Contents
1 The Sieve of Eratosthenes
2 Primality Testing by Trial Division
3 Factorization by Trial Division
4 Miller-Rabin Pseudoprimality Checking
5 Factorization by Pollard’s Rho Method
6 Going Further
Appendices
C
Haskell
Java
Python
Scheme

Prime numbers are those integers greater than one that are divisible only by themselves and one; an integer greater than one that is not prime is composite. Prime numbers have fascinated mathematicians since the days of the ancient Greek mathematicians, and remain an object of study today. The sequence of prime numbers begins 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, … and continues to infinity, which Euclid, the famous teacher of geometry, proved about twenty-three centuries ago:

Assume for the moment that the number of primes is finite, and make a list of them: p1, p2, …, pk. Now compute the number n = p1 · p2 · … · pk + 1. Certainly n is not evenly divisible by any of the primes p1, p2, …, pk, because division by any them leaves a remainder of 1. Thus either n is prime, or n is composite but has two or more prime factors not on the list p1, p2, …, pk of prime numbers. In either case the assumption that the number of primes is finite is contradicted, thus proving the infinitude of primes. — Euclid, Elements, Book IX, Proposition 20, circa 300 B.C.

In this essay we will examine three problems related to prime numbers: enumerating the prime numbers, determining if a given number is prime or composite, and factoring a composite number into its prime factors. We describe in detail five relevant functions: one that makes a list of the prime numbers less than a given number using the Sieve of Eratosthenes; two that determine whether a given number is prime or composite, one using trial division and the other using an algorithm developed by Gary Miller and Michael Rabin; and two that find the unique factorization of a given composite number, one using trial division and the other using John Pollard’s rho algorithm. We first describe the algorithms in the body of the essay, then describe actual implementations in five languages — C, Haskell, Java, Python and Scheme — in a series of appendices.

Our goals are modest. Our purpose is pedagogical, so we are primarily interested in the clarity of the code. We describe algorithms that are well known and implement them carefully. And we hope that careful reading will lead you to be a better programmer in addition to learning something about prime numbers. Even so, our functions are genuinely useful for a variety of purposes beyond simple study.

[ Return to top ]

 

1 The Sieve of Eratosthenes

The method that is in common use today to make a list of the prime numbers less than a given input n was invented about two hundred years before Christ by Eratosthenes of Cyrene, who was an astronomer, geographer and mathematician, as well as the third chief librarian of Ptolemy’s Great Library at Alexandria; he calculated the distance from Earth to Sun, the tilt of the Earth’s axis, and the circumference of the Earth, and invented the leap day and a system of latitude and longitude. His method begins by making a list of all the numbers from 2 to the desired maximum prime number n. Then the method enters an iterative phase. At each step, the smallest uncrossed number that hasn’t yet been considered is identified, and all multiples of that number are crossed out; this is repeated until no uncrossed numbers remain unconsidered. All the remaining uncrossed numbers are prime.

Thus, the first step crosses out all multiples of 2: 4, 6, 8, 10 and so on. At the second step, the smallest uncrossed number is 3, and multiples of 3 are crossed out: 6, 9, 12, 15 and so on; note that some numbers, such as 6, might be crossed out multiple times. At this point 4 has been crossed out, so the next smallest uncrossed number is 5, and its multiples 10, 15, 20, 25 and so on are also crossed out. The process continues until all uncrossed numbers have been considered. Thus, each prime is used to “sift” its multiples out of the original list, so that only primes are left in the sieve. Or you may prefer the ditty:

Strike the twos and strike the threes,
The Sieve of Eratosthenes!
When the multiples sublime,
The numbers that remain are prime.

Although this is the basic algorithm, there are three optimizations that are routinely applied. First, since 2 is the only even prime, it is best to handle 2 separately and sieve only on odd numbers, reducing the size of the sieve by half. Second, instead of starting the crossing-out at the smallest multiple of the current sieving prime, it is possible to start at the square of the multiple, since all smaller numbers will have already been crossed out; we saw that in the sample when 6 was already crossed out as a multiple of 2 when we were crossing out multiples of 3. Third, as a consequence of the second optimization, sieving can stop as soon as the square of the sieving prime is greater than n, since there is nothing else to do. Here is a formal statement of the sieve of Eratosthenes:

Algorithm 1. Sieve of Eratosthenes: Generate the primes not exceeding n > 1:
1. [Initialization.] Set m ← ⌊(n−1)/2⌋. Create a bitarray B[0..m−1] with each item set to TRUE. Set i ← 0. Set p ← 3. Output the prime 2.
2. [Sieving complete?] If n < p2, go to Step 5.
3. [Found prime?] If B[i] = FALSE, set ii + 1, set pp + 2, and go to Step 2. Otherwise, output the prime p and set j ← 2i2 + 6i + 3 (or j ← (p2 − 3) / 2).
4. [Sift on p.] If j < m, set B[j] ← FALSE, set jj + 2i + 3 (or jj + p) and go to Step 4. Otherwise, set ii + 1, set pp + 2, and go to Step 2.
5. [Terminate?] If i = m, stop.
6. [Report remaining primes.] If B[i] = TRUE, output the prime p. Then, regardless of the value of B[i], set ii + 1, set pp + 2, and go to Step 5.

The calculation of j in Step 3 is interesting. Since the bitarray contains odd numbers starting from 3, an index i of the bitarray corresponds to the number p = 2i+3; for instance, the fifth item in the bitarray, at index 4, is 11. Sifting starts from the square of the current prime, so to sift the prime 11 at index 4 we start from 112 = 121 at index 59, calculated as (i−3)/2. Thus, to compute the starting index j, we calculate ((2i+3)2−3)/2, which with a little bit of algebra simplifies to the formula in Step 3. You may prefer the alternate calculation (p2−3) / 2 which exploits the identity 2i+3 = p.

As an example, we show the calculation of the primes less than a hundred. The first iteration notes that 3 is the smallest uncrossed number, and crosses out every third number starting from 3 · 3: 9, 15, 21, 27, 33, 39, 45, 51, 57, 63, 69, 75, 81, 87, 93, 99.

3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Now 5 is the next smallest uncrossed number, so we cross out every fifth number starting from 5 · 5: 25, 35, 45, 55, 65, 75, 85, 95. Note that 45 and 75 were previously crossed out, so only six additional numbers are now crossed.

3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Now 7 is the next smallest uncrossed number, so we cross out every seventh number starting from 7 · 7: 49, 63, 77, 91. Note that 63 was previously crossed out, so only three additional numbers are crossed.

3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 79 81 83 85 87 89 91 93 95 97 99

Now the next smallest uncrossed number is 11, but 11 · 11 = 121 is greater than 100, so sieving is complete. The complete list of primes, including 2 followed by the remaining uncrossed numbers, is: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, and 97.

The algorithm outputs primes in Step 1 (the only even prime 2), Step 3 (the sieving primes), and Step 6 (sweeping up the primes that survived the sieve). The word “output” can mean anything. It is common to collect the primes in a list, but depending on your needs, you could count them, sum them, or use them in many other ways.

The Sieve of Eratosthenes runs in time O(n log log n), and because the only operations in the inner loop (Step 4) are a single comparison, addition and crossing-out, it is very fast in practice.

There are other ways to make lists of prime numbers. If memory is constrained, or if you want only the primes on a limited range from m to n, you may be interested in the segmented Sieve of Eratosthenes, which finds the primes in blocks; the sieving primes are those less than the square root of n, and the minimum multiple of each sieving prime in each segment is reset at the beginning of the next segment. A. O. L. Atkin, an IBM researcher, invented a sieve that is faster than the Sieve of Eratosthenes, as it crosses out multiples of the squares of the sieving primes after some precomputations. Paul Pritchard, an Australian mathematician, has developed several methods of sieving using wheels that have time complexity O(n / log log n), which is asymptotically faster than the Sieve of Eratosthenes, though in practice Pritchard’s sieves are somewhat slower since the bookkeeping in each step of the inner loop is more involved.

Sometimes instead of the primes less than n you need the first x primes. The simplest method is to estimate the value of n using the Prime Number Theorem, first conjectured by Carl Friedrich Gauss in 1792 or 1793 (at the age of fifteen) and proved independently by Jacques Hadamard and Charles Jean de la Vallée-Poussin in 1896, which states that there are approximately x / loge x primes less than x; estimate the value of n that gives the desired value of x, add a buffer of 20% or thereabouts, compute the primes less than n, and throw away the excess.

By the way, many people implement a function to list the prime numbers to a limit that they call the Sieve of Eratosthenes, but really isn’t; their functions use trial division instead. If you use a modulo operator, or division in any form, your algorithm is not the Sieve of Eratosthenes, and will run much slower than the algorithm described above. On a recent-vintage personal computer, a properly-implemented Sieve of Eratosthenes should be able to list the primes less than a million in less than a second.

[ Return to top ]

 

2 Primality Testing by Trial Division

We turn next to the problem of classifying a number n as prime or composite. The oldest method, and for nearly two thousand years the only method, was trial division. If n has no remainder when divided by 2, it is composite. Otherwise, if n has no remainder when divided by 3, it is composite. Otherwise, if n has no remainder when divided by 4, it is composite. Otherwise, …. And so on. Iteration stops, and the number is declared prime, when the trial divisor is greater than the square root of n. We can see that this is so because, if n = p · q, then one of p or q must be less than the square root of n while the other is greater than the square root of n (unless p and q are equal, and n is a perfect square). A simple optimization notes that if the number is even it is composite, and if the number is odd any factor must be odd, so it is necessary to divide only by the odd numbers greater than 2, not by the even numbers.

Algorithm 2. Primality Testing by Trial Division: Determine if an integer n > 1 is prime or composite:
1. [Finished if even] If n is even, return COMPOSITE and stop.
2. [Initialize for odd] Set d ← 3.
3. [Terminate if prime] If d2 > n, return PRIME and stop.
4. [Trial division] If n mod d = 0, return COMPOSITE and stop.
5. [Iterate] Set dd + 2. Go to Step 3.

As an example, consider the number 100524249167. It is not even. Dividing by 3 gives a remainder of 2. Dividing by 5 gives a remainder of 2. Dividing by 7 gives a remainder of 7. Dividing by 9 gives a remainder of 5. Dividing by 11 gives a remainder of 1. Dividing by 13 gives a remainder of 4. Dividing by 15 gives a remainder of 2. Dividing by 17 gives a remainder of 8. Dividing by 19 gives a remainder of 3. Dividing by 21 gives a remainder of 20. Dividing by 23 gives a remainder of 23 and, in Step 4, demonstrates that 100524249167 = 23 · 127 · 239 · 311 · 463 is composite.

There are other optimizations possible. If you have a list of prime numbers at hand, you can use only the primes as trial divisors and skip the composites, which makes things go quite a bit faster; this works because, if you’ve already tested its component primes, it is not possible for a composite to be a divisor. If you can’t afford the space to store a list of primes, you can use one of Pritchard’s wheels, just as with the sieve.

Trial division iterates until it reaches either the smallest prime factor of a number or its square root. Most composites are identified fairly quickly; it’s the primes that take longer, as all the trial divisors fail one by one. In general, the time complexity of trial division is O(√n), and if n is large it will take a very long time. Thus, trial division is best limited to cases where n is small, less than a billion or thereabouts. If n is large, it is common to use probabilistic methods to distinguish primes from composites, as the next section will show.

