nss-3.41/nss/lib/freebl/mpi/doc/mul.txt

Multiplication

This describes the multiplication algorithm used by the MPI library.

This is basically a standard "schoolbook" algorithm.  It is slow --
O(mn) for m = #a, n = #b -- but easy to implement and verify.
Basically, we run two nested loops, as illustrated here (R is the
radix):

k = 0
for j <- 0 to (#b - 1)
  for i <- 0 to (#a - 1)
    w = (a[j] * b[i]) + k + c[i+j]
    c[i+j] = w mod R
    k = w div R
  endfor
  c[i+j] = k;
  k = 0;
endfor

It is necessary that 'w' have room for at least two radix R digits.
The product of any two digits in radix R is at most:

	(R - 1)(R - 1) = R^2 - 2R + 1

Since a two-digit radix-R number can hold R^2 - 1 distinct values,
this insures that the product will fit into the two-digit register.

To insure that two digits is enough for w, we must also show that
there is room for the carry-in from the previous multiplication, and
the current value of the product digit that is being recomputed.
Assuming each of these may be as big as R - 1 (and no larger,
certainly), two digits will be enough if and only if:

	(R^2 - 2R + 1) + 2(R - 1) <= R^2 - 1

Solving this equation shows that, indeed, this is the case:

	R^2 - 2R + 1 + 2R - 2 <= R^2 - 1

	R^2 - 1 <= R^2 - 1

This suggests that a good radix would be one more than the largest
value that can be held in half a machine word -- so, for example, as
in this implementation, where we used a radix of 65536 on a machine
with 4-byte words.  Another advantage of a radix of this sort is that
binary-level operations are easy on numbers in this representation.

Here's an example multiplication worked out longhand in radix-10,
using the above algorithm:

   a =     999
   b =   x 999
  -------------
   p =   98001

w = (a[jx] * b[ix]) + kin + c[ix + jx]
c[ix+jx] = w % RADIX
k = w / RADIX
                                                               product
ix	jx	a[jx]	b[ix]	kin	w	c[i+j]	kout	000000
0	0	9	9	0	81+0+0	1	8	000001
0	1	9	9	8	81+8+0	9	8	000091
0	2	9	9	8	81+8+0	9	8	000991
				8			0	008991
1	0	9	9	0	81+0+9	0	9	008901
1	1	9	9	9	81+9+9	9	9	008901
1	2	9	9	9	81+9+8	8	9	008901
				9			0	098901
2	0	9	9	0	81+0+9	0	9	098001
2	1	9	9	9	81+9+8	8	9	098001
2	2	9	9	9	81+9+9	9	9	098001

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