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mersenne.cal
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1995-07-17
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833b
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45 lines
/*
* Copyright (c) 1993 David I. Bell
* Permission is granted to use, distribute, or modify this source,
* provided that this copyright notice remains intact.
*
* Perform a primality test of 2^p-1, for prime p>1.
*/
define mersenne(p)
{
local u, i, p_mask;
/* firewall */
if (! isint(p))
quit "p is not an integer";
/* two is a special case */
if (p == 2)
return 1;
/* if p is not prime, then 2^p-1 is not prime */
if (! ptest(p,10))
return 0;
/* calculate 2^p-1 for later mods */
p_mask = 2^p - 1;
/* lltest: u(i+1) = u(i)^2 - 2 mod 2^p-1 */
u = 4;
for (i = 2; i < p; ++i) {
u = u^2 - 2;
u = u&p_mask + u>>p;
if (u > p_mask)
u = u&p_mask + 1;
}
/* 2^p-1 is prime iff u(p) = 0 mod 2^p-1 */
return (u == p_mask);
}
global lib_debug;
if (lib_debug >= 0) {
print "mersenne(p) defined";
}