[ Return to top ]

 

3 Factorization by Trial Division

The problem of breaking down a composite integer into its prime factors has fascinated mathematicians from the ancient Greeks to modern times. The ancient Greek mathematicians proved the Fundamental Theorem of Arithmetic that the factorization of a number is unique, ignoring the order of the factors. Modern mathematicians use the difficulty of the factorization process to provide cryptographic security for the internet. Factoring integers is a hard and honorable problem.

Euclid gave the proof of the Fundamental Theorem of Arithmetic in his book Elements. We assume without proof that if p divides ab then either p divides a or p divides b; this assertion is known as Euclid’s Lemma and was proved in Elements, Book VII, Proposition 7. The proof is in two parts: first a proof that all positive integers greater than 1 can be written as the product of primes, and second a proof that the factorization is unique.

Suppose that n is the smallest positive integer greater than 1 that cannot be writen as the product of primes. Now n cannot be prime because such a number is the product of a single prime, itself. Thus the composite is n = a · b, where both a and b are positive integers less than n. Since n is the smallest number that cannot be written as the product of primes, both a and b must be able to be written as the product of primes. But then n = a · b can be written as the product of primes simply by combining the factorizations of a and b. But that contradicts our supposition. Therefore, all positive integers greater than 1 can be written as the product of primes.

Furthermore, that factorization is unique, ignoring the order in which the primes are written. Now suppose that s is the smallest positive integer greater than 1 that can be written as two different products of prime numbers, so that s = p1 · p2 · … · pm = q1 · q2 · … · qn. By Euclid’s lemma either p1 divides q1 or p1 divides q2 · … · qn. Therefore p1 = qk for some k. But removing p1 and qk from the initial equivalence leaves a smaller integer that can be factored in two ways, contradicting the initial supposition. Thus there can be no such s, and all integers greater than 1 have a unique factorization. — Euclid, Elements, Book IX, Proposition 14, circa 300 B.C.

There are many algorithms for factoring integers, of which the simplest is trial division. Divide the number being factored by 2, then 3, then 4, and so on. When a factor divides evenly, record it, divide the original number by the factor, then continue the process until the remaining cofactor is prime. A simple optimization notes that once the factors of 2 have been removed from a number, it is odd, and all its factors will be odd, so it is only necessary to perform trial division by the odd numbers starting from 3, thus halving the work required to be done. A second optimization stops the trial division when the factor being tried exceeds the square root of the current cofactor, indicating that the current cofactor is prime and is thus the final factor of the original number. Here is a formal statement of the trial division factorization algorithm:

Algorithm 3. Integer Factorization by Trial Division: Find the prime factors of a composite integer n > 1:
1. [Remove factors of 2] If n is even, output the factor 2, set n ← n ÷ 2, and go to Step 1.
2. [Initialize for odd factors] If n = 1, stop. Otherwise, set f ← 3.
3. [Terminate if prime] If n < f · f, output the factor n and stop.
4. [Trial division] Calculate the quotient q and remainder r when dividing n by f, so that n = q f + r with 0 ≤ r < f.
5. [Loop on odd integers] If r > 0, set ff + 2 and go to Step 3. Otherwise, output the factor f, set nq, and go to Step 3.

As an example, we find the factors of n = 13195. Since 13195 is odd, Step 1 does nothing and we go to Step 2, where f = 3. Since 3 · 3 < 13195, we calculate q = 13195 ÷ 3 = 4398 and r = 1 in Step 4, so in Step 5 we set f = 3 + 2 = 5 and go to Step 3. Since 5 · 5 < 13195, we calculate q = 13195 ÷ 5 = 2639 and r = 0 in Step 4, so in Step 5 we output the factor 5, set n = 2639, and go to Step 3. Since 5 · 5 < 2639, we calculate q = 2639 ÷ 5 = 527 and r = 4 in Step 4, so in Step 5 we set f = 5 + 2 = 7 and go to Step 3. Since 7 · 7 < 2639, we calculate q = 2639 ÷ 7 = 377 and r = 0 in Step 4, so in Step 5 we output the factor 7, set n = 377, and go to Step 3. Since 7 · 7 < 377, we calculate q = 377 ÷ 7 = 53 and r = 6 in Step 4, so in Step 5 we set f = 7 + 2 = 9 and go to Step 3. Since 9 · 9 < 377, we calculate q = 377 ÷ 9 = 41 and r = 8 in Step 4, so in Step 5 we set f = 9 + 2 = 11 and go to Step 3. Since 11 · 11 < 377, we calculate q = 377 ÷ 11 = 34 and r = 3 in Step 4, so in Step 5 we set f = 11 + 2 = 13 and go to Step 3. Since 13 · 13 < 377, we calculate q = 377 ÷ 13 = 29 and r = 0 in Step 4, so in Step 5 we output the factor 13, set n = 29, and go to Step 3. Since 29 < 13 · 13, we output the factor 29 in Step 3 and stop. The complete factorization is 13195 = 5 · 7 · 13 · 29.

Step 1 removes factors of 2 from n. If you like, you can extend that to other primes: remove factors of 3, then factors of 5, then factors of 7, and so on, never wasting the time to trial-divide by a composite. Of course, that requires you to pre-compute and store the primes up to the square root of n, which may be inconvenient. If you prefer, there is a method known as wheel factorization, akin to Pritchard’s sieving wheels, that achieves most of the benefits of trial division by primes but requires only a small, constant amount of extra space to store the wheel.

The time complexity of trial division is O(√n), as all trial divisors up to the square root of the number being factored must potentially be tried. In practice, you will generally want to choose a bound on the maximum divisor you are willing to test, depending on the speed of your hardware and on your patience; generally speaking, the bound should be fairly small, perhaps somewhere between a thousand and a million. Then, if you have reached the bound without completing the factorization, you can turn to a different, more potent, method of integer factorization.

[ Return to top ]

 

4 Miller-Rabin Pseudoprimality Checking

As we saw above, trial division can be used to determine if a number n is prime or composite, but is very slow. Often it is sufficient to show that n is probably prime, which is a very much faster calculation; the French number theorist Henri Cohen calls numbers that pass a probable-prime test industrial-grade primes. The most common method of testing an industrial-grade prime is based on a theorem that dates to 1640.

Pierre de Fermat was a French lawyer at the Parlement in Toulouse, a jurist, and an amateur mathematician who worked in number theory, probability, analytic geometry, differential calculus, and optics. Fermat’s Little Theorem states that if p is a prime number, then for any integer a it is true that apa (mod p), which is frequently stated in the equivalent form ap−1 ≡ 1 (mod p) by dividing both sides of the congruence by a, assuming a ≠ 0. As was his habit, Fermat gave no proof for his theorem, which was proved by Gottfried Wilhelm von Leibniz forty years later and first published by Leonhard Euler in 1736. This formula gives us a way to distinguish primes from composites: if we can find an a for which Fermat’s Little Theorem fails, then p must be composite.

But there is a problem. There are some numbers, known as Carmichael numbers, that are composite but pass Fermat’s test for all a; the smallest Carmichael number is 561 = 3 · 11 · 17, and the sequence begins 561, 1105, 1729, 2465, 2821, 6601, 8911, 10585, 15841, 29341, …. In 1976, Gary Lee Miller, a computer science professor at Carnegie Mellon University, developed an alternate test for which there are no strong liars, that is, numbers for which there is no a that distinguishes prime from composite. A strong pseudoprime to base a is an odd composite number n = d · 2s + 1 with d odd for which either ad ≡ 1 (mod n) or ad·2r ≡ −1 (mod n) for some r = 0, 1, …, s−1. This works because n = 2m + 1 is odd, so we can rewrite Fermat’s Little Theorem as a2m − 1 ≡ (am − 1)(am + 1) ≡ 0 (mod n). If n is prime, it must divide one of the factors, but can’t divide both because it would then divide their difference (am + 1) − (am − 1) = 2. Miller’s observation leads to the following algorithm:

Algorithm 4.A. Strong-Pseudoprime Test: Determine if a on the range 1 < a < n is a witness to the compositeness of an odd integer n > 2.
1. [Initialize] Set dn − 1. Set s ← 0.
2. [Reduce while even] If d is even, set dd / 2, set ss + 1, and go to Step 2.
3. [Easy return?] Set tad (mod n). If t = 1 or t = n − 1, output PROBABLY PRIME and stop.
4. [Terminate?] Set ss − 1. If s = 0, output COMPOSITE and stop.
5. [Square and test] Set t = t2 (mod n). If t = n − 1, output PROBABLY PRIME and stop. Otherwise, go to Step 4.

The algorithm is stated differently than the math given above, though the result is the same. In the math, we calculated ad·2r, which is initially ad when r = 0, then a2d, then a4d, and so on; in other words, ad is squared at each step. Step 5 thus reduces the modular operation from exponentiation to multiplication; the strength reduction makes the code simpler and faster.

As an example, consider the prime number 73 = 23 · 9 + 1; at the end of Step 2, d = 9 and s = 3. If the witness is 2, then 29 ≡ 1 (mod 73) and 73 is declared PROBABLY PRIME in Step 3. If the witness is 3, then 39 ≡ 46 (mod 73) and the test of Step 3 is indeterminate, but 32·9 ≡ 72 ≡ −1 (mod 73) in the second iteration of Step 5, and 73 is declared PROBABLY PRIME. On the other hand, the composite number 75 = 21 · 37 + 1 is declared COMPOSITE with witness 2 because 237 ≡ 47 (mod 75) in Step 3. One more example is the composite number 2047 = 23 · 89;, which is declared PROBABLY PRIME by the witness 2 but COMPOSITE by the witness 3; 2047 is the smallest number for which 2 is a strong liar to its compositeness.

Miller proved that n must be prime if no a from 2 to 70 (loge n)2 is a witness to the compositeness of n; Eric Bach, a professor at the University of Wisconsin in Madison, later reduced the constant from 70 to 2. Unfortunately, the proof assumes the Riemann Hypothesis and can’t be relied upon because the Riemann Hypothesis remains unproven. However, Michael O. Rabin, an Israeli computer scientist and recipient of the Turing Award, used Miller’s strong pseudo-prime test to build a probabilistic primality test. Rabin proved that for any odd composite n, at least 3/4 of the bases a are witnesses to the compositeness of n; although that’s the proven lower bound, in practice the proportion is much higher than 3/4. Thus, the Miller-Rabin method performs k strong pseudo-prime tests, each with a different a, and if all the tests pass the method concludes that n is prime with probability at least 4k, and in practice much higher; a common value of k is 25, which gives a maximum probability of 1 error in 1017.

Algorithm 4.B. Miller-Rabin Pseudo-Primality Test: Determine if an odd integer n > 2 is probably prime by performing k strong pseudo-prime tests:
1. [Terminate?] If k = 0, output PROBABLY PRIME and stop.
2. [Strong pseudo-prime test] Choose a random number a such that 1 < a < n. Perform a strong pseudo-prime test using Algorithm 4.A. to determine if a is a witness to the compositeness of n.
3. [Pseudo-prime?] If the strong pseudo-prime test indicates a is a witness to the compositeness of n, output COMPOSITE and stop. Otherwise, set kk − 1 and go to Step 1.

