read_line_number.rb

def readA io, line_num
  line_num.times{io.gets}
  io.gets
end

def readB io, line_num
  size=65536
  buf=??*size
  while true
    io.read size, buf
    count = buf.count "\n" # こういうの速かったはず
    if line_num > count
      line_num -= count
    else
      lines = buf.lines
      line = lines[line_num]
      if lines.size == line_num + 1 && line[-1] != "\n"
        return line + io.gets.to_s
      else
        return line
      end
    end
  end
end

def create_file
  file_name = 'file'
  return if File.exist? file_name
  srand 0
  File.open file_name, 'w' do |f|
    1000000.times do |i|
      size = rand 0..100
      f.puts ['[', i, ' ', (i%10).to_s * size, ']'].join
    end
    f.write "last-line"
  end
end

def withio
  io = File.open 'file', 'r'
  yield io
ensure
  io.close
end
create_file

require 'benchmark'
p :readA
p Benchmark.realtime { withio { |io| readA(io, 10000) } }
p Benchmark.realtime { withio { |io| readA(io, 100000) } }
p Benchmark.realtime { withio { |io| readA(io, 1000000) } }

p :readB
p Benchmark.realtime { withio { |io| readB(io, 10000) } }
p Benchmark.realtime { withio { |io| readB(io, 100000) } }
p Benchmark.realtime { withio { |io| readB(io, 1000000) } }

p :sed
p Benchmark.realtime { `sed -n '10001{p;q;}' file` }
p Benchmark.realtime { `sed -n '100001{p;q;}' file` }
p Benchmark.realtime { `sed -n '1000001{p;q;}' file` }

__END__
:readA
0.00426099996548146
0.04655800003092736
0.4983699999283999
:readB
0.0007429999532178044
0.0027410000329837203
0.02360800001770258
:sed
0.01061000011395663
0.039450000040233135
0.32519200001843274