@geonet/geohash/src/encode.js

Geohash decode/encoder

/** ****************************************************************************************************
 * File: encode.js
 * Project: geohash
 * @author Nick Soggin <iSkore@users.noreply.github.com> on 19-Feb-2019
 *******************************************************************************************************/
'use strict';

const
	{
		BASE32,
		ENCODE_AUTO,
		MIN_LNG,
		MIN_LAT,
		MAX_LNG,
		MAX_LAT
	} = require( './variables' );

const
	{
		determinePrecision,
		isNumber
	} = require( './utils' );

/**
 * encode
 * Encodes latitude/longitude to geohash, either to specified precision or to automatically
 * evaluated precision.
 *
 * @param   {number} lng - Longitude in degrees.
 * @param   {number} lat - Latitude in degrees.
 * @param   {number} [precision=ENCODE_AUTO] - Number of characters in resulting geohash.
 * @returns {string} Geohash of supplied latitude/longitude.
 * @throws  Invalid geohash.
 *
 * @example
 *     const geohash = Geohash.encode(52.205, 0.119, 7); // geohash: 'u120fxw'
 */
function encode( lng, lat, precision = ENCODE_AUTO ) {
	lng       = +lng;
	lat       = +lat;
	precision = +precision;

	if ( !isNumber( lng ) ) {
		throw new Error( 'Invalid value for `lng`' );
	}
	else if ( !isNumber( lat ) ) {
		throw new Error( 'Invalid value for `lat`' );
	}
	else if ( !isNumber( precision ) ) {
		throw new Error( 'Invalid value for `precision`' );
	}

	precision = determinePrecision( lng, lat, precision );

	let
		geohash = '',
		hash    = 0, // index into BASE32 map
		bit     = 0, // each char holds 5 bits
		evenBit = true,
		lngMin  = MIN_LNG,
		latMin  = MIN_LAT,
		lngMax  = MAX_LNG,
		latMax  = MAX_LAT,
		mid     = 0;

	while ( geohash.length < precision ) {
		if ( evenBit ) {
			// bisect E-W longitude
			mid = ( lngMin + lngMax ) / 2;
			if ( lng >= mid ) {
				hash   = ( hash << 1 ) + 1;
				lngMin = mid;
			}
			else {
				hash   = hash << 1;
				lngMax = mid;
			}
		}
		else {
			// bisect N-S latitude
			mid = ( latMin + latMax ) / 2;
			if ( lat >= mid ) {
				hash   = ( hash << 1 ) + 1;
				latMin = mid;
			}
			else {
				hash   = hash << 1;
				latMax = mid;
			}
		}

		evenBit = !evenBit;

		if ( ++bit === 5 ) {
			// 5 bits gives us a character: append it and start over
			geohash += BASE32[ hash ];
			bit  = 0;
			hash = 0;
		}
	}

	return geohash;
}

module.exports = encode;