Simple approach of parsing a large logfile

parser_1.rb

require 'time'

class LogfileParser

  # Regexp to match the timestamps in the apache common log format
  TIME_REGEXP   = /\[\d{2}\/\w{3}\/\d{4}\:\d{2}:\d{2}:\d{2}\s.{5}\]/

  def initialize path, starting_at
    raise ArgumentError unless File.exists?( path )

    @log          = File.open( path )
    @starting_at  = Time.parse( starting_at )
    @ending_at    = (@starting_at + 300)
  end

  def emit &block
    @log.each_line do |line|
      next unless timestamp = line.match( TIME_REGEXP )

      current_time = Time.parse( timestamp[0].sub(":", " ") )

      if current_time >= @starting_at && current_time <= @ending_at
        yield line
      end

      if current_time > @ending_at
        return
      end
    end
  end

end

parser = LogfileParser.new( *ARGV )

parser.emit {|l| puts l}