// Not `Map.groupBy`: the relay runs this core on Node 18, which lacks it.
export function groupedBy<T, K>(items: readonly T[], keyOf: (item: T) => K): Map<K, T[]> {
  const groups = new Map<K, T[]>()
  for (const item of items) {
    const key = keyOf(item)
    const group = groups.get(key)
    if (group) {
      group.push(item)
    } else {
      groups.set(key, [item])
    }
  }
  return groups
}