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Merged
tompng merged 1 commit into from
Jul 17, 2025
Merged

Use minimum necessary division precision in BigDecimal_DoDivmod #371

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95 changes: 42 additions & 53 deletions ext/bigdecimal/bigdecimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -174,6 +174,14 @@ check_allocation_count_nonzero(void)
# define check_allocation_count_nonzero() /* nothing */
#endif /* BIGDECIMAL_DEBUG */

/* VpMult VpDivd helpers */
#define VPMULT_RESULT_PREC(a, b) (a->Prec + b->Prec + 1)
/* To calculate VpDivd with n-digits precision, quotient needs n+2*BASE_FIG-1 digits space */
/* In the worst precision case 0001_1111_1111 / 9999 = 0000_0001_1112, there are 2*BASE_FIG-1 leading zeros */
#define VPDIVD_QUO_DIGITS(required_digits) ((required_digits) + 2 * BASE_FIG - 1)
/* Required r.MaxPrec for calculating VpDivd(c, r, a, b) */
#define VPDIVD_REM_PREC(a, b, c) Max(a->Prec, b->Prec + c->MaxPrec - 1)

static NULLABLE_BDVALUE
CreateFromString(size_t mx, const char *str, VALUE klass, bool strict_p, bool raise_exception);

Expand Down Expand Up @@ -222,16 +230,6 @@ rbd_allocate_struct_decimal_digits(size_t const decimal_digits, bool limit_preci

static VALUE BigDecimal_wrap_struct(VALUE obj, Real *vp);

static BDVALUE
rbd_reallocate_struct(BDVALUE value, size_t const internal_digits)
{
size_t const size = rbd_struct_size(internal_digits);
Real *new_real = (Real *)ruby_xrealloc(value.real, size);
new_real->MaxPrec = internal_digits;
BigDecimal_wrap_struct(value.bigdecimal, new_real);
return (BDVALUE) { value.bigdecimal, new_real };
}

static void
rbd_free_struct(Real *real)
{
Expand Down Expand Up @@ -1843,7 +1841,6 @@ BigDecimal_mult(VALUE self, VALUE r)
{
ENTER(5);
BDVALUE a, b, c;
size_t mx;

GUARD_OBJ(a, GetBDValueMust(self));
if (RB_TYPE_P(r, T_FLOAT)) {
Expand All @@ -1859,8 +1856,7 @@ BigDecimal_mult(VALUE self, VALUE r)
b = bdvalue_nonnullable(b2);
}

mx = a.real->Prec + b.real->Prec;
GUARD_OBJ(c, NewZeroWrapLimited(1, (mx + 1) * BASE_FIG));
GUARD_OBJ(c, NewZeroWrapLimited(1, VPMULT_RESULT_PREC(a.real, b.real) * BASE_FIG));
VpMult(c.real, a.real, b.real);
return CheckGetValue(c);
}
Expand Down Expand Up @@ -1941,9 +1937,9 @@ static VALUE
BigDecimal_DoDivmod(VALUE self, VALUE r, NULLABLE_BDVALUE *div, NULLABLE_BDVALUE *mod, bool truncate)
{
ENTER(8);
BDVALUE a, b, c, d, e, res;
ssize_t a_prec, b_prec;
size_t mx;
BDVALUE a, b, dv, md, res;
ssize_t a_exponent, b_exponent;
size_t mx, rx;

GUARD_OBJ(a, GetBDValueMust(self));

Expand Down Expand Up @@ -1997,39 +1993,36 @@ BigDecimal_DoDivmod(VALUE self, VALUE r, NULLABLE_BDVALUE *div, NULLABLE_BDVALUE
return Qtrue;
}