Although the algorithm given above specifies random numbers for the bases of the strong pseudo-prime test, it is common to fix the bases in advance, based on the value of n. If n is a 32-bit integer, it is sufficient to test on the three bases 2, 7, and 61; all the odd numbers less than 232 have been tested and no errors in the determination of primality exist. If n is less than a trillion, it is sufficient to test to the bases 2, 13, 23, and 1662803. Gerhard Jaesche used the first seven primes as bases and determined that the first false positive is 341550071728321. And Zhenxiang Zhang plausibly conjectures that there are no errors less than 1036 when using the first twenty primes as bases.

As an example, we determine the primality of 2149−1. Algorithm 4.A returns PROBABLY PRIME for witness 2 but COMPOSITE for witness 3, so 2149−1 = 86656268566282183151 · 8235109336690846723986161 is composite. That determination would be impossible for trial division, at least in any reasonable time frame, as the Prime Number Theorem suggests there are approximately 1.9 · 1018 primes to be tested.

Step 5 of Algorithm 4.A. requires modular exponentiation. Some languages provide modular exponentiation as a built-in function, but others don’t. If your language doesn’t, you will have to write your own function. You should not write your function by first performing the exponentiation and then performing the modulo operation, as the intermediate result of the exponentiation can be very large. Instead, use the square-and-multiply algorithm.

Algorithm 4.C. Modular Exponentiation: Compute be (mod m) with b, e and m all positive integers by the square-and-multiply algorithm:
1. [Initialize] Set r ← 1.
2. [Terminate when finished] If e = 0, return r and stop.
3. [Multiply if odd] If e is odd, set rr · b (mod n).
4. [Square and iterate] Set e ← ⌊e / 2⌋. Set bb2 (mod n). Go to Step 2.

Consider the calculation 43713 (mod 1741) = 819. Initially b = 437, e = 13, r = 1 and the test in Step 2 fails. Since e is odd, r = 1 · 437 = 437 in Step 3, then e = 13 / 2 = 6 and b = 4372 (mod 1741) = 1200 in Step 4 and the test in Step 2 fails. Since e is even, r = 437 is unchanged in Step 3, then e = 6 / 2 = 3 and b = 12002 (mod 1741) = 193 in Step 4 and the test in Step 2 fails. Since e is odd, r = 437 · 193 (mod 1741) = 773 in Step 3, then e = 3 / 2 = 1 and b = 1932 (mod 1741) = 688 in Step 4 and the test in Step 2 fails. Since e is odd, r = 773 · 688 (mod 1741) = 819 in Step 3, then e = 1 / 2 = 0 and b = 6882 (mod 1741) = 1533 in Step 4. At this point the test in Step 2 succeeds and the result r = 819 is returned. By the way, the intermediate calculation results in the very large number 43713 = 21196232792890476235164446315006597, so you can see why Algorithm 4.C is preferable.

The time complexity of the Miller-Rabin primality checker is O(1), which is vastly better than trial division. The time for the strong pseudo-prime test depends on the number of factors of 2 found in n − 1, which is independent of n. Likewise, the number k of strong pseudo-prime tests is independent of n, so it contributes to the implied constant, not to the overall order. If n is large, the arithmetic takes time O(log log n), but we ignore that in our analysis.

There are other methods for quickly checking the primality of a number, including the Baillie-Wagstaff method that combines a strong pseudoprime test base 2 with a Lucas pseudoprime test and the method of Mathematica that adds a strong pseudoprime test base 3 to the Baillie-Wagstaff method; both methods are faster than the Miller-Rabin method, and also give fewer false positives. If a slight chance of error is too much for you, and you need to prove the primality of a number, you can use the trial division of Algorithm 2, there is a method of Pocklington that uses the factorization of n − 1, a method using Jacobi sums (the APR-CL method), a method using elliptic curves due to Atkin and Morain, and the new AKS method, which operates in proven polynomial time but is not yet practical.

[ Return to top ]

 

5 Factorization by Pollard’s Rho Method

In 1975, British mathematician John Pollard invented a method of integer factorization that finds factors in time O(n1/4), which is the square root of the O(√n) time complexity of trial division. The method is simple to program and takes only a small amount of auxiliary space. Before we explain Pollard’s algorithm, we discuss two elements of mathematics on which it relies, the Chinese Remainder Theorem and the birthday paradox.

The original version of the Chinese Remainder Theorem was stated in the third-century by the Chinese mathematician Sun Zi in his book Sun Zi suanjing (literally, “The Mathematical Classic of Sun Zi”), and was proved by the Indian mathematician Aryabhata in the sixth century:

Let r and s be positive integers which are relatively prime and let a and b be any two integers. Then there exists an integer n such that na (mod r) and nb (mod s). Furthermore, n is unique modulo the product r · s.

Sun Zi gave an example: When a number is divided by 3, the remainder is 2. When the same number is divided by 5 the remainder is 3. And when the same number is divided by 7, the remainder is 2. The smallest number that satisfies all three criteria is 23, which you can verify easily. And since the least common multiple of 3, 5, and 7 is 105, any number of the form 23 + 105k, from the arithmetic progression 23, 128, 233, 338, …, is also a solution. Sun Zi used this method to count the men in his emperor’s armies; arrange them in columns of 11, then 12, then 13, take the remainder at each step, and calculate the number of soldiers.

In the theory of probability, the “birthday paradox” calculates the likelihood that in a group of p people two of them will have the same birthday. Obviously, in a group of 367 people the probability is 100%, since there are only 366 possible birthdays. What is surprising is that there is a 99% probability of a matching pair in a group as small as 57 people and a 50% probability of a matching pair in a group as small as 23 people. If, instead of birthdays, we consider integers modulo n, there is a 50% probability that two integers are congruent modulo n in a group of 1.177 √n integers.

Pollard’s rho algorithm uses the quadratic congruential random-number generator x2 + c (mod n) with c ∉ {0, −2} to generate a series of random integers xk. By the Chinese Remainder Theorem, if n = p · q, then x (mod n) corresponds uniquely to the pair of integers x (mod p) and x (mod q). Furthermore, the xk sequence also follows the Chinese Remainder Theorem, so that xk+1 = [xk (mod p)]2 + c (mod p) and xk+1 = [xk (mod q)]2 + c (mod q) so that the sequence of xk falls into a much shorter cycle of length √p by the birthday paradox. Thus p is identified when xk and xk+1 are congruent modulo p, which can be determined when gcd(|xkxk+1|, n) = p is between 1 and n.

Depending on the values of p, q and c, it is possible that the random-number generator may reach a cycle before a factor is found. Thus, Pollard used Robert Floyd’s tortoise-and-hare cycle-detection method. The sequence of xs starts with two values the same, call them t and h. Then each time t is incremented, h is incremented twice; the hare runs twice as fast as the tortoise. If the hare reaches the tortoise, that is, t = h (mod n), before a factor is found, then a cycle has been reached and further work is pointless. At that point, either the factorization attempt can be abandoned or a new random-number generator can be tried by using a different c.

Pollard called his method “Monte Carlo factorization” because of the use of random numbers. The algorithm is now called the rho algorithm because the sequence of x values has an initial tail followed by a cycle, giving it the shape of the Greek letter rho ρ. Fortunately the algorithm is much simpler than the explanation.

Algorithm 5.A. Pollard’s Rho Method: Find a factor of an odd composite integer n > 1:
1. [Initialization] Set t ← 2, h ← 2, and c ← 1. Define the function f(x) = x2 + c (mod n).
2. [Iteration] Set tf(t), hf(f(h)), and d ← gcd(th, n). If d = 1, go to Step 2.
3. [Termination] If d < n, output d and stop. Otherwise, either stop with failure or continue by setting t ← 2, h ← 2, and cc + 1, redefining the function f(x) using the new value of c and going to Step 2.

As an example, we consider the factorization of 8051. Initially, t=2, h=2, and c=1. After one iteration of Step 2, t=22+1=5, h=(22+1)2+1=26, and d=gcd(5−26,8051)=1. After the second iteration of Step 2, t=52+1=26, h=(262+1)2+1=7474, and d=gcd(26−7474,8051)=1. After the third iteration of Step 2, t=262+1=677, h=(74742+1)2+1=871 (mod 8051), and d=gcd(677-871,8051)=97, which is a factor of 8051; the complete factorization is 8051=83·97.

Be sure before you begin that n is composite; if n is prime, then d will always be 1 (because if n is prime it is always coprime to every other number) and the algorithm will loop forever. As with trial division, it is probably wise to set some bound on the maximum number of steps you are willing to take in the iteration of Step 2, because large factors can take a long time to find using this algorithm. You should also be careful not to let c be 0 or -2, because in those cases the random numbers aren’t very random. Note that the factor found in Step 3 may not be prime, in which case you can apply the algorithm again to the reduced factor, using a different c. And of course, once you have one factor, you can continue by factoring the remaining cofactor. Algorithm 5.B gives the complete factorization of a number.

Algorithm 5.B. Integer Factorization by Pollard’s Rho Method: Find all the prime factors of a composite integer n greater than 1:
1. [Remove factors of 2] If n is even, output the factor 2, set nn ÷ 2, and go to Step 1.
2. [Terminate if prime] If n is prime by the method of Algorithm 2.B, output the factor n and stop.
3. [Find a factor] Use Algorithm 5.A to find a factor of n and call it f. Output the factor f, set nn ÷ f, and go to Step 2.

There are two ways in which Pollard’s algorithm can be improved. First, it should bother you that each number in the random sequence is computed twice; it bothered the Australian mathematician Richard Brent, who devised a cycle-finding algorithm based on powers of 2 that computes each number in the random sequence only once, and it is Brent’s variant that is most often used today. A second improvement notes that for any a, b, and n, gcd(ab,n) > 1 if and only if at least one of gcd(a,n) > 1 or gcd(b,n) > 1, and accumulates the products of the elements of the t and h sequences for several steps (for large n, 100 steps is common) before computing the gcd, thus saving much time; if the gcd is n, then it is possible either that a cycle has been found or that two factors were found since the last gcd, in which case it is necessary to return to values saved from the previous gcd calculation and iterate one step at a time.

The time complexity of Pollard’s rho algorithm depends on the unknown factor d. By the birthday paradox, in the average case it will take 1.177 √d steps to find the factor, or O(√d). Thus, if n is the product of two primes, it will take O(n 1/4) to perform the factorization, assuming the two primes are roughly the same size. In other words, a million iterations of trial division will find factors up to a million, while a million iterations of Pollard’s rho method will find factors up to a trillion; that’s why you want to switch from trial division to Pollard’s rho method at a fairly low bound.

[ Return to top ]

 

6 Going Further

Although there is more to programming with prime numbers, we will stop here, since our small library has fulfilled our modest goals. The five appendices give implementations in C, Haskell, Java, Python, and Scheme, and the savvy reader will study all of them, because while they all implement exactly the same algorithms, each does so in a different way, and the differences are enlightening, about both the algorithms and the languages. The C appendix describes the tasteful use of the GMP multi-precision number library. The Haskell and Scheme appendices describe some of the syntax and semantics of those languages, on the assumption that they are unfamiliar to many readers. The Java appendix is the most faithful of all the appendices to the exact structure of the algorithms, including error-checking on the inputs as described in the preambles of each of the algorithms. The Python and Scheme appendices are the most “real-world” implementations, as they include error-checking on the inputs, bounds-checking to stop calculations that take too long, and even a non-mathematical but highly useful extension of the domain of the factoring functions.

Although our goals were modest, we have accomplished much. It’s hard to improve on the Sieve of Eratosthenes, and the Miller-Rabin primality checker will handle inputs of virtually unlimited size. The rho algorithm will find most factors up to a dozen digits or more, regardless of the size of the number being factored.

