Prototype string replacer that parses the input character-by-character. Final implementation to be written in Go.

file_string_replacer.rb

# To test, create a text file called test.txt in the current directory with the following content:
#
# Hello foo...
#
# Then run: ruby file_string_replacer.rb
# It will output send the modified version of the file contents to STDOUT

class FileStringReplacer
  attr_accessor :file_path, :char

  def initialize(file_path)
    self.file_path = file_path
    @chunk_index = 0
    @match_index = nil
  end

  def file
    @file ||= File.open(file_path, 'r')
  end

  def next_chunk
    if file.eof?
      @eof = true
      return nil
    end
    @chunk_index = 0
    @chunk = file.read(4)
    @eof = true if file.eof?
    @chunk
  end

  def eof?
    @eof ? true : false
  end

  def next_char
    if @chunk.nil? || @chunk_index >= @chunk.length
      @chunk = next_chunk
    end

    return nil if @chunk.nil?

    @char = @chunk[@chunk_index]
    @chunk_index += 1
    @char
  end

  def replace(pattern, replacement)
    while ch = next_char
      quit unless ch
      if ch == pattern[0]
        parse_match(pattern, replacement)
      else
        send(ch)
      end
    end
    quit
  end

  def send(str)
    print str
  end

  def quit
    file.close
  end

  def parse_match(pattern, replacement)
    match_buf = ''
    @match_index = 0
    while char && pattern[@match_index] && char == pattern[@match_index]
      match_buf += char
      @match_index += 1
      next_char
    end

    if match_buf.length == pattern.length
      # we found a match
      send(replacement)
    end

    send(char) if char
  end
end

r = FileStringReplacer.new('test.txt')
r.replace('foo', '0')