import { Enumerable, IEnumerable } from '../enumerable'
import { EqualityComparer, IEqualityComparer } from '../equality-comparer'
import { throws } from '../utils'
import * as assistance from './assistance'

export function union<TSource>(first: Iterable<TSource>, second: Iterable<TSource>, comparer?: IEqualityComparer<TSource>): IEnumerable<TSource> {
  throws.ThrowIfNull('first', first)
  throws.ThrowIfNull('second', second)
  comparer = comparer || EqualityComparer.Default()
  return Enumerable.AsEnumerable(assistance.distinct(assistance.concat(first, second), (x, y) => comparer.Equals(x, y)))
}