# Class: Prime::TrialDivision

Inherits:
Object
• Object
show all
Includes:
Singleton
Defined in:
lib/prime.rb

## Overview

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

## Instance Method Summary collapse

• Returns the indexth prime number.

• constructor

:nodoc:.

## Constructor Details

### #initialize ⇒ TrialDivision

:nodoc:

 ``` 381 382 383 384 385 386 387 388 389 390 391 392``` ```# File 'lib/prime.rb', line 381 def initialize # :nodoc: # These are included as class variables to cache them for later uses. If memory # usage is a problem, they can be put in Prime#initialize as instance variables. # There must be no primes between @primes[-1] and @next_to_check. @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] # @next_to_check % 6 must be 1. @next_to_check = 103 # @primes[-1] - @primes[-1] % 6 + 7 @ulticheck_index = 3 # @primes.index(@primes.reverse.find {|n| # n < Math.sqrt(@@next_to_check) }) @ulticheck_next_squared = 121 # @primes[@ulticheck_index + 1] ** 2 end```

## Instance Method Details

### #[](index) ⇒ Object

Returns the indexth prime number.

`index` is a 0-based index.

 ``` 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415``` ```# File 'lib/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```