If Pollard’s rho algorithm won’t crack your composite, there are more powerful algorithms available, though they are beyond our modest aspirations. The elliptic curve method will find factors up to about thirty or forty digits (even fifty or sixty digits if you are patient). The quadratic sieve will split semi-primes up to about 90 digits on a single personal computer or 120 digits on a modest network of personal computers, and the number field sieve will split semi-primes up to about 200 digits on that same network. At the time this of this writing, the current record factorization is 231 decimal digits (768 bits), which took a team of experts about 2000 PC-years, and about eight months of calendar time, on a “network” of computers around the world connected via email.

If your goal isn’t self-study and you really want to factor a large number, and the rho technique fails, you have several options. A good first step is Dario Alpern’s factorization applet at http://­www.­alpertron.­com.­ar/­ECM.­HTM. Paul-Zimmermann’s gmp-ecm program at http://­ecm.­gforge.­inria.­fr uses a combination of trial division, Pollard’s rho algorithm, another algorithm of Pollard known as p−1, and Hendrik Lenstra’s elliptic curve method to find factors. Jason Papadopoulos’ msieve program at http://­www.­boo.­net/­~jasonp/­qs.­html uses both the quadratic sieve of Carl Pomerance and the number field sieve of John Pollard.

There is much more to prime numbers and integer factorization than we have discussed here; for instance, there are methods other than trial division for proving the primality of large numbers (several hundred digits) and methods other than enumeration with a sieve for counting the primes less than a given input number. At the end of each algorithm above was a discussion of alternatives; the interested reader will find that web searches for the topics mentioned will be fruitful and interesting. A superb reference for programmers is the book Prime Numbers: A Computational Perspective by Richard Crandall and Carl B. Pomerance (be sure to look for the second edition, which includes discussion of the new AKS primality prover); beware that although the approach is computational, there is still heavy mathematical content in the book. You may also be interested in the Programming Praxis web site at http://­programmingpraxis.­com, which has many exercises on the theme of prime numbers.

[ Return to top ]

 

Appendix: C

C is a small language with limited data types; integers are limited to what the underlying hardware provides, there are no lists, and there are no bitarrays, so we have to depend on libraries to provide those things for us. Since we are interested in prime numbers and not lists or bitarrays, we will write the smallest libraries that are necessary to get a working program. We begin with bitarrays, which are represented as arrays of characters in which we set and clear individual bits using macros:

#define ISBITSET(x, i) (( x[i>>3] & (1<<(i&7)) ) != 0)
#define SETBIT(x, i) x[i>>3] |= (1<<(i&7));
#define CLEARBIT(x, i) x[i>>3] &= (1<<(i&7)) ^ 0xFF;

To declare a bitarray b of length 8n, say char b[n], and to initialize each bit to 0, say memset(b, 0, sizeof(b)); change the 0 to 255 to set each bit initially to 1. Here is our minimal list library:

typedef struct list {
    void *data;
    struct list *next;
} List;

List *insert(void *data, List *next)
{
    List *new;

    new = malloc(sizeof(List));
    new->data = data;
    new->next = next;
    return new;
}

List *insert_in_order(void *x, List *xs)
{
    if (xs == NULL || mpz_cmp(x, xs->data) < 0)
    {
        return insert(x, xs);
    }
    else
    {
        List *head = xs;
        while (xs->next != NULL && mpz_cmp(x, xs->next->data) > 0)
        {
            xs = xs->next;
        }
        xs->next = insert(x, xs->next);
        return head;
    }
}

List *reverse(List *list) {
    List *new = NULL;
    List *next;

    while (list != NULL)
    {
        next = list->next;
        list->next = new;
        new = list;
        list = next;
    }

    return new;
}

int length(List *xs)
{
    int len = 0;
    while (xs != NULL)
    {
        len += 1;
        xs = xs->next;
    }
    return len;
}

Lists are represented as structs of two members; the empty list is represented as NULL. The trickiest function is reverse, which operates in-place to make each list item point to its predecessor. The list functions leave all notion of memory management to the caller.

With that out of the way, we are ready to begin work on the prime number functions. Our version of the Sieve of Eratosthenes uses b for the bitarray, i indexes into the bitarray, and p = 2i + 3 is the number represented at location i of the bitarray. The first while loop identifies the sieving primes and performs the sieving in an inner while, and the third while loop sweeps up the remaining primes that survive the sieve.

List *primes(long n)
{
    int m = (n-1) / 2;
    char b[m/8+1];
    int i = 0;
    int p = 3;
    List *ps = NULL;
    int j;

    ps = insert((void *) 2, ps);

    memset(b, 255, sizeof(b));

    while (p*p < n)
    {
        if (ISBITSET(b,i))
        {
            ps = insert((void *) p, ps);
            j = (p*p - 3) / 2;
            while (j < m)
            {
                CLEARBIT(b, j);
                j += p;
            }
        }
        i += 1; p += 2;
    }

    while (i < m)
    {
        if (ISBITSET(b,i))
        {
            ps = insert((void *) p, ps);
        }
        i += 1; p += 2;
    }

    return reverse(ps);
}

We look next at the two algorithms that use trial division; we’ll look at them together because they are so similar. We used long integers for the Sieve of Eratosthenes because they are almost certainly big enough, but we will use long long unsigned integers for the two trial division functions because that extends the range of the inputs that we can consider. The function that tests primality using trial division uses an if to identify even numbers, then a while ranges over the odd numbers d from 3 to the square root of n.

int td_prime(long long unsigned n)
{
    if (n % 2 == 0)
    {
        return n == 2;
    }

    long long unsigned d = 3;

    while (d*d <= n)
    {
        if (n % d == 0)
        {
            return 0;
        }
        d += 2;
    }

    return 1;
}

The td_factors function is very similar to the td_prime function. The initial if becomes a while, because we no longer want to quit as soon as we find a single factor, and the body of the second while also changes so that it collects all the factors instead of quitting as soon as it finds a single factor; the factors are stacked in increasing order as they are discovered, hence the reversal. The list of factors, which will contain only the original input n if it is prime, is returned as the value of the function.

List *td_factors(long long unsigned n)
{
    List *fs = NULL;

    while (n % 2 == 0)
    {
        fs = insert((void *) 2, fs);
        n /= 2;
    }

    if (n == 1)
    {
        return reverse(fs);
    }

    long long unsigned f = 3;

    while (f*f <= n)
    {
        if (n % f == 0)
        {
            fs = insert((void *) f, fs);
            n /= f;
        }
        else
        {
            f += 2;
        }
    }

    fs = insert((void *) n, fs);
    return reverse(fs);
}

We used long integers for the Sieve of Eratosthenes and long long unsigned integers for the two trial-division algorithms. Those native integer types are sufficient for those applications; usually only small n are required for the Sieve of Eratosthenes, and trial division is just too slow for large n. But many applications require much larger numbers, so we need a big-integer library, and we choose the GMP library from GNU, which is well-known for its useful interface and fast, bug-free implementation. You can obtain GMP from gmplib.org; to use it in your program, include the line #include <gmp.h> at the top of your program, and link with the option -lgmp.

We look next at Gary Miller’s strong pseudo-prime test. The first while computes d and s, then the if checks for an early return, the second while computes and tests the powers of a, and the default return is composite.

int is_spsp(mpz_t n, mpz_t a)
{
    mpz_t d, n1, t;
    mpz_inits(d, n1, t, NULL);
    mpz_sub_ui(n1, n, 1);
    mpz_set(d, n1);
    int s = 0;

    while (mpz_even_p(d))
    {
        mpz_divexact_ui(d, d, 2);
        s += 1;
    }

    mpz_powm(t, a, d, n);
    if (mpz_cmp_ui(t, 1) == 0 || mpz_cmp(t, n1) == 0)
    {
        mpz_clears(d, n1, t, NULL);
        return 1;
    }

    while (--s > 0)
    {
        mpz_mul(t, t, t);
        mpz_mod(t, t, n);
        if (mpz_cmp(t, n1) == 0)
        {
            mpz_clears(d, n1, t, NULL);
            return 1;
        }
    }

    mpz_clears(d, n1, t, NULL);
    return 0;
}

Let’s take a moment for a quick lesson in GMP. The datatype of big integers is given by mpz_t, where the mp is for multi-precision, z is for integer (from the German word Zahlen, for number), and _t is to indicate a type variable. All mpz_t variables must be initialized and cleared; in exchange for this effort, GMP takes care of all memory management automatically. The basic operations are given as mpz_add, mpz_mul, mpz_powm and the like, and they all return void, with the result given in the first argument (by analogy to an assignment, which puts the result on the left); the various division operators have their own naming conventions. Most of the operators have _ui variants in which the second operand (third argument) is a long unsigned integer instead of an mpz_t integer. Comparisons take two values and return a negative integer if the first is less than the second, a positive integer is the first is greater than the second, and 0 if the two are equal.

To determine whether a given integer is prime or composite we use 25 random integers, saving the GMP random state in a static variable that persists from one call of the function to the next. The algorithm expects an integer greater than 2, so that is our first test. Then we check that n is odd, and additionally that it is not divisible by 3, 5 or 7; those tests aren’t strictly part of the algorithm, but they eliminate about three-quarters of all positive integers, and if they determine the compositeness of n, they are much cheaper than the full Miller-Rabin test. Finally, if we don’t yet have an answer, we proceed with the full algorithm with k counting down to 0.

int is_prime(mpz_t n)
{
    static gmp_randstate_t gmpRandState;
    static int is_seeded = 0;

    if (! is_seeded)
    {
        gmp_randinit_default(gmpRandState);
        gmp_randseed_ui(gmpRandState, time(NULL));
        is_seeded = 1;
    }

    mpz_t a, n3, t;
    mpz_inits(a, n3, t, NULL);
    mpz_sub_ui(n3, n, 3);
    int i;
    int k = 25;
    long unsigned ps[] = { 2, 3, 5, 7 };

    if (mpz_cmp_ui(n, 2) < 0)
    {
        mpz_clears(a, n3, t, NULL);
        return 0;
    }

    for (i = 0; i < sizeof(ps) / sizeof(long unsigned); i++)
    {
        mpz_mod_ui(t, n, ps[i]);
        if (mpz_cmp_ui(t, 0) == 0)
        {
            mpz_clears(a, n3, t, NULL);
            return mpz_cmp_ui(n, ps[i]) == 0;
        }
    }

    while (k > 0)
    {
        mpz_urandomm(a, gmpRandState, n3);
        mpz_add_ui(a, a, 2);
        if (! is_spsp(n, a))
        {
            mpz_clears(a, n3, t, NULL);
            return 0;
        }
        k -= 1;
    }

    mpz_clears(a, n3, t, NULL);
    return 1;
}

The default GMP random number generator is the Mersenne Twister, which has good randomness properties and a very long period; we initialize the internal state of the random number generator with the current time (number of seconds since the epoch). The function mpz_urandomm returns in its first argument a uniformly-distributed pseudo-random non-negative integer less than its third argument, using and resetting the internal state of the random number generator in its second argument. GMP provides our is_prime function under the name mpz_probab_prime_p, but we give our own implementation anyway, so you can see how it is done.

