/*
 * randmprime - generate a random prime of the form h*2^n-1
 *
 * Copyright (C) 1999  Landon Curt Noll
 *
 * Calc is open software; you can redistribute it and/or modify it under
 * the terms of the version 2.1 of the GNU Lesser General Public License
 * as published by the Free Software Foundation.
 *
 * Calc is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU Lesser General
 * Public License for more details.
 *
 * A copy of version 2.1 of the GNU Lesser General Public License is
 * distributed with calc under the filename COPYING-LGPL.  You should have
 * received a copy with calc; if not, write to Free Software Foundation, Inc.
 * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 *
 * Under source code control:	1994/03/14 23:11:21
 * File existed as early as:	1994
 *
 * chongo <was here> /\oo/\	http://www.isthe.com/chongo/
 * Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
 */


/* obtain our required libs */
read -once "lucas.cal"

/*
 * randmprime - find a random prime of the form h*2^n-1 of a given size
 *
 * given:
 *	bits	minimum bits in prime to return
 *	seed	random seed for srandom()
 *	[dbg]	if given, enable debugging
 *
 * returns:
 *	a prime of the form h*2^n-1
 */
define
randmprime(bits, seed, dbg)
{
    local n;		/* n as in h*2^n-1 */
    local h;		/* h as in h*2^n-1 */
    local plush;	/* value added to h since the beginning */
    local init;		/* initial cpu time */
    local start;	/* cpu time before last test */
    local stop;		/* cpu time afte last test */
    local tmp;		/* just a tmp place holder value */
    local ret;		/* h*2^n-1 that is prime */

    /* firewall */
    if (param(0) < 2 || param(0) > 3) {
	quit "bad usage: rndprime(dig, seed [,dbg])";
    }
    if (!isint(bits) || bits < 0 || !isint(seed) || seed < 0) {
	quit "args must be non-negative integers";
    }
    if (bits < 1) {
	bits = 1;
    }
    if (param(0) == 2 || dbg < 0) {
	dbg = 0;
    }

    /* seed generator */
    tmp = srandom(seed, 13);

    /* determine initial h and n values */
    n = random(bits>>1, highbit(bits)+bits>>1+1);
    h = randombit(n);
    h += iseven(h);
    while (highbit(h) >= n) {
       ++n;
    }
    if (dbg >= 1) {
	print "DEBUG3: initial h =", h;
	print "DEBUG3: initial n =", n;
    }

    /*
     * loop until we find a prime
     */
    if (dbg >= 1) {
	start = usertime();
	init = usertime();
	plush = 0;
	print "DEBUG1: testing (h+" : plush : ")*2^" : n : "-1";
    }
    while (lucas(h,n) == 0) {

	/* bump h, and n if needed */
	if (dbg >= 2) {
	    stop = usertime();
	    print "DEBUG2: last test:", stop-start, "	total time:", stop-init;
	}
	if (dbg >= 1) {
	    print "DEBUG1: composite: (h+" : plush : ")*2^" : n : "-1";
	    plush += 2;
	}
	h += 2;
	while (highbit(h) >= n) {
	   ++n;
	}
	if (dbg >= 1) {
	    print "DEBUG1: testing (h+" : plush : ")*2^" : n : "-1";
	    start = stop;
	}
    }

    /* found a prime */
    if (dbg >= 2) {
	stop = usertime();
	print "DEBUG2: last test:", stop-start, "   total time:", stop-init;
	print "DEBUG3: " : h : "*2^" : n : "-1 is prime";
    }
    if (dbg >= 1) {
	print "DEBUG1: prime: (h+" : plush : ")*2^" : n : "-1";
    }
    ret = h*2^n-1;
    if (dbg >= 3) {
	print "DEBUG3: highbit(h):", highbit(h);
	print "DEBUG3: digits(h):", digits(h);
	print "DEBUG3: highbit(n):", highbit(n);
	print "DEBUG3: digits(2^n):", int(n*ln(10)/ln(2)+1);
	print "DEBUG3: highbit(h*2^n-1):", highbit(ret);
	print "DEBUG3: digits(h*2^n)-1:", digits(ret);
    }
    return ret;
}
