58 lines
1.5 KiB
C++
58 lines
1.5 KiB
C++
/**
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* PANDA 3D SOFTWARE
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* Copyright (c) Carnegie Mellon University. All rights reserved.
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*
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* All use of this software is subject to the terms of the revised BSD
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* license. You should have received a copy of this license along
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* with this source code in a file named "LICENSE."
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*
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* @file primeNumberGenerator.cxx
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* @author drose
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* @date 2001-03-22
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*/
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#include "primeNumberGenerator.h"
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/**
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*
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*/
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PrimeNumberGenerator::
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PrimeNumberGenerator() {
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_primes.push_back(2);
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}
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/**
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* Returns the nth prime number. this[0] returns 2, this[1] returns 3;
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* successively larger values of n return larger prime numbers, up to the
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* largest prime number that can be represented in an int.
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*/
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int PrimeNumberGenerator::
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operator [] (int n) {
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nassertr(n >= 0, 0);
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// Compute the prime numbers between the last-computed prime number and n.
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int candidate = _primes.back() + 1;
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while ((int)_primes.size() <= n) {
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// Is candidate prime? It is not if any one of the already-found prime
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// numbers (up to its square root) divides it evenly.
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bool maybe_prime = true;
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int j = 0;
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while (maybe_prime && _primes[j] * _primes[j] <= candidate) {
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if ((_primes[j] * (candidate / _primes[j])) == candidate) {
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// This one is not prime.
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maybe_prime = false;
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}
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j++;
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nassertr(j < (int)_primes.size(), 0);
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}
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if (maybe_prime) {
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// Hey, we found a prime!
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_primes.push_back(candidate);
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}
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candidate++;
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}
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return _primes[n];
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}
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