/*      $NetBSD: mfp_mul.c,v 1.5 2020/05/25 20:47:25 christos Exp $     */

/*
* /src/NTP/ntp4-dev/libparse/mfp_mul.c,v 4.9 2005/07/17 20:34:40 kardel RELEASE_20050717_A
*
* mfp_mul.c,v 4.9 2005/07/17 20:34:40 kardel RELEASE_20050717_A
*
* $Created: Sat Aug 16 20:35:08 1997 $
*
* Copyright (c) 1997-2005 by Frank Kardel <kardel <AT> ntp.org>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
*    notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
*    notice, this list of conditions and the following disclaimer in the
*    documentation and/or other materials provided with the distribution.
* 3. Neither the name of the author nor the names of its contributors
*    may be used to endorse or promote products derived from this software
*    without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
*/
#include <config.h>
#include <stdio.h>
#include "ntp_stdlib.h"
#include "ntp_types.h"
#include "ntp_fp.h"

#define LOW_MASK  (u_int32)((1<<(FRACTION_PREC/2))-1)
#define HIGH_MASK (u_int32)(LOW_MASK << (FRACTION_PREC/2))

/*
* for those who worry about overflows (possibly triggered by static analysis tools):
*
* Largest value of a 2^n bit number is 2^n-1.
* Thus the result is: (2^n-1)*(2^n-1) = 2^2n - 2^n - 2^n + 1 < 2^2n
* Here overflow can not happen for 2 reasons:
* 1) the code actually multiplies the absolute values of two signed
*    64bit quantities.thus effectively multiplying 2 63bit quantities.
* 2) Carry propagation is from low to high, building principle is
*    addition, so no storage for the 2^2n term from above is needed.
*/

void
mfp_mul(
       int32   *o_i,
       u_int32 *o_f,
       int32    a_i,
       u_int32  a_f,
       int32    b_i,
       u_int32  b_f
       )
{
 int32 i, j;
 u_int32  f;
 u_long a[4];                  /* operand a */
 u_long b[4];                  /* operand b */
 u_long c[5];                  /* result c - 5 items for performance - see below */
 u_long carry;

 int neg = 0;

 if (a_i < 0)                  /* examine sign situation */
   {
     neg = 1;
     M_NEG(a_i, a_f);
   }

 if (b_i < 0)                  /* examine sign situation */
   {
     neg = !neg;
     M_NEG(b_i, b_f);
   }

 a[0] = a_f & LOW_MASK;        /* prepare a operand */
 a[1] = (a_f & HIGH_MASK) >> (FRACTION_PREC/2);
 a[2] = a_i & LOW_MASK;
 a[3] = (a_i & HIGH_MASK) >> (FRACTION_PREC/2);

 b[0] = b_f & LOW_MASK;        /* prepare b operand */
 b[1] = (b_f & HIGH_MASK) >> (FRACTION_PREC/2);
 b[2] = b_i & LOW_MASK;
 b[3] = (b_i & HIGH_MASK) >> (FRACTION_PREC/2);

 c[0] = c[1] = c[2] = c[3] = c[4] = 0;

 for (i = 0; i < 4; i++)       /* we do assume 32 * 32 = 64 bit multiplication */
   for (j = 0; j < 4; j++)
     {
       u_long result_low, result_high;
       int low_index = (i+j)/2;      /* formal [0..3]  - index for low long word */
       int mid_index = 1+low_index;  /* formal [1..4]! - index for high long word
                                        will generate unecessary add of 0 to c[4]
                                        but save 15 'if (result_high) expressions' */
       int high_index = 1+mid_index; /* formal [2..5]! - index for high word overflow
                                        - only assigned on overflow (limits range to 2..3) */

       result_low = (u_long)a[i] * (u_long)b[j];       /* partial product */

       if ((i+j) & 1)          /* splits across two result registers */
         {
           result_high   = result_low >> (FRACTION_PREC/2);
           result_low  <<= FRACTION_PREC/2;
           carry         = (unsigned)1<<(FRACTION_PREC/2);
         }
       else
         {                     /* stays in a result register - except for overflows */
           result_high = 0;
           carry       = 1;
         }

       if (((c[low_index] >> 1) + (result_low >> 1) + ((c[low_index] & result_low & carry) != 0)) &
           (u_int32)((unsigned)1<<(FRACTION_PREC - 1))) {
         result_high++;        /* propagate overflows */
       }

       c[low_index]   += result_low; /* add up partial products */

       if (((c[mid_index] >> 1) + (result_high >> 1) + ((c[mid_index] & result_high & 1) != 0)) &
           (u_int32)((unsigned)1<<(FRACTION_PREC - 1))) {
         c[high_index]++;              /* propagate overflows of high word sum */
       }

       c[mid_index] += result_high;  /* will add a 0 to c[4] once but saves 15 if conditions */
     }

#ifdef DEBUG
 if (debug > 6)
   printf("mfp_mul: 0x%04lx%04lx%04lx%04lx * 0x%04lx%04lx%04lx%04lx = 0x%08lx%08lx%08lx%08lx\n",
        a[3], a[2], a[1], a[0], b[3], b[2], b[1], b[0], c[3], c[2], c[1], c[0]);
#endif

 if (c[3])                     /* overflow */
   {
     i = ((unsigned)1 << (FRACTION_PREC-1)) - 1;
     f = ~(unsigned)0;
   }
 else
   {                           /* take produkt - discarding extra precision */
     i = c[2];
     f = c[1];
   }

 if (neg)                      /* recover sign */
   {
     M_NEG(i, f);
   }

 *o_i = i;
 *o_f = f;

#ifdef DEBUG
 if (debug > 6)
   printf("mfp_mul: %s * %s => %s\n",
          mfptoa((u_long)a_i, a_f, 6),
          mfptoa((u_long)b_i, b_f, 6),
          mfptoa((u_long)i, f, 6));
#endif
}

/*
* History:
*
* mfp_mul.c,v
* Revision 4.9  2005/07/17 20:34:40  kardel
* correct carry propagation implementation
*
* Revision 4.8  2005/07/12 16:17:26  kardel
* add explanation why we do not write into c[4]
*
* Revision 4.7  2005/04/16 17:32:10  kardel
* update copyright
*
* Revision 4.6  2004/11/14 15:29:41  kardel
* support PPSAPI, upgrade Copyright to Berkeley style
*
* Revision 4.3  1999/02/21 12:17:37  kardel
* 4.91f reconcilation
*
* Revision 4.2  1998/12/20 23:45:28  kardel
* fix types and warnings
*
* Revision 4.1  1998/05/24 07:59:57  kardel
* conditional debug support
*
* Revision 4.0  1998/04/10 19:46:38  kardel
* Start 4.0 release version numbering
*
* Revision 1.1  1998/04/10 19:27:47  kardel
* initial NTP VERSION 4 integration of PARSE with GPS166 binary support
*
* Revision 1.1  1997/10/06 21:05:46  kardel
* new parse structure
*
*/

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