Union by Array#&, Set#&, and Set#intersection

union-array-ampersand-vs-set.rb

require 'benchmark/ips'

X = [5, 5, 1, 1]
Y = [1, 2, 5]

def fast
  X & Y
end

def slow
  require "set"
  Set.new(X) & Set.new(Y)
end

def slowest
  require "set"
  Set.new(X).intersection Set.new(Y)
end

Benchmark.ips do |x|
  x.report('Union by Array#&')          { fast    }
  x.report('Union by Set#&')            { slow    }
  x.report('Union by Set#intersection') { slowest }
  x.compare!
end

$ ruby -v union-array-vs-set.rb
ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin14]

Calculating -------------------------------------
    Union by Array#&      73.748k i/100ms
      Union by Set#&       9.078k i/100ms
Union by Set#intersection  8.956k i/100ms
-------------------------------------------------
    Union by Array#&        1.402M (± 7.4%) i/s -      7.006M
      Union by Set#&      100.719k (± 7.5%) i/s -    508.368k
Union by Set#intersection 105.286k (± 4.8%) i/s -    528.404k

Comparison:
    Union by Array#&:        1401564.1 i/s
Union by Set#intersection:    105285.9 i/s - 13.31x slower
      Union by Set#&:         100719.1 i/s - 13.92x slower