tree command in Ruby

tree.rb

#! /usr/bin/env ruby
# sri, Sun Aug  3 22:28:22 PDT 2014

# The tree command in Ruby.
# Mimics the output of tree.
module Tree
  class Directory
    attr_reader :pathname
    def initialize(pathname)
      @pathname = pathname
      @state = nil
      @given_last = nil
      ancestors = @pathname.ancestors + [self]
      entries = Dir.entries(@pathname.abs_path).sort_by(&:downcase)
      entries -= %w{. ..}
      @entries = entries.map do |e|
        full = File.join(@pathname.abs_path, e)
        Pathname.new(full, ancestors)
      end
      @entries.reverse!
    end
    # When next_entry is called, we update the state.
    def next_entry
      p = @entries.pop
      if @state == :last
        @state = :beyond_last
      elsif @entries.empty?
        @state = :last
      end
      p
    end
    # When dir_prefix is called, we update the state.
    # If state is last, "`" is given to the client
    # only once.
    def dir_prefix
      if @state == :last
        result = @given_last ? " " : "`"
        @given_last = true
        result
      else
        "|"
      end
    end
  end

  class Pathname
    attr_reader :abs_path, :ancestors
    def initialize(abs_path, ancestors = [])
      @abs_path = abs_path
      @ancestors = ancestors
    end
    def directory?
      File.directory?(abs_path)
    end
    def as_directory
      Directory.new(self)
    end
    def basename
      File.basename(abs_path)
    end
    def line
      result = [ancestors.first.dir_prefix]
      parents = ancestors[1..-1]
      while !parents.empty?
        result << "   " << parents.shift.dir_prefix
      end
      result << "-- " << basename
      result.join
    end
  end

  def self.tree(d)
    f = d.next_entry
    return if f.nil?
    puts f.line
    tree(f.as_directory) if f.directory?
    tree(d)
  end

  def self.from_cmdline
    arg = ARGV[0] || "."
    p = Tree::Pathname.new(arg)
    puts arg
    tree(p.as_directory)
  end
end


if $0 == __FILE__
  Tree.from_cmdline
end