Contents

require 'eratostene_sieve/version'
require 'eratostene_sieve/railtie' if defined? Rails

# Utility to get:
# - nth_prime(n): the nth prime number.
# - enumerator: an enumerator on prime number serie.
module EratosteneSieve
  Inf = 1.0 / 0.0

  def self.nth_prime(index)
    return first_primes[index - 1] if index <= first_primes.size
    counter = first_primes.size
    ((first_primes.last + 2)..Inf).step(2) do |candidate|
      (2..((candidate**0.5).to_i)).each do |m|
        break if candidate % m == 0
        counter += 1 if m == (candidate**0.5).to_i
      end
      return candidate.to_i if counter == index
    end
  end

  def self.enumerator
    Enumerator.new do |y|
      a = 2
      b = 3
      index = 3
      loop do
        y << a
        a, b = b, nth_prime(index)
        index += 1
      end
    end
  end

  def self.serie(options = {})
    from = options[:from] || 1
    up_to = options[:up_to] || 10
    fail "from option must be an integer, given #{from}." unless from.is_a?(Integer)
    fail "up_to option must be an integer, given #{up_to}." unless up_to.is_a?(Integer)
    fail "from option must be > 0, given #{from}." if from < 1
    serie = []
    enumerator = EratosteneSieve.enumerator
    lowest_index = from < up_to ? from : up_to
    (lowest_index - 1).times { enumerator.next }
    ((from - up_to).abs + 1).times { serie << enumerator.next }
    serie.reverse! if from > up_to
    serie
  end

  private

  def self.first_primes
    @first_primes ||= [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31]
  end
end

Version data entries

1 entries across 1 versions & 1 rubygems

Version	Path
eratostene_sieve-0.0.3.alpha	lib/eratostene_sieve.rb