MapComplete/Logic/ExtraFunction.ts

import {GeoOperations} from "./GeoOperations";
import {UIElement} from "../UI/UIElement";
import Combine from "../UI/Base/Combine";

export class ExtraFunction {


    static readonly intro = `<h2>Calculating tags with Javascript</h2>

<p>In some cases, it is useful to have some tags calculated based on other properties. Some useful tags are available by default (e.g. <b>lat</b>, <b>lon</b>, <b>_country</b>), as detailed above.</p>

<p>It is also possible to calculate your own tags - but this requires some javascript knowledge. </p>

Before proceeding, some warnings:

<ul>
<li> DO NOT DO THIS AS BEGINNER</li>
<li> <b>Only do this if all other techniques fail</b>. This should <i>not</i> be done to create a rendering effect, only to calculate a specific value</li>
<li> <b>THIS MIGHT BE DISABLED WITHOUT ANY NOTICE ON UNOFFICIAL THEMES</b>. As unofficial themes might be loaded from the internet, this is the equivalent of injecting arbitrary code into the client. It'll be disabled if abuse occurs.</li>
</ul>
In the layer object, add a field <b>calculatedTags</b>, e.g.:

<div class="code">
  "calculatedTags": [
    "_someKey=javascript-expression",
    "name=feat.properties.name ?? feat.properties.ref ?? feat.properties.operator",
    "_distanceCloserThen3Km=feat.distanceTo( some_lon, some_lat) < 3 ? 'yes' : 'no'" 
  ]
</div>

The above code will be executed for every feature in the layer. The feature is accessible as <b>feat</b> and is an amended geojson object:
- <b>area</b> contains the surface area (in square meters) of the object
- <b>lat</b> and <b>lon</b> contain the latitude and longitude

Some advanced functions are available on <b>feat</b> as well:

`
    private static OverlapFunc = new ExtraFunction(
        "overlapWith",
        "Gives a list of features from the specified layer which this feature overlaps with, the amount of overlap in m². The returned value is <b>{ feat: GeoJSONFeature, overlap: number}</b>",
        ["...layerIds - one or more layer ids  of the layer from which every feature is checked for overlap)"],
        (featuresPerLayer, feat) => {
            return (...layerIds: string[]) => {
                const result = []
                for (const layerId of layerIds) {
                    const otherLayer = featuresPerLayer.get(layerId);
                    if (otherLayer === undefined) {
                        continue;
                    }
                    if (otherLayer.length === 0) {
                        continue;
                    }
                    result.push(...GeoOperations.calculateOverlap(feat, otherLayer));
                }
                return result;
            }
        }
    )
    private static DistanceToFunc = new ExtraFunction(
        "distanceTo",
        "Calculates the distance between the feature and a specified point",
        ["longitude", "latitude"],
        (featuresPerLayer, feature) => {
            return (lon, lat) => {
                // Feature._lon and ._lat is conveniently place by one of the other metatags
                return GeoOperations.distanceBetween([lon, lat], [feature._lon, feature._lat]);
            }
        }
    )
    private static readonly allFuncs: ExtraFunction[] = [ExtraFunction.DistanceToFunc, ExtraFunction.OverlapFunc];
    private readonly _name: string;
    private readonly _args: string[];
    private readonly _doc: string;
    private readonly _f: (featuresPerLayer: Map<string, any[]>, feat: any) => any;

    constructor(name: string, doc: string, args: string[], f: ((featuresPerLayer: Map<string, any[]>, feat: any) => any)) {
        this._name = name;
        this._doc = doc;
        this._args = args;
        this._f = f;

    }

    public static FullPatchFeature(featuresPerLayer: Map<string, any[]>, feature) {
        for (const func of ExtraFunction.allFuncs) {
            func.PatchFeature(featuresPerLayer, feature);
        }
    }

    public static HelpText(): UIElement {
        return new Combine([
            ExtraFunction.intro,
            "<ul>",
            ...ExtraFunction.allFuncs.map(func => 
            new Combine([
                "<li>", func._name, "</li>"
            ])
            ),
            "</ul>",
            ...ExtraFunction.allFuncs.map(func =>
                new Combine([
                    "<h3>" + func._name + "</h3>",
                    func._doc,
                    "<ul>",
                    ...func._args.map(arg => "<li>" + arg + "</li>"),
                    "</ul>"
                ])
            )
        ]);
    }

    public PatchFeature(featuresPerLayer: Map<string, any[]>, feature: any) {
        feature[this._name] = this._f(featuresPerLayer, feature);
    }
}