#!/usr/bin/ruby

require 'rubygems/package'

module RubyGemsReq

  module Helpers

    # Expands '~>' and '!=' gem requirements.

    def self.expand_requirement(requirements)

      requirements.inject([]) do |output, r|

        output.concat case r.first

        when '~>'

          expand_pessimistic_requirement(r)

        when '!='

          # If there is only the conflict requirement, we still need to depend

          # on the specified gem.

          if requirements.size == 1

            Gem::Requirement.default.requirements

          else

            []

          end

        else

          [r]

        end

      end.reject {|r| r.empty? }

    end

    # Expands the pessimistic version operator '~>' into equivalent '>=' and

    # '<' pair.

    def self.expand_pessimistic_requirement(requirement)

      next_version = Gem::Version.create(requirement.last).bump

      return ['>=', requirement.last], ['<', next_version]

    end

    # Converts Gem::Requirement into array of requirements strings compatible

    # with RPM .spec file.

    def self.requirement_versions_to_rpm(requirement)

      self.expand_requirement(requirement.requirements).map do |op, version|

        version == Gem::Version.new(0) ? "" : " #{op} #{version}"

      end

    end

    # Compose dependency together with its requirements in RPM rich dependency

    # string.

    def self.compose_dependency_string(name, requirements)

      dependency_strings = requirements.map { |requirement| name + requirement }

      dependency_string = dependency_strings.join(' with ')

      dependency_string.prepend('(').concat(')') if dependency_strings.length > 1

      dependency_string

    end

  end

  # Report RubyGems dependency, versioned if required.

  def self.rubygems_dependency(specification)

    dependency_name = "ruby(rubygems)"

    requirements = Helpers::requirement_versions_to_rpm(specification.required_rubygems_version)

    puts Helpers::compose_dependency_string(dependency_name, requirements)

  end

  # Report all gem dependencies including their version.

  def self.gem_depenencies(specification)

    specification.runtime_dependencies.each do |dependency|

      dependency_name = "rubygem(#{dependency.name})"

      requirements = Helpers::requirement_versions_to_rpm(dependency.requirement)

      puts Helpers::compose_dependency_string(dependency_name, requirements)

    end

  end

  # Reports all requirements specified by all provided .gemspec files.

  def self.requires

    while filename = gets

      filename.strip!

      begin

        specification = Gem::Specification.load filename

        rubygems_dependency(specification)

        gem_depenencies(specification)

      rescue => e

        # Ignore all errors.

      end

    end

  end

end

if __FILE__ == $0

  RubyGemsReq::requires

end