3 * Efficient reduction modulo sparse binary polynomials
5 * (c) 2004 Straylight/Edgeware
8 /*----- Licensing notice --------------------------------------------------*
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28 /*----- Header files ------------------------------------------------------*/
30 #include <mLib/alloc.h>
31 #include <mLib/darray.h>
32 #include <mLib/macros.h>
36 #include "gfreduce-exp.h"
40 /*----- Data structures ---------------------------------------------------*/
42 DA_DECL(instr_v, gfreduce_instr);
44 /*----- Main code ---------------------------------------------------------*/
46 /* --- What's going on here? --- *
48 * Let's face it, @gfx_div@ sucks. It works (I hope), but it's not in any
49 * sense fast. Here, we do efficient reduction modulo sparse polynomials.
51 * Suppose we have a polynomial @X@ we're trying to reduce mod @P@. If we
52 * take the topmost nonzero word of @X@, call it @w@, then we can eliminate
53 * it by subtracting off @w P x^{k}@ for an appropriate value of @k@. The
54 * trick is in observing that if @P@ is sparse we can do this multiplication
55 * and subtraction efficiently, just by XORing appropriate shifts of @w@ into
58 * The first tricky bit is in working out when to stop. I'll use eight-bit
59 * words to demonstrate what I'm talking about.
61 * xxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
62 * 001ppppp pppppppp pppppppp pppppppp
64 * |<------------ bp ------------->|
65 * |<------------ nw --------------->|
67 * The trick of taking whole words off of @X@ stops working when there are
68 * only @nw@ words left. Then we have to mask off the bottom bits of @w@
72 /* --- @gfreduce_create@ --- *
74 * Arguments: @gfreduce *r@ = structure to fill in
75 * @mp *x@ = a (hopefully sparse) polynomial
79 * Use: Initializes a context structure for reduction.
82 void gfreduce_create(gfreduce *r, mp *p)
91 size_t w, ww, wi, wl, ll, bb;
93 /* --- Sort out the easy stuff --- */
95 d = mp_bits(p); assert(d); d--;
101 r->mask = MPW(((mpw)-1) << dw);
106 /* --- Stash a new instruction --- */
108 #define INSTR(op_, arg_) do { \
110 DA(&iv)[DA_LEN(&iv)].op = (op_); \
111 DA(&iv)[DA_LEN(&iv)].arg = (arg_); \
117 w = (d + MPW_BITS - 1)/MPW_BITS;
123 for (i = 0, mp_scan(&sc, p); mp_step(&sc) && i < d; i++) {
126 ww = (d - i + MPW_BITS - 1)/MPW_BITS;
129 INSTR(GFRI_STORE, w);
133 INSTR(GFRI_LOAD, ww);
135 INSTR(GFRI_LOAD, w - 1);
136 for (; wi < wl; wi++) {
138 assert(ip->op == GFRI_LSL);
140 INSTR(GFRI_LSR, MPW_BITS - ip->arg);
143 INSTR(GFRI_STORE, w - 1);
144 INSTR(GFRI_LOAD, ww);
151 INSTR(GFRI_LSL, (bb + i)%MPW_BITS);
152 if ((bb + i)%MPW_BITS)
156 INSTR(GFRI_STORE, w);
160 INSTR(GFRI_LOAD, w - 1);
161 for (; wi < wl; wi++) {
163 assert(ip->op == GFRI_LSL);
165 INSTR(GFRI_LSR, MPW_BITS - ip->arg);
167 INSTR(GFRI_STORE, w - 1);
173 r->iv = xmalloc(r->in * sizeof(gfreduce_instr));
175 memcpy(r->iv, DA(&iv), r->in * sizeof(gfreduce_instr));
179 /* --- @gfreduce_destroy@ --- *
181 * Arguments: @gfreduce *r@ = structure to free
185 * Use: Reclaims the resources from a reduction context.
188 void gfreduce_destroy(gfreduce *r)
194 /* --- @gfreduce_dump@ --- *
196 * Arguments: @gfreduce *r@ = structure to dump
197 * @FILE *fp@ = file to dump on
201 * Use: Dumps a reduction context.
204 void gfreduce_dump(gfreduce *r, FILE *fp)
208 fprintf(fp, "poly = "); mp_writefile(r->p, fp, 16);
209 fprintf(fp, "\n lim = %lu; mask = %lx\n",
210 (unsigned long)r->lim, (unsigned long)r->mask);
211 for (i = 0; i < r->in; i++) {
212 static const char *opname[] = { "load", "lsl", "lsr", "store" };
213 assert(r->iv[i].op < N(opname));
214 fprintf(fp, " %s %lu\n",
216 (unsigned long)r->iv[i].arg);
220 /* --- @gfreduce_do@ --- *
222 * Arguments: @gfreduce *r@ = reduction context
223 * @mp *d@ = destination
226 * Returns: Destination, @x@ reduced modulo the reduction poly.
