tildefriends/apps/gg/polyline.js

/**
 * Based off of [the offical Google document](https://developers.google.com/maps/documentation/utilities/polylinealgorithm)
 *
 * Some parts from [this implementation](http://facstaff.unca.edu/mcmcclur/GoogleMaps/EncodePolyline/PolylineEncoder.js)
 * by [Mark McClure](http://facstaff.unca.edu/mcmcclur/)
 *
 * @module polyline
 */

var polyline = {};

function py2_round(value) {
	// Google's polyline algorithm uses the same rounding strategy as Python 2, which is different from JS for negative values
	return Math.floor(Math.abs(value) + 0.5) * (value >= 0 ? 1 : -1);
}

function encode(current, previous, factor) {
	current = py2_round(current * factor);
	previous = py2_round(previous * factor);
	var coordinate = (current - previous) * 2;
	if (coordinate < 0) {
		coordinate = -coordinate - 1;
	}
	var output = '';
	while (coordinate >= 0x20) {
		output += String.fromCharCode((0x20 | (coordinate & 0x1f)) + 63);
		coordinate /= 32;
	}
	output += String.fromCharCode((coordinate | 0) + 63);
	return output;
}

/**
 * Decodes to a [latitude, longitude] coordinates array.
 *
 * This is adapted from the implementation in Project-OSRM.
 *
 * @param {String} str
 * @param {Number} precision
 * @returns {Array}
 *
 * @see https://github.com/Project-OSRM/osrm-frontend/blob/master/WebContent/routing/OSRM.RoutingGeometry.js
 */
polyline.decode = function (str, precision) {
	var index = 0,
		lat = 0,
		lng = 0,
		coordinates = [],
		shift = 0,
		result = 0,
		byte = null,
		latitude_change,
		longitude_change,
		factor = Math.pow(10, Number.isInteger(precision) ? precision : 5);

	// Coordinates have variable length when encoded, so just keep
	// track of whether we've hit the end of the string. In each
	// loop iteration, a single coordinate is decoded.
	while (index < str.length) {
		// Reset shift, result, and byte
		byte = null;
		shift = 1;
		result = 0;

		do {
			byte = str.charCodeAt(index++) - 63;
			result += (byte & 0x1f) * shift;
			shift *= 32;
		} while (byte >= 0x20);

		latitude_change = result & 1 ? (-result - 1) / 2 : result / 2;

		shift = 1;
		result = 0;

		do {
			byte = str.charCodeAt(index++) - 63;
			result += (byte & 0x1f) * shift;
			shift *= 32;
		} while (byte >= 0x20);

		longitude_change = result & 1 ? (-result - 1) / 2 : result / 2;

		lat += latitude_change;
		lng += longitude_change;

		coordinates.push([lat / factor, lng / factor]);
	}

	return coordinates;
};

/**
 * Encodes the given [latitude, longitude] coordinates array.
 *
 * @param {Array.<Array.<Number>>} coordinates
 * @param {Number} precision
 * @returns {String}
 */
polyline.encode = function (coordinates, precision) {
	if (!coordinates.length) {
		return '';
	}

	var factor = Math.pow(10, Number.isInteger(precision) ? precision : 5),
		output =
			encode(coordinates[0][0], 0, factor) +
			encode(coordinates[0][1], 0, factor);

	for (var i = 1; i < coordinates.length; i++) {
		var a = coordinates[i],
			b = coordinates[i - 1];
		output += encode(a[0], b[0], factor);
		output += encode(a[1], b[1], factor);
	}

	return output;
};

function flipped(coords) {
	var flipped = [];
	for (var i = 0; i < coords.length; i++) {
		var coord = coords[i].slice();
		flipped.push([coord[1], coord[0]]);
	}
	return flipped;
}

/**
 * Encodes a GeoJSON LineString feature/geometry.
 *
 * @param {Object} geojson
 * @param {Number} precision
 * @returns {String}
 */
polyline.fromGeoJSON = function (geojson, precision) {
	if (geojson && geojson.type === 'Feature') {
		geojson = geojson.geometry;
	}
	if (!geojson || geojson.type !== 'LineString') {
		throw new Error('Input must be a GeoJSON LineString');
	}
	return polyline.encode(flipped(geojson.coordinates), precision);
};

/**
 * Decodes to a GeoJSON LineString geometry.
 *
 * @param {String} str
 * @param {Number} precision
 * @returns {Object}
 */
polyline.toGeoJSON = function (str, precision) {
	var coords = polyline.decode(str, precision);
	return {
		type: 'LineString',
		coordinates: flipped(coords),
	};
};

let polyline_decode = polyline.decode;
export {polyline_decode as decode};