arithmoi-0.13.4.0: Math/NumberTheory/Primes/Testing/Probabilistic.hs
-- |
-- Module: Math.NumberTheory.Primes.Testing.Probabilistic
-- Copyright: (c) 2011 Daniel Fischer, 2017 Andrew Lelechenko
-- Licence: MIT
-- Maintainer: Daniel Fischer <daniel.is.fischer@googlemail.com>
--
-- Probabilistic primality tests, Miller-Rabin and Baillie-PSW.
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Math.NumberTheory.Primes.Testing.Probabilistic
( isPrime
, millerRabinV
, bailliePSW
, isStrongFermatPP
, isFermatPP
, lucasTest
) where
import Data.Bits
import Data.Mod
import Data.Proxy
import GHC.Num.BigNat
import GHC.Num.Integer
import GHC.Exts (Word(..), Int(..), (-#), (<#), isTrue#)
import GHC.TypeNats (KnownNat, SomeNat(..), someNatVal)
import Math.NumberTheory.Moduli.JacobiSymbol
import Math.NumberTheory.Primes.Small
import Math.NumberTheory.Roots
import Math.NumberTheory.Utils
-- | @isPrime n@ tests whether @n@ is a prime (negative or positive).
-- It is a combination of trial division and Baillie-PSW test.
--
-- If @isPrime n@ returns @False@ then @n@ is definitely composite.
-- There is a theoretical possibility that @isPrime n@ is @True@,
-- but in fact @n@ is not prime. However, no such numbers are known
-- and none exist below @2^64@. If you have found one, please report it,
-- because it is a major discovery.
isPrime :: Integer -> Bool
isPrime n
| n < 0 = isPrime (-n)
| n < 2 = False
| n < 4 = True
| otherwise = millerRabinV 0 n -- trial division test
&& bailliePSW n
-- | Miller-Rabin probabilistic primality test. It consists of the trial
-- division test and several rounds of the strong Fermat test with different
-- bases. The choice of trial divisors and bases are
-- implementation details and may change in future silently.
--
-- First argument stands for the number of rounds of strong Fermat test.
-- If it is 0, only trial division test is performed.
--
-- If @millerRabinV k n@ returns @False@ then @n@ is definitely composite.
-- Otherwise @n@ may appear composite with probability @1/4^k@.
millerRabinV :: Int -> Integer -> Bool
millerRabinV k n
| n < 0 = millerRabinV k (-n)
| n < 2 = False
| n < 4 = True
| otherwise = go smallPrimes
where
go (p:ps)
| p*p > n = True
| otherwise = (n `rem` p /= 0) && go ps
go [] = all (isStrongFermatPP n) (take k smallPrimes)
smallPrimes = map toInteger $ smallPrimesFromTo minBound maxBound
-- | @'isStrongFermatPP' n b@ tests whether non-negative @n@ is
-- a strong Fermat probable prime for base @b@.
--
-- Apart from primes, also some composite numbers have the tested
-- property, but those are rare. Very rare are composite numbers
-- having the property for many bases, so testing a large prime
-- candidate with several bases can identify composite numbers
-- with high probability. An odd number @n > 3@ is prime if and
-- only if @'isStrongFermatPP' n b@ holds for all @b@ with
-- @2 <= b <= (n-1)/2@, but of course checking all those bases
-- would be less efficient than trial division, so one normally
-- checks only a relatively small number of bases, depending on
-- the desired degree of certainty. The probability that a randomly
-- chosen base doesn't identify a composite number @n@ is less than
-- @1/4@, so five to ten tests give a reasonable level of certainty
-- in general.
--
-- Please consult <https://miller-rabin.appspot.com Deterministic variants of the Miller-Rabin primality test>
-- for the best choice of bases.
isStrongFermatPP :: Integer -> Integer -> Bool
isStrongFermatPP n b
| n < 0 = error "isStrongFermatPP: negative argument"
| n <= 1 = False
| n == 2 = True
| otherwise = case someNatVal (fromInteger n) of
SomeNat (_ :: Proxy t) -> isStrongFermatPPMod (fromInteger b :: Mod t)
isStrongFermatPPMod :: KnownNat n => Mod n -> Bool
isStrongFermatPPMod b = b == 0 || a == 1 || go t a
where
m = -1
(t, u) = shiftToOddCount $ unMod m
a = b ^% u
go 0 _ = False
go k x = x == m || go (k - 1) (x * x)
-- | @'isFermatPP' n b@ tests whether @n@ is a Fermat probable prime
-- for the base @b@, that is, whether @b^(n-1) `mod` n == 1@.
-- This is a weaker but simpler condition. However, more is lost
-- in strength than is gained in simplicity, so for primality testing,
-- the strong check should be used. The remarks about
-- the choice of bases to test from @'isStrongFermatPP'@ apply
-- with the modification that if @a@ and @b@ are Fermat bases
-- for @n@, then @a*b@ /always/ is a Fermat base for @n@ too.
