChars(x uint64) string {
	var s string
	for i := 59; i >= 0; i -= 5 {
		s += string(TEXTREP[(x&(((1<<5)-1)<<i))>>i])
	}
	return s
}

// Map each 5 bit to TEXTREP and produce a string representation of the geohash.
func toCharsUnrolled(x uint64) string {
	var s string
	s += string(TEXTREP[(59&(((1<<5)-1)<<59))>>59])
	s += string(TEXTREP[(54&(((1<<5)-1)<<54))>>54])
	s += string(TEXTREP[(49&(((1<<5)-1)<<49))>>49])
	s += string(TEXTREP[(44&(((1<<5)-1)<<44))>>44])
	s += string(TEXTREP[(39&(((1<<5)-1)<<39))>>39])
	s += string(TEXTREP[(34&(((1<<5)-1)<<34))>>34])
	s += string(TEXTREP[(29&(((1<<5)-1)<<29))>>29])
	s += string(TEXTREP[(24&(((1<<5)-1)<<24))>>24])
	s += string(TEXTREP[(19&(((1<<5)-1)<<19))>>19])
	s += string(TEXTREP[(15&(((1<<5)-1)<<15))>>15])
	s += string(TEXTREP[(9&(((1<<5)-1)<<9))>>9])
	s += string(TEXTREP[(4&(((1<<5)-1)<<4))>>4])
	return s
}

// For a latitude and longitude value, compute its GeoHash.
func GeoHashEncode(lat, lon float64) uint64 {
	var latOffset, lonOffset float64

	latOffset = (lat - MIN_LATITUDE) / (MAX_LATITUDE - MIN_LATITUDE)
	lonOffset = (lon - MIN_LONGITUDE) / (MAX_LONGITUDE - MIN_LONGITUDE)

	latOffset *= 1 << STEP
	lonOffset *= 1 << STEP

	return interleave64(uint32(latOffset), uint32(lonOffset))
}