snail.rb

class Snail
  def initialize(data)
    @data = data
    @direction = :right
  end

  def call
    values = []
    values << get(x, y) while move
    values
  end

  private

  attr_accessor :data, :x, :y

  def move
    if @x.nil?
      @x = 0
      @y = 0
      return true
    end

    change_direction unless fetch(@y + move_y, @x + move_x)

    @x += move_x
    @y += move_y

    # Return false if ended
    fetch(@y, @x)
  end

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

  def fetch(y, x)
    return if x < 0 || y < 0

    data.dig(y, x)
  end

  def get(x, y)
    value = fetch(y, x) || return

    data[y][x] = nil

    value
  end

  def move_x
    case @direction
    when :right then 1
    when :left then -1
    else
      0
    end
  end

  def move_y
    case @direction
    when :down then 1
    when :up then -1
    else
      0
    end
  end
end

p 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],
]).call