229 static void run(const gfreduce_instr *i, const gfreduce_instr *il,
234 for (; i < il; i++) {
236 case GFRI_LOAD: w = *(v - i->arg); break;
237 case GFRI_LSL: w ^= z << i->arg; break;
238 case GFRI_LSR: w ^= z >> i->arg; break;
239 case GFRI_STORE: *(v - i->arg) = MPW(w); break;
245 mp *gfreduce_do(gfreduce *r, mp *d, mp *x)
248 const gfreduce_instr *il;
251 /* --- Try to reuse the source's space --- */
255 MP_DEST(x, MP_LEN(x), x->f);
257 /* --- Do the reduction --- */
260 if (MP_LEN(x) >= r->lim) {
267 run(r->iv, il, vl, z);
271 while (*vl & r->mask) {
274 run(r->iv, il, vl, z);
285 /* --- @gfreduce_sqrt@ --- *
287 * Arguments: @gfreduce *r@ = pointer to reduction context
288 * @mp *d@ = destination
289 * @mp *x@ = some polynomial
291 * Returns: The square root of @x@ modulo @r->p@, or null.
294 mp *gfreduce_sqrt(gfreduce *r, mp *d, mp *x)
297 mp *z, *spare = MP_NEW;
298 unsigned long m = mp_bits(r->p) - 1;
301 for (i = 0; i < m - 1; i++) {
302 mp *t = gf_sqr(spare, y);
304 y = gfreduce_do(r, t, t);
306 z = gf_sqr(spare, y);
307 z = gfreduce_do(r, z, z);
317 /* --- @gfreduce_trace@ --- *
319 * Arguments: @gfreduce *r@ = pointer to reduction context
320 * @mp *x@ = some polynomial
322 * Returns: The trace of @x@. (%$\Tr(x)=x + x^2 + \cdots + x^{2^{m-1}}$%
323 * if %$x \in \gf{2^m}$%).
326 int gfreduce_trace(gfreduce *r, mp *x)
330 unsigned long m = mp_bits(r->p) - 1;
334 for (i = 0; i < m - 1; i++) {
335 mp *t = gf_sqr(spare, y);
337 y = gfreduce_do(r, t, t);
346 /* --- @gfreduce_halftrace@ --- *
348 * Arguments: @gfreduce *r@ = pointer to reduction context
349 * @mp *d@ = destination
350 * @mp *x@ = some polynomial
352 * Returns: The half-trace of @x@.
353 * (%$\HfTr(x)= x + x^{2^2} + \cdots + x^{2^{m-1}}$%
354 * if %$x \in \gf{2^m}$% with %$m$% odd).
357 mp *gfreduce_halftrace(gfreduce *r, mp *d, mp *x)
361 unsigned long m = mp_bits(r->p) - 1;
365 for (i = 0; i < m - 1; i += 2) {
366 mp *t = gf_sqr(spare, y);
368 y = gfreduce_do(r, t, t);
369 t = gf_sqr(spare, y);
371 y = gfreduce_do(r, t, t);
378 /* --- @gfreduce_quadsolve@ --- *
380 * Arguments: @gfreduce *r@ = pointer to reduction context
381 * @mp *d@ = destination
382 * @mp *x@ = some polynomial
384 * Returns: A polynomial @y@ such that %$y^2 + y = x$%, or null.
387 mp *gfreduce_quadsolve(gfreduce *r, mp *d, mp *x)
389 unsigned long m = mp_bits(r->p) - 1;
394 d = gfreduce_halftrace(r, d, x);
396 mp *z, *w, *rho = MP_NEW;
398 grand *fr = fibrand_create(0);
402 rho = mprand(rho, m, fr, 0);
405 for (i = 0; i < m - 1; i++) {
406 t = gf_sqr(spare, z); spare = z; z = gfreduce_do(r, t, t);
407 t = gf_sqr(spare, w); spare = w; w = gfreduce_do(r, t, t);
408 t = gf_mul(spare, w, x); t = gfreduce_do(r, t, t); spare = t;
410 w = gf_add(w, w, rho);
421 fr->ops->destroy(fr);
425 t = gf_sqr(MP_NEW, d); t = gfreduce_do(r, t, t); t = gf_add(t, t, d);
432 if (d) d->v[0] &= ~(mpw)1;
436 /* --- @gfreduce_exp@ --- *
438 * Arguments: @gfreduce *gr@ = pointer to reduction context
439 * @mp *d@ = fake destination
443 * Returns: Result, %$a^e \bmod m$%.
