5 * Efficient reduction modulo sparse binary polynomials
7 * (c) 2004 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Catacomb.
14 * Catacomb is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU Library General Public License as
16 * published by the Free Software Foundation; either version 2 of the
17 * License, or (at your option) any later version.
19 * Catacomb is distributed in the hope that it will be useful,
20 * but WITHOUT ANY WARRANTY; without even the implied warranty of
21 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 * GNU Library General Public License for more details.
24 * You should have received a copy of the GNU Library General Public
25 * License along with Catacomb; if not, write to the Free
26 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
30 /*----- Header files ------------------------------------------------------*/
32 #include <mLib/alloc.h>
33 #include <mLib/darray.h>
34 #include <mLib/macros.h>
38 #include "gfreduce-exp.h"
42 /*----- Data structures ---------------------------------------------------*/
44 DA_DECL(instr_v, gfreduce_instr);
46 /*----- Main code ---------------------------------------------------------*/
48 /* --- What's going on here? --- *
50 * Let's face it, @gfx_div@ sucks. It works (I hope), but it's not in any
51 * sense fast. Here, we do efficient reduction modulo sparse polynomials.
53 * Suppose we have a polynomial @X@ we're trying to reduce mod @P@. If we
54 * take the topmost nonzero word of @X@, call it @w@, then we can eliminate
55 * it by subtracting off @w P x^{k}@ for an appropriate value of @k@. The
56 * trick is in observing that if @P@ is sparse we can do this multiplication
57 * and subtraction efficiently, just by XORing appropriate shifts of @w@ into
60 * The first tricky bit is in working out when to stop. I'll use eight-bit
61 * words to demonstrate what I'm talking about.
63 * xxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
64 * 001ppppp pppppppp pppppppp pppppppp
66 * |<------------ bp ------------->|
67 * |<------------ nw --------------->|
69 * The trick of taking whole words off of @X@ stops working when there are
70 * only @nw@ words left. Then we have to mask off the bottom bits of @w@
74 /* --- @gfreduce_create@ --- *
76 * Arguments: @gfreduce *r@ = structure to fill in
77 * @mp *x@ = a (hopefully sparse) polynomial
81 * Use: Initializes a context structure for reduction.
84 void gfreduce_create(gfreduce *r, mp *p)
93 size_t w, ww, wi, wl, ll;
95 /* --- Sort out the easy stuff --- */
97 d = mp_bits(p); assert(d); d--;
103 r->mask = MPW(((mpw)-1) << dw);
108 /* --- Stash a new instruction --- */
110 #define INSTR(op_, arg_) do { \
112 DA(&iv)[DA_LEN(&iv)].op = (op_); \
113 DA(&iv)[DA_LEN(&iv)].arg = (arg_); \
119 w = (d + MPW_BITS - 1)/MPW_BITS;
124 for (i = 0, mp_scan(&sc, p); mp_step(&sc) && i < d; i++) {
127 ww = (d - i + MPW_BITS - 1)/MPW_BITS;
130 INSTR(GFRI_STORE, w);
134 INSTR(GFRI_LOAD, ww);
136 INSTR(GFRI_LOAD, w - 1);
137 for (; wi < wl; wi++) {
139 assert(ip->op == GFRI_LSL);
141 INSTR(GFRI_LSR, MPW_BITS - ip->arg);
144 INSTR(GFRI_STORE, w - 1);
145 INSTR(GFRI_LOAD, ww);
152 INSTR(GFRI_LSL, (MPW_BITS + i - d)%MPW_BITS);
153 if ((MPW_BITS + i - d)%MPW_BITS)
157 INSTR(GFRI_STORE, w);
161 INSTR(GFRI_LOAD, w - 1);
162 for (; wi < wl; wi++) {
164 assert(ip->op == GFRI_LSL);
166 INSTR(GFRI_LSR, MPW_BITS - ip->arg);
168 INSTR(GFRI_STORE, w - 1);
174 r->iv = xmalloc(r->in * sizeof(gfreduce_instr));
176 memcpy(r->iv, DA(&iv), r->in * sizeof(gfreduce_instr));
180 /* --- @gfreduce_destroy@ --- *
182 * Arguments: @gfreduce *r@ = structure to free
186 * Use: Reclaims the resources from a reduction context.
189 void gfreduce_destroy(gfreduce *r)
195 /* --- @gfreduce_dump@ --- *
197 * Arguments: @gfreduce *r@ = structure to dump
198 * @FILE *fp@ = file to dump on
202 * Use: Dumps a reduction context.
205 void gfreduce_dump(gfreduce *r, FILE *fp)
209 fprintf(fp, "poly = "); mp_writefile(r->p, fp, 16);
210 fprintf(fp, "\n lim = %lu; mask = %lx\n",
211 (unsigned long)r->lim, (unsigned long)r->mask);
212 for (i = 0; i < r->in; i++) {
213 static const char *opname[] = { "load", "lsl", "lsr", "store" };
214 assert(r->iv[i].op < N(opname));
215 fprintf(fp, " %s %lu\n",
217 (unsigned long)r->iv[i].arg);
221 /* --- @gfreduce_do@ --- *
223 * Arguments: @gfreduce *r@ = reduction context
224 * @mp *d@ = destination
227 * Returns: Destination, @x@ reduced modulo the reduction poly.