There are two functions that implement integer factorization by pollard rho: rho_factor finds a single factor, and rho_factors performs the complete factorization. The rho_factor function assumes that n is odd and composite; t is the tortoise, h is the hare, d is the greatest common divisor, and r is a temporary working variable holding the difference between t and h. The function keeps cycling until it finds a prime factor, calling itself recursively with the next greater c if it reaches a cycle or finds a composite factor.

void rho_factor(mpz_t f, mpz_t n, long long unsigned c)
{
    mpz_t t, h, d, r;

    mpz_init_set_ui(t, 2);
    mpz_init_set_ui(h, 2);
    mpz_init_set_ui(d, 1);
    mpz_init_set_ui(r, 0);

    while (mpz_cmp_si(d, 1) == 0)
    {
        mpz_mul(t, t, t);
        mpz_add_ui(t, t, c);
        mpz_mod(t, t, n);

        mpz_mul(h, h, h);
        mpz_add_ui(h, h, c);
        mpz_mod(h, h, n);

        mpz_mul(h, h, h);
        mpz_add_ui(h, h, c);
        mpz_mod(h, h, n);

        mpz_sub(r, t, h);
        mpz_gcd(d, r, n);
    }

    if (mpz_cmp(d, n) == 0) /* cycle */
    {
        rho_factor(f, n, c+1);
    }
    else if (mpz_probab_prime_p(d, 25)) /* success */
    {
        mpz_set(f, d);
    }
    else /* found composite factor */
    {
        rho_factor(f, d, c+1);
    }

    mpz_clears(t, h, d, r, NULL);
}

The rho-factors function extracts factors of 2 in the first while, then calls rho-factor repeatedly in the second while until the remaining cofactor is prime. Rho-factors returns the list of factors in its first argument, like all the GMP functions.

void rho_factors(List **fs, mpz_t n)
{

    while (mpz_even_p(n))
    {
        mpz_t *f = malloc(sizeof(*f));
        mpz_init_set_ui(*f, 2);
        *fs = insert(*f, *fs);
        mpz_divexact_ui(n, n, 2);
    }

    if (mpz_cmp_ui(n, 1) == 0) return;

    while (! (mpz_probab_prime_p(n, 25)))
    {
        mpz_t *f = malloc(sizeof(*f));
        mpz_init_set_ui(*f, 0);

        rho_factor(*f, n, 1);
        *fs = insert_in_order(*f, *fs);
        mpz_divexact(n, n, *f);
    }

    *fs = insert_in_order(n, *fs);
}

We demonstrate the functions shown above with this main function:

int main(int argc, char *argv[])
{
    mpz_t n;
    mpz_init(n);
    List *ps = NULL;
    List *fs = NULL;

    ps = primes(100); /* 2 3 5 7 11 13 17 19 23 29 31 37 41 */
    while (ps != NULL) /* 43 47 53 59 61 67 71 73 79 83 89 97 */
    {
        printf("%ld%s", (long) ps->data,
            (ps->next == NULL) ? "\n" : " ");
        ps = ps->next;
    }

    printf("%d\n", length(primes(1000000))); /* 78498 */

    printf("%d\n", td_prime(600851475143LL)); /* composite */

    fs = td_factors(600851475143LL); /* 71 839 1471 6857 */
    while (fs != NULL)
    {
        printf("%llu%s", (unsigned long long int) fs->data,
            (fs->next == NULL) ? "\n" : " ");
        fs = fs->next;
    }

    mpz_t a;
    mpz_init(a);
    mpz_set_str(n, "2047", 10);
    mpz_set_str(a, "2", 10);
    printf("%d\n", is_spsp(n, a)); /* pseudo-prime */

    mpz_set_str(n, "600851475143", 10); /* composite */
    printf("%d\n", is_prime(n));

    mpz_set_str(n, "2305843009213693951", 10); /* prime */
    printf("%d\n", is_prime(n));

    mpz_set_str(n, "600851475143", 10);
    rho_factors(&fs, n); /* 71 839 1471 6857 */
    while (fs != NULL) {
        printf("%s%s", mpz_get_str(NULL, 10, fs->data),
            (fs->next == NULL) ? "\n" : " ");
        fs = fs->next;
    }
}

To compile the program, say gcc prime.c -lgmp -o prime , and to run the program say ./prime. If you get any warnings about the cast to void * you can safely ignore them, as it is always permissible to cast to void. Here is the output from the program:

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
78498
0
71 839 1471 6857
1
0
1
71 839 1471 6857

You can see the program assembled at http://­ideone.­com/­VHDJL, but you won’t be able to run it because the environment doesn’t provide the GMP library. An abbreviated version of the program that provides only the Sieve of Eratosthenes and the two trial-division algorithms that use native integers is available at http://­ideone.­com/­MOGsZ.

[ Return to top ]

 

Appendix: Haskell

Haskell is the classic purely functional language, far different from C. We begin our look at Haskell by examining a function that is frequently cited as the Sieve of Eratosthenes. Many texts include a definition something like this:

primes = sieve [2..]

sieve (p:xs) p : sieve [x | x <- xs, x `mod` p > 0]

But this is not the Sieve of Eratosthenes because it is based on division (the mod operator) rather than addition. It will quickly become slow as the primes grow larger; try, for instance, to extract a list of the primes less than a hundred thousand. Unfortunately, a proper implementation of the Sieve of Eratosthenes is a little bit ugly, since Haskell and arrays don’t easily mix.

Most Haskell programs begin by importing various functions from Haskell’s Standard Libraries, and ours is no exception; all imports must appear before any executable code. ST is the Haskell state-transformer monad, which provides mutable data structures, including Control.Monad.ST and Data.Array.ST. Control.Monad provides imperative-style for and when, and Data.Array.Unboxed provides arrays that store data directly, rather than with a pointer, as long as the data is a suitable type; we will be using arrays of booleans, which are suitable. Finally, Data.List provides a sort function for use by Pollard’s rho algorithm.

import Control.Monad (forM_, when)
import Control.Monad.ST
import Data.Array.ST
import Data.Array.Unboxed
import Data.List (sort)

The Sieve of Eratosthenes is implemented using exactly the same algorithm as all the other languages, though it looks somewhat foreign to imperative-trained eyes. Functions in Haskell optionally begin with a declaration of the type of the function, and we will include one in each of our functions. Thus, sieve :: Int -> UArray Int Bool declares an object named sieve that has type (the double colon) that is a function (the arrow) that takes a value of type Int and returns a value of type UArray Int Bool. Int is some fixed-size integer based on the native machine type. UArray is an unboxed array; Int is the type of its indices and Bool is the type of its values. Note that typenames always begin with a capital letter, as opposed to simple variables or function names that begin with a lower-case letter.

The body of the sieve function is fairly atypical of Haskell code, due to its use of arrays. The first line, runSTUArray $ do (that parses as run, ST for the state-transformer monad, UArray for the unboxed array), sets up the array processing; the array is initialized with indices from 0 to m−1, with all m values True, and assigned to variable bits. An expression like forM_ [0..x] $ \i do would be rendered in C as for (i=0; i<=x; i++); the expression [0 .. x] expands to 0, 1, … x, and is evaluated lazily, as if by a list generator, so the whole list is never reified all at once. Functions readArray and writeArray fetch and store elements of an array. Variable isPrime is assigned either True or False, depending on the value of the element of the bits array with value i. In the inner loop, iteration starts at 2*i*i+6*i+3, and each iteration steps by 2*i+3, which is the difference between the first and second elements of the list, continuing until j is greater than m−1.

sieve :: Int -> UArray Int Bool
sieve n = runSTUArray $ do
    let m = (n-1) `div` 2
        r = floor . sqrt $ fromIntegral n
    bits <- newArray (0, m-1) True
    forM_ [0 .. r `div` 2 - 1] $ \i -> do
        isPrime <- readArray bits i
        when isPrime $ do
            forM_ [2*i*i+6*i+3, 2*i*i+8*i+6 .. (m-1)] $ \j -> do
                writeArray bits j False
    return bits

The primes function is simple; assocs collects the elements of the bits in order, paired with their index, and those that are True are included in the out-going list of primes. The type signature indicates that the function takes an Int and returns a list of Int values, as indicated by the square brackets. The overall structure of the function is a list which has 2 as its head joined by a colon : to a list comprehension between square brackets []. The list comprehension has two parts. The expression 2*i+3 before the vertical bar | defines the elements of the output list. The generator after the bar assigns to the tuple all of the elements returned by the assocs function and keeps only those where the second element of the tuple is True, binding the first element of the tuple to the variable i that is used in the result expression.

primes :: Int -> [Int]
primes n = 2 : [2*i+3 | (i, True) <- assocs $ sieve n]

It is simple to test primality by trial division because Haskell offers a simple way of generating the list of 2 followed by odd numbers. The colon operator, pronounced “cons,” is the list constructor. The expression [3,5..] is a list constructor (anything surrounded by square brackets is a list) with 3 as its first element, 5 as its second element, and so on in an arithmetic progression that increases by 5 − 3 = 2 at each step. The .. operator at the end of the list expression signifies that the list goes on forever; if there is a value after the .. operator, that is the ending value included in list.

tdPrime :: Int -> Bool
tdPrime n = prime (2:[3,5..])
    where prime (d:ds)
            | n < d * d      = True
            | n `mod` d == 0 = False
            | otherwise      = prime ds

Factorization by trial division is expressed more simply in Haskell than in the other languages because Haskell provides an easy way to build the list of trial divisors: the expression 2:[3,5..] is an infinite list generator that returns 2 followed by the odd integers. The guard expressions of the local facts function makes tdFactors very easy to read. Note that we used a where clause, but could equally have used a letin; in this case the choice is a matter of personal preference, though there are other situations where one or the other is required.

tdFactors :: Int -> [Int]
tdFactors n = facts n (2:[3,5..])
    where facts n (f:fs)
            | n < f * f      = [n]
            | n `mod` f == 0 = f : facts (n `div` f) (f:fs)
            | otherwise      = facts n fs

As in C, we have gone as far as we can using native integers, and we’ll switch at this point to big integers; note that Haskell has no long integers, so it’s more restrictive than C. The switch is simpler for Haskell than for C, since big integers are provided directly in the language, in the Integer datatype. For the Miller-Rabin primality test, we first need to write the function to perform modular exponentiation, since Haskell doesn’t provide one in any of its standard libraries:

powmod :: Integer -> Integer -> Integer -> Integer
powmod b e m =
    let times p q = (p*q) `mod` m
        pow b e x
            | e == 0    = x
            | even e    = pow (times b b) (e `div` 2) x
            | otherwise = pow (times b b) (e `div` 2) (times b x)
    in  pow b e 1

This function is rather more typical of Haskell than the sieve function that used arrays. The signature indicates that the function takes three Integer values and returns an Integer value. The function is written as if it is three functions because all functions in Haskell are curried, so powmod is actually a function that takes an integer b and returns a function that takes an integer e that returns a function that takes an integer m and returns an integer; thus, it is only a colloquialism, and frankly wrong, to say that powmod is a function that takes three integers and returns an integer. The letin … defines local values. Local function times performs modular multiplication mod m. Local function pow has three definition, each with a guard (the predicate between the vertical bar | and the equal sign =); the expression corresponding to the first matching guard predicate is calculated and returned as the value of the function. Here, the first guard expression checks for termination and the other two expressions call the pow function recursively.