446 mp *gfreduce_exp(gfreduce *gr, mp *d, mp *a, mp *e)
449 mp *spare = (e->f & MP_BURN) ? MP_NEWSEC : MP_NEW;
457 a = gf_modinv(a, a, gr->p);
458 if (MP_LEN(e) < EXP_THRESH)
469 /*----- Test rig ----------------------------------------------------------*/
473 #define MP(x) mp_readstring(MP_NEW, #x, 0, 0)
475 static int vreduce(dstr *v)
477 mp *d = *(mp **)v[0].buf;
478 mp *n = *(mp **)v[1].buf;
479 mp *r = *(mp **)v[2].buf;
484 gfreduce_create(&rr, d);
485 c = gfreduce_do(&rr, MP_NEW, n);
487 fprintf(stderr, "\n*** reduction failed\n*** ");
488 gfreduce_dump(&rr, stderr);
489 fprintf(stderr, "\n*** n = "); mp_writefile(n, stderr, 16);
490 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
491 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
492 fprintf(stderr, "\n");
495 gfreduce_destroy(&rr);
496 mp_drop(n); mp_drop(d); mp_drop(r); mp_drop(c);
497 assert(mparena_count(MPARENA_GLOBAL) == 0);
501 static int vmodexp(dstr *v)
503 mp *p = *(mp **)v[0].buf;
504 mp *g = *(mp **)v[1].buf;
505 mp *x = *(mp **)v[2].buf;
506 mp *r = *(mp **)v[3].buf;
511 gfreduce_create(&rr, p);
512 c = gfreduce_exp(&rr, MP_NEW, g, x);
514 fprintf(stderr, "\n*** modexp failed\n*** ");
515 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
516 fprintf(stderr, "\n*** g = "); mp_writefile(g, stderr, 16);
517 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
518 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
519 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
520 fprintf(stderr, "\n");
523 gfreduce_destroy(&rr);
524 mp_drop(p); mp_drop(g); mp_drop(r); mp_drop(x); mp_drop(c);
525 assert(mparena_count(MPARENA_GLOBAL) == 0);
529 static int vsqrt(dstr *v)
531 mp *p = *(mp **)v[0].buf;
532 mp *x = *(mp **)v[1].buf;
533 mp *r = *(mp **)v[2].buf;
538 gfreduce_create(&rr, p);
539 c = gfreduce_sqrt(&rr, MP_NEW, x);
541 fprintf(stderr, "\n*** sqrt failed\n*** ");
542 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
543 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
544 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
545 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
546 fprintf(stderr, "\n");
549 gfreduce_destroy(&rr);
550 mp_drop(p); mp_drop(r); mp_drop(x); mp_drop(c);
551 assert(mparena_count(MPARENA_GLOBAL) == 0);
555 static int vtr(dstr *v)
557 mp *p = *(mp **)v[0].buf;
558 mp *x = *(mp **)v[1].buf;
559 int r = *(int *)v[2].buf, c;
563 gfreduce_create(&rr, p);
564 c = gfreduce_trace(&rr, x);
566 fprintf(stderr, "\n*** trace failed\n*** ");
567 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
568 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
569 fprintf(stderr, "\n*** c = %d", c);
570 fprintf(stderr, "\n*** r = %d", r);
571 fprintf(stderr, "\n");
574 gfreduce_destroy(&rr);
575 mp_drop(p); mp_drop(x);
576 assert(mparena_count(MPARENA_GLOBAL) == 0);
580 static int vhftr(dstr *v)
582 mp *p = *(mp **)v[0].buf;
583 mp *x = *(mp **)v[1].buf;
584 mp *r = *(mp **)v[2].buf;
589 gfreduce_create(&rr, p);
590 c = gfreduce_halftrace(&rr, MP_NEW, x);
592 fprintf(stderr, "\n*** halftrace failed\n*** ");
593 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
594 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
595 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
596 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
597 fprintf(stderr, "\n");
600 gfreduce_destroy(&rr);
601 mp_drop(p); mp_drop(r); mp_drop(x); mp_drop(c);
602 assert(mparena_count(MPARENA_GLOBAL) == 0);
606 static int vquad(dstr *v)
608 mp *p = *(mp **)v[0].buf;
609 mp *x = *(mp **)v[1].buf;
610 mp *r = *(mp **)v[2].buf;
615 gfreduce_create(&rr, p);
616 c = gfreduce_quadsolve(&rr, MP_NEW, x);
618 fprintf(stderr, "\n*** quadsolve failed\n*** ");
619 fprintf(stderr, "\n*** p = "); mp_writefile(p, stderr, 16);
620 fprintf(stderr, "\n*** x = "); mp_writefile(x, stderr, 16);
621 fprintf(stderr, "\n*** c = "); mp_writefile(c, stderr, 16);
622 fprintf(stderr, "\n*** r = "); mp_writefile(r, stderr, 16);
623 fprintf(stderr, "\n");
626 gfreduce_destroy(&rr);
627 mp_drop(p); mp_drop(r); mp_drop(x); mp_drop(c);
628 assert(mparena_count(MPARENA_GLOBAL) == 0);
632 static test_chunk defs[] = {
633 { "reduce", vreduce, { &type_mp, &type_mp, &type_mp, 0 } },
634 { "modexp", vmodexp, { &type_mp, &type_mp, &type_mp, &type_mp, 0 } },
635 { "sqrt", vsqrt, { &type_mp, &type_mp, &type_mp, 0 } },
636 { "trace", vtr, { &type_mp, &type_mp, &type_int, 0 } },
637 { "halftrace", vhftr, { &type_mp, &type_mp, &type_mp, 0 } },
638 { "quadsolve", vquad, { &type_mp, &type_mp, &type_mp, 0 } },
642 int main(int argc, char *argv[])
644 test_run(argc, argv, defs, SRCDIR"/t/gfreduce");
650 /*----- That's all, folks -------------------------------------------------*/