proposal: slices: add Intersect and IntersectFunc

[Delta456]

Proposal Details

Utility functions like Intersect and IntersectFunc for slices packages will be handy for several cases.

// Intersect returns the common elements between two slices.
func Intersect[S ~[]E](s1, s2 S) (S ~[]E) {
        var result S
	for _, item := range s2 {
		if slices.Contains(s1, item) {
			result = append(result, item)
		}
	}
}

// IntersectFunc returns the elements between two slices according to the functions.
func IntersectFunc[S ~[]E, E comparable](s1, s2 S, f func(s1, s2 E) bool) S {
	var result S
	for _, item := range s2 {
		for _, subItem := range s1 {
			if f(subItem, item) {
				result = append(result, item)
			}
		}
	}
	return result
}

[earthboundkid]

This is O(N^2). It should not be in the standard library.

[Delta456]

This is O(N^2). It should not be in the standard library.

So only less than O(N) are allowed in the standard library so that it doesn't affect the performance?

EDIT: I wonder if this can be optimized further because what I wrote is just the prototype.

[gophun]

They belong in a set package.

[Delta456]

They belong in a set package.

Yes, I saw that earlier but I didn't see it being further discussed so I thought no conclusion was drawn.

[gophun]

It's on hold pending the completion of the iterator design (#61897).