The strong pseudo-prime test is implemented by three local functions in the isSpsp function: getDandS extracts the powers of 2 from n−1, spsp takes the tuple returned by getDandS as input and performs the easy-return test, and doSpsp computes and tests the powers of a. Note that mod and div are curried prefix functions; the back-quotes turn them into binary infix functions. Note also that if is an expression in Haskell, as opposed to a statement in imperative languages, which means that it returns a value instead of controlling program flow; thus there may be no else-less if, and both consequents of the if must have the same type.

isSpsp :: Integer -> Integer -> Bool
isSpsp n a =
    let getDandS d s =
            if even d then getDandS (d `div` 2) (s+1) else (d, s)
        spsp (d, s) =
            let t = powmod a d n
            in  if t == 1 then True else doSpsp t s
        doSpsp t s
            | s == 0     = False
            | t == (n-1) = True
            | otherwise  = doSpsp ((t*t) `mod` n) (s-1)
    in  spsp $ getDandS (n-1) 0

Haskell makes it difficult to work with random numbers (it’s possible, though inconvenient, in the same way that arrays were inconvenient in the Sieve of Eratosthenes) because they require a state to be maintained from one call to the next, so we use the primes less than a hundred as the bases for the Miller-Rabin primality test.

isPrime :: Integer -> Bool
isPrime n =
    let ps = [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]
    in  n `elem` ps || all (isSpsp n) ps

The rhoFactor function finds a single factor by the rho method. Function f is the random-number generator. Function fact implements the tortoise-and-hare loop recursively; the computations in the where clause are done prior to the guard expressions in the body of the function. The rhoFactor function is called recursively if the loop falls into a cycle (the d == n clause) or finds a composite factor (the otherwise clause).

rhoFactor :: Integer -> Integer -> Integer
rhoFactor n c =
    let f x = (x*x+c) `mod` n
        fact t h
            | d == 1    = fact t' h'
            | d == n    = rhoFactor n (c+1)
            | isPrime d = d
            | otherwise = rhoFactor d (c+1)
            where
                t' = f t
                h' = f (f h)
                d = gcd (t' - h') n
    in  fact 2 2

Function rhoFactors calls rhoFactor repeatedly until it completes the factorization of n. The first two clauses extract factors of 2, the third clause tests primality of a remaining cofactor, and the fourth clause adjusts n and the list of factors after calling rhoFactor. Note that we solved the problem of factoring a perfect power of 2 differently than in the C version of the program, stopping as soon as n is reduced to 2.

rhoFactors :: Integer -> [Integer]
rhoFactors n =
    let facts n
            | n == 2    = [2]
            | even n    = 2 : facts (n `div` 2)
            | isPrime n = [n]
            | otherwise = let f = rhoFactor n 1
                          in  f : facts (n `div` f)
    in  sort $ facts n

A main program that exercises the functions defined above is shown below. To compile the program with the GHC compiler, assuming it is stored in file primes.hs, say ghc -o prime prime.hs, and to run the program say ./prime.

main = do
    print $ primes 100
    print $ length $ primes 1000000
    print $ tdPrime 716151937
    print $ tdFactors 8051
    print $ powmod 437 13 1741
    print $ isSpsp 2047 2
    print $ isPrime 600851475143
    print $ isPrime 2305843009213693951
    print $ rhoFactors 600851475143

The output is the same as the C version of the program, except for the change in the input to tdFactors. You can run the program at http://­ideone.­com/­uc0M8.

[ Return to top ]

 

Appendix: Java

Java is an object-oriented language, widely used, with a very large collection of libraries. Like Haskell, Java provides big integers, linked lists and bit arrays natively, so we can quickly jump in to the coding. The functions are shown below, but we leave it to you to package them into classes as you wish; in the sample code we put all the functions into class Main. We will be more careful here than in the two prior versions to ensure that we validate all the input arguments. We begin with the Sieve of Eratosthenes, which we limit to ints, but if you prefer a larger data type you are free to change it.

public static LinkedList primes(int n)
{
    if (n < 2)
    {
        throw new IllegalArgumentException("must be greater than one");
    }

    int m = (n-1) / 2;
    BitSet b = new BitSet(m);
    b.set(0, b.size(), true);

    int i = 0;
    int p = 3;
    LinkedList ps = new LinkedList();
    ps.add(2);

    while (p * p < n)
    {
        if (b.get(i))
        {
            ps.add(p);
            int j = 2*i*i + 6*i + 3;
            while (j < m)
            {
                b.clear(j);
                j = j + 2*i + 3;
            }
        }
        i += 1; p += 2;
    }

    while (i < m)
    {
        if (b.get(i))
        {
            ps.add(p);
        }
        i += 1; p += 2;
    }

    return ps;
}

We used the built-in exception IllegalArgumentException instead of creating our own exception; it’s easier, and just as clear. We also used the built-in data types BitSet and LinkedList; indeed, it is one of the benefits of programming in Java that the standard libraries provide so much useful code.

Another of the libraries that Java provides is the BigInteger library, and we switch from normal integers to BigInteger for the rest of our functions; int is sufficient for the Sieve of Eratosthenes, because sieving with a large n produces too much output to be useful, but for the other functions BigInteger is definitely useful. The tdPrime function validates its input in the first if, checks for even numbers in the second if statement, and checks for odd divisors in the body of the while.

public static Boolean tdPrime(BigInteger n)
{
    BigInteger two = BigInteger.valueOf(2);
    
    if (n.compareTo(two) < 0)
    {
        throw new IllegalArgumentException("must be greater than one");
    }

    if (n.mod(two).equals(BigInteger.ZERO))
    {
        return n.equals(two);
    }

    BigInteger d = BigInteger.valueOf(3);

    while (d.multiply(d).compareTo(n) <= 0)
    {
        if (n.mod(d).equals(BigInteger.ZERO))
        {
            return false;
        }
        d = d.add(two);
    }

    return true;
}

The tdFactors function domain-checks the input, removes factors of two, and, if the remaining cofactor is not 1, begins a loop over the odd numbers starting from 3, trying each odd number in turn until it finds factors and the remaining cofactor is greater than the square of the current factor. As with the GMP functions in C, the messiness of doing arithmetic by calling functions hides the simplicity of the algorithm.

public static LinkedList tdFactors(BigInteger n)
{
    BigInteger two = BigInteger.valueOf(2);
    LinkedList fs = new LinkedList();

    if (n.compareTo(two) < 0)
    {
        throw new IllegalArgumentException("must be greater than one");
    }

    while (n.mod(two).equals(BigInteger.ZERO))
    {
        fs.add(two);
        n = n.divide(two);
    }

    if (n.compareTo(BigInteger.ONE) > 0)
    {
        BigInteger f = BigInteger.valueOf(3);
        while (f.multiply(f).compareTo(n) <= 0)
        {
            if (n.mod(f).equals(BigInteger.ZERO))
            {
                fs.add(f);
                n = n.divide(f);
            }
            else
            {
                f = f.add(two);
            }
        }
        fs.add(n);
    }

    return fs;
}

It is annoying that the add method is overloaded, with the same method name referring to the addition of two BigIntegers when used as f.add(two) and to the insertion of an item in a LinkedList when used as fs.add(f). Such usage may not be confusing to the compiler, because it keeps track of the types of all variables, but it can be confusing to the programmer who writes and reads the code and has to make sense of it.

The isSpsp function computes d and s in the first while loop, checks for an early termination in the if, then counts down s in the second while loop. Note that the early termination test is different in the Java version than the Haskell version; the Haskell version separates the early termination test from the n−1 tests, but the Java version combines the early termination test with the first loop of the n−1 tests. Both versions of the function get the right answer; the choice is based on the convenience of the programmer.

private static Boolean isSpsp(BigInteger n, BigInteger a)
{
    BigInteger two = BigInteger.valueOf(2);
    BigInteger n1 = n.subtract(BigInteger.ONE);
    BigInteger d = n1;
    int s = 0;

    while (d.mod(two).equals(BigInteger.ZERO))
    {
        d = d.divide(two);
        s += 1;
    }

    BigInteger t = a.modPow(d, n);

    if (t.equals(BigInteger.ONE) || t.equals(n1))
    {
        return true;
    }

    while (--s > 0)
    {
        t = t.multiply(t).mod(n);
        if (t.equals(n1))
        {
            return true;
        }
    }

    return false;
}

After using a predefined list of bases in the Haskell version of the function, we’re back to using random bases in the isPrime function. The two if tests check the input domain and exit quickly if the input is even, then the while loop performs 25 strong pseudo-prime tests.

public static Boolean isPrime(BigInteger n)
{
    Random r = new Random();
    BigInteger two = BigInteger.valueOf(2);
    BigInteger n3 = n.subtract(BigInteger.valueOf(3));
    BigInteger a;
    int k = 25;

    if (n.compareTo(two) < 0)
    {
        return false;
    }

    if (n.mod(two).equals(BigInteger.ZERO))
    {
        return n.equals(two);
    }

    while (k > 0)
    {
        a = new BigInteger(n.bitLength(), r).add(two);
        while (a.compareTo(n) >= 0)
        {
            a = new BigInteger(n.bitLength(), r).add(two);
        }

        if (! isSpsp(n, a))
        {
            return false;
        }

        k -= 1;
    }

    return true;
}

Note that Java’s BigInteger library includes a function isProbablePrime that performs this computation in exactly the same way.

The rhoFactor function races the tortoise and hare until the gcd is greater than 1, then the ifelse chain either returns a prime factor or retries the factorization with a different random function.

private static BigInteger rhoFactor(BigInteger n, BigInteger c)
{
    BigInteger t = BigInteger.valueOf(2);
    BigInteger h = BigInteger.valueOf(2);
    BigInteger d = BigInteger.ONE;

    while (d.equals(BigInteger.ONE))
    {
        t = t.multiply(t).add(c).mod(n);
        h = h.multiply(h).add(c).mod(n);
        h = h.multiply(h).add(c).mod(n);
        d = t.subtract(h).gcd(n);
    }

    if (d.equals(n)) /* cycle */
    {
        return rhoFactor(n, c.add(BigInteger.ONE));
    }
    else if (isPrime(d)) /* success */
    {
        return d;
    }
    else /* found composite factor */
    {
        return rhoFactor(d, c.add(BigInteger.ONE));
    }
}

The rhoFactors function first validates its input, then extracts factors of 2 in the first while, and, unless the input is a power of 2, calls rhoFactor repeatedly in the second while until the remaining cofactor is prime, sorting the list of factors before returning it. The built-in isProbablePrime function is called rather than the one we defined above.

public static LinkedList rhoFactors(BigInteger n)
{
    BigInteger f;
    BigInteger two = BigInteger.valueOf(2);
    LinkedList fs = new LinkedList();

    if (n.compareTo(two) < 0)
    {
        return fs;
    }

    while (n.mod(two).equals(BigInteger.ZERO))
    {
        n = n.divide(two);
        fs.add(two);
    }

    if (n.equals(BigInteger.ONE))
    {
        return fs;
    }

    while (! n.isProbablePrime(25))
    {
        f = rhoFactor(n, BigInteger.ONE);
        n = n.divide(f);
        fs.add(f);
    }

    fs.add(n);
    Collections.sort(fs);
    return fs;
}

To show examples of the use of these functions, we have to create a complete program with all of its imports and a class declaration. The program shown below is decidedly simple-minded, sufficient only to show a few examples; you will surely want to do arrange the class differently in your own programs. For sake of brevity, the function bodies are elided below.

import java.util.LinkedList;
import java.util.BitSet;
import java.math.BigInteger;
import java.lang.Exception;
import java.lang.Boolean;
 
class Main {
 
    public static LinkedList primes(int n) { ... }
    public static Boolean tdPrime(BigInteger n) { ... }
    public static LinkedList tdFactors(BigInteger n) { ... }
    private static Boolean isSpsp(BigInteger n, BigInteger a) { ... }
    public static Boolean isPrime(BigInteger n) { ... }
    private static Boolean rhoFactor(BigInteger n, BigInteger c) { ... }
    public static LinkedList rhoFactors(BigInteger n) { ... }
 
    public static void main (String[] args)
    {
        System.out.println(primes(100));
        System.out.println(primes(1000000).size());
        System.out.println(tdPrime(new BigInteger("600851475143")));
        System.out.println(tdFactors(new BigInteger("600851475143")));
        System.out.println(isPrime(new BigInteger("600851475143")));
        System.out.println(isPrime(new BigInteger("2305843009213693951")));
        System.out.println(rhoFactors(new BigInteger("600851475143")));
    }
 
}

Output from the program is the same as all the other implementations. You can run the program at http://­ideone.­com/­jHVJd.

