mapbox/vendor/polyline.js

interface to mapbox services

'use strict';

/*
 * polyline
 *
 * https://github.com/mapbox/polyline
 *
 * by John Firebaugh, Tom MacWright, and contributors
 * licensed under BSD 3-clause
 */

/*
 * 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 encode(coordinate, factor) {
    coordinate = Math.round(coordinate * factor);
    coordinate <<= 1;
    if (coordinate < 0) {
        coordinate = ~coordinate;
    }
    var output = '';
    while (coordinate >= 0x20) {
        output += String.fromCharCode((0x20 | (coordinate & 0x1f)) + 63);
        coordinate >>= 5;
    }
    output += String.fromCharCode(coordinate + 63);
    return output;
}

/**
 * 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], factor) + encode(coordinates[0][1], 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);
};

module.exports = polyline;