package linq

// Except produces the set difference of two sequences.
// The set difference is the members of the first sequence
// that don't appear in the second sequence.
func (q Query) Except(q2 Query) Query {
	return Query{
		Iterate: func() Iterator {
			next := q.Iterate()

			next2 := q2.Iterate()
			set := make(map[interface{}]bool)
			for i, ok := next2(); ok; i, ok = next2() {
				set[i] = true
			}

			return func() (item interface{}, ok bool) {
				for item, ok = next(); ok; item, ok = next() {
					if _, has := set[item]; !has {
						return
					}
				}

				return
			}
		},
	}
}

// ExceptBy invokes a transform function on each element of a collection
// and produces the set difference of two sequences.
// The set difference is the members of the first sequence
// that don't appear in the second sequence.
func (q Query) ExceptBy(
	q2 Query, selector func(interface{}) interface{}) Query {
	return Query{
		Iterate: func() Iterator {
			next := q.Iterate()

			next2 := q2.Iterate()
			set := make(map[interface{}]bool)
			for i, ok := next2(); ok; i, ok = next2() {
				s := selector(i)
				set[s] = true
			}

			return func() (item interface{}, ok bool) {
				for item, ok = next(); ok; item, ok = next() {
					s := selector(item)
					if _, has := set[s]; !has {
						return
					}
				}

				return
			}
		},
	}
}