/* mpz_bin_uiui - compute n over k. Copyright 2011 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 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. 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 Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */ #include "gmp.h" #include "gmp-impl.h" #include "longlong.h" /* Algorithm: Accumulate chunks of factors first limb-by-limb (using one of mul0-mul8) which are then accumulated into mpn numbers. The first inner loop accumulates divisor factors, the 2nd inner loop accumulates exactly the same number of dividend factors. We avoid accumulating more for the divisor, even with its smaller factors, since we else cannot guarantee divisibility. Since we know each division will yield an integer, we compute the quotient using Hensel norm: If the quotient is limited by 2^t, we compute A / B mod 2^t. Improvements: (1) An obvious improvement to this code would be to compute mod 2^t everywhere. Unfortunately, we cannot determine t beforehand, unless we invoke some approximation, such as Stirling's formula. Of course, we don't need t to be tight. However, it is not clear that this would help much, our numbers are kept reasonably small already. (2) Compute nmax/kmax semi-accurately, without scalar division or a loop. Extracting the 3 msb, then doing a table lookup using cnt*8+msb as index, would make it both reasonably accurate and fast. (We could use a table stored into a limb, perhaps.) The table should take the removed factors of 2 into account (those done on-the-fly in mulN). */ /* This threshold determines how large divisor to accumulate before we call bdiv. Perhaps we should never call bdiv, and accumulate all we are told, since we are just basecase code anyway? Presumably, this depends on the relative speed of the asymptotically fast code ad this code. */ #define SOME_THRESHOLD 20 /* Multiply-into-limb functions. These remove factors of 2 on-the-fly. FIXME: All versions of MAXFACS don't take this 2 removal into account now, meaning that then, shifting just adds some overhead. (We remove factors from the completed limb anyway.) */ static mp_limb_t mul1 (mp_limb_t m) { return m; } static mp_limb_t mul2 (mp_limb_t m) { mp_limb_t m01 = (m + 0) * (m + 1) >> 1; return m01; } static mp_limb_t mul3 (mp_limb_t m) { mp_limb_t m01 = (m + 0) * (m + 1) >> 1; mp_limb_t m2 = (m + 2); return m01 * m2; } static mp_limb_t mul4 (mp_limb_t m) { mp_limb_t m01 = (m + 0) * (m + 1) >> 1; mp_limb_t m23 = (m + 2) * (m + 3) >> 1; return m01 * m23 >> 1; } static mp_limb_t mul5 (mp_limb_t m) { mp_limb_t m012 = (m + 0) * (m + 1) * (m + 2) >> 1; mp_limb_t m34 = (m + 3) * (m + 4) >> 1; return m012 * m34 >> 1; } static mp_limb_t mul6 (mp_limb_t m) { mp_limb_t m01 = (m + 0) * (m + 1); mp_limb_t m23 = (m + 2) * (m + 3); mp_limb_t m45 = (m + 4) * (m + 5) >> 1; mp_limb_t m0123 = m01 * m23 >> 3; return m0123 * m45; } static mp_limb_t mul7 (mp_limb_t m) { mp_limb_t m01 = (m + 0) * (m + 1); mp_limb_t m23 = (m + 2) * (m + 3); mp_limb_t m456 = (m + 4) * (m + 5) * (m + 6) >> 1; mp_limb_t m0123 = m01 * m23 >> 3; return m0123 * m456; } static mp_limb_t mul8 (mp_limb_t m) { mp_limb_t m01 = (m + 0) * (m + 1); mp_limb_t m23 = (m + 2) * (m + 3); mp_limb_t m45 = (m + 4) * (m + 5); mp_limb_t m67 = (m + 6) * (m + 7); mp_limb_t m0123 = m01 * m23 >> 3; mp_limb_t m4567 = m45 * m67 >> 3; return m0123 * m4567 >> 1; } typedef mp_limb_t (* mulfunc_t) (mp_limb_t); static mulfunc_t mulfunc[] = {0,mul1,mul2,mul3,mul4,mul5,mul6,mul7,mul8}; #define M (numberof(mulfunc)-1) /* Number of factors-of-2 removed by the corresponding mulN functon. */ static unsigned char tcnttab[] = {0,0,1,1,3,3,4,4,7}; /* Rough computation of how many factors we can multiply together without spilling a limb. */ #if 0 /* This variant is inaccurate and relies on an expensive division. */ #define