BigDecimal_count_precision_and_scale(self, &a_prec, NULL);
BigDecimal_count_precision_and_scale(rr, &b_prec, NULL);
a_exponent = VpExponent10(a.real);
b_exponent = VpExponent10(b.real);
mx = a_exponent > b_exponent ? a_exponent - b_exponent + 1 : 1;
GUARD_OBJ(dv, NewZeroWrapLimited(1, VPDIVD_QUO_DIGITS(mx)));

mx = (a_prec > b_prec) ? a_prec : b_prec;
mx *= 2;
/* res is reused for VpDivd remainder and VpMult result */
rx = VPDIVD_REM_PREC(a.real, b.real, dv.real);
mx = VPMULT_RESULT_PREC(dv.real, b.real);
GUARD_OBJ(res, NewZeroWrapNolimit(1, Max(rx, mx) * BASE_FIG));
/* AddSub needs one more prec */
GUARD_OBJ(md, NewZeroWrapLimited(1, (res.real->MaxPrec + 1) * BASE_FIG));

if (2*BIGDECIMAL_DOUBLE_FIGURES > mx)
mx = 2*BIGDECIMAL_DOUBLE_FIGURES;
VpDivd(dv.real, res.real, a.real, b.real);
VpMidRound(dv.real, VP_ROUND_DOWN, 0);
VpMult(res.real, dv.real, b.real);
VpAddSub(md.real, a.real, res.real, -1);

GUARD_OBJ(c, NewZeroWrapLimited(1, mx + 2*BASE_FIG));
GUARD_OBJ(res, NewZeroWrapNolimit(1, mx*2 + 2*BASE_FIG));
VpDivd(c.real, res.real, a.real, b.real);

mx = c.real->Prec * BASE_FIG;
GUARD_OBJ(d, NewZeroWrapLimited(1, mx));
VpActiveRound(d.real, c.real, VP_ROUND_DOWN, 0);

VpMult(res.real, d.real, b.real);
VpAddSub(c.real, a.real, res.real, -1);

if (!truncate && !VpIsZero(c.real) && (VpGetSign(a.real) * VpGetSign(b.real) < 0)) {
if (!truncate && !VpIsZero(md.real) && (VpGetSign(a.real) * VpGetSign(b.real) < 0)) {
/* result adjustment for negative case */
res = rbd_reallocate_struct(res, d.real->MaxPrec);
res.real->MaxPrec = d.real->MaxPrec;
VpAddSub(res.real, d.real, VpOne(), -1);
GUARD_OBJ(e, NewZeroWrapLimited(1, GetAddSubPrec(c.real, b.real) * 2*BASE_FIG));
VpAddSub(e.real, c.real, b.real, 1);
*div = bdvalue_nullable(res);
*mod = bdvalue_nullable(e);
BDVALUE dv2, md2;
GUARD_OBJ(dv2, NewZeroWrapLimited(1, (dv.real->MaxPrec + 1) * BASE_FIG));
GUARD_OBJ(md2, NewZeroWrapLimited(1, (GetAddSubPrec(md.real, b.real) + 1) * BASE_FIG));
VpAddSub(dv2.real, dv.real, VpOne(), -1);
VpAddSub(md2.real, md.real, b.real, 1);
*div = bdvalue_nullable(dv2);
*mod = bdvalue_nullable(md2);
}
else {
*div = bdvalue_nullable(d);
*mod = bdvalue_nullable(c);
*div = bdvalue_nullable(dv);
*mod = bdvalue_nullable(md);
}
return Qtrue;

Expand Down Expand Up @@ -2121,7 +2114,7 @@ BigDecimal_div2(VALUE self, VALUE b, VALUE n)
ENTER(5);
SIGNED_VALUE ix;
BDVALUE av, bv, cv, res;
size_t mx, pl;
size_t pl;

if (NIL_P(n)) { /* div in Float sense */
NULLABLE_BDVALUE div;
Expand Down Expand Up @@ -2158,11 +2151,9 @@ BigDecimal_div2(VALUE self, VALUE b, VALUE n)
ix = 2 * BIGDECIMAL_DOUBLE_FIGURES;
}

