Source: encode.js

  1. /** ****************************************************************************************************
  2.  * File: encode.js
  3.  * Project: geohash
  4.  * @author Nick Soggin <iSkore@users.noreply.github.com> on 19-Feb-2019
  5.  *******************************************************************************************************/
  6. 'use strict';
  8. const
  9. 	{
  10. 		BASE32,
  11. 		ENCODE_AUTO,
  12. 		MIN_LNG,
  13. 		MIN_LAT,
  14. 		MAX_LNG,
  15. 		MAX_LAT
  16. 	} = require( './variables' );
  18. const
  19. 	{
  20. 		determinePrecision,
  21. 		isNumber
  22. 	} = require( './utils' );
  24. /**
  25.  * encode
  26.  * Encodes latitude/longitude to geohash, either to specified precision or to automatically
  27.  * evaluated precision.
  28.  *
  29.  * @param   {number} lng - Longitude in degrees.
  30.  * @param   {number} lat - Latitude in degrees.
  31.  * @param   {number} [precision=ENCODE_AUTO] - Number of characters in resulting geohash.
  32.  * @returns {string} Geohash of supplied latitude/longitude.
  33.  * @throws  Invalid geohash.
  34.  *
  35.  * @example
  36.  *     const geohash = Geohash.encode(52.205, 0.119, 7); // geohash: 'u120fxw'
  37.  */
  38. function encode( lng, lat, precision = ENCODE_AUTO ) {
  39. 	lng       = +lng;
  40. 	lat       = +lat;
  41. 	precision = +precision;
  43. 	if ( !isNumber( lng ) ) {
  44. 		throw new Error( 'Invalid value for `lng`' );
  45. 	}
  46. 	else if ( !isNumber( lat ) ) {
  47. 		throw new Error( 'Invalid value for `lat`' );
  48. 	}
  49. 	else if ( !isNumber( precision ) ) {
  50. 		throw new Error( 'Invalid value for `precision`' );
  51. 	}
  53. 	precision = determinePrecision( lng, lat, precision );
  55. 	let
  56. 		geohash = '',
  57. 		hash    = 0, // index into BASE32 map
  58. 		bit     = 0, // each char holds 5 bits
  59. 		evenBit = true,
  60. 		lngMin  = MIN_LNG,
  61. 		latMin  = MIN_LAT,
  62. 		lngMax  = MAX_LNG,
  63. 		latMax  = MAX_LAT,
  64. 		mid     = 0;
  66. 	while ( geohash.length < precision ) {
  67. 		if ( evenBit ) {
  68. 			// bisect E-W longitude
  69. 			mid = ( lngMin + lngMax ) / 2;
  70. 			if ( lng >= mid ) {
  71. 				hash   = ( hash << 1 ) + 1;
  72. 				lngMin = mid;
  73. 			}
  74. 			else {
  75. 				hash   = hash << 1;
  76. 				lngMax = mid;
  77. 			}
  78. 		}
  79. 		else {
  80. 			// bisect N-S latitude
  81. 			mid = ( latMin + latMax ) / 2;
  82. 			if ( lat >= mid ) {
  83. 				hash   = ( hash << 1 ) + 1;
  84. 				latMin = mid;
  85. 			}
  86. 			else {
  87. 				hash   = hash << 1;
  88. 				latMax = mid;
  89. 			}
  90. 		}
  92. 		evenBit = !evenBit;
  94. 		if ( ++bit === 5 ) {
  95. 			// 5 bits gives us a character: append it and start over
  96. 			geohash += BASE32[ hash ];
  97. 			bit  = 0;
  98. 			hash = 0;
  99. 		}
  100. 	}
  102. 	return geohash;
  103. }
  105. module.exports = encode;