- prime.rb

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

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Returns the +index+th prime number.

`index`

is a 0-based index.

# File prime.rb, line 397 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