// VpDivd needs 2 extra DECDIGs for rounding.
GUARD_OBJ(cv, NewZeroWrapLimited(1, ix + 2 * VpBaseFig()));

mx = Max(av.real->Prec, bv.real->Prec + cv.real->MaxPrec - 1);
GUARD_OBJ(res, NewZeroWrapNolimit(1, mx * VpBaseFig()));
// Needs to calculate 1 extra digit for rounding.
GUARD_OBJ(cv, NewZeroWrapLimited(1, VPDIVD_QUO_DIGITS(ix + 1)));
GUARD_OBJ(res, NewZeroWrapNolimit(1, VPDIVD_REM_PREC(av.real, bv.real, cv.real) * BASE_FIG));
VpDivd(cv.real, res.real, av.real, bv.real);
VpSetPrecLimit(pl);
if (!VpIsZero(res.real)) {
Expand Down Expand Up @@ -4221,9 +4212,8 @@ BigDecimal_vpdivd(VALUE self, VALUE r, VALUE cprec) {
size_t cn = NUM2INT(cprec);
a = GetBDValueMust(self);
b = GetBDValueMust(r);
size_t dn = Max(a.real->Prec, b.real->Prec + cn - 1);
c = NewZeroWrapLimited(1, cn * BASE_FIG);
d = NewZeroWrapLimited(1, dn * BASE_FIG);
d = NewZeroWrapLimited(1, VPDIVD_REM_PREC(a.real, b.real, c.real) * BASE_FIG);
VpDivd(c.real, d.real, a.real, b.real);
VpNmlz(c.real);
VpNmlz(d.real);
Expand All @@ -4237,8 +4227,7 @@ BigDecimal_vpmult(VALUE self, VALUE v) {
BDVALUE a,b,c;
a = GetBDValueMust(self);
b = GetBDValueMust(v);
size_t cn = a.real->Prec + b.real->Prec + 1;
c = NewZeroWrapLimited(1, cn * BASE_FIG);
c = NewZeroWrapLimited(1, VPMULT_RESULT_PREC(a.real, b.real) * BASE_FIG);
VpMult(c.real, a.real, b.real);
VpNmlz(c.real);
RB_GC_GUARD(a.bigdecimal);
Expand Down
30 changes: 30 additions & 0 deletions test/bigdecimal/test_bigdecimal.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -1107,6 +1107,14 @@ def test_div_various_precisions
end
end

def test_div_round_worst_precision_case
x = BigDecimal(5)
y = BigDecimal(9 * BASE / 10)
(BASE_FIG * 2..BASE_FIG * 4).each do |prec|
assert_equal(BigDecimal('0.' + '5' * (prec - 1) + "6E-#{BASE_FIG - 1}"), x.div(y, prec))
end
end

def test_div_rounding_with_small_remainder
BigDecimal.mode(BigDecimal::ROUND_MODE, BigDecimal::ROUND_HALF_UP)
assert_equal(BigDecimal('0.12e1'), BigDecimal('1.25').div(BigDecimal("1.#{'0' * 30}1"), 2))
Expand Down Expand Up @@ -1210,6 +1218,28 @@ def test_divmod_precision
q, r = a.divmod(b)
assert_equal((a/b).round(0, :down) - 1, q)
assert_equal((a - q*b), r)

a = BigDecimal('3e100')
b = BigDecimal('-1.7e-100')
q, r = a.divmod(b)
assert_include(0...-b, -r)
assert_equal((a - q*b), r)

a = BigDecimal('0.32e23')
b = BigDecimal('-0.1999999999e-23')
q, r = a.divmod(b)
assert_include(0...-b, -r)
assert_equal((a - q*b), r)

a = BigDecimal('199.9999999999999999999999999')
q, r = a.divmod(1)
assert_equal([199, a - 199], [q, r])

a = BigDecimal('0.30000000000000000000000000000000000000000000000000000000000000001e91')
b = BigDecimal('0.1e20')
q, r = a.divmod(b)
assert_include(0...b, r)
assert_equal((a - q*b), r)
end

def test_divmod_error
Expand Down
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