@ahamove/polygon-lookup/README.md

A fork from pelias/polygon-lookup with updated dependencies

# polygon-lookup

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## Inspired by [Pelias Polygon Lookup](https://github.com/pelias/polygon-lookup) with updated dependencies and ESM

A data-structure for performing fast, accurate point-in-polygon intersections against (potentially very large) sets of
polygons. `PolygonLookup` builds an [R-tree](http://en.wikipedia.org/wiki/R-tree), or bounding-box spatial index, for its
polygons and uses it to quickly narrow down the set of candidate polygons for any given point. If there are any
ambiguities, it'll perform point-in-polygon intersections to identify the one that *really* intersects. `PolygonLookup`
operates entirely in memory, and works best for polygons with little overlap.

## API

##### `PolygonLookup(featureCollection)`

* `featureCollection` (**optional**): A GeoJSON collection to optionally immediately load with `.loadFeatureCollection()`.

##### `PolygonLookup.search(x, y, limit)`

Narrows down the candidate polygons by bounding-box, and then performs point-in-polygon intersections to identify the first n container polygon (with n = limit, even if more polygons really do intersect).

* `x`: the x-coordinate to search for
* `y`: the y-coordinate to search for
* `limit` **optional**: the upper bound for number of intersecting polygon found (default value is 1, -1 to return all intersecting polygons)
* `return`: the intersecting polygon if one was found; a GeoJson FeatureCollection if multiple polygons were found and limit > 1; otherwise, `undefined`.

##### `PolygonLookup.loadFeatureCollection(featureCollection)`

Stores a feature collection in this `PolygonLookup`, and builds a spatial index for it. The polygons and rtree can be
accessed via the `.polygons` and `.rtree` properties.

* `featureCollection` (**optional**): A GeoJSON collection containing some Polygons/MultiPolygons. Note that
    MultiPolygons will get expanded into multiple polygons.

## example usage

```javascript
import PolygonLookup = from 'polygon-lookup';
const featureCollection = {
 type: 'FeatureCollection',
 features: [{
  type: 'Feature',
  properties: { id: 'bar' },
  geometry: {
   type: 'Polygon',
   coordinates: [ [ [ 0, 1 ], [ 2, 1 ], [ 3, 4 ], [ 1, 5 ] ] ]
  }
 }]
};
const lookup = new PolygonLookup( featureCollection );
const poly = lookup.search( 1, 2 );
console.log( poly.properties.id ); // bar
```