rorate_cells.rb

class Cell
  attr_accessor :x,:y,:chr
  def initialize(x,y,chr)
    @x,@y,@chr = x,y,chr
  end

  def ==(c)
    @chr == c
  end

  def inspect
    @chr
  end
end

class Sheet
  def initialize(size = 5)
    @size = size
    chr = 'a'
    @cells = @size.times.map{|y|
      @size.times.map{|x|
        cell = Cell.new(x, y, chr.dup)
        chr.succ!
        cell
      }
    }
  end

  def get(x,y)
    return nil if x < 0 || x >= @size || y < 0 || y >= @size
    @cells[y][x]
  end

  def around(x,y)
    a = [[-1,-1],[0,-1],[1,-1], [1,0],[1, 1],[0, 1],[-1, 1],[-1,0]]
    a.map{|ax,ay|
      get(x+ax, y+ay)
    }.compact
  end

  def rotate!(x,y, is_reverse=false)
    cells = around(x,y)
    copy = cells.map{|cl| cl.dup}
    cells.size.times{|i|
      idx = is_reverse ? i+1 : i-1
      idx %= cells.size
      cells[i].chr = copy[idx].chr
    }
  end

  def find(c)
    down_c = c.downcase
    @cells.flatten.find{|e| e == down_c}
  end

  def rotate_by_chr!(c)
    cell = find(c)
    is_upper = c != c.downcase
    rotate!(cell.x, cell.y, is_upper)
  end

  def rotate_by_string!(str)
    str.each_char{|c| rotate_by_chr!(c)}
  end
end

def answer(str)
  sheet = Sheet.new
  sheet.rotate_by_string!(str)
  cell = sheet.find(str[-1,1])
  sheet.around(cell.x, cell.y).map{|cl| cl.chr}.sort.join
end

if __FILE__ == $0
  p answer("a")
  p answer("b")
  p answer("m")
  p answer("mg")
  p answer("Mg")
end