I have JavaScript code that creates geohash with base-32 system.
var BASE32_CODES = "0123456789bcdefghjkmnpqrstuvwxyz";
var BASE32_CODES_DICT = {};
for (var i = 0; i < BASE32_CODES.length; i++) {
BASE32_CODES_DICT[BASE32_CODES.charAt(i)] = i;
}
var ENCODE_AUTO = 'auto';
var SIGFIG_HASH_LENGTH = [0, 5, 7, 8, 11, 12, 13, 15, 16, 17, 18];
var encode = function (latitude, longitude, numberOfChars) {
if (numberOfChars === ENCODE_AUTO) {
if (typeof(latitude) === 'number' || typeof(longitude) === 'number') {
throw new Error('string notation required for auto precision.');
}
var decSigFigsLat = latitude.split('.')[1].length;
var decSigFigsLong = longitude.split('.')[1].length;
var numberOfSigFigs = Math.max(decSigFigsLat, decSigFigsLong);
numberOfChars = SIGFIG_HASH_LENGTH[numberOfSigFigs];
} else if (numberOfChars === undefined) {
numberOfChars = 9;
}
var chars = [],
bits = 0,
bitsTotal = 0,
hash_value = 0,
maxLat = 90,
minLat = -90,
maxLon = 180,
minLon = -180,
mid;
while (chars.length < numberOfChars) {
if (bitsTotal % 2 === 0) {
mid = (maxLon + minLon) / 2;
if (longitude > mid) {
hash_value = (hash_value << 1) + 1;
minLon = mid;
} else {
hash_value = (hash_value << 1) + 0;
maxLon = mid;
}
} else {
mid = (maxLat + minLat) / 2;
if (latitude > mid) {
hash_value = (hash_value << 1) + 1;
minLat = mid;
} else {
hash_value = (hash_value << 1) + 0;
maxLat = mid;
}
}
bits++;
bitsTotal++;
if (bits === 5) {
var code = BASE32_CODES[hash_value];
chars.push(code);
bits = 0;
hash_value = 0;
}
}
return chars.join('');
};
Actually, I took it from ngeohash npm module (source code).
I'm not good at numeral systems binary data, etc. I don't know how to change encoding to the base-16 system ('0123456789abcdef'). I'll be happy if someone changes it or just points me what do I need to change. Thanks.
So I think you'll only have to change three things:
BASE32_CODES
in your snippet), to have the base 16 character set instead.Since we're moving from 5 -> 4 bits per char, we would need additional characters to reach the same precision (5bits/char * 9chars = 45bits, so we need at least 12 characters in base 16 for the same precision (4bits/char * 12chars = 48bits, so we actually get some extra precision with that character count)
So your snippet would become:
var BASE16_CODES = "0123456789abcdef"; // <-- changed this
var BASE16_CODES_DICT = {};
for (var i = 0; i < BASE16_CODES.length; i++) {
BASE16_CODES_DICT[BASE16_CODES.charAt(i)] = i;
}
var ENCODE_AUTO = 'auto';
var SIGFIG_HASH_LENGTH = [0, 5, 7, 8, 11, 12, 13, 15, 16, 17, 18];
var encode = function (latitude, longitude, numberOfChars) {
if (numberOfChars === ENCODE_AUTO) {
if (typeof(latitude) === 'number' || typeof(longitude) === 'number') {
throw new Error('string notation required for auto precision.');
}
var decSigFigsLat = latitude.split('.')[1].length;
var decSigFigsLong = longitude.split('.')[1].length;
var numberOfSigFigs = Math.max(decSigFigsLat, decSigFigsLong);
numberOfChars = SIGFIG_HASH_LENGTH[numberOfSigFigs];
} else if (numberOfChars === undefined) {
numberOfChars = 12; // <-- and this
}
var chars = [],
bits = 0,
bitsTotal = 0,
hash_value = 0,
maxLat = 90,
minLat = -90,
maxLon = 180,
minLon = -180,
mid;
while (chars.length < numberOfChars) {
if (bitsTotal % 2 === 0) {
mid = (maxLon + minLon) / 2;
if (longitude > mid) {
hash_value = (hash_value << 1) + 1;
minLon = mid;
} else {
hash_value = (hash_value << 1) + 0;
maxLon = mid;
}
} else {
mid = (maxLat + minLat) / 2;
if (latitude > mid) {
hash_value = (hash_value << 1) + 1;
minLat = mid;
} else {
hash_value = (hash_value << 1) + 0;
maxLat = mid;
}
}
bits++;
bitsTotal++;
if (bits === 4) { // <-- and finally this
var code = BASE16_CODES[hash_value];
chars.push(code);
bits = 0;
hash_value = 0;
}
}
return chars.join('');
};
console.log(encode(...[40.676843, -73.935769]))
EDIT: Come to think of it, if you're using the ENCODE_AUTO
feature that this function has, you'll need to increase the values in the SIGFIG_HASH_LENGTH
array as well (for the same reason as before, more chars are needed for the same or greater precision). So it should end up as:
var SIGFIG_HASH_LENGTH = [0, 7, 9, 10, 14, 15, 17, 19, 20, 22, 23]