[ Return to top ]

 

Appendix: Python

Python is a commonly-used scripting language with a reputation of being easy to read and write and a mixed imperative/object-oriented flavor. We’ll take the opportunity with Python to extend the domain of integer factorization beyond the integers greater than 1 that mathematicians generally consider. Specifically, we’ll consider −1, 0 and 1 to be prime, so they factor as themselves, and we’ll factor negative numbers by adding −1 to the factors of the corresponding positive number. This isn’t entirely correct, but it isn’t entirely incorrect, either, and is actually useful in some cases. And we’re in good company; Wolfram|Alpha calculates factors the same way we do.

We begin, as we did with Haskell, with a one-liner that purports to be the Sieve of Eratosthenes:

print [x for x in range(2,100) if not [y for y in range(2, int(x**0.5)+1) if x%y == 0]]

That prints the primes less than a hundred. But the expression if x%y == 0 at the end gives the game away: it’s really trial division (the % operator for modulo), so it’s not a sieve. Don’t be fooled by cute one-liners!

We begin with the Sieve of Eratosthenes. After validating the input, the first while loop collects the sieving primes and performs the sieving, and the second while loop collects the remaining primes that survived the sieve. Note that append adds an element after the target, unlike the function we wrote in C that inserts an element before the target.

def primes(n):
    if type(n) != int and type(n) != long:
        raise TypeError('must be integer')
    if n < 2:
        raise ValueError('must be greater than one')
    m = (n-1) // 2
    b = [True] * m
    i, p, ps = 0, 3, [2]
    while p*p < n:
        if b[i]:
            ps.append(p)
            j = 2*i*i + 6*i + 3
            while j < m:
                b[j] = False
                j = j + 2*i + 3
        i += 1; p += 2
    while i < m:
        if b[i]:
            ps.append(p)
        i += 1; p += 2
    return ps

The next function, td_prime, has three possible return values: PRIME, COMPOSITE, and UNKNOWN if the limit is exceeded. We use True and False for PRIME and COMPOSITE, and raise an OverflowError exception if the limit is exceeded, requiring some effort for the user to trap the error and respond accordingly. After validating the input, the if checks even numbers and the while loop checks odd numbers. Python’s optional-argument syntax is simple and convenient; the argument limit is optional, and is given a default value if not specified.

def td_prime(n, limit=1000000):
    if type(n) != int and type(n) != long:
        raise TypeError('must be integer')
    if n % 2 == 0:
        return n == 2
    d = 3
    while d * d <= n:
        if limit < d:
            raise OverflowError('limit exceeded')
        if n % d == 0:
            return False
        d += 2
    return True

The td_factors function has the same problem with the limit argument as td_prime, and we solve it the same way, using an OverflowError exception. The if becomes a while on the factors of 2, and the function collects factors instead of returning, but otherwise td_factors is very similar to td_prime.

def td_factors(n, limit=1000000):
    if type(n) != int and type(n) != long:
        raise TypeError('must be integer')
    fs = []
    while n % 2 == 0:
        fs += [2]
        n /= 2
    if n == 1:
        return fs
    f = 3
    while f * f <= n:
        if limit < f:
            raise OverflowError('limit exceeded')
        if n % f == 0:
            fs += [f]
            n /= f
        else:
            f += 2
    return fs + [n]

The Miller-Rabin primality checker makes use of Python’s ability to nest functions to hide the strong pseudo-prime checker inside the is_prime function. It also uses the built-in modular exponentiation function; with two arguments, pow(b,e) computes be, but with three arguments, pow(b,e,m) computes be (mod m). Python offers random numbers, but we prefer to test a fixed set of bases.

def is_prime(n):
    if type(n) != int and type(n) != long:
        raise TypeError('must be integer')
    if n < 2:
        return False
    ps = [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]
    def is_spsp(n, a):
        d, s = n-1, 0
        while d%2 == 0:
            d /= 2; s += 1
        t = pow(a,d,n)
        if t == 1:
            return True
        while s > 0:
            if t == n-1:
                return True
            t = (t*t) % n
            s -= 1
        return False
    if n in ps: return True
    for p in ps:
        if not is_spsp(n,p):
            return False
    return True

Like is_prime, the rho_factors function reduces namespace pollution by hiding local functions; notice the lambda, which is an alternate way of creating a local function. Again, as with Java, the + function is overloaded for both addition and list construction. The code is similar to Java, even if it looks quite different.

def rho_factors(n, limit=1000000):
    if type(n) != int and type(n) != long:
        raise TypeError('must be integer')
    def gcd(a,b):
        while b: a, b = b, a%b
        return abs(a)
    def rho_factor(n, c, limit):
        f = lambda(x): (x*x+c) % n
        t, h, d = 2, 2, 1
        while d == 1:
            if limit == 0:
                raise OverflowError('limit exceeded')
            t = f(t); h = f(f(h)); d = gcd(t-h, n)
        if d == n:
            return rho_factor(n, c+1, limit)
        if is_prime(d):
            return d
        return rho_factor(d, c+1, limit)
    if -1 <= n <= 1: return [n]
    if n < -1: return [-1] + rho_factors(-n, limit)
    fs = []
    while n % 2 == 0:
        n = n // 2; fs = fs + [2]
    if n == 1: return fs
    while not is_prime(n):
        f = rho_factor(n, 1, limit)
        n = n / f
        fs = fs + [f]
    return sorted(fs + [n])

We could import the gcd function from the fractions library, but instead we implement it ourselves because it gives us the chance to discuss this famous algorithm. Donald E. Knuth, in Volume 2, Section 4.5.2 of his book The Art of Computer Programming, calls this the “granddaddy” of all algorithms because it is the oldest nontrivial algorithm that has survived to the present day. The algorithm is commonly called the Euclidean algorithm because it was described in Book 7, Propositions 1 and 2 of Euclid’s Elements, but scholars believe the algorithm dates to about two hundred years before Euclid, sometime around 500 B.C. Knuth gives the entire history of the algorithm, and an extensive analysis of its time complexity, which is well worth your time. Euclid’s version of the algorithm worked by repeatedly subtracting the smaller amount from the larger until they are the same; the modern version of the algorithm replaces subtraction with division (the modulo operator).

Here are some sample calls to the functions defined above; the answers are the same as all the other implementations.

print primes(100)
print len(primes(1000000))
print td_prime(600851475143)
print td_factors(600851475143)
print is_prime(600851475143)
print is_prime(2305843009213693951)
print rho_factors(600851475143)

You can run the program at http://­ideone.­com/­OBOZQ.

[ Return to top ]

 

Appendix: Scheme

Scheme is primarily an academic language, useful for expressing algorithms in imperative, functional, and message-passing styles, with a fully-parenthesized prefix syntax derived from Lisp. Scheme provides big integers natively, and also lists, but has no bit arrays, so our implementation of the Sieve of Eratosthenes uses a vector of booleans, which uses eight bits per element instead of one but works perfectly well.

(define (primes n)
  (if (or (not (integer? n)) (< n 2))
      (error 'primes "must be integer greater than one")
      (let* ((len (quotient (- n 1) 2))
             (bits (make-vector len #t)))
        (let loop ((i 0) (p 3) (ps (list 2)))
          (cond ((< n (* p p))
                  (do ((i i (+ i 1)) (p p (+ p 2))
                       (ps ps (if (vector-ref bits i) (cons p ps) ps)))
                      ((= i len) (reverse ps))))
                ((vector-ref bits i)
                  (do ((j (+ (* 2 i i) (* 6 i) 3) (+ j p)))
                      ((<= len j) (loop (+ i 1) (+ p 2) (cons p ps)))
                    (vector-set! bits j #f)))
                (else (loop (+ i 1) (+ p 2) ps)))))))

Let’s take a moment for a quick lesson in Scheme. An expression like (let ((var1 value1) ...) body) establishes a local binding for each of several var/value pairs that is active in the body of the let; a let* expression is the same, except that the bindings are executed left-to-right, and each binding is available to those that follow.

There are two looping constructs. The named-let variant of let, given by (let name ((var1 value1) ...) body), is like let, but additionally binds name to a function with arguments vark whose code is the body of the let, which executes loops when it is called recursively; by convention, the variable name is often called loop, but it is sometimes convenient to use other names.

The other looping construct is do, which is similar to the for of C. The form of the do loop is (do ((var1 value1 next1) ...) (done? ret-value) body ...). Each var/value/next triplet in the first do clause specifies a variable name, a value for the variable when the do is initialized, and an expression evaluated at each step of the do; there may be multiple var/value/next triplets, in which case each is executed simultaneously, rather like a comma operator in a C for statement. The done? predicate terminates the do loop when it becomes true; this is the opposite of a C for loop, which terminates when the condition becomes false. The return value is optional; if it is not given, the return value of the do loop is unspecified. The statements in the body of the do loop are optional, and are evaluated only for their side effects.

Scheme also provides two conditional constructs. The first is (if cond then else), which first evaluates the condition then evaluates one of the two succeeding clauses; like Haskell, an if is an expression, not a control-flow statement. Cond is similar to a nested set of if statements; each clause consists of a condition and body, the conditions are read in order until one is true, when the corresponding body is evaluated.

In the example above, len is the length of the bitarray, called m in the description of the algorithm, and bits is the bitarray itself, a vector of booleans. The cond has three clauses. The first clause is actually the termination clause of Step 5 and Step 6 that is executed last; the body-less do sweeps up the primes after sieving is complete, and the return value is the list of primes, which must be reversed because each newly-found prime is pushed to the front, not the back, of an accumulating list of primes. The second clause sifts each prime, as in Step 4; this do has a body, which clears the jth element of the bitarray, and the return value is an expression that calls the named-let recursively to advance to the next sieving prime. The else clause recurs when p is not prime.

Our td-prime? function follows the Scheme convention that predicates (functions that return a boolean) have names that end in a question mark. An error is signaled if limit is less than the smallest prime factor of n; this isn’t as convenient as raising an exception in Python, because standard Scheme has no way to trap the error, but most implementations provide some kind of error trapping.

