gistfile1.rb

require 'fileutils'

File.open("./numeric.c", "r") do |klass|
  File.open("./numeric.tmp", "w") do |tmpfile|
    while line = klass.gets
      tmpfile.write line
      if line =~ /fix_to_s\(int argc, VALUE \*argv, VALUE x\)/
        2.times {tmpfile.write klass.readline}
        tmpfile.write <<PATCH
    int val;
    val = FIX2INT(x);
    if (val % 15 == 0) return rb_str_new2("FizzBuzz");
    if (val %  3 == 0) return rb_str_new2("Fizz");
    if (val %  5 == 0) return rb_str_new2("Buzz");
PATCH
      end
    end
  end
end

FileUtils.mv("numeric.tmp", "numeric.c")

system('make')
system('./ruby -ve "(1..100).each{|i| puts i}"')