inflector.rb

class Parts
  def initialize(parts)
    @parts = parts
  end

  def join(part_divider, word_divider='')
    @parts.map do |word|
      word.join(word_divider)
    end.join(part_divider)
  end

  def transform(transformer, part_divider, word_divider='')
    @parts.map do |word|
      word.reduce('') do |result, word|
        word = transformer.call(word)
        result += word_divider + word
      end
    end.join(part_divider)
  end

  def last
    [@parts.last]
  end
end

class InternalString
  MODULE_DIVIDER = '::'.freeze
  PATH_DIVIDER = '/'.freeze

  attr_reader :parts

  def initialize(value)
    if value.kind_of?(Array)
      @parts = Parts.new(value)
    else
      words = value.to_s.split(/#{MODULE_DIVIDER}|#{PATH_DIVIDER}/)
      @parts = Parts.new(words.map { |word| word.split(/\_|(?!(^|[a-z]|$))/) })
    end
  end

  def camelize
    @parts.transform(:capitalize.to_proc, MODULE_DIVIDER)
  end

  def underscore
    @parts.join(PATH_DIVIDER, '_').downcase
  end

  def demodulize
    @parts.last.join
  end

  def foreign_key
    self.class.new(@parts.last).underscore + '_id'
  end
end

original = 'active_support/inflector'
str = InternalString.new('active_support/inflector')

puts "Original: #{original}"
puts "Underscore: #{str.underscore}"
puts "Camelize: #{str.camelize}"
puts "Demodulize: #{str.demodulize}"
puts "Foreign key: #{str.foreign_key}"