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January 23, 2024 21:48
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/* | |
* Copyright 2018 Uber Technologies, Inc. | |
* | |
* Licensed under the Apache License, Version 2.0 (the "License"); | |
* you may not use this file except in compliance with the License. | |
* You may obtain a copy of the License at | |
* | |
* http://www.apache.org/licenses/LICENSE-2.0 | |
* | |
* Unless required by applicable law or agreed to in writing, software | |
* distributed under the License is distributed on an "AS IS" BASIS, | |
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | |
* See the License for the specific language governing permissions and | |
* limitations under the License. | |
*/ | |
/** | |
* @module geojson2h3 | |
*/ | |
const h3 = require('h3-js'); | |
const FEATURE = 'Feature'; | |
const FEATURE_COLLECTION = 'FeatureCollection'; | |
const POLYGON = 'Polygon'; | |
const MULTI_POLYGON = 'MultiPolygon'; | |
// ---------------------------------------------------------------------------- | |
// Private utilities | |
/** | |
* Utility for efficient flattening of arrays. This mutates input, | |
* flattening into the first array in the list. | |
* @private | |
* @param {String[][]} arrays Arrays to flatten | |
* @return {String} Single array with all values from all input arrays | |
*/ | |
function flatten(arrays) { | |
let out = null; | |
for (let i = 0; i < arrays.length; i++) { | |
if (out !== null) { | |
for (let j = 0; j < arrays[i].length; j++) { | |
out.push(arrays[i][j]); | |
} | |
} else { | |
out = arrays[i]; | |
} | |
} | |
return Array.from(new Set(out)); | |
} | |
/** | |
* Utility to compute the centroid of a polygon, based on @turf/centroid | |
* @private | |
* @param {Number[][][]} polygon Polygon, as an array of loops | |
* @return {Number[]} lngLat Lng/lat centroid | |
*/ | |
function centroid(polygon) { | |
let lngSum = 0; | |
let latSum = 0; | |
let count = 0; | |
const loop = polygon[0]; | |
for (let i = 0; i < loop.length; i++) { | |
lngSum += loop[i][0]; | |
latSum += loop[i][1]; | |
count++; | |
} | |
return [lngSum / count, latSum / count]; | |
} | |
/** | |
* Convert a GeoJSON feature collection to a set of hexagons. Only hexagons whose centers | |
* fall within the features will be included. | |
* @private | |
* @param {Object} feature GeoJSON FeatureCollection | |
* @param {Number} resolution Resolution of hexagons, between 0 and 15 | |
* @return {String[]} H3 indexes | |
*/ | |
function featureCollectionToH3Set(featureCollection, resolution) { | |
const {features} = featureCollection; | |
if (!features) { | |
throw new Error('No features found'); | |
} | |
return flatten(features.map(feature => featureToH3Set(feature, resolution))); | |
} | |
// ---------------------------------------------------------------------------- | |
// Public API functions | |
/** | |
* Convert a GeoJSON feature to a set of hexagons. *Only hexagons whose centers | |
* fall within the feature will be included.* Note that conversion from GeoJSON | |
* is lossy; the resulting hexagon set only approximately describes the original | |
* shape, at a level of precision determined by the hexagon resolution. | |
* | |
* If the polygon is small in comparison with the chosen resolution, there may be | |
* no cell whose center lies within it, resulting in an empty set. To fall back | |
* to a single H3 cell representing the centroid of the polygon in this case, use | |
* the `ensureOutput` option. | |
* | |
* ![featureToH3Set](./doc-files/featureToH3Set.png) | |
* @static | |
* @param {Object} feature Input GeoJSON: type must be either `Feature` or | |
* `FeatureCollection`, and geometry type must be | |
* either `Polygon` or `MultiPolygon` | |
* @param {Number} resolution Resolution of hexagons, between 0 and 15 | |
* @param {Object} [options] Options | |
* @param {Boolean} [options.ensureOutput] Whether to ensure that at least one | |
* cell is returned in the set | |
* @return {String[]} H3 indexes | |
*/ | |
function featureToH3Set(feature, resolution, options = {}) { | |
const {type, geometry} = feature; | |
const geometryType = geometry && geometry.type; | |
