Reversing Many to Many Relationship array

Question

var x = {one:[1,2,3],two:[3],three:[2]}
var y = {1:['one'], 2:['three', 'one'], 3:['one', 'two']}

How can I turn var x to var y? Basically, x is of the form set->user_arrays , set->user_arrays , when I want to 'reverse' them and get user->set_arrays . Preferably the fastest solution, and functional (map etc.)

Answer 1

var x = {one:[1,2,3],two:[3],three:[2]}
var y = {};

for (var k in x) {
  for (var i = 0; i < x[k].length; i++) {
    y[x[k][i]] = y[x[k][i]] || [];
    y[x[k][i]].push(k);
  }
}

THE WORKING DEMO.