class Prime::EratosthenesSieve

Internal use. An implementation of Eratosthenes' sieve

Public Class Methods

new() click to toggle source
# File lib/prime.rb, line 422
def initialize
  @primes = [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, 101]
  # @max_checked must be an even number
  @max_checked = @primes.last + 1
end

Public Instance Methods

get_nth_prime(n) click to toggle source
# File lib/prime.rb, line 428
def get_nth_prime(n)
  compute_primes while @primes.size <= n
  @primes[n]
end

Private Instance Methods

compute_primes() click to toggle source
# File lib/prime.rb, line 434
  def compute_primes
    # max_segment_size must be an even number
    max_segment_size = 1e6.to_i
    max_cached_prime = @primes.last
    # do not double count primes if #compute_primes is interrupted
    # by Timeout.timeout
    @max_checked = max_cached_prime + 1 if max_cached_prime > @max_checked

    segment_min = @max_checked
    segment_max = [segment_min + max_segment_size, max_cached_prime * 2].min
    root = Integer.sqrt(segment_max)

    segment = ((segment_min + 1) .. segment_max).step(2).to_a

    (1..Float::INFINITY).each do |sieving|
      prime = @primes[sieving]
      break if prime > root
      composite_index = (-(segment_min + 1 + prime) / 2) % prime
      while composite_index < segment.size do
        segment[composite_index] = nil
        composite_index += prime
      end
    end

    @primes.concat(segment.compact!)

    @max_checked = segment_max
  end
end