1 /* mpn_tdiv_qr -- Divide the numerator (np,nn) by the denominator (dp,dn) and
2 write the nn-dn+1 quotient limbs at qp and the dn remainder limbs at rp. If
3 qxn is non-zero, generate that many fraction limbs and append them after the
4 other quotient limbs, and update the remainder accordingly. The input
5 operands are unaffected.
8 1. The most significant limb of of the divisor must be non-zero.
9 2. nn >= dn, even if qxn is non-zero. (??? relax this ???)
11 The time complexity of this is O(qn*qn+M(dn,qn)), where M(m,n) is the time
12 complexity of multiplication.
14 Copyright 1997, 2000, 2001, 2002, 2005, 2009 Free Software Foundation, Inc.
16 This file is part of the GNU MP Library.
18 The GNU MP Library is free software; you can redistribute it and/or modify
19 it under the terms of the GNU Lesser General Public License as published by
20 the Free Software Foundation; either version 3 of the License, or (at your
21 option) any later version.
23 The GNU MP Library is distributed in the hope that it will be useful, but
24 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
25 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
26 License for more details.
28 You should have received a copy of the GNU Lesser General Public License
29 along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */
37 mpn_tdiv_qr (mp_ptr qp, mp_ptr rp, mp_size_t qxn,
38 mp_srcptr np, mp_size_t nn, mp_srcptr dp, mp_size_t dn)
40 ASSERT_ALWAYS (qxn == 0);
44 ASSERT (dn == 0 || dp[dn - 1] != 0);
45 ASSERT (! MPN_OVERLAP_P (qp, nn - dn + 1 + qxn, np, nn));
46 ASSERT (! MPN_OVERLAP_P (qp, nn - dn + 1 + qxn, dp, dn));
55 rp[0] = mpn_divrem_1 (qp, (mp_size_t) 0, np, nn, dp[0]);
65 if ((dp[1] & GMP_NUMB_HIGHBIT) == 0)
69 count_leading_zeros (cnt, dp[1]);
72 d2p[1] = (dp[1] << cnt) | (dp[0] >> (GMP_NUMB_BITS - cnt));
73 d2p[0] = (dp[0] << cnt) & GMP_NUMB_MASK;
74 n2p = TMP_ALLOC_LIMBS (nn + 1);
75 cy = mpn_lshift (n2p, np, nn, cnt);
77 qhl = mpn_divrem_2 (qp, 0L, n2p, nn + (cy != 0), d2p);
79 qp[nn - 2] = qhl; /* always store nn-2+1 quotient limbs */
80 rp[0] = (n2p[0] >> cnt)
81 | ((n2p[1] << (GMP_NUMB_BITS - cnt)) & GMP_NUMB_MASK);
82 rp[1] = (n2p[1] >> cnt);
87 n2p = TMP_ALLOC_LIMBS (nn);
88 MPN_COPY (n2p, np, nn);
89 qhl = mpn_divrem_2 (qp, 0L, n2p, nn, d2p);
90 qp[nn - 2] = qhl; /* always store nn-2+1 quotient limbs */
104 adjust = np[nn - 1] >= dp[dn - 1]; /* conservative tests for quotient size */
105 if (nn + adjust >= 2 * dn)
111 qp[nn - dn] = 0; /* zero high quotient limb */
112 if ((dp[dn - 1] & GMP_NUMB_HIGHBIT) == 0) /* normalize divisor */
114 count_leading_zeros (cnt, dp[dn - 1]);
115 cnt -= GMP_NAIL_BITS;
116 d2p = TMP_ALLOC_LIMBS (dn);
117 mpn_lshift (d2p, dp, dn, cnt);
118 n2p = TMP_ALLOC_LIMBS (nn + 1);
119 cy = mpn_lshift (n2p, np, nn, cnt);
127 n2p = TMP_ALLOC_LIMBS (nn + 1);
