ioquatix/process-daemon

# Copyright, 2014, by Samuel G. D. Williams. <http://www.codeotaku.com>
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.

module Process
    class Daemon
        # This is a special file instance which provides the ability to read a log file from the end backwards.
        class LogFile < File
            # Yields the lines of a log file in reverse order, once the yield statement returns true, stops, and returns the lines in order.
            def tail_log
                lines = []

                seek_end
                
                reverse_each_line do |line|
                    lines << line
                    
                    break if block_given? and yield line
                end

                return lines.reverse
            end
            
            private
            
            # Seek to the end of the file
            def seek_end(offset = 0)
                seek(offset, IO::SEEK_END)
            end
    
            # Read a chunk of data and then move the file pointer backwards.
            #
            # Calling this function multiple times will return new data and traverse the file backwards.
            #
            def read_reverse(length)
                offset = tell

                if offset == 0
                    return nil
                end

                start = [0, offset-length].max

                seek(start, IO::SEEK_SET)

                buf = read(offset-start)

                seek(start, IO::SEEK_SET)

                return buf
            end

            REVERSE_BUFFER_SIZE = 128

            # This function is very similar to gets but it works in reverse.
            #
            # You can use it to efficiently read a file line by line backwards.
            # 
            # It returns nil when there are no more lines.
            def reverse_gets(sep_string=$/)
                end_pos = tell

                offset = nil
                buf = ""

                while offset == nil
                    chunk = read_reverse(REVERSE_BUFFER_SIZE)

                    return (buf == "" ? nil : buf) if chunk == nil

                    buf = chunk + buf

                    # Don't consider the last newline.
                    offset = buf.rindex(sep_string, -(sep_string.length + 1))
                end

                # Don't include newline:
                offset += 1

                line = buf[offset...buf.size]

                seek((end_pos - buf.size) + offset, IO::SEEK_SET)

                return line
            end

            # Similar to each_line but works in reverse. Don't forget to call
            # seek_end before you start!
            def reverse_each_line(sep_string=$/, &block)
                return to_enum(:reverse_each_line) unless block_given?
        
                line = reverse_gets(sep_string)

                while line != nil
                    yield line

                    line = reverse_gets(sep_string)
                end
            end
        end
    end
end