import { ensureEntryInMap } from "@sanjo/ensure-entry-in-map";

export function groupByToMap<T, GroupingType>(
  set: Set<T>,
  predicate: (element: T) => GroupingType
): Map<GroupingType, Set<T>> {
  const groups = new Map<GroupingType, Set<T>>();
  const createDefaultValue = () => new Set<T>();
  for (const element of set) {
    const groupValue = predicate(element);
    ensureEntryInMap(groups, groupValue, createDefaultValue);
    const group = groups.get(groupValue)!;
    group.add(element);
  }
  return groups;
}