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Using an Array or Other Modifiable Object as a Hash Key
CodeExpectedActual
coordinates = [10, 5]
treasure_map = { coordinates => 'jewels' }
treasure_map[coordinates]
 "jewels" "jewels" 
coordinates << -5
coordinates
 [10, 5, -5] [10, 5, -5] 
treasure_map[coordinates]
 nil nil 
treasure_map.rehash
treasure_map[coordinates]
 "jewels" "jewels" 
module ReliablyHashable
  def hash
    return object_id
  end
end
class ReliablyHashableArray < Array
  include ReliablyHashable
end
coordinates = ReliablyHashableArray.new([10,5])
treasure_map = { coordinates => 'jewels' }
treasure_map[coordinates]
 "jewels" "jewels" 
coordinates.push(-5)
treasure_map[coordinates]
 "jewels" "jewels" 
a = [1,2]
b = a.clone
a.hash
 11 11 
b.hash
 11 11 
a  = ReliablyHashableArray.new([1,2])
b = a.clone
a.hash
 -606031406 -606028286 
b.hash
 -606034266 -606031146