Sieve of Eratosthenes

inc_prime.rb

prime = -> a=[false, false], z=1 do
  n = a.size + 1
  b = a + [true]
  (2..Math.sqrt(a.size)).each do |i|
    if b[i]
      x = 0
      while (y = i**2+x*i) < n
        b[y] = false
        x += 1
      end
    end
  end
  r = (2..(n-1)).select{ |i| b[i] }.last
  if z == r
    prime[b, z]
  else
    puts r
    -> { prime[b,r] }
  end
end

# Output
> prime[][][][][][][][][][][][][][][][][][][][][][][][][]
2
3
5
7
11
13
17
19
23
29
31
37
41
43
47
53
59
61
67
71
73
79
83
89
97

prime.rb

def all_prime_in(n)
  a = [true]*n
  (2..Math.sqrt(n)).each do |i|
    if a[i]
      x = 0
      while (y = i**2+x*i) < n
        a[y] = false
        x += 1
      end
    end
  end
  (2..(n-1)).select { |i| a[i] }
end

def max_prime_in(n)
  all_prime_in(n).last
end