test out truncating a file in place

truncate_in_place.rb

#!/usr/bin/env ruby

log = "big.log"
words = %w(foo bar baz blech mumble what no ack aiee)

File.open(log, 'w') do |file|
  1000.times do |n|
    content = (0..(rand(100) + 10)).map { words[rand(words.size)] }
    file.puts "#{n} #{content.join ' '}"
  end
end

reader = File.open(log, 'r')
writer = File.open(log, 'r+')
writer.rewind

# assume no overlap between reading and writing!
reader.each_line.with_index do |line, n|
  next if n < 10
  writer.puts line
end

writer.truncate(writer.pos)

reader.close
writer.close