http://nabetani.sakura.ne.jp/yokohamarb/103mask/ の解答

a.rb

require "minitest/autorun"
require "json"

def solve(src)
  src_i = src.to_i(16)
  out = []
  1.upto src_i do |i|
    if (src_i & i ) == i
      out << i
    end
  end
  if out.length > 15
    out[0..12].join(",") + ",...," + out[-2].to_s + "," + out[-1].to_s
  else
    out.join(",")
  end
end

if ! ARGV[0] || ! File.exist?( ARGV[0] )
  raise "you should specify json file as ARGV[0]"
end

class TestYokohamaRb103 < Minitest::Test
  json_string = File.open( ARGV[0], &:read )
  data = JSON.parse( json_string, symbolize_names:true )
  data[:test_data].each do | number:, src:, expected: |
    define_method( :"test_#{number}" ) do
      actual = solve(src)
      assert_equal( expected, actual )
    end
  end
end