snail.rb

class Snail
  include Enumerable

  def initialize(data)
    @data = data
    @direction = :right
    @x = 0
    @y = 0
  end

  def each
    while (value = fetch(@x, @y))
      yield value
      move
    end
  end

  private

  def fetch(x, y)
    @data.dig(y, x) if x >= 0 && y >= 0
  end

  def move
    # Remove value for the following loops
    @data[@y][@x] = nil

    @direction = next_direction unless fetch(*next_position)
    @x, @y = next_position
  end

  def next_position
    case @direction
    when :right then [@x + 1, @y]
    when :left  then [@x - 1, @y]
    when :down  then [@x,     @y + 1]
    when :up    then [@x,     @y - 1]
    end
  end

  def next_direction
    directions = %i[right down left up]
    directions.fetch(directions.index(@direction) + 1, directions.first)
  end
end

snail = Snail.new([
  %w[A  B  C  D  E  F  G  H],
  %w[I  J  K  L  M  N  O  P],
  %w[Q  R  S  T  U  V  W  X],
  %w[Y  Z  0  1  2  3  4  5],
  %w[6  7  8  9  10 11 12 13],
  %w[14 15 16 17 18 19 20 21],
])

p snail.to_a