230 static void run(const gfreduce_instr *i, const gfreduce_instr *il,
235 for (; i < il; i++) {
237 case GFRI_LOAD: w = *(v - i->arg); break;
238 case GFRI_LSL: w ^= z << i->arg; break;
239 case GFRI_LSR: w ^= z >> i->arg; break;
240 case GFRI_STORE: *(v - i->arg) = MPW(w); break;
246 mp *gfreduce_do(gfreduce *r, mp *d, mp *x)
249 const gfreduce_instr *il;
252 /* --- Try to reuse the source's space --- */
256 MP_DEST(x, MP_LEN(x), x->f);
258 /* --- Do the reduction --- */
261 if (MP_LEN(x) >= r->lim) {
268 run(r->iv, il, vl, z);
272 while (*vl & r->mask) {
275 run(r->iv, il, vl, z);
286 /* --- @gfreduce_sqrt@ --- *
288 * Arguments: @gfreduce *r@ = pointer to reduction context
289 * @mp *d@ = destination
290 * @mp *x@ = some polynomial
292 * Returns: The square root of @x@ modulo @r->p@, or null.
295 mp *gfreduce_sqrt(gfreduce *r, mp *d, mp *x)
298 mp *z, *spare = MP_NEW;
299 unsigned long m = mp_bits(r->p) - 1;
302 for (i = 0; i < m - 1; i++) {
303 mp *t = gf_sqr(spare, y);
305 y = gfreduce_do(r, t, t);
307 z = gf_sqr(spare, y);
308 z = gfreduce_do(r, z, z);
318 /* --- @gfreduce_trace@ --- *
320 * Arguments: @gfreduce *r@ = pointer to reduction context
321 * @mp *x@ = some polynomial
323 * Returns: The trace of @x@. (%$\Tr(x)=x + x^2 + \cdots + x^{2^{m-1}}$%
324 * if %$x \in \gf{2^m}$%).
327 int gfreduce_trace(gfreduce *r, mp *x)
331 unsigned long m = mp_bits(r->p) - 1;
335 for (i = 0; i < m - 1; i++) {
336 mp *t = gf_sqr(spare, y);
338 y = gfreduce_do(r, t, t);
347 /* --- @gfreduce_halftrace@ --- *
349 * Arguments: @gfreduce *r@ = pointer to reduction context
350 * @mp *d@ = destination
351 * @mp *x@ = some polynomial
353 * Returns: The half-trace of @x@.
354 * (%$\HfTr(x)= x + x^{2^2} + \cdots + x^{2^{m-1}}$%
355 * if %$x \in \gf{2^m}$% with %$m$% odd).
358 mp *gfreduce_halftrace(gfreduce *r, mp *d, mp *x)
362 unsigned long m = mp_bits(r->p) - 1;
366 for (i = 0; i < m - 1; i += 2) {
367 mp *t = gf_sqr(spare, y);
369 y = gfreduce_do(r, t, t);
370 t = gf_sqr(spare, y);
372 y = gfreduce_do(r, t, t);
379 /* --- @gfreduce_quadsolve@ --- *
381 * Arguments: @gfreduce *r@ = pointer to reduction context
382 * @mp *d@ = destination
383 * @mp *x@ = some polynomial
385 * Returns: A polynomial @y@ such that %$y^2 + y = x$%, or null.
388 mp *gfreduce_quadsolve(gfreduce *r, mp *d, mp *x)
390 unsigned long m = mp_bits(r->p) - 1;
395 d = gfreduce_halftrace(r, d, x);
397 mp *z, *w, *rho = MP_NEW;
399 grand *fr = fibrand_create(0);
403 rho = mprand(rho, m, fr, 0);
406 for (i = 0; i < m - 1; i++) {
407 t = gf_sqr(spare, z); spare = z; z = gfreduce_do(r, t, t);
408 t = gf_sqr(spare, w); spare = w; w = gfreduce_do(r, t, t);
409 t = gf_mul(spare, w, x); t = gfreduce_do(r, t, t); spare = t;
411 w = gf_add(w, w, rho);
422 fr->ops->destroy(fr);
426 t = gf_sqr(MP_NEW, d); t = gfreduce_do(r, t, t); t = gf_add(t, t, d);
433 if (d) d->v[0] &= ~(mpw)1;
437 /* --- @gfreduce_exp@ --- *
439 * Arguments: @gfreduce *gr@ = pointer to reduction context
440 * @mp *d@ = fake destination
444 * Returns: Result, %$a^e \bmod m$%.