MAXFACS(max,l) \ do { \ int __cnt, __logb; \ count_leading_zeros (__cnt, (mp_limb_t) (l)); \ __logb = GMP_LIMB_BITS - __cnt; \ (max) = (GMP_NUMB_BITS + 1) / __logb; /* FIXME: remove division */ \ } while (0) #else /* This variant is exact but uses a loop. It takes the 2 removal of mulN into account. */ #if GMP_NUMB_BITS == 64 static unsigned long ftab[] = /* 1 to 8 factors per iteration */ {0xfffffffffffffffful,0x100000000ul,0x32cbfe,0x16a0b,0x24c4,0xa16,0x34b,0x1b2 /*,0xdf,0x8d */}; #endif #if GMP_NUMB_BITS == 32 static unsigned long ftab[] = /* 1 to 7 factors per iteration */ {0xffffffff,0x10000,0x801,0x16b,0x71,0x42,0x26 /* ,0x1e */}; #endif #define MAXFACS(max,l) \ do { \ int __i; \ for (__i = numberof (ftab) - 1; l > ftab[__i]; __i--) \ ; \ (max) = __i + 1; \ } while (0) #endif void mpz_bdiv_bin_uiui (mpz_ptr r, unsigned long int n, unsigned long int k) { int nmax, kmax, nmaxnow, numfac; mp_ptr np, kp; mp_size_t nn, kn, alloc; mp_limb_t i, j, t, iii, jjj, cy, dinv; mp_bitcnt_t i2cnt, j2cnt; int cnt; mp_size_t maxn; TMP_DECL; TMP_MARK; maxn = 1 + n / GMP_NUMB_BITS; /* absolutely largest result size (limbs) */ /* FIXME: This allocation might be insufficient, but is usually way too large. */ alloc = SOME_THRESHOLD + MAX (3 * maxn / 2, SOME_THRESHOLD); np = TMP_ALLOC_LIMBS (alloc); kp = TMP_ALLOC_LIMBS (alloc); MAXFACS (nmax, n); nmax = MIN (nmax, M); MAXFACS (kmax, k); kmax = MIN (kmax, M); j = 1; i = n - k + 1; nmax = MIN (nmax, k); kmax = MIN (kmax, k); np[0] = 1; nn = 1; i2cnt = 0; /* total low zeros in dividend */ j2cnt = 0; /* total low zeros in divisor */ while (kmax != 0) { numfac = j; jjj = mulfunc[kmax] (j); j += kmax; /* number of factors used */ count_trailing_zeros (cnt, jjj); /* count low zeros */ jjj >>= cnt; /* remove remaining low zeros */ j2cnt += tcnttab[kmax] + cnt; /* update low zeros count */ kp[0] = jjj; /* store new factors */ kn = 1; t = k - j + 1; kmax = MIN (kmax, t); while (kmax != 0 && kn < SOME_THRESHOLD) { jjj = mulfunc[kmax] (j); j += kmax; /* number of factors used */ count_trailing_zeros (cnt, jjj); /* count low zeros */ jjj >>= cnt; /* remove remaining low zeros */ j2cnt += tcnttab[kmax] + cnt; /* update low zeros count */ cy = mpn_mul_1 (kp, kp, kn, jjj); /* accumulate new factors */ kp[kn] = cy; kn += cy != 0; t = k - j + 1; kmax = MIN (kmax, t); } numfac = j - numfac; while (numfac != 0) { nmaxnow = MIN (nmax, numfac); iii = mulfunc[nmaxnow] (i); i += nmaxnow; /* number of factors used */ count_trailing_zeros (cnt, iii); /* count low zeros */ iii >>= cnt; /* remove remaining low zeros */ i2cnt += tcnttab[nmaxnow] + cnt; /* update low zeros count */ cy = mpn_mul_1 (np, np, nn, iii); /* accumulate new factors */ np[nn] = cy; nn += cy != 0; numfac -= nmaxnow; } ASSERT (nn < alloc); binvert_limb (dinv, kp[0]); nn += (np[nn - 1] >= kp[kn - 1]); nn -= kn; mpn_sbpi1_bdiv_q (np, np, nn, kp, MIN(kn,nn), -dinv); } /* Put back the right number of factors of 2. */ cnt = i2cnt - j2cnt; if (cnt != 0) { ASSERT (cnt < GMP_NUMB_BITS); /* can happen, but not for intended use */ cy = mpn_lshift (np, np, nn, cnt); np[nn] = cy; nn += cy != 0; } nn -= np[nn - 1] == 0; /* normalisation */ MPZ_REALLOC (r, nn); SIZ(r) = nn; MPN_COPY (PTR(r), np, nn); TMP_FREE; } #ifdef TEST #include #include int main (int argc, char **argv) { unsigned long n, k; mpz_t r, ref; mpz_inits (r, ref, 0); for (k = 1; ; k++) { printf ("\r%ld", k); fflush (stdout); for (n = 2 * k; n < 5 * k; n++) { mpz_bin_uiui (ref, n, k); mpz_bdiv_bin_uiui (r, n, k); if (mpz_cmp (ref, r) != 0) { printf ("\r%ld over %ld fails", n, k); abort (); } } } return 0; } #endif #ifdef MAIN #include int main (int argc, char **argv) { unsigned long n, k; mpz_t r; n = strtoul (argv[1], 0, 0); k = strtoul (argv[2], 0, 0); mpz_init (r); mpz_bdiv_bin_uiui (r, n, k); gmp_printf ("%Zx\n", r); return 0; } #endif