gistfile1.rb

require 'benchmark'

F = Struct.new(:name, :size) do
  def grouping
    "#{name}/#{size}"
  end
end

f0 = F.new("a.txt", 110)
f1 = F.new("a.txt", 10)
f2 = F.new("b.txt", 10)
f3 = F.new("a.txt", 110)
f4 = F.new("d.txt", 11)

FILES = [f0, f1, f2, f3, f4]

N = 100_000

Benchmark.bm do |x|
  x.report("block")        { N.times { Hash[FILES.group_by{|e|"#{e.name}/#{e.size}"}.select{|k,v| v.size > 1}.collect{|k,v| [k.split('/')[0],v]}] } }
  x.report("method")       { N.times { Hash[FILES.group_by(&:grouping).select{|k,v| v.size > 1}.collect{|k,v| [k.split('/')[0],v]}] } }
  x.report("block reject") { N.times { Hash[FILES.group_by{|e|"#{e.name}/#{e.size}"}.reject{|k,v| v.size <= 1}.collect{|k,v| [k.split('/')[0],v]}] } }
  x.report("inject")       { N.times { FILES.group_by{|e|"#{e.name}/#{e.size}"}.inject({}){|res, (k, v)| res[k.split('/')[0]] = v if v.size > 1; res}  } }
end