Yokohama.rb 70回で出題された問題の回答です。

yokohamarb_70.rb

// coding: utf-8
// http://nabetani.sakura.ne.jp/yokohamarb/2016.07.ront/
Tiles = [
  {top: :left, left: :top, right: :down, down: :right},
  {top: :right, left: :down, right: :top, down: :left},
  {top: :down, left: :right, right: :left, down: :top},
  {top: :death, left: :death, right: :death, down: :death}
]

def input2tiles(input)
  array_field = input.split("/").map { |e| e.split(//) }

  field = []
  array_field.each_with_index do |row, i|
    field[i] = []
    row.each_with_index do |col, j|
      field[i][j] = Tiles[col.to_i]
    end
  end
  field
end

def position2char(row, col)
  ('a'.ord + (4 * row + col)).chr
end

def in_area?(row, col)
  0 <= row && row <= 3 && 0 <= col && col <= 3
end

def solve(input)
  field = input2tiles(input)

  row_position = 0
  col_position = 1
  face = :top

  answer = "b"

  begin
    case field[row_position][col_position][face]
    when :top
      row_position -= 1
      face = :down
    when :left
      col_position -= 1
      face = :right
    when :right
      col_position += 1
      face = :left
    when :down
      row_position += 1
      face = :top
    when :death
      break
    end

    if in_area?(row_position, col_position)
      answer << position2char(row_position, col_position)
    else
      break
    end
  end while true

  answer
end

def test(input, expect)
  if solve(input) != expect
    raise "Test failed. input = #{input} expect = #{expect} actual = #{solve(input)}"
  end
end

test( "0113/1201/2201/2100", "bcgfeabfjnoklpo" ) # 0
test( "2110/2013/2210/0122", "bcgh" ) # 1
test( "2222/2130/2121/2002", "bfg" ) # 2
test( "0021/2212/2102/1220", "baeimnoplhdcbfjkgfe" ) # 3
test( "0213/1221/0220/1103", "bfjnokgcbaefghlkjimn" ) # 4
test( "3201/3120/3333/3333", "bfghdcgk" ) # 5
test( "3233/3233/3133/3333", "bfjk" ) # 6
test( "3333/3333/3333/3333", "b" ) # 7
test( "1212/1201/2123/2220", "bfjkl" ) # 8
test( "2212/3102/1002/2100", "bfgcba" ) # 9
test( "0023/2221/1102/0031", "baeijnm" ) # 10
test( "1121/3120/0212/1120", "bcdhgfba" ) # 11
test( "2202/3211/2120/1210", "bfjklhgcd" ) # 12
test( "3201/3211/1111/0100", "bfjkonjie" ) # 13
test( "0121/1120/1111/1211", "bcdhgfbaefjkop" ) # 14
test( "1212/1213/2103/0213", "bfjkgfea" ) # 15
test( "1121/2212/1323/3031", "bcdhl" ) # 16
test( "0030/1230/1121/0031", "baefg" ) # 17
test( "2223/1211/0002/1200", "bfjimnokl" ) # 18
test( "3210/0033/0201/0130", "bfei" ) # 19
test( "0213/2220/0021/3002", "bfjim" ) # 20
test( "2121/2112/1110/1010", "bcdhlkgfba" ) # 21
test( "0113/1003/2303/2220", "bcgfj" ) # 22
test( "2202/1110/1302/0313", "bfgkj" ) # 23
test( "1211/3202/2102/0222", "bfjkgh" ) # 24
test( "3113/0002/0112/1022", "bcgfjko" ) # 25
test( "1200/3000/0121/0121", "bfe" ) # 26
test( "2221/1122/1031/2200", "bfgh" ) # 27
test( "1202/0121/0222/1300", "bfghlpo" ) # 28
test( "0002/1012/1021/3300", "baefbc" ) # 29
test( "0211/1200/2220/2103", "bfjnokghdc" ) # 30