128 MPN_COPY (n2p, np, nn);
133 invert_pi1 (dinv, d2p[dn - 1], d2p[dn - 2]);
134 if (BELOW_THRESHOLD (dn, DC_DIV_QR_THRESHOLD))
135 mpn_sbpi1_div_qr (qp, n2p, nn, d2p, dn, dinv.inv32);
136 else if (BELOW_THRESHOLD (dn, MUPI_DIV_QR_THRESHOLD) || /* fast condition */
137 BELOW_THRESHOLD (nn, 2 * MU_DIV_QR_THRESHOLD) || /* fast condition */
138 (double) (2 * (MU_DIV_QR_THRESHOLD - MUPI_DIV_QR_THRESHOLD)) * dn /* slow... */
139 + (double) MUPI_DIV_QR_THRESHOLD * nn > (double) dn * nn) /* ...condition */
140 mpn_dcpi1_div_qr (qp, n2p, nn, d2p, dn, &dinv);
143 mp_size_t itch = mpn_mu_div_qr_itch (nn, dn, 0);
144 mp_ptr scratch = TMP_ALLOC_LIMBS (itch);
145 mpn_mu_div_qr (qp, rp, n2p, nn, d2p, dn, scratch);
150 mpn_rshift (rp, n2p, dn, cnt);
152 MPN_COPY (rp, n2p, dn);
157 /* When we come here, the numerator/partial remainder is less
158 than twice the size of the denominator. */
163 Divide a numerator N with nn limbs by a denominator D with dn
164 limbs forming a quotient of qn=nn-dn+1 limbs. When qn is small
165 compared to dn, conventional division algorithms perform poorly.
166 We want an algorithm that has an expected running time that is
167 dependent only on qn.
169 Algorithm (very informally stated):
171 1) Divide the 2 x qn most significant limbs from the numerator
172 by the qn most significant limbs from the denominator. Call
173 the result qest. This is either the correct quotient, but
174 might be 1 or 2 too large. Compute the remainder from the
175 division. (This step is implemented by a mpn_divrem call.)
177 2) Is the most significant limb from the remainder < p, where p
178 is the product of the most significant limb from the quotient
179 and the next(d)? (Next(d) denotes the next ignored limb from
180 the denominator.) If it is, decrement qest, and adjust the
181 remainder accordingly.
183 3) Is the remainder >= qest? If it is, qest is the desired
184 quotient. The algorithm terminates.
186 4) Subtract qest x next(d) from the remainder. If there is
187 borrow out, decrement qest, and adjust the remainder
190 5) Skip one word from the denominator (i.e., let next(d) denote
191 the next less significant limb. */
198 mp_limb_t quotient_too_large;
202 qp[qn] = 0; /* zero high quotient limb */
203 qn += adjust; /* qn cannot become bigger */
207 MPN_COPY (rp, np, dn);
212 in = dn - qn; /* (at least partially) ignored # of limbs in ops */
213 /* Normalize denominator by shifting it to the left such that its
214 most significant bit is set. Then shift the numerator the same
215 amount, to mathematically preserve quotient. */
216 if ((dp[dn - 1] & GMP_NUMB_HIGHBIT) == 0)
218 count_leading_zeros (cnt, dp[dn - 1]);
219 cnt -= GMP_NAIL_BITS;
221 d2p = TMP_ALLOC_LIMBS (qn);
222 mpn_lshift (d2p, dp + in, qn, cnt);
223 d2p[0] |= dp[in - 1] >> (GMP_NUMB_BITS - cnt);
