module Gem

  class << self

    ##

    # Returns a string representing that part or the directory tree that is

    # common to all specified directories.

    def common_path(dirs)

      paths = dirs.collect {|dir| dir.split(File::SEPARATOR)}

      uncommon_idx = paths.transpose.each_with_index.find {|dirnames, idx| dirnames.uniq.length > 1}.last

      paths[0][0 ... uncommon_idx].join(File::SEPARATOR)

    end

    private :common_path

    ##

    # Default gems locations allowed on FHS system (/usr, /usr/share).

    # The locations are derived from directories specified during build

    # configuration.

    def default_locations

      @default_locations ||= {

        :system => common_path([ConfigMap[:vendorlibdir], ConfigMap[:vendorarchdir]]),

        :local => common_path([ConfigMap[:sitelibdir], ConfigMap[:sitearchdir]])

      }

    end

    ##

    # For each location provides set of directories for binaries (:bin_dir)

    # platform independent (:gem_dir) and dependent (:ext_dir) files.

    def default_dirs

      @default_dirs ||= Hash[default_locations.collect do |destination, path|

        [destination, {

          :bin_dir => File.join(path, ConfigMap[:bindir].split(File::SEPARATOR).last),

          :gem_dir => File.join(path, ConfigMap[:datadir].split(File::SEPARATOR).last, 'gems'),

          :ext_dir => File.join(path, ConfigMap[:libdir].split(File::SEPARATOR).last, 'gems')

        }]

      end]

    end

    ##

    # RubyGems default overrides.

    def default_dir

      if Process.uid == 0

        Gem.default_dirs[:local][:gem_dir]

      else

        Gem.user_dir

      end

    end

    def default_path

      path = default_dirs.collect {|location, paths| paths[:gem_dir]}

      path.unshift Gem.user_dir if File.exist? Gem.user_home

    end

    def default_bindir

      if Process.uid == 0

        Gem.default_dirs[:local][:bin_dir]

      else

        File.join [Dir.home, 'bin']

      end

    end

    def default_ext_dir_for base_dir

      dirs = Gem.default_dirs.detect {|location, paths| paths[:gem_dir] == base_dir}

      dirs && File.join(dirs.last[:ext_dir], 'exts')

    end

  end

end