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'use strict';

/**

* 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) * Math.sign(value);

}

function encode(current, previous, factor) {

current = py2_round(current * factor);

previous = py2_round(previous * factor);

var coordinate = current - previous;

coordinate <<= 1;

if (current - previous < 0) {

coordinate = ~coordinate;

}

var output = '';

while (coordinate >= 0x20) {

output += String.fromCharCode((0x20 | (coordinate & 0x1f)) + 63);

coordinate >>= 5;

}

output += String.fromCharCode(coordinate + 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, 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 = 0;

result = 0;

do {

byte = str.charCodeAt(index++) - 63;

result |= (byte & 0x1f) << shift;

shift += 5;

} while (byte >= 0x20);

latitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

shift = result = 0;

do {

byte = str.charCodeAt(index++) - 63;

result |= (byte & 0x1f) << shift;

shift += 5;

} while (byte >= 0x20);

longitude_change = ((result & 1) ? ~(result >> 1) : (result >> 1));

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, 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++) {

flipped.push(coords[i].slice().reverse());

}

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)

};

};

if (typeof module === 'object' && module.exports) {

module.exports = polyline;

}