/**
 * Return an array equal to the given one, but with duplicate items removed.
 * @param items The array to deduplicate. Not mutated.
 * @param getItemId Get the identifier to compare sameness of two items.
 * Defaults to the item itself, so sameness means strict equality
 * (`item1 === item2`).
 * @param merge Callback function to merge duplicate items if desired; the
 * returned value will take the place of the first item.
 * @returns A new array.
 */
export function unique<T>(
  items: T[],
  getItemId: (item: T) => any = (item) => item,
  merge?: (item1: T, item2: T) => T,
): T[] {
  const seen = new Set();
  const newItems: T[] = [];
  for (const item of items) {
    const id = getItemId(item);
    if (id === undefined || !seen.has(id)) {
      newItems.push(item);
      seen.add(id);
    } else if (merge) {
      const indexOfConflictingItem = newItems.findIndex(
        (newItem) => getItemId(newItem) === id,
      );
      newItems[indexOfConflictingItem] = merge(
        newItems[indexOfConflictingItem],
        item,
      );
    }
  }
  return newItems;
}