add Intersect() to compute the intersection of multiple slices

[ndrpnt]

Hi, I needed to compute the intersection of several slices. Algorithm is probably not optimal, but I thought that might interest you nonetheless.
Feel free to reword, rename, add an IntersectFunc[T comparable](eq func(T, T) bool, in ...[]T) []T variant…

[bradenaw]

Interesting. What was your use of this?

For what it's worth, you could make this a bit more efficient by using two map[T]int, one for the current list and one for the previous, and taking the min between the two at each step. Cuts down on needing a whole make([]int, len(in)) allocation for each element.

[ndrpnt]

For a given user, I had some (short) lists of permissions and resources, e.g.

{
  {
    "permission": "read",
    "resources": ["foo", "bar", "baz"]
  },
  {
    "permission": "write",
    "resources": ["bar", "baz"]
  },
  ...
}

and needed to compute the list of resources for which the user has both read and write permissions. In the end I went with a naive/obvious O(n^2) implementation. 🙈

Later on I stumbled upon your library and thought I'd write something better for fun and contribute it. Thanks for the optimization btw, should I update the PR with it?