(define (td-prime? n . args)
  (if (or (not (integer? n)) (< n 2))
      (error 'td-prime? "must be integer greater than one")
      (let ((limit (if (pair? args) (car args) 1000000)))
        (if (even? n) (= n 2)
          (let loop ((d 3))
            (cond ((< limit d)
                    (error 'td-prime? "limit exceeded"))
                  ((< n (* d d)) #t)
                  ((zero? (modulo n d)) #f)
                  (else (loop (+ d 2)))))))))

Td-factors is similar to td-prime?, except that the first if becomes a loop, on twos, and both loops collect the factors that they find instead of stopping on the first factor. Here is a case where the names twos and odds in the named-let provide documentation of the nature of the loop, making the function clearer to the reader.

(define (td-factors n . args)
  (if (or (not (integer? n)) (< n 2))
      (error 'td-factors "must be integer greater than one")
      (let ((limit (if (pair? args) (car args) 1000000)))
        (let twos ((n n) (fs '()))
          (if (even? n)
              (twos (/ n 2) (cons 2 fs))
              (let odds ((n n) (d 3) (fs fs))
                (cond ((< limit d)
                        (error 'td-factors "limit exceeded"))
                      ((< n (* d d))
                        (reverse (cons n fs)))
                      ((zero? (modulo n d))
                        (odds (/ n d) d (cons d fs)))
                      (else (odds n (+ d 2) fs)))))))))

We’ve been using lists but haven’t mentioned how they work. A list is either null or is a pair with an item in its car and another list in its cdr; the terms car and cdr are pre-historic. An item x is inserted at the front of a list xs by (cons x xs); the word cons is short for construct. Predicates (null? xs) and (pair? xs) distinguish empty lists from non-empty ones. The null list is represented as '(), and the order of items in a list is reversed by (reverse xs).

The function prime? that implements the Miller-Rabin primality checker illustrates two more features of Scheme. We have been introducing functions with the notation (define (name args …) body), but the alternate notation is (define name (lambda (args …) body)). We use it here because we want variable seed to persist from one invocation of prime? to the next. Since the let is inside the define but outside the lambda, the variable bound by the let retains its value from one call of the function to the next, just like static variables in some programming languages. Thus, prime? is a closure, not just a function, because it encloses the seed variable. And while we’re talking about define, even though it doesn’t apply here, we have been using the dot-notation for some of our argument lists; a construct like (define (f args ... . rest) body) provides a variable-arity argument list, with all arguments after the dot collected into a list rest.

The other Scheme feature is internal-define, which is used to provide local functions that don’t pollute the global namespace. We define three local functions, rand that returns random numbers, expm that performs modular exponentiation (the name is a variant of expt that Scheme provides for the normal powering function) and spsp? that checks if a is a witness to the compositeness of n. And we’re not done; the internal definition expm has its own internal definition times for modular multiplication. An internal-define must appear immediately after another define, a lambda, or a let.

(define prime?
  (let ((seed 3141592654))
    (lambda (n)
      (define (rand)
        (set! seed (modulo (+ (* 69069 seed) 1234567) 4294967296))
        (+ (quotient (* seed (- n 2)) 4294967296) 2))
      (define (expm b e m)
        (define (times x y) (modulo (* x y) m))
        (let loop ((b b) (e e) (r 1))
          (if (zero? e) r
            (loop (times b b) (quotient e 2)
                  (if (odd? e) (times b r) r)))))
      (define (spsp? n a)
        (do ((d (- n 1) (/ d 2)) (s 0 (+ s 1)))
            ((odd? d)
              (let ((t (expm a d n)))
                (if (or (= t 1) (= t (- n 1))) #t
                  (do ((s (- s 1) (- s 1))
                       (t (expm t 2 n) (expm t 2 n)))
                      ((or (zero? s) (= t (- n 1)))
                        (positive? s))))))))
      (if (not (integer? n))
          (error 'prime? "must be integer")
          (if (< n 2) #f
            (do ((a (rand) (rand)) (k 25 (- k 1)))
                ((or (zero? k) (not (spsp? n a)))
                  (zero? k))))))))

In the prime? function we define our own random number generator, since standard Scheme doesn’t provide one; the static variable seed maintains the current state of the random number generator, which is of a type known as a linear-congruential generator. The multiplier 69069 is due to Knuth. Note that the seed is reset with each call to rand; the witness a is set to the range 1 to n, exclusive.

There are three do loops in the function. The first, the outer do in spsp?, binds two variables, d and s, and iterates until d is odd, performing Step 2 of Algorithm 4.A. The inner do in spsp? binds the two variables s and t and implements the loop of Step 4 and Step 5 of Algorithm 4.A, performing the modular squaring and terminating when s is zero or t is n−1. The result of the do-loop is computed by the predicate (positive? s), which is #t if tn−1 (mod n) and #f when s reaches 0 without finding tn−1 (mod n). The do in the main body of the function uses the same idiom of having two terminating conditions and using the finishing predicate to differentiate the two.

Our final function implements integer factorization by Pollard’s rho algorithm. The two internal definitions are cons<, which inserts an item into a list in ascending order instead of at the front, and rho, which implements the rho algorithm. The cons< function turns a vice into a virtue; since standard Scheme lacks a sort function, we insert the factors in order as we find them, instead of writing a sort function to sort them at the end, which gives the virtue of simpler code and is probably faster, given the short length of most lists of factors. The body of the function does error checking, extracts factors of 2, and assembles the complete factorization.

(define (rho-factors n . args)
  (define (cons< x xs)
    (cond ((null? xs) (list x))
          ((< x (car xs)) (cons x xs))
          (else (cons (car xs) (cons< x (cdr xs))))))
  (define (rho n limit)
    (let loop ((t 2) (h 2) (d 1) (c 1) (limit limit))
      (define (f x) (modulo (+ (* x x) c) n))
      (cond ((zero? limit) (error 'rho-factors "limit exceeded"))
            ((= d 1) (let ((t (f t)) (h (f (f h))))
                       (loop t h (gcd (- t h) n) c (- limit 1))))
            ((= d n) (loop 2 2 1 (+ c 1) (- limit 1)))
            ((prime? d) d)
            (else (rho d (- limit 1))))))
  (if (not (integer? n))
      (error 'rho-factors "must be integer")
      (let ((limit (if (pair? args) (car args) 1000)))
        (cond ((<= -1 n 1) (list n))
              ((negative? n) (cons -1 (rho-factors (- n) limit)))
              ((even? n)
                (if (= n 2) (list 2)
                  (cons 2 (rho-factors (/ n 2) limit))))
              (else (let loop ((n n) (fs '()))
                      (if (prime? n)
                          (cons< n fs)
                          (let ((f (rho n limit)))
                            (loop (/ n f) (cons< f fs))))))))))

Here are some examples:

> (primes 100)
(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)
> (length (primes 1000000))
78498
> (td-prime? 600851475143)
#f
> (td-factors 600851475143)
(71 839 1471 6857)
> (prime? 600851475143)
#f
> (prime? 2305843009213693951)
#t
> (rho-factors 600851475143)
(71 839 1471 6857)

You can run the program at http://­ideone.­com/­QC8bC.

As your reward for reading this far, we discuss a fast implementation of Pollard’s rho algorithm. We make two changes, one algorithmic and one code-tuning.

The first change is algorithmic: we replace Floyd’s turtle-and-hare cycle-finding algorithm, which Pollard used in his original version of the rho algorithm, with Brent’s powers-of-two cycle-finding algorithm. Each time the step-counter i is a power of two, the value of xi is saved; if a subsequent xj = xi is found before j = 2i, a cycle has been identified. Brent’s cycle-finding algorithm requires only one modular multiplication per step, instead of the three modular multiplications required by Floyd’s cycle-finding algorithm, so even though Brent’s method typically requires more steps that Floyd’s method, in practice the number of modular multiplications is generally about a quarter less than Floyd’s method, giving a welcome speed-up to Pollard’s factoring algorithm.

For instance, consider the sequence 1, 2, 3, 4, 5, 6, 3, 4, 5, 6, …. Initially j = 1, xj = 1, q = 2j = 2 and the saved x = 1. Then j = 2, xj = 2, q is reset to 4 and the saved x is reset to 2. Then j = 3, then j = 4, and q is reset to 8 and the saved x is reset to 4. The iteration continues until j = 8 and xj = 4, which equals the saved x, thus identifying the cycle.

The second change is code-tuning: we replace the gcd at each step with a gcd that is calculated only periodically, performing instead a modular multiplication at each step, which is much faster than a gcd calculation. This is done by taking the product of all the |xi+1xi| modulo n for several steps, then taking a gcd of the product at the end of the several steps. If the gcd is 1, then all the intermediate gcd calculations were also 1. If the gcd is prime, it is a factor of n. If the gcd is composite (including the case where the gcd is equal to n) it is necessary to retreat to the saved value of x from the prior gcd calculation and proceed step-by-step through the gcd calculations. The number of steps between successive gcd calculations varies with the size of n (bigger n means less frequent gcd calculations) and the number of trial divisions performed before starting the rho algorithm (more trial divisions means less frequent gcd calculations); values between 10 and 250 may be appropriate depending on the circumstances.

(define (brent n c limit)
  (define (f y) (modulo (+ (* y y) c) n))
  (define (g p x y) (modulo (* p (abs (- x y))) n))
  (let loop1 ((x 2) (y (+ 4 c)) (z (+ 4 c)) (j 1) (q 2) (p 1))
    (if (= j limit) (error 'brent "timeout")
      (if (= x y) (brent n (+ c 1) (- limit j)) ; cycle
        (if (= j q) (let ((t (f y))) (loop1 y (f y) z (+ j 1) (* q 2) (g p y t)))
          (if (positive? (modulo j 25)) (loop1 x (f y) z (+ j 1) q (g p x y))
            (let ((d (gcd p n)))
              (if (= d 1) (loop1 x (f y) y (+ j 1) q (g p x y))
                (if (< 1 d n) d ; factor
                  (let loop2 ((k 1) (z (f z)))
                    (if (= k 25) (brent n (+ c 1) (- limit j))
                      (let ((d (gcd (- z x) n)))
                        (if (= d 1) (loop2 (+ k 1) (f z))
                          (if (= d n) (brent n (+ c 1) (- limit j))
                            d))))))))))))))

Our function keeps three values of the random-number sequence: x is the running value, y is the value at the last gcd calculation, and z is the value at the last power of two. The main loop also defines j as the step counter, q as the next power of two, and p as the current product for the short-circuit gcd calculation. Function f delivers the next value in the random-number sequence and function g accumulates the current product for the short-circuit gcd calculation. Loop1 is the main body of the function, and loop2 reruns the short-circuit gcd calculation when necessary; both call the function recursively if they find a cycle. We arbitrarily choose 25 as the number of steps between successive gcd calculations.

[ Return to top ]

Copyright © 2012 by Programming Praxis of Saint Louis, Missouri, USA. All rights reserved. This work is licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. To view a copy of this license, visit http://­creativecommons.­org/­licenses/­by-nc-sa/­3.0/­us/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA.

This essay can be found in html format at http://­programmingpraxis.­com/­programming-with-prime-numbers/ and
in pdf format at http://­programmingpraxis.­files.­wordpress.­com/­2012/­02/­programming-with-prime-numbers.­pdf.
The author can be reached at programmingpraxis@gmail.com.

One Response to “Programming with Prime Numbers”

  1. […] Programming with Prime Numbers | Programming Praxis – Prime numbers are those integers greater than one that are divisible only by themselves and one; an integer greater than one that is not prime is composite. […]

%d bloggers like this: