feat: borders

src/borders.js

@@ -0,0 +1,40 @@
+import * as topojsonclient from "topojson-client";
+import * as topojsonserver from "topojson-server";
+const topojson = Object.assign({}, topojsonclient, topojsonserver);
+
+export function borders({ geojson, id, values }) {
+  const topo = topojson.topology({ d: geojson });
+  const ids = geojson.features.map((d) => d.properties[id]);
+  const neighbors = topojson.neighbors(topo.objects["d"].geometries);
+  const valbyid = new Map(
+    geojson.features.map((d) => [d.properties[id], d.properties[values]])
+  );
+
+  let result = [];
+  ids.forEach((e) => {
+    let r = neighbors[ids.indexOf(e)].map((i) => ({
+      properties: {
+        i: e,
+        j: ids[i],
+        var_i: +valbyid.get(e),
+        var_j: +valbyid.get(ids[i]),
+        rel: +valbyid.get(e) / +valbyid.get(ids[i]),
+        abs: +valbyid.get(e) - +valbyid.get(ids[i])
+      }
+    }));
+    r.forEach((e) => {
+      let geom = topojson.mesh(
+        topo,
+        topo.objects["d"],
+        (a, b) =>
+          (a.properties[id] == e.properties.i) &
+          (b.properties[id] == e.properties.j)
+      );
+
+      let output = Object.assign({ type: "Feature" }, e);
+      result.push(Object.assign(output, { geometry: geom }));
+    });
+  });
+
+  return { type: "FeatureCollection", features: result };
+}

src/index.js

@@ -4,6 +4,7 @@ export {merge} from "./merge.js";
 export {quickdraw} from "./quickdraw.js";
 export {table2geo} from "./table2geo.js";
 export {links} from "./links.js";
+export {borders} from "./borders.js";
 
 // layer functions