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mruby-bigint: extract core subtraction algorithm to eliminate duplication

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Extract multi-limb subtraction algorithm from usub() and usub_pool()
into shared usub_core() helper function. Both functions now use the
same core subtraction logic with borrow propagation, eliminating
duplicated algorithm code.

Benefits:
- Eliminates ~14 lines of duplicated subtraction algorithm code
- Single source of truth for multi-limb subtraction with borrow handling
- Reduces maintenance burden for future optimizations
- Maintains all existing functionality and performance

Co-authored-by: Claude <noreply@anthropic.com>
This commit is contained in:
Yukihiro "Matz" Matsumoto
2025-07-25 12:18:22 +09:00
parent 2e45af37a8
commit 131d86629b
1 changed files with 32 additions and 29 deletions
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mrbgems/mruby-bigint/core/bigint.c
+32 -29
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@@ -405,26 +405,42 @@ uadd_pool(mrb_state *mrb, mpz_t *z, mpz_t *x, mpz_t *y, mpz_pool_t *pool)
return 1; /* Success - used pool memory */
}
/* Core subtraction algorithm - extracted from usub/usub_pool duplication */
static void
usub_core(mpz_t *z, mpz_t *y, mpz_t *x)
{
/* Core multi-limb subtraction with borrow propagation */
mp_dbl_limb_signed b = 0;
size_t i;
/* Subtract overlapping limbs from both operands */
for (i = 0; i < x->sz; i++) {
b += (mp_dbl_limb_signed)y->p[i];
b -= (mp_dbl_limb_signed)x->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
}
/* Process remaining limbs from minuend with borrow */
for (; i < y->sz; i++) {
b += y->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
}
/* Normalize result size */
z->sz = digits(z);
}
/* z = y - x, ignoring sign */
/* precondition: abs(y) >= abs(x) */
static void
usub(mrb_state *mrb, mpz_t *z, mpz_t *y, mpz_t *x)
{
mpz_realloc(mrb, z, (size_t)(y->sz));
mp_dbl_limb_signed b = 0;
size_t i;
for (i=0;i<x->sz;i++) {
b += (mp_dbl_limb_signed)y->p[i];
b -= (mp_dbl_limb_signed)x->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
}
for (;i<y->sz; i++) {
b += y->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
}
z->sz = digits(z);
/* Use core subtraction algorithm */
usub_core(z, y, x);
}
/* Pool-based unsigned subtraction - uses stack memory for result */
@@ -439,21 +455,8 @@ usub_pool(mrb_state *mrb, mpz_t *z, mpz_t *y, mpz_t *x, mpz_pool_t *pool)
/* Try to allocate in pool */
MPZ_POOL_ALLOC(mrb, *z, pool, y->sz);
/* Perform subtraction using pool memory */
mp_dbl_limb_signed b = 0;
size_t i;
for (i=0;i<x->sz;i++) {
b += (mp_dbl_limb_signed)y->p[i];
b -= (mp_dbl_limb_signed)x->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
}
for (;i<y->sz; i++) {
b += y->p[i];
z->p[i] = LOW(b);
b = HIGH(b);
}
z->sz = digits(z);
/* Use core subtraction algorithm */
usub_core(z, y, x);
return 1; /* Success - used pool memory */
}