+mp *mpbarrett_exp(mpbarrett *mb, mp *d, mp *a, mp *e)
+{
+ mp **tab;
+ mp *a2;
+ mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
+ mp *x = MP_ONE;
+ unsigned i, sq = 0;
+ mpscan sc;
+
+ /* --- Do we bother? --- */
+
+ MP_SHRINK(e);
+ if (MP_LEN(e) == 0)
+ goto exit;
+ if (MP_LEN(e) < THRESH) {
+ x->ref--;
+ return (exp_simple(mb, d, a, e));
+ }
+
+ /* --- Do the precomputation --- */
+
+ a2 = mp_sqr(MP_NEW, a);
+ a2 = mpbarrett_reduce(mb, a2, a2);
+ tab = xmalloc(TABSZ * sizeof(mp *));
+ tab[0] = MP_COPY(a);
+ for (i = 1; i < TABSZ; i++) {
+ mp *x = mp_mul(MP_NEW, tab[i - 1], a2);
+ tab[i] = mpbarrett_reduce(mb, x, x);
+ }
+ 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 = mpbarrett_reduce(mb, y, y);
+ spare = x; x = y;
+ sq--;
+ }
+
+ /* --- Do the multiply --- */
+
+ { mp *y = mp_mul(spare, x, tab[i]); spare = x;
+ x = mpbarrett_reduce(mb, y, 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 = mpbarrett_reduce(mb, y, y);
+ spare = x; x = y;
+ sq--;
+ }
+
+ /* --- Done --- */
+
+ for (i = 0; i < TABSZ; i++)
+ mp_drop(tab[i]);
+ xfree(tab);
+exit:
+ mp_drop(d);
+ mp_drop(spare);
+ return (x);
+}
+