Method for splitting array into (element => remaining elements) pairs

Question

Given an array literal, I would like to create a hash where the keys are the elements from the array and the values are arrays containing the other / remaining elements.

Input:

[1, 2, 3]

Output:

{1=>[2, 3], 2=>[1, 3], 3=>[1, 2]}

It's easy if I introduce a variable:

arr = [1, 2, 3]
arr.map { |i| [i, arr - [i]] }.to_h

But with an array literal, the only solution I could come up with involves instance_exec or instance_eval, which seems hackish:

[1, 2, 3].instance_exec { map { |i| [i, self - [i]] } }.to_h

Am I overlooking a built-in method or an obvious solution? group_by, combination, permutation and partition don't seem to help.

Answer 1

I've come up with something like this:

[1,2,3].permutation.to_a.map{ |e| [e.shift, e] }.to_h

However this has a flaw: it assigns the same key many times, but since you don't care about the sequence of the elements inside this might be a "good enough" solution.