gfm_code_block.rb

#!/usr/bin/env ruby

# Converts a markdown file from 4-space code blocks to ``` gated code blocks.
# aka, normal markdown code blocks to Github Formatted markdown code blocks.
#
# Usage:
#
#   ruby gfm_code_block.rb path/to/file.md
#

require "tempfile"
require "pathname"

DELIM = "    "

def gfmify(input, output)
  input.each_line.each_cons(3) do |(prev_line, line, next_line)|
    # If we've not written anything, print out prev line
    # Bug - we don't handle `    ` being on the first line of file
    if output.pos.zero?
      output.print prev_line
    end

    if line.start_with?(DELIM)
      # Ticks go before first line in block
      output.puts "```" unless prev_line.start_with?(DELIM)

      # De-indent one tabstop
      line = line[4..-1]

      # Ticks come after last line in block
      line << "```\n" unless next_line.start_with?(DELIM)
    end

    output.print line

    # Make sure to output the last line
    output.print next_line if input.eof?
  end
end

ARGV.each do |path|
  path = Pathname.new(path)
  Tempfile.open(path.basename.to_s) do |tmpfile|
    gfmify path.open("r"), tmpfile

    FileUtils.mv tmpfile.path, path.to_s
  end
end