NAME
    rcmul - REDC multiplication

SYNOPSIS
    rcmul(x, y, m)

TYPES
    x		integer
    y		integer
    m		odd positive integer

    return	integer v, 0 <= v < m.

DESCRIPTION
    Let B be the base calc uses for representing integers internally
    (B = 2^16 for 32-bit machines, 2^32 for 64-bit machines)
    and N the number of words (base-B digits) in the representation
    of m.  Then rcmul(x,y,m) returns the value of B^-N * x * y % m,
    where the inverse implicit in B^-N is modulo m
    and the modulus operator % gives the least non-negative residue.

    The normal use of rcmul() may be said to be that of multiplying modulo m
    values encoded by rcin() and REDC functions, as in:

	    rcin(x * y, m) = rcmul(rcin(x,m), rcin(y,m), m),

    or with only one factor encoded:

	    x * y % m = rcmul(rcin(x,m), y, m).

RUNTIME
    If the value of m in rcmul(x,y,m) is being used for the first time
    in a REDC function, the information required for the REDC
    algorithms is calculated and stored for future use, possibly
    replacing an already stored valued, in a table covering up to 5
    (i.e. MAXREDC) values of m.	 The runtime required for this is about
    two times that required for multiplying two N-word integers.

    Two algorithms are available for evaluating rcmul(x,y,m), the one
    which is usually faster for small N is used when N <
    config("redc2"); the other is usually faster for larger N. If
    config("redc2") is set at about 90 and x and y have both been
    reduced modulo m, the runtime required for rcmul(x,y,m) is at most
    about f times the runtime required for an N-word by N-word
    multiplication, where f increases from about 1.3 for N = 1 to near
    3 for N > 90.  More runtime may be required if x and y have to be
    reduced modulo m.

EXAMPLE
    Using a 64-bit machine with B = 2^32:

    > print rcin(4 * 5, 9), rcmul(rcin(4,9), rcin(5,9), 9), rcout(8, 9);
    8 8 2

LIMITS
    none

LINK LIBRARY
    void zredcmul(REDC *rp, ZVALUE z1, ZVALUE z2, ZVALUE *res)

SEE ALSO
    rcin, rcout, rcsq, rcpow

## 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.
## 59 Temple Place, Suite 330, Boston, MA  02111-1307, USA.
##
## @(#) $Revision: 29.2 $
## @(#) $Id: rcmul,v 29.2 2000/06/07 14:02:33 chongo Exp $
## @(#) $Source: /usr/local/src/cmd/calc/help/RCS/rcmul,v $
##
## Under source code control:	1996/02/25 02:22:21
## File existed as early as:	1996
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
