Issue #11815 has been updated by marcandre (Marc-Andre Lafortune).

Note that `Enumerable#tally` make many of these tasks based on the number of appearances reasonably easy to do. In the examples of the original poster:

Determine if two arrays of the same size contain the same elements

```ruby
a = [2,1,4,2,5,3,3,1]
b = [3,4,1,1,2,3,5,2]
a.difference(b).empty?
#=> true

# Use tally:
a.tally == b.tally
# => true
# assuming they can't be sorted, otherwise using sort works too:
a.sort == b.sort
```

Identify an array's unique elements

```ruby
a = [1,3,2,4,3,4]
u = a.uniq #=> [1, 2, 3, 4]
u - a.difference(u) #=> [1, 2]

# Use tally:
a.tally.select {|k, nb| nb == 1}.keys # => [1, 2]
# Makes it easy to select elements on a diffent criteria, e.g. == 2 for exactly duplicated, etc.
```

Identify a maximal number of 1-1 matches between the elements of two arrays and return an array of all elements from both arrays that were not matched

```
a = [1, 2, 4, 2, 1, 7, 4, 2, 9]
b = [4, 7, 3, 2, 2, 7]
a.difference(b).concat(b.difference(a))
#=> [1, 1, 4, 2, 9, 3, 7]

a.tally.merge(b.tally) { |_, nb_a, nb_b| nb_a - nb_b }.flat_map { |obj, n| [obj] * n.abs }
```

----------------------------------------
Feature #11815: Proposal for method `Array#difference`
https://bugs.ruby-lang.org/issues/11815#change-86626

* Author: CaryInVictoria (Cary Swoveland)
* Status: Open
* Priority: Normal
----------------------------------------
I propose that a method `Array#difference` be added to the Ruby core. *(`Array#difference`, which appears to be an alias of `Array#-`, was added to Ruby 2.6. Accordingly, I would propose that this method be named `remove`, but I will not make any changes here.)*  It is similar to [Array#-](http://ruby-doc.org/core-2.2.0/Array.html#method-i-2D) but for each element of the (array) argument it removes only one matching element from the receiver. For example:

a = [1,2,3,4,3,2,2,4]
b = [2,3,4,4,4]

a - b #=> [1]
c = a.difference b #=> [1, 3, 2, 2]

As you see, `a` contains three `2`'s and `b` contains `1`, so the first `2` in `a` has been removed from `a` in constructing `c`. When `b` contains as least as many instances of an element as does `a`, `c` contains no instances of that element.

It could be implemented as follows:

class Array
def difference(other)
h = other.each_with_object(Hash.new(0)) { |e,h| h[e] += 1 }
reject { |e| h[e] > 0 && h[e] -= 1 }
end
end

Here are a few examples of its use:

*Determine if two arrays of the same size contain the same elements*

a = [2,1,4,2,5,3,3,1]
b = [3,4,1,1,2,3,5,2]
a.difference(b).empty?
#=> true

*Identify an array's unique elements*

a = [1,3,2,4,3,4]
u = a.uniq #=> [1, 2, 3, 4]
u - a.difference(u) #=> [1, 2]

*Identify a maximal number of 1-1 matches between the elements of two arrays and return an array of all elements from both arrays that were not matched*

a = [1, 2, 4, 2, 1, 7, 4, 2, 9]
b = [4, 7, 3, 2, 2, 7]
a.difference(b).concat(b.difference(a))
#=> [1, 1, 4, 2, 9, 3, 7]

To remove elements from `a` starting at the end (rather the beginning) of `a`:

a = [1,2,3,4,3,2,2,4]
b = [2,3,4,4,4]

a.reverse.difference(b).reverse #=> [1,2,3,2]

`Array#difference!` could be defined in the obvious way.