Index: node_modules/d3-array/src/group.js
===================================================================
--- node_modules/d3-array/src/group.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-array/src/group.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,65 @@
+import {InternMap} from "internmap";
+import identity from "./identity.js";
+
+export default function group(values, ...keys) {
+  return nest(values, identity, identity, keys);
+}
+
+export function groups(values, ...keys) {
+  return nest(values, Array.from, identity, keys);
+}
+
+function flatten(groups, keys) {
+  for (let i = 1, n = keys.length; i < n; ++i) {
+    groups = groups.flatMap(g => g.pop().map(([key, value]) => [...g, key, value]));
+  }
+  return groups;
+}
+
+export function flatGroup(values, ...keys) {
+  return flatten(groups(values, ...keys), keys);
+}
+
+export function flatRollup(values, reduce, ...keys) {
+  return flatten(rollups(values, reduce, ...keys), keys);
+}
+
+export function rollup(values, reduce, ...keys) {
+  return nest(values, identity, reduce, keys);
+}
+
+export function rollups(values, reduce, ...keys) {
+  return nest(values, Array.from, reduce, keys);
+}
+
+export function index(values, ...keys) {
+  return nest(values, identity, unique, keys);
+}
+
+export function indexes(values, ...keys) {
+  return nest(values, Array.from, unique, keys);
+}
+
+function unique(values) {
+  if (values.length !== 1) throw new Error("duplicate key");
+  return values[0];
+}
+
+function nest(values, map, reduce, keys) {
+  return (function regroup(values, i) {
+    if (i >= keys.length) return reduce(values);
+    const groups = new InternMap();
+    const keyof = keys[i++];
+    let index = -1;
+    for (const value of values) {
+      const key = keyof(value, ++index, values);
+      const group = groups.get(key);
+      if (group) group.push(value);
+      else groups.set(key, [value]);
+    }
+    for (const [key, values] of groups) {
+      groups.set(key, regroup(values, i));
+    }
+    return map(groups);
+  })(values, 0);
+}