# Primes and Primality

# Some algorithms come from Bressoud, Factorization and Primality
# Testing (see Genius bibliography FIXME: we don't have that yet)

#####################################
#### (Strong) Pseudoprime tests #####
#####################################
# FIXME: references and definitions!

# The ``pseudoprime test'':
# Returns true iff b^(n-1) == 1 (mod n).
# FIXME: Should we take |n| if n is negative?
# FIXME: is there a good name for an integer that is EITHER a pseudoprime OR a prime?
SetHelp("PseudoprimeTest","number_theory","Pseudoprime test, true if and only if b^(n-1) == 1 (mod n)");
function PseudoprimeTest(n,b) =
 (
  b^(|n|-1) == 1 mod |n|
 )

# An integer is said to be a PseudoPrime with base b iff it is:
# odd, composite, and satisfies b^(n-1) == 1 (mod n).
# We rearrange the order below to take advantage of lazy evaluation
# (IsOdd and PseudoprimeTest are MUCH faster than IsPrime)
function IsPseudoprime(n,b) =
 (
  IsOdd(n) and PseudoprimeTest(n,b) and not(IsPrime(n))
 )
SetHelp("IsPseudoprime","number_theory","If n is a pseudoprime base b but not a prime, that is if b^(n-1) == 1 mod n");

function IsStrongPseudoprime(n,b) =
 (
  IsOdd(n) and gcd(n,b)==1 and StrongPseudoprimeTest(n,b) and not (IsPrime(n))
 )
SetHelp("IsStrongPseudoprime","number_theory","Test if n is a strong pseudoprime to base b but not a prime");

#######################################
############## Misc ###################
#######################################

SetHelp("LucasLehmer","number_theory","Test if Mp is a Mersenne prime using the Lucas-Lehmer test");
function LucasLehmer(p) = (
        if IsMatrix(p) then
                return ApplyOverMatrix(p, LucasLehmer)
	else if not IsPositiveInteger(p) then
		(error("LucasLehmer: argument not an integer larger than 0");bailout);
	if p==1 then return false
	else if p==2 then return true;
	S = 4;
	n = 1;
	Mp = (2^p) - 1;
	while n<p-1 do (
		S = ((S^2) - 2) % Mp;
		increment n
	);
	S == 0
);

# See http://primes.utm.edu/mersenne/index.html
# Or http://www.mersenne.org
SetHelp("MersennePrimeExponents", "number_theory", "Vector with the known Mersenne prime exponents");
MersennePrimeExponents = [2
3
5
7
13
17
19
31
61
89
107
127
521
607
1279
2203
2281
3217
4253
4423
9689
9941
11213
19937
21701
23209
44497
86243
110503
132049
216091
756839
859433
1257787
1398269
2976221
3021377
6972593
13466917
20996011
24036583
25964951
30402457
32582657
37156667
42643801
43112609
57885161
74207281
77232917
82589933
136279841];

# See http://www.mersenne.org
SetHelp("IsMersennePrimeExponent","number_theory","Test if Mp is a Mersenne prime using a table");
function IsMersennePrimeExponent(p) = (
        if IsMatrix(p) then
                return ApplyOverMatrix(p, IsMersennePrimeExponent)
	else if not IsPositiveInteger(p) then
		(error("IsMersennePrimeExponent: argument not an integer larger than 0");bailout);

	# A prime
	if IsIn (p, MersennePrimeExponents) then
		return true;

	# http://www.mersenne.org / GIMPS doublechecked everything up
	# to 80 053 537 (on Feb 21, 2026)
	if p < 80053537 then
		return false;

	error("IsMersennePrimeExponent: Number too large (known values up to: " + 80053537 + ")");
	bailout
);
