google_recruit

お題

https://qiita.com/daddygongon/items/ba94b0f2a73990fc6a07

解答

# frozen_string_literal: true

require './assert_equal'

def prime?(n)
  return true if n == 2

  # ある数 n に対し、初期値 m=1, l=nとして、mを増やしながら、lを減らしながら n =
  # m*l となる(m,l)を探すとき、m=sqrt(n)に到達した以降の(m,l)は、m=sqrt(n) に到
  # 達する以前の(l,m)と同じであるから、探索はm=sqrt(n)で打ちきることができる
  max_i = Math.sqrt(n).to_i + 1

  [*2..max_i].each do |i|
    mod = n % i
    break if mod.zero?
    return true if i == max_i
  end
  false
end

def read_exp
  gets.to_s.chomp
end

def prime_test
  expected = [2, 3, 5, 7]
  results = []

  [*2..10].each do |num|
    results << num if prime?(num)
  end
  assert_equal(expected, results)
end

def read_exp_test
  exp1 = read_exp

  assert_equal(7_182_818_284, exp1[2..2 + 10 - 1].to_i)
end

def main
  exp1 = read_exp

  [*2..exp1.length - 10].each do |i|
    num = exp1[i..i + 10 - 1].to_i
    if prime?(num)
      puts num
      break
    end
  end
end

main if __FILE__ == $PROGRAM_NAME

output

7427466391

類題

Congratulations. You've made it to level 2. Go to www.Linux.org and enter Bobsyouruncle as the login and the answer to this equation as the password.

f(1)=7182818284
f(2)=8182845904
f(3)=8747135266
f(4)=7427466391
f(5)=__________

ctf っぽい。数列を眺め、試行錯誤してたら法則性を偶然発見した。

# frozen_string_literal: true

require './assert_equal'

def read_exp
  gets.to_s.chomp
end

def g_eq?(n)
  sum = 0
  until n.zero?
    sum += n % 10
    n = (n / 10).floor
  end
  sum == 49
end

def main
  exp1 = read_exp

  [*2..exp1.length - 10].each do |i|
    num = exp1[i..i + 10 - 1].to_i
    puts num if g_eq?(num)
  end
end

main if __FILE__ == $PROGRAM_NAME

output

7182818284
8182845904
8747135266
7427466391
5966290435
2952605956

余談

Go to www.Linux.org and enter Bobsyouruncle as the login and the answer to thisequation as the password. っていうとおりにlogin を試みたが、失敗した。実は上の回答が間違っているか、もうgoogle_recruit は時間が経ちすぎていて、無効になっているか…多分後者だと思うけど。

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• source ~/ghq/github.com/TeamNishitani/grad_members_20f/members/syasin-5d/google_recruit.org