Prime::TrialDivision

Parent:ObjectIncluded modules:Singleton

Internal use. An implementation of prime table by trial division method.

Public Instance Methods

Show source

Returns the +index+th prime number.

index is a 0-based index.

# File lib/prime.rb, line 400
def [](index)
  while index >= @primes.length
    # Only check for prime factors up to the square root of the potential primes,
    #   but without the performance hit of an actual square root calculation.
    if @next_to_check + 4 > @ulticheck_next_squared
      @ulticheck_index += 1
      @ulticheck_next_squared = @primes.at(@ulticheck_index + 1) ** 2
    end
    # Only check numbers congruent to one and five, modulo six. All others

    #   are divisible by two or three.  This also allows us to skip checking against
    #   two and three.
    @primes.push @next_to_check if @primes[2..@ulticheck_index].find {|prime| @next_to_check % prime == 0 }.nil?
    @next_to_check += 4
    @primes.push @next_to_check if @primes[2..@ulticheck_index].find {|prime| @next_to_check % prime == 0 }.nil?
    @next_to_check += 2
  end
  @primes[index]
end

cache() Show source

Returns the cached prime numbers.

# File lib/prime.rb, line 391
def cache
  @primes
end

Also aliased as: primes, primes_so_far

primes()

Alias for: cache

primes_so_far()

Alias for: cache

 Ruby Core © 1993–2017 Yukihiro Matsumoto

Licensed under the Ruby License.

Ruby Standard Library © contributors

Licensed under their own licenses.