How do I do an integer list intersection while keeping duplicates?

Question

I'm working on a Greatest Common Factor and Least Common Multiple assignment and I have to list the common factors. Intersection() won't work because that removes duplicates. Contains() won't work because if it sees the int in the second list it returns all matching ints from the first list. Is there a way to do an Intersection that is not Distinct?

edit: sorry for not providing an example, here is what I meant:

if I have the sets:

{1, 2, 2, 2, 3, 3, 4, 5}
{1, 1, 2, 2, 3, 3, 3, 4, 4}

I would want the output

{1, 2, 2, 3, 3, 4}

Answer 1

I wrote this extension to solve the problem:

public static IEnumerable<T> Supersect<T>(this IEnumerable<T> a, ICollection<T> b)
              => a.Where(b.Remove);

example:

var a = new List<int> { 1, 2, 2, 2, 3, 3, 4, 5 };
var b = new List<int> { 1, 1, 2, 2, 3, 3, 3, 4, 4};

var result = a.Supersect(b);

result:

{ 1, 2, 2, 3, 3, 4 }

Answer 2

ILookup<int, int> lookup1 = list1.ToLookup(i => i);
ILookup<int, int> lookup2 = list2.ToLookup(i => i);

int[] result =
(
  from group1 in lookup1
  let group2 = lookup2[group1.Key]
  where group2.Any()
  let smallerGroup = group1.Count() < group2.Count() ? group1 : group2
  from i in smallerGroup
  select i
).ToArray();

The where expression is technically optional, I feel it makes the intent clearer.

---

If you want more terse code:

ILookup<int, int> lookup2 = list2.ToLookup(i => i);

int[] result =
(
  from group1 in list1.GroupBy(i => i)
  let group2 = lookup2[group1.Key]
  from i in (group1.Count() < group2.Count() ? group1 : group2)
  select i
).ToArray();