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bigint.c: fix division by zero in udiv normalization

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after left-shifting the divisor in udiv(), trailing zero limbs could
remain, causing division by zero. added trim(&y) after ulshift() to
remove zero limbs, and safety check to handle edge cases where divisor
becomes zero after normalization.

Co-authored-by: Claude <noreply@anthropic.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2025-11-18 09:33:24 +09:00
parent a6b55e741e
commit 8113c0d24a
1 changed files with 18 additions and 1 deletions
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mrbgems/mruby-bigint/core/bigint.c
+18 -1
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@@ -1365,6 +1365,7 @@ udiv(mpz_ctx_t *ctx, mpz_t *qq, mpz_t *rr, mpz_t *xx, mpz_t *yy)
mrb_assert(yy->sn != 0); /* divided by zero */
mrb_assert(yy->sz > 0); /* divided by zero */
mrb_assert(!uzero_p(yy)); /* divided by zero */
/* Use new context architecture with automatic pool/heap management */
size_t pool_state = pool_save(ctx);
@@ -1377,11 +1378,27 @@ udiv(mpz_ctx_t *ctx, mpz_t *qq, mpz_t *rr, mpz_t *xx, mpz_t *yy)
size_t ns = lzb(yy->p[yd-1]);
ulshift(ctx, &x, xx, ns);
ulshift(ctx, &y, yy, ns);
trim(&y); /* Trim after shift to remove any zero limbs */
size_t xd = digits(&x);
yd = digits(&y);
/* Handle edge case: divisor became zero after normalization */
if (yd == 0 || y.p[yd-1] == 0) {
/* This should not happen with valid inputs, but handle gracefully */
zero(qq);
zero(rr);
mpz_clear(ctx, &q);
mpz_clear(ctx, &x);
mpz_clear(ctx, &y);
pool_restore(ctx, pool_state);
return;
}
mpz_realloc(ctx, &q, xd-yd+1); // Quotient has xd-yd+1 digits maximum
/* Core Knuth Algorithm D division loop */
mp_dbl_limb z = y.p[yd-1];
mrb_assert(z != 0); /* Divisor high limb must be non-zero after normalization */
if (xd >= yd) {
for (size_t j = xd - yd;; j--) {
@@ -1501,7 +1518,7 @@ udiv(mpz_ctx_t *ctx, mpz_t *qq, mpz_t *rr, mpz_t *xx, mpz_t *yy)
if (j == 0) break;
}
}
x.sz = yy->sz;
x.sz = yd;
urshift(ctx, rr, &x, ns);
trim(&q);
mpz_move(ctx, qq, &q);