Convert a Heroku .gems file to a Bundler Gemfile

gemfile-from-dotgems.rb

#!/usr/bin/env ruby
# encoding: UTF-8
require 'strscan'

(puts DATA.read.gsub(/\$0/, File.basename($0)); exit 1) unless ARGV.empty? # handled -h and anything else since we don't take args

# tokenize lines
lines = STDIN.read.strip.split(/\n+/)
gems  = lines.inject({}) do |h, line|
  next h if line =~ /^\s*(#.*)?$/
  ss      = StringScanner.new(line.strip)
  name    = ss.scan(/\S+/) or raise "Don't know what to do with: #{line}"
  h[name] = {}
  until ss.eos?
    ss.skip(/\s+/)
    if ss.scan(/-\w|--\w\w+/) and key = ss[0]
      ss.skip(/\s*(=\s*)?/)
      ss.scan(/'(.*?)(?!<\\)'/) or
      ss.scan(/"(.*?)(?!<\\)"/) or
      ss.scan(/(\S+)/)          or raise "Expected value for key #{key} in #{ss.rest}"
      h[name][key] = ss[1]
    else ss.skip(/#.*/) or raise "Unexpected: #{ss.rest}"
    end
  end
  h
end

# semantic validation
gems.each do |name, options|
  s = options.delete("-s") || options.delete("--source")  and options[:source]  = s
  v = options.delete("-v") || options.delete("--version") and options[:version] = v
  invalid_keys = options.keys - [:source, :version]
  invalid_keys.empty? or raise "Invalid keys for gem '#{name}': #{invalid_keys.join(",")}"
end

# output Gemfile
puts "source :rubygems"
gems.values.each do |options|
  next unless source = options.delete(:source)
  puts "source #{source.inspect}"
end
puts
gems.each do |name, options|
  name_version = [name, options.delete(:version)].compact
  puts "gem #{name_version.map(&:inspect).join(', ')}"
end


__END__
Convert Heroku gem manifest format (.gems) to Bundler format (Gemfile)

  Usage:
    $0 < .gems
    $0 < .gems > Gemfile

Now also as a real github project: http://github.com/kch/gemfile-from-dotgems