225 n2p = TMP_ALLOC_LIMBS (2 * qn + 1);
226 cy = mpn_lshift (n2p, np + nn - 2 * qn, 2 * qn, cnt);
234 n2p[0] |= np[nn - 2 * qn - 1] >> (GMP_NUMB_BITS - cnt);
240 d2p = (mp_ptr) dp + in;
242 n2p = TMP_ALLOC_LIMBS (2 * qn + 1);
243 MPN_COPY (n2p, np + nn - 2 * qn, 2 * qn);
251 /* Get an approximate quotient using the extracted operands. */
255 udiv_qrnnd (q0, r0, n2p[1], n2p[0] << GMP_NAIL_BITS, d2p[0] << GMP_NAIL_BITS);
256 n2p[0] = r0 >> GMP_NAIL_BITS;
260 mpn_divrem_2 (qp, 0L, n2p, 4L, d2p); /* FIXME: obsolete function */
263 invert_pi1 (dinv, d2p[qn - 1], d2p[qn - 2]);
264 if (BELOW_THRESHOLD (qn, DC_DIV_QR_THRESHOLD))
265 mpn_sbpi1_div_qr (qp, n2p, 2 * qn, d2p, qn, dinv.inv32);
266 else if (BELOW_THRESHOLD (qn, MU_DIV_QR_THRESHOLD))
267 mpn_dcpi1_div_qr (qp, n2p, 2 * qn, d2p, qn, &dinv);
270 mp_size_t itch = mpn_mu_div_qr_itch (2 * qn, qn, 0);
271 mp_ptr scratch = TMP_ALLOC_LIMBS (itch);
273 if (np == r2p) /* If N and R share space, put ... */
274 r2p += nn - qn; /* intermediate remainder at N's upper end. */
275 mpn_mu_div_qr (qp, r2p, n2p, 2 * qn, d2p, qn, scratch);
276 MPN_COPY (n2p, r2p, qn);
281 /* Multiply the first ignored divisor limb by the most significant
282 quotient limb. If that product is > the partial remainder's
283 most significant limb, we know the quotient is too large. This
284 test quickly catches most cases where the quotient is too large;
285 it catches all cases where the quotient is 2 too large. */
295 #if GMP_NAIL_BITS == 0
296 x = (dp[in - 1] << cnt) | ((dl >> 1) >> ((~cnt) % GMP_LIMB_BITS));
298 x = (dp[in - 1] << cnt) & GMP_NUMB_MASK;
300 x |= dl >> (GMP_NUMB_BITS - cnt);
302 umul_ppmm (h, dummy, x, qp[qn - 1] << GMP_NAIL_BITS);
308 mpn_decr_u (qp, (mp_limb_t) 1);
309 cy = mpn_add_n (n2p, n2p, d2p, qn);
312 /* The partial remainder is safely large. */
319 quotient_too_large = 0;
324 /* Append partially used numerator limb to partial remainder. */
325 cy1 = mpn_lshift (n2p, n2p, rn, GMP_NUMB_BITS - cnt);
326 n2p[0] |= np[in - 1] & (GMP_NUMB_MASK >> cnt);
328 /* Update partial remainder with partially used divisor limb. */
329 cy2 = mpn_submul_1 (n2p, qp, qn, dp[in - 1] & (GMP_NUMB_MASK >> cnt));
332 ASSERT_ALWAYS (n2p[qn] >= cy2);
337 n2p[qn] = cy1 - cy2; /* & GMP_NUMB_MASK; */
339 quotient_too_large = (cy1 < cy2);
344 /* True: partial remainder now is neutral, i.e., it is not shifted up. */
346 tp = TMP_ALLOC_LIMBS (dn);
352 MPN_COPY (rp, n2p, rn);
353 ASSERT_ALWAYS (rn == dn);
356 mpn_mul (tp, qp, qn, dp, in);
359 mpn_mul (tp, dp, in, qp, qn);
361 cy = mpn_sub (n2p, n2p, rn, tp + in, qn);
362 MPN_COPY (rp + in, n2p, dn - in);
363 quotient_too_large |= cy;
364 cy = mpn_sub_n (rp, np, tp, in);
365 cy = mpn_sub_1 (rp + in, rp + in, rn, cy);
366 quotient_too_large |= cy;
368 if (quotient_too_large)
370 mpn_decr_u (qp, (mp_limb_t) 1);
371 mpn_add_n (rp, rp, dp, dn);