階段昇降

stairs.rb

# 毎日『m段上ってn段下がる』を繰り返したら、x日目にd段目に初めて着いた。
# xをd,m,nで表せ。
# 文字はすべて正の整数で、m>nとする。
# https://twitter.com/hyuki/status/1134018952416321536

class Stairs
  def initialize(dest, up, down)
    @dest = dest
    @up = up
    @down = down
  end

  def to_s
    "d=#{@dest}, m=#{@up}, n=#{@down}"
  end

  def solve
    [1, Rational(@dest - @down, @up - @down).ceil].max
  end

  def simulate(verbose=true)
    day = 0
    current = 0
    done = false
    until done
      day += 1
      print "#{day}日目: " if verbose
      step = []
      @up.times do
        current += 1
        step.push current
        if current >= @dest
          done = true
          break
        end
      end
      unless done
        @down.times do
          current -= 1
          step.push current
        end
      end
      puts step.join(', ') if verbose
    end
    day
  end
end

def test(dest, up, down)
  problem = Stairs.new(dest, up, down)
  puts problem.to_s

  answer = problem.solve
  puts "x=#{answer}"

  simulated = problem.simulate
  raise "wrong answer." if answer != simulated
end

test(7, 5, 4)
test(7, 5, 2)
test(7, 8, 6)
test(7, 11, 8)
test(15, 3, 2)
test(15, 15, 14)