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mruby-bigint: add mpn-style shift functions

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Add mpn_rshift and mpn_lshift functions that operate directly on limb
arrays, following GMP's mpn layer design. These functions support
in-place operation and return shifted-out bits.

Refactor urshift and ulshift to use these new mpn functions, simplifying
the code and improving reusability.

Co-authored-by: Claude <noreply@anthropic.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2026-01-13 07:57:51 +09:00
parent 8b0641f97a
commit c9905ffaa4
1 changed files with 67 additions and 18 deletions
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mrbgems/mruby-bigint/core/bigint.c
+67 -18
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@@ -1707,6 +1707,67 @@ lzb(mp_limb x)
return j;
}
/*
* mpn-style low-level shift functions.
* These operate directly on limb arrays without mpz_t overhead.
*/
/*
* Right shift limb array by cnt bits (0 < cnt < DIG_SIZE).
* rp[0..n-1] = ap[0..n-1] >> cnt
* Returns the bits shifted out from the low end.
* Supports in-place operation (rp == ap).
*
* Processing order: LOW to HIGH
* - rp[i] depends on ap[i] and ap[i+1]
* - Writing rp[i] doesn't affect ap[i+1], so in-place is safe
*/
static mp_limb
mpn_rshift(mp_limb *rp, const mp_limb *ap, size_t n, unsigned int cnt)
{
mp_limb shifted_out;
size_t i;
mrb_assert(cnt > 0 && cnt < DIG_SIZE);
shifted_out = (ap[0] << (DIG_SIZE - cnt)) & DIG_MASK;
for (i = 0; i < n - 1; i++) {
rp[i] = ((ap[i] >> cnt) | (ap[i + 1] << (DIG_SIZE - cnt))) & DIG_MASK;
}
rp[n - 1] = (ap[n - 1] >> cnt) & DIG_MASK;
return shifted_out;
}
/*
* Left shift limb array by cnt bits (0 < cnt < DIG_SIZE).
* rp[0..n-1] = ap[0..n-1] << cnt (lower n limbs)
* Returns the bits shifted out from the high end (carry).
* Supports in-place operation (rp == ap).
*
* Processing order: HIGH to LOW
* - rp[i] depends on ap[i] and ap[i-1]
* - Writing rp[i] doesn't affect ap[i-1], so in-place is safe
*/
static mp_limb
mpn_lshift(mp_limb *rp, const mp_limb *ap, size_t n, unsigned int cnt)
{
mp_limb carry;
size_t i;
mrb_assert(cnt > 0 && cnt < DIG_SIZE);
carry = (ap[n - 1] >> (DIG_SIZE - cnt)) & DIG_MASK;
for (i = n - 1; i > 0; i--) {
rp[i] = ((ap[i] << cnt) | (ap[i - 1] >> (DIG_SIZE - cnt))) & DIG_MASK;
}
rp[0] = (ap[0] << cnt) & DIG_MASK;
return carry;
}
/* c1 = a>>n */
/* n must be < DIG_SIZE */
static void
@@ -1722,15 +1783,8 @@ urshift(mpz_ctx_t *ctx, mpz_t *c1, mpz_t *a, size_t n)
zero(c1);
}
else {
mp_limb cc = 0;
mp_dbl_limb rm = (((mp_dbl_limb)1<<n) - 1);
mpz_realloc(ctx, c1, a->sz);
for (size_t i=a->sz-1;; i--) {
c1->p[i] = ((a->p[i] >> n) | cc) & DIG_MASK;
cc = (a->p[i] & rm) << (DIG_SIZE - n);
if (i == 0) break;
}
mpn_rshift(c1->p, a->p, a->sz, (unsigned int)n);
c1->sz = a->sz;
trim(c1);
}
@@ -1750,18 +1804,13 @@ ulshift(mpz_ctx_t *ctx, mpz_t *c1, mpz_t *a, size_t n)
zero(c1);
}
else {
mp_limb cc = 0;
mpz_t c;
mp_limb rm = (((mp_dbl_limb)1<<n) - 1) << (DIG_SIZE-n);
mp_limb carry;
mpz_init_heap(ctx, &c, a->sz+1);
size_t i;
for (i=0; i<a->sz; i++) {
c.p[i] = ((a->p[i] << n) | cc) & DIG_MASK;
cc = (a->p[i] & rm) >> (DIG_SIZE-n);
}
c.p[i] = cc;
mpz_init_heap(ctx, &c, a->sz + 1);
carry = mpn_lshift(c.p, a->p, a->sz, (unsigned int)n);
c.p[a->sz] = carry;
c.sz = a->sz + 1;
trim(&c);
mpz_move(ctx, c1, &c);
}