-- A /Charmichael number/ is a composite number @n@ which is a
-- Fermat probable prime for all bases @b@ coprime to @n@. By the
-- above, only primes @p <= n/2@ not dividing @n@ need to be tested
-- to identify Carmichael numbers (however, testing all those
-- primes would be less efficient than determining Carmichaelness
-- from the prime factorisation; but testing an appropriate number
-- of prime bases is reasonable to find out whether it's worth the
-- effort to undertake the prime factorisation).
isFermatPP :: Integer -> Integer -> Bool
isFermatPP n b = case someNatVal (fromInteger n) of
SomeNat (_ :: Proxy t) -> (fromInteger b :: Mod t) ^% (n-1) == 1
-- | Primality test after Baillie, Pomerance, Selfridge and Wagstaff.
-- The Baillie-PSW test consists of a strong Fermat probable primality
-- test followed by a (strong) Lucas primality test. This implementation
-- assumes that the number @n@ to test is odd and larger than @3@.
-- Even and small numbers have to be handled before. Also, before
-- applying this test, trial division by small primes should be performed
-- to identify many composites cheaply (although the Baillie-PSW test is
-- rather fast, about the same speed as a strong Fermat test for four or
-- five bases usually, it is, for large numbers, much more costly than
-- trial division by small primes, the primes less than @1000@, say, so
-- eliminating numbers with small prime factors beforehand is more efficient).
--
-- The Baillie-PSW test is very reliable, so far no composite numbers
-- passing it are known, and it is known (Gilchrist 2010) that no
-- Baillie-PSW pseudoprimes exist below @2^64@. However, a heuristic argument
-- by Pomerance indicates that there are likely infinitely many Baillie-PSW
-- pseudoprimes. On the other hand, according to
-- <http://mathworld.wolfram.com/Baillie-PSWPrimalityTest.html> there is
-- reason to believe that there are none with less than several
-- thousand digits, so that for most use cases the test can be
-- considered definitive.
bailliePSW :: Integer -> Bool
bailliePSW n = isStrongFermatPP n 2 && lucasTest n
-- precondition: n odd, > 3 (no small prime factors, typically large)
-- | The Lucas-Selfridge test, including square-check, but without
-- the Fermat test. For package-internal use only.
lucasTest :: Integer -> Bool
lucasTest n
| isSquare n || d == 0 = False
| d == 1 = True
| otherwise = uo == 0 || go t vo qo
where
d = find True 5
find !pos cd = case jacobi (n `rem` cd) cd of
MinusOne -> if pos then cd else (-cd)
Zero -> if cd == n then 1 else 0
One -> find (not pos) (cd+2)
q = (1-d) `quot` 4
(t,o) = shiftToOddCount (n+1)
(uo, vo, qo) = testLucas n q o
go 0 _ _ = False
go s vn qn = vn == 0 || go (s-1) ((vn*vn-2*qn) `rem` n) ((qn*qn) `rem` n)
-- n odd positive, n > abs q, index odd
testLucas :: Integer -> Integer -> Integer -> (Integer, Integer, Integer)
testLucas n q (IS i#) = look (finiteBitSize (0 :: Word) - 2)
where
j = I# i#
look k
| testBit j k = go (k-1) 1 1 1 q
| otherwise = look (k-1)
go k un un1 vn qn
| k < 0 = (un, vn, qn)
| testBit j k = go (k-1) u2n1 u2n2 v2n1 q2n1
| otherwise = go (k-1) u2n u2n1 v2n q2n
where
u2n = (un*vn) `rem` n
u2n1 = (un1*vn-qn) `rem` n
u2n2 = ((un1-q*un)*vn-qn) `rem` n
v2n = (vn*vn-2*qn) `rem` n
v2n1 = ((un1 - (2*q)*un)*vn-qn) `rem` n
q2n = (qn*qn) `rem` n
q2n1 = (qn*qn*q) `rem` n
testLucas n q (IP bn#) = test (s# -# 1#)
where
s# = bigNatSize# bn#
test j# = case bigNatIndex# bn# j# of
0## -> test (j# -# 1#)
w# -> look (j# -# 1#) (W# w#) (finiteBitSize (0 :: Word) - 1)
look j# w i
| testBit w i = go j# w (i - 1) 1 1 1 q
| otherwise = look j# w (i-1)
go k# w i un un1 vn qn
| i < 0 = if isTrue# (k# <# 0#)
then (un,vn,qn)
else go (k# -# 1#) (W# (bigNatIndex# bn# k#)) (finiteBitSize (0 :: Word) - 1) un un1 vn qn
| testBit w i = go k# w (i-1) u2n1 u2n2 v2n1 q2n1
| otherwise = go k# w (i-1) u2n u2n1 v2n q2n
where
u2n = (un*vn) `rem` n
u2n1 = (un1*vn-qn) `rem` n
u2n2 = ((un1-q*un)*vn-qn) `rem` n
v2n = (vn*vn-2*qn) `rem` n
v2n1 = ((un1 - (2*q)*un)*vn-qn) `rem` n
q2n = (qn*qn) `rem` n
q2n1 = (qn*qn*q) `rem` n
-- Listed as a precondition of lucasTest
testLucas _ _ _ = error "lucasTest: negative argument"