Y-shaped road tour (how to write)

http://nabetani.sakura.ne.jp/hena/ord3ynode/

When A Graph is given a continuous instruction of "turn right/go forward/turn left/go back", it is a problem to output a column of passing nodes.

Ruby

module YshapedRoadTour
  Graph = {"A"=>"DCB", "B"=>"CEA", "C"=>"FBA",
           "D"=>"EFA", "E"=>"DBF", "F"=>"ECD"}
           
  def self.solve(input)
    input = input.each_char
    result = ""
    
    go = ->(s, e) {
      result << e
      idx = Graph[e].index(s)
      case input.next
      when "b"
        go.(e, s)
      when "r"
        i = (idx - 1) % 3
      when "l"
        i = (idx + 1) % 3
      end
      go.(e, Graph[e][i])
    }
    go.("B", "A")
  rescue StopIteration
    result
  end
end


if __FILE__ == $0
  require 'minitest/autorun'
  describe 'YshapedRoadTour' do
    [
      ["b", "AB"],
      ["l", "AD"],
      ["r", "AC"],
      ["bbb", "ABAB"],
      ["rrr", "ACBA"],
      ["blll", "ABCAB"],
      ["llll", "ADEBA"],
      ["rbrl", "ACADE"],
      ["brrrr", "ABEDAB"],
      ["llrrr", "ADEFDE"],
      ["lrlll", "ADFEDF"],
      ["lrrrr", "ADFCAD"],
      ["rllll", "ACFDAC"],
      ["blrrrr", "ABCFEBC"],
      ["brllll", "ABEFCBE"],
      ["bbbrrlrl", "ABABEDFCB"],
      ["rbllrrrr", "ACABCFEBC"],
      ["lbrlrrblr", "ADABCFEFCA"],
      ["rlbrrrrbl", "ACFCADFCFD"],
      ["bllrlrbrrb", "ABCADEFEBCB"],
      ["rllrllllbb", "ACFDEBADEDE"],
      ["blblrlrrlbr", "ABCBEDFCABAC"],
      ["lrlrrrrrbrb", "ADFEBCFEBEDE"],
      ["rblllrlrrlrr", "ACABCADEFDABE"],
      ["rbrrlrblrllb", "ACADFEBEFDACA"],
      ["lrrrlrllrrllr", "ADFCABEFCADEBC"],
      ["rrlblllrrlrrb", "ACBEBADEFDABEB"],
      ["brbllrrbbrlrll", "ABEBADFCFCABEFC"],
      ["rrrbbrlbrlblrb", "ACBABACFCABADFD"],
      ["lllllllllllblrr", "ADEBADEBADEBEFDE"],
      ["llllllrllrlbrrr", "ADEBADEFCBADABED"]
    ].each do |input, expect|
      it input do
        assert_equal YshapedRoadTour.solve(input), expect 
      end
    end
  end
end

It's difficult to express a right turn or a left turn. Here, the graph is represented by Hash, but if it is {"A "=>" DCB "}, it means that it is D, C, B clockwise around node A.

Input is converted to Enumerator with each_char and extracted character by character with next. When there is nothing to retrieve, an exception StopIteration will occur, so I try to catch it with rescue and return the result.

go. (S, e) means going from s to e.

I'm using recursion here, but I think loops are better. But I somehow like recursion (laughs).

Loop version

Is it like this with a loop? After all this is better (laughs).

module YshapedRoadTour
  Graph = {"A"=>"DCB", "B"=>"CEA", "C"=>"FBA",
           "D"=>"EFA", "E"=>"DBF", "F"=>"ECD"}
           
  def self.solve(input)
    input = input.each_char
    result = ""
    s, e = "B", "A"
    
    loop do
      result << e
      idx = Graph[e].index(s)
      case input.next
      when "b"
        s, e = e, s
      when "r"
        i = (idx - 1) % 3
        s, e = e, Graph[e][i]
      when "l"
        i = (idx + 1) % 3
        s, e = e, Graph[e][i]
      end
    end
    result
  end
end

Note that loop do ~ end catches the exception StopIteration, so Enumerator # next can be written straightforwardly.