if (type === FEATURE_COLLECTION) { | |
return featureCollectionToH3Set(feature, resolution); | |
} | |
if (type !== FEATURE) { | |
throw new Error(`Unhandled type: ${type}`); | |
} | |
if (geometryType !== POLYGON && geometryType !== MULTI_POLYGON) { | |
throw new Error(`Unhandled geometry type: ${geometryType}`); | |
} | |
// Normalize to MultiPolygon | |
const polygons = geometryType === POLYGON ? [geometry.coordinates] : geometry.coordinates; | |
// Polyfill each polygon and flatten the results | |
return flatten( | |
polygons.map(polygon => { | |
const result = h3.polyfill(polygon, resolution, true); | |
if (result.length || !options.ensureOutput) { | |
return result; | |
} | |
// If we got no results, index the centroid | |
const [lng, lat] = centroid(polygon); | |
return [h3.geoToH3(lat, lng, resolution)]; | |
}) | |
); | |
} | |
/** | |
* Convert a single H3 hexagon to a `Polygon` feature | |
* @static | |
* @param {String} hexAddress Hexagon address | |
* @param {Object} [properties] Optional feature properties | |
* @return {Feature} GeoJSON Feature object | |
*/ | |
function h3ToFeature(h3Index, properties = {}) { | |
// Wrap in an array for a single-loop polygon | |
const coordinates = [h3.h3ToGeoBoundary(h3Index, true)]; | |
return { | |
type: FEATURE, | |
id: h3Index, | |
properties, | |
geometry: { | |
type: POLYGON, | |
coordinates | |
} | |
}; | |
} | |
/** | |
* Convert a set of hexagons to a GeoJSON `Feature` with the set outline(s). The | |
* feature's geometry type will be either `Polygon` or `MultiPolygon` depending on | |
* the number of outlines required for the set. | |
* | |
* ![h3SetToFeature](./doc-files/h3SetToFeature.png) | |
* @static | |
* @param {String[]} hexagons Hexagon addresses | |
* @param {Object} [properties] Optional feature properties | |
* @return {Feature} GeoJSON Feature object | |
*/ | |
function h3SetToFeature(hexagons, properties = {}) { | |
const polygons = h3.h3SetToMultiPolygon(hexagons, true); | |
// See if we can unwrap to a simple Polygon. | |
const isMultiPolygon = polygons.length > 1; | |
const type = isMultiPolygon ? MULTI_POLYGON : POLYGON; | |
// MultiPolygon, single polygon, or empty array for an empty hex set | |
const coordinates = isMultiPolygon ? polygons : polygons[0] || []; | |
return { | |
type: FEATURE, | |
properties, | |
geometry: { | |
type, | |
coordinates | |
} | |
}; | |
} | |
/** | |
* Convert a set of hexagons to a GeoJSON `MultiPolygon` feature with the | |
* outlines of each individual hexagon. | |
* | |
* ![h3SetToMultiPolygonFeature](./doc-files/h3SetToFeatureCollection.png) | |
* @static | |
* @param {String[]} hexagons Hexagon addresses | |
* @param {Object} [properties] Optional feature properties | |
* @return {Feature} GeoJSON Feature object | |
*/ | |
function h3SetToMultiPolygonFeature(hexagons, properties = {}) { | |
const coordinates = hexagons.map(h3Index => | |
// Wrap in an array for a single-loop polygon | |
[h3.h3ToGeoBoundary(h3Index, {geoJson: true})] | |
); | |
return { | |
type: FEATURE, | |
properties, | |
geometry: { | |
type: MULTI_POLYGON, | |
coordinates | |
} | |
}; | |
} | |
/** | |
* Convert a set of hexagons to a GeoJSON `FeatureCollection` with each hexagon | |
* in a separate `Polygon` feature with optional properties. | |
* | |
* ![h3SetToFeatureCollection](./doc-files/h3SetToFeatureCollection.png) | |
* @static | |
* @param {String[]} hexagons Hexagon addresses | |
* @param {Function} [getProperties] Optional function returning properties | |
* for a hexagon: f(h3Index) => Object | |
* @return {FeatureCollection} GeoJSON FeatureCollection object | |
*/ | |
function h3SetToFeatureCollection(hexagons, getProperties) { | |
const features = []; | |
for (let i = 0; i < hexagons.length; i++) { | |
const h3Index = hexagons[i]; | |
const properties = getProperties ? getProperties(h3Index) : {}; | |
features.push(h3ToFeature(h3Index, properties)); | |
} | |
return { | |
type: FEATURE_COLLECTION, | |
features | |
}; | |
} | |
module.exports = { | |
featureToH3Set, | |
h3ToFeature, | |
h3SetToFeature, | |
h3SetToMultiPolygonFeature, | |
h3SetToFeatureCollection | |
}; |
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