90 lines
2.4 KiB
C
90 lines
2.4 KiB
C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is library that provides for multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library is designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tomstdenis@iahu.ca, http://libtommath.iahu.ca
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*/
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#include <tommath.h>
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/* low level squaring, b = a*a, HAC pp.596-597, Algorithm 14.16 */
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int
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s_mp_sqr (mp_int * a, mp_int * b)
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{
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mp_int t;
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int res, ix, iy, pa;
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mp_word r, u;
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mp_digit tmpx, *tmpt;
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/* can we use the fast multiplier? */
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if (((a->used * 2 + 1) < 512)
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&& a->used <
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(1 << ((CHAR_BIT * sizeof (mp_word)) - (2 * DIGIT_BIT) - 1))) {
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res = fast_s_mp_sqr (a, b);
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return res;
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}
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pa = a->used;
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if ((res = mp_init_size (&t, pa + pa + 1)) != MP_OKAY) {
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return res;
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}
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t.used = pa + pa + 1;
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for (ix = 0; ix < pa; ix++) {
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/* first calculate the digit at 2*ix */
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/* calculate double precision result */
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r =
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((mp_word) t.dp[ix + ix]) +
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((mp_word) a->dp[ix]) * ((mp_word) a->dp[ix]);
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/* store lower part in result */
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t.dp[ix + ix] = (mp_digit) (r & ((mp_word) MP_MASK));
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/* get the carry */
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u = (r >> ((mp_word) DIGIT_BIT));
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/* left hand side of A[ix] * A[iy] */
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tmpx = a->dp[ix];
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/* alias for where to store the results */
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tmpt = &(t.dp[ix + ix + 1]);
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for (iy = ix + 1; iy < pa; iy++) {
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/* first calculate the product */
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r = ((mp_word) tmpx) * ((mp_word) a->dp[iy]);
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/* now calculate the double precision result, note we use
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* addition instead of *2 since its easier to optimize
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*/
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r = ((mp_word) * tmpt) + r + r + ((mp_word) u);
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/* store lower part */
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*tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
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/* get carry */
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u = (r >> ((mp_word) DIGIT_BIT));
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}
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r = ((mp_word) * tmpt) + u;
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*tmpt = (mp_digit) (r & ((mp_word) MP_MASK));
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u = (r >> ((mp_word) DIGIT_BIT));
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/* propagate upwards */
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++tmpt;
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while (u != ((mp_word) 0)) {
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r = ((mp_word) * tmpt) + ((mp_word) 1);
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*tmpt++ = (mp_digit) (r & ((mp_word) MP_MASK));
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u = (r >> ((mp_word) DIGIT_BIT));
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}
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}
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mp_clamp (&t);
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mp_exch (&t, b);
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mp_clear (&t);
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return MP_OKAY;
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}
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