- goto exit;
- if (MP_LEN(e) < THRESH) {
- x->ref--;
- return (exp_simple(mm, d, a, e));
- }
-
- /* --- Do the precomputation --- */
-
- ar = mpmont_mul(mm, MP_NEW, a, mm->r2);
- a2 = mp_sqr(MP_NEW, ar);
- a2 = mpmont_reduce(mm, a2, a2);
- tab = xmalloc(TABSZ * sizeof(mp *));
- tab[0] = ar;
- for (i = 1; i < TABSZ; i++)
- tab[i] = mpmont_mul(mm, MP_NEW, tab[i - 1], a2);
- mp_drop(a2);
- mp_rscan(&sc, e);
-
- /* --- Skip top-end zero bits --- *
- *
- * If the initial step worked, there must be a set bit somewhere, so keep
- * stepping until I find it.
- */
-
- MP_RSTEP(&sc);
- while (!MP_RBIT(&sc)) {
- MP_RSTEP(&sc);
- }
-
- /* --- Now for the main work --- */
-
- for (;;) {
- unsigned l = 0;
- unsigned z = 0;
-
- /* --- The next bit is set, so read a window index --- *
- *
- * Reset @i@ to zero and increment @sq@. Then, until either I read
- * @WINSZ@ bits or I run out of bits, scan in a bit: if it's clear, bump
- * the @z@ counter; if it's set, push a set bit into @i@, shift it over
- * by @z@ bits, bump @sq@ by @z + 1@ and clear @z@. By the end of this
- * palaver, @i@ is an index to the precomputed value in @tab@.
- */
-
- i = 0;
- sq++;
- for (;;) {
- l++;
- if (l >= WINSZ || !MP_RSTEP(&sc))
- break;
- if (!MP_RBIT(&sc))
- z++;
- else {
- i = ((i << 1) | 1) << z;
- sq += z + 1;
- z = 0;
- }
- }
-
- /* --- Do the squaring --- *
- *
- * Remember that @sq@ carries over from the zero-skipping stuff below.
- */
-
- while (sq) {
- mp *y;
- y = mp_sqr(spare, x);
- y = mpmont_reduce(mm, y, y);
- spare = x; x = y;
- sq--;
- }
-
- /* --- Do the multiply --- */
-
- { mp *y = mpmont_mul(mm, spare, x, tab[i]); spare = x; x = y; }
-
- /* --- Now grind along through the rest of the bits --- */
-
- sq = z;
- for (;;) {
- if (!MP_RSTEP(&sc))
- goto done;
- if (MP_RBIT(&sc))
- break;
- sq++;
- }
- }
-
- /* --- Do a final round of squaring --- */
-
-done:
- while (sq) {
- mp *y;
- y = mp_sqr(spare, x);
- y = mpmont_reduce(mm, y, y);
- spare = x; x = y;
- sq--;
- }
-
- /* --- Done --- */
-
- for (i = 0; i < TABSZ; i++)
- mp_drop(tab[i]);
- xfree(tab);
-exit:
- if (d != MP_NEW)
- mp_drop(d);
- if (spare)
- mp_drop(spare);