offsetRange

src/mark.js

@@ -31,7 +31,7 @@ export class Mark {
         index = [], data = [];
         for (const facet of facets) {
           const facetData = this.transform(take(this.data, facet));
-          const facetIndex = facetData === undefined ? undefined : range(facetData).map(i => i + data.length); // TODO optimize
+          const facetIndex = facetData === undefined ? undefined : offsetRange(facetData, data.length);
           index.push(facetIndex);
           data.push(...facetData); // TODO optimize
         }
@@ -161,6 +161,12 @@ export function range(data) {
   return Uint32Array.from(data, indexOf);
 }
 
+// Returns a Uint32Array with elements [k, k + 1, … k + data.length - 1].
+export function offsetRange(data, k) {
+  k = Math.floor(k);
+  return Uint32Array.from(data, (_, i) => i + k);
+}
+
 // Returns an array [values[index[0]], values[index[1]], …].
 export function take(values, index) {
   return Array.from(index, i => values[i]);

src/transforms/bin.js

@@ -1,5 +1,5 @@
 import {bin as binner, cross, sum} from "d3-array";
-import {valueof, first, second, maybeValue, range} from "../mark.js";
+import {valueof, first, second, maybeValue, range, offsetRange} from "../mark.js";
 
 export function bin1(options = {}) {
   let {value, domain, thresholds, cumulative} = maybeValue(options);
@@ -16,7 +16,7 @@ export function bin1(options = {}) {
         let b = bins.map(bin => binsubset(bin, facet));
         b = cumulative ? accumulate(cumulative < 0 ? b.reverse() : b) : b;
         b = b.filter(nonempty);
-        index.push(range(b).map(i => i + data.length)); // TODO optimize
+        index.push(offsetRange(b, data.length));
         data.push(...b); // TODO optimize
       }
       return {index, data};