447 mp *gfreduce_exp(gfreduce *gr, mp *d, mp *a, mp *e)
450 mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
458 a = gf_modinv(a, a, gr->p);
459 if (MP_LEN(e) < EXP_THRESH)
470 /*----- Test rig ----------------------------------------------------------*/
474 #define MP(x) mp_readstring(MP_NEW, #x, 0, 0)
476 static int vreduce(dstr *v)
478 mp *d = *(mp **)v[0].buf;
479 mp *n = *(mp **)v[1].buf;
480 mp *r = *(mp **)v[2].buf;
485 gfreduce_create(&rr, d);
486 c = gfreduce_do(&rr, MP_NEW, n);
488 fprintf(stderr, "\n*** reduction failed\n*** ");
489 gfreduce_dump(&rr, stderr);
490 fprintf(stderr, "\n*** n = "); mp_writefile(n, stderr, 16);
491 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
492 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
493 fprintf(stderr, "\n");
496 gfreduce_destroy(&rr);
497 mp_drop(n); mp_drop(d); mp_drop(r); mp_drop(c);
498 assert(mparena_count(MPARENA_GLOBAL) == 0);
502 static int vmodexp(dstr *v)
504 mp *p = *(mp **)v[0].buf;
505 mp *g = *(mp **)v[1].buf;
506 mp *x = *(mp **)v[2].buf;
507 mp *r = *(mp **)v[3].buf;
512 gfreduce_create(&rr, p);
513 c = gfreduce_exp(&rr, MP_NEW, g, x);
515 fprintf(stderr, "\n*** modexp failed\n*** ");
516 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
517 fprintf(stderr, "\n*** g = "); mp_writefile(g, stderr, 16);
518 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
519 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
520 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
521 fprintf(stderr, "\n");
524 gfreduce_destroy(&rr);
525 mp_drop(p); mp_drop(g); mp_drop(r); mp_drop(x); mp_drop(c);
526 assert(mparena_count(MPARENA_GLOBAL) == 0);
530 static int vsqrt(dstr *v)
532 mp *p = *(mp **)v[0].buf;
533 mp *x = *(mp **)v[1].buf;
534 mp *r = *(mp **)v[2].buf;
539 gfreduce_create(&rr, p);
540 c = gfreduce_sqrt(&rr, MP_NEW, x);
542 fprintf(stderr, "\n*** sqrt failed\n*** ");
543 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
544 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
545 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
546 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
547 fprintf(stderr, "\n");
550 gfreduce_destroy(&rr);
551 mp_drop(p); mp_drop(r); mp_drop(x); mp_drop(c);
552 assert(mparena_count(MPARENA_GLOBAL) == 0);
556 static int vtr(dstr *v)
558 mp *p = *(mp **)v[0].buf;
559 mp *x = *(mp **)v[1].buf;
560 int r = *(int *)v[2].buf, c;
564 gfreduce_create(&rr, p);
565 c = gfreduce_trace(&rr, x);
567 fprintf(stderr, "\n*** trace failed\n*** ");
568 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
569 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
570 fprintf(stderr, "\n*** c = %d", c);
571 fprintf(stderr, "\n*** r = %d", r);
572 fprintf(stderr, "\n");
575 gfreduce_destroy(&rr);
576 mp_drop(p); mp_drop(x);
577 assert(mparena_count(MPARENA_GLOBAL) == 0);
581 static int vhftr(dstr *v)
583 mp *p = *(mp **)v[0].buf;
584 mp *x = *(mp **)v[1].buf;
585 mp *r = *(mp **)v[2].buf;
590 gfreduce_create(&rr, p);
591 c = gfreduce_halftrace(&rr, MP_NEW, x);
593 fprintf(stderr, "\n*** halftrace failed\n*** ");
594 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
595 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
596 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
597 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
598 fprintf(stderr, "\n");
601 gfreduce_destroy(&rr);
602 mp_drop(p); mp_drop(r); mp_drop(x); mp_drop(c);
603 assert(mparena_count(MPARENA_GLOBAL) == 0);
607 static int vquad(dstr *v)
609 mp *p = *(mp **)v[0].buf;
610 mp *x = *(mp **)v[1].buf;
611 mp *r = *(mp **)v[2].buf;
616 gfreduce_create(&rr, p);
617 c = gfreduce_quadsolve(&rr, MP_NEW, x);
619 fprintf(stderr, "\n*** quadsolve failed\n*** ");
620 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
621 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
622 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
623 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
624 fprintf(stderr, "\n");
627 gfreduce_destroy(&rr);
628 mp_drop(p); mp_drop(r); mp_drop(x); mp_drop(c);
629 assert(mparena_count(MPARENA_GLOBAL) == 0);
633 static test_chunk defs[] = {
634 { "reduce", vreduce, { &type_mp, &type_mp, &type_mp, 0 } },
635 { "modexp", vmodexp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
636 { "sqrt", vsqrt, { &type_mp, &type_mp, &type_mp, 0 } },
637 { "trace", vtr, { &type_mp, &type_mp, &type_int, 0 } },
638 { "halftrace", vhftr, { &type_mp, &type_mp, &type_mp, 0 } },
639 { "quadsolve", vquad, { &type_mp, &type_mp, &type_mp, 0 } },
643 int main(int argc, char *argv[])
645 test_run(argc, argv, defs, SRCDIR"/tests/gfreduce");
651 /*----- That's all, folks -------------------------------------------------*/