Add a contributor counting actor

Logic/ContributorCount.ts

@@ -0,0 +1,21 @@
+/// Given a feature source, calculates a list of OSM-contributors who mapped the latest versions
+import FeatureSource from "./FeatureSource/FeatureSource";
+import {UIEventSource} from "./UIEventSource";
+
+export default class ContributorCount {
+
+    public readonly Contributors: UIEventSource<Map<string, number>>;
+
+    constructor(featureSource: FeatureSource) {
+        this.Contributors = featureSource.features.map(features => {
+            const hist = new Map<string, number>();
+            for (const feature of features) {
+                const contributor = feature.feature.properties["_last_edit:contributor"]
+                const count = hist.get(contributor) ?? 0;
+                hist.set(contributor, count + 1)
+            }
+            return hist;
+        })
+    }
+
+}