WIP on feature deduplication across tile boundaries

It seems like the ids stored in the features are not reliable, so I'm trying
to generate ids

modules/util/util.js

@@ -266,3 +266,19 @@ export function utilNoAuto(selection) {
         .attr('autocapitalize', 'off')
         .attr('spellcheck', isText ? 'true' : 'false');
 }
+
+
+// https://stackoverflow.com/questions/194846/is-there-any-kind-of-hash-code-function-in-javascript
+// https://werxltd.com/wp/2010/05/13/javascript-implementation-of-javas-string-hashcode-method/
+export function utilHashcode(str) {
+    var hash = 0;
+    if (str.length === 0) {
+        return hash;
+    }
+    for (var i = 0; i < str.length; i++) {
+        var char = str.charCodeAt(i);
+        hash = ((hash << 5) - hash) + char;
+        hash = hash & hash; // Convert to 32bit integer
+    }
+    return hash;
+}