Yields the last 5 minutes of a pound log file

parser_5.rb

require 'time'
require 'stringio'

class LogfileParser

  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 )
    @ending_at      = Time.parse( starting_at )
    @starting_at    = (@ending_at - 300)
  end

  def backwards
    filesize      = @log.stat.size
    buffer_size   = 128000
    offset        = buffer_size

    @log.seek(0, File::SEEK_END)

    while @log.tell > 0
      @log.seek(-offset, File::SEEK_END)

      # Making the buffer a StringIO to get #tell() for calculating the offset
      buffer = StringIO.new( @log.read( buffer_size ) )

      minute      = @starting_at.strftime("%M").chars.first
      time_range  = Regexp.new(
        @starting_at.strftime(
          "\\[%d\\/%b\\/%Y\\:%H\\:%M\\:%S\\s.{5}\\]"
        )
      )

      buffer.each_line do |line|
        if timestamp = line.match( time_range )

          # Calculate the final offset
          offset = filesize-([email protected])-buffer_size+buffer.tell
          @log.seek(offset, File::SEEK_SET)
          return
        end
      end

      offset += buffer_size

      return if offset > filesize
    end

  end

  def emit &block
    backwards

    @log.each_line do |line|
      yield line
    end
  end

end

parser = LogfileParser.new( *ARGV )
parser.emit {|l| puts l }