bigqrx.c revision 65c4736d9c0ebc6d9b1d991593b55566909da9cd
/* Copyright (C) RSA Data Security, Inc. created 1990, 1996. This is an
unpublished work protected as such under copyright law. This work
contains proprietary, confidential, and trade secret information of
RSA Data Security, Inc. Use, disclosure or reproduction without the
express written authorization of RSA Data Security, Inc. is
prohibited.
*/
#include "global.h"
#include "bigmath.h"
/* BigQrx (q, r, b, c, cInv, n) -- compute quotient and remainder fast.
-- computes q and r s.t. b = q * c + r with 0 <= r < c.
-- assumes b and c are positive integers.
-- assumes q, r, c have n words, cInv has n+2 words, b has 2*n words.
-- assumes cInv previously computed with BigInv.
*/
unsigned int n;
{
int uwsw3;
register unsigned int i;
/* 2**(cl-1) <= c < 2**cl
2**(u-cl) <= cInv <= 2**(u-cl+1) */
/* u is in bits, uw is in words */
uw = u/16;
/* sw is in words, s is is bits */
if (uwsw3 < 0)
uwsw3 = 0;
/* Copy b to local register.
*/
/* Compute qsc = cInv * floor (b/ (2**s)).
qsc an approximation to (b/c) * (2**(u-s))
2**((u-cl)+ (bl-1-s)) <= qsc 2**((u-cl+1)+ (bl-s))
2**(u-cl+bl-s-1) <= qsc <= 2 ** (u-cl+bl-s+1)
(Actually, we only compute a "high-order" approximation
to qsc, by using BigPmpyh.)
*/
/* Divide by 2**(u-s) to get initial estimate for quotient q
2**(bl-cl-1) <= q <= 2**(bl-cl+1) (unless q = 0).
*/
for (i = 0; i < n; i++)
/* compute qc = low-order part of q * c
2 ** (bl - 2) <= qc <= 2 ** (bl + 1) */
/* subtract qc from b to get initial estimate for remainder r */
/* Adjust to be exactly right by repeated subtraction.
*/
while (BigCmp (r, c, n) >= 0) {
BigSub (r, r, c, n);
BigInc (q, n);
}
}