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+/* mpn_broot -- Compute hensel sqrt
+
+ Contributed to the GNU project by Niels Möller
+
+ THE FUNCTIONS IN THIS FILE ARE INTERNAL WITH MUTABLE INTERFACES. IT IS ONLY
+ SAFE TO REACH THEM THROUGH DOCUMENTED INTERFACES. IN FACT, IT IS ALMOST
+ GUARANTEED THAT THEY WILL CHANGE OR DISAPPEAR IN A FUTURE GMP RELEASE.
+
+Copyright 2012 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of either:
+
+ * the GNU Lesser General Public License as published by the Free
+ Software Foundation; either version 3 of the License, or (at your
+ option) any later version.
+
+or
+
+ * the GNU General Public License as published by the Free Software
+ Foundation; either version 2 of the License, or (at your option) any
+ later version.
+
+or both in parallel, as here.
+
+The GNU MP Library is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+for more details.
+
+You should have received copies of the GNU General Public License and the
+GNU Lesser General Public License along with the GNU MP Library. If not,
+see https://www.gnu.org/licenses/. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+
+/* Computes a^e (mod B). Uses right-to-left binary algorithm, since
+ typical use will have e small. */
+static mp_limb_t
+powlimb (mp_limb_t a, mp_limb_t e)
+{
+ mp_limb_t r = 1;
+ mp_limb_t s = a;
+
+ for (r = 1, s = a; e > 0; e >>= 1, s *= s)
+ if (e & 1)
+ r *= s;
+
+ return r;
+}
+
+/* Computes a^{1/k - 1} (mod B^n). Both a and k must be odd.
+
+ Iterates
+
+ r' <-- r - r * (a^{k-1} r^k - 1) / n
+
+ If
+
+ a^{k-1} r^k = 1 (mod 2^m),
+
+ then
+
+ a^{k-1} r'^k = 1 (mod 2^{2m}),
+
+ Compute the update term as
+
+ r' = r - (a^{k-1} r^{k+1} - r) / k
+
+ where we still have cancellation of low limbs.
+
+ */
+void
+mpn_broot_invm1 (mp_ptr rp, mp_srcptr ap, mp_size_t n, mp_limb_t k)
+{
+ mp_size_t sizes[GMP_LIMB_BITS * 2];
+ mp_ptr akm1, tp, rnp, ep;
+ mp_limb_t a0, r0, km1, kp1h, kinv;
+ mp_size_t rn;
+ unsigned i;
+
+ TMP_DECL;
+
+ ASSERT (n > 0);
+ ASSERT (ap[0] & 1);
+ ASSERT (k & 1);
+ ASSERT (k >= 3);
+
+ TMP_MARK;
+
+ akm1 = TMP_ALLOC_LIMBS (4*n);
+ tp = akm1 + n;
+
+ km1 = k-1;
+ /* FIXME: Could arrange the iteration so we don't need to compute
+ this up front, computing a^{k-1} * r^k as (a r)^{k-1} * r. Note
+ that we can use wraparound also for a*r, since the low half is
+ unchanged from the previous iteration. Or possibly mulmid. Also,
+ a r = a^{1/k}, so we get that value too, for free? */
+ mpn_powlo (akm1, ap, &km1, 1, n, tp); /* 3 n scratch space */
+
+ a0 = ap[0];
+ binvert_limb (kinv, k);
+
+ /* 4 bits: a^{1/k - 1} (mod 16):
+
+ a % 8
+ 1 3 5 7
+ k%4 +-------
+ 1 |1 1 1 1
+ 3 |1 9 9 1
+ */
+ r0 = 1 + (((k << 2) & ((a0 << 1) ^ (a0 << 2))) & 8);
+ r0 = kinv * r0 * (k+1 - akm1[0] * powlimb (r0, k & 0x7f)); /* 8 bits */
+ r0 = kinv * r0 * (k+1 - akm1[0] * powlimb (r0, k & 0x7fff)); /* 16 bits */
+ r0 = kinv * r0 * (k+1 - akm1[0] * powlimb (r0, k)); /* 32 bits */
+#if GMP_NUMB_BITS > 32
+ {
+ unsigned prec = 32;
+ do
+ {
+ r0 = kinv * r0 * (k+1 - akm1[0] * powlimb (r0, k));
+ prec *= 2;
+ }
+ while (prec < GMP_NUMB_BITS);
+ }
+#endif
+
+ rp[0] = r0;
+ if (n == 1)
+ {
+ TMP_FREE;
+ return;
+ }
+
+ /* For odd k, (k+1)/2 = k/2+1, and the latter avoids overflow. */
+ kp1h = k/2 + 1;
+
+ /* FIXME: Special case for two limb iteration. */
+ rnp = TMP_ALLOC_LIMBS (2*n + 1);
+ ep = rnp + n;
+
+ /* FIXME: Possible to this on the fly with some bit fiddling. */
+ for (i = 0; n > 1; n = (n + 1)/2)
+ sizes[i++] = n;
+
+ rn = 1;
+
+ while (i-- > 0)
+ {
+ /* Compute x^{k+1}. */
+ mpn_sqr (ep, rp, rn); /* For odd n, writes n+1 limbs in the
+ final iteration. */
+ mpn_powlo (rnp, ep, &kp1h, 1, sizes[i], tp);
+
+ /* Multiply by a^{k-1}. Can use wraparound; low part equals r. */
+
+ mpn_mullo_n (ep, rnp, akm1, sizes[i]);
+ ASSERT (mpn_cmp (ep, rp, rn) == 0);
+
+ ASSERT (sizes[i] <= 2*rn);
+ mpn_pi1_bdiv_q_1 (rp + rn, ep + rn, sizes[i] - rn, k, kinv, 0);
+ mpn_neg (rp + rn, rp + rn, sizes[i] - rn);
+ rn = sizes[i];
+ }
+ TMP_FREE;
+}
+
+/* Computes a^{1/k} (mod B^n). Both a and k must be odd. */
+void
+mpn_broot (mp_ptr rp, mp_srcptr ap, mp_size_t n, mp_limb_t k)
+{
+ mp_ptr tp;
+ TMP_DECL;
+
+ ASSERT (n > 0);
+ ASSERT (ap[0] & 1);
+ ASSERT (k & 1);
+
+ if (k == 1)
+ {
+ MPN_COPY (rp, ap, n);
+ return;
+ }
+
+ TMP_MARK;
+ tp = TMP_ALLOC_LIMBS (n);
+
+ mpn_broot_invm1 (tp, ap, n, k);
+ mpn_mullo_n (rp, tp, ap, n);
+
+ TMP_FREE;
+}