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Allocate minimum necessary digits for quotient and remainder #326

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f7c294e
Allocate minimum necessary digits for quotient and remainder
tompng May 24, 2025
952f481
Fix large remainder allocation in BigDecimal_div2
tompng May 25, 2025
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17 changes: 9 additions & 8 deletions ext/bigdecimal/bigdecimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1825,7 +1825,7 @@ BigDecimal_divide(VALUE self, VALUE r, Real **c, Real **res, Real **div)
ENTER(5);
Real *a, *b;
ssize_t a_prec, b_prec;
size_t mx;
size_t mx, res_maxprec;

TypedData_Get_Struct(self, Real, &BigDecimal_data_type, a);
SAVE(a);
Expand Down Expand Up @@ -1854,14 +1854,15 @@ BigDecimal_divide(VALUE self, VALUE r, Real **c, Real **res, Real **div)

BigDecimal_count_precision_and_scale(self, &a_prec, NULL);
BigDecimal_count_precision_and_scale(rr, &b_prec, NULL);
mx = (a_prec > b_prec) ? a_prec : b_prec;
mx *= 2;
mx = ((a_prec > b_prec) ? a_prec : b_prec) + BIGDECIMAL_DOUBLE_FIGURES;

if (2*BIGDECIMAL_DOUBLE_FIGURES > mx)
mx = 2*BIGDECIMAL_DOUBLE_FIGURES;
Comment on lines +1857 to 1860

@tompng tompng May 24, 2025

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Precision was max(a_prec*2, b_prec*2, 32).
I think we normally need max(a_prec, b_prec) + extra precision, and the extra part was ≧16 before.


GUARD_OBJ((*c), NewZeroWrapNolimit(1, mx + 2*BASE_FIG));
GUARD_OBJ((*res), NewZeroWrapNolimit(1, (mx + 1)*2 + 2*BASE_FIG));
res_maxprec = b->Prec + (*c)->MaxPrec - 1;
if (res_maxprec <= a->Prec) res_maxprec = a->Prec + 1;
GUARD_OBJ((*res), NewZeroWrapNolimit(1, res_maxprec * BASE_FIG));
VpDivd(*c, *res, a, b);

return Qnil;
Expand Down Expand Up @@ -2218,9 +2219,9 @@ BigDecimal_div2(VALUE self, VALUE b, VALUE n)
b_prec = BIGDECIMAL_DOUBLE_FIGURES;
}
GUARD_OBJ(bv, GetVpValueWithPrec(b, b_prec, 1));
mx = av->Prec + bv->Prec + 2;
if (mx <= cv->MaxPrec) mx = cv->MaxPrec + 1;
GUARD_OBJ(res, NewZeroWrapNolimit(1, (mx * 2 + 2)*VpBaseFig()));
mx = bv->Prec + cv->MaxPrec - 1;
if (mx <= av->Prec) mx = av->Prec + 1;
GUARD_OBJ(res, NewZeroWrapNolimit(1, mx * VpBaseFig()));
VpDivd(cv, res, av, bv);
VpSetPrecLimit(pl);
VpLeftRound(cv, VpGetRoundMode(), ix);
Expand Down Expand Up @@ -6164,7 +6165,7 @@ VpDivd(Real *c, Real *r, Real *a, Real *b)
word_c = c->MaxPrec;
word_r = r->MaxPrec;

if (word_a >= word_r) goto space_error;
if (word_a >= word_r || word_b + word_c - 2 >= word_r) goto space_error;

@tompng tompng May 24, 2025

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There are two more goto space_error in this function.
One is on L6218

ind_r = ind_c + ind_b;
if (ind_r >= word_r) goto space_error;

And another in L6253

ind_r = b->Prec + ind_c - 1; // L6242
ind_r = b->Prec + ind_c; // L6250 (ind_c + 1 < word_c is ensured in L6247)
if (ind_r >= word_r) goto space_error;

Both has the same requirement: (word_b-1) + (word_c-1) < word_r


ind_r = 1;
r->frac[0] = 0;
Expand Down
32 changes: 31 additions & 1 deletion test/bigdecimal/test_bigdecimal.rb
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Expand Up @@ -982,7 +982,7 @@ def test_div
def test_div_gh220
x = BigDecimal("1.0")
y = BigDecimal("3672577333.6608990499165058135986328125")
c = BigDecimal("0.272288343892592687909520102748926752911779209181321744700032723729015151607289998e-9")
c = BigDecimal("0.272288343892592687909520102748926752911779209181321745e-9")
assert_equal(c, x / y, "[GH-220]")
end

Expand All @@ -995,6 +995,36 @@ def test_div_precision
"(101/0.9163472602589686).precision >= (0.9163472602589686).precision #{bug13754}")
end

def test_div_various_precisions
a_precs = [5, 20, 70]
b_precs = [*5..80]
exponents = [0, 5]
a_precs.product(exponents, b_precs, exponents).each do |prec_a, ex_a, prec_b, ex_b|
a = BigDecimal('7.' + '1' * (prec_a - 1) + "e#{ex_a}")
b = BigDecimal('3.' + '1' * (prec_b - 1) + "e#{ex_b}")
c = a / b
max = [prec_a, prec_b, BigDecimal.double_fig].max
# Precision must be enough and not too large
precision_min = max + BigDecimal.double_fig / 2
precision_max = max + 2 * BigDecimal.double_fig
assert_includes(precision_min..precision_max, c.n_significant_digits)
end
end

def test_div2_various_precisions
a_precs = [5, 20, 70, 100]
b_precs = [5, 20, 70, 100]
c_precs = [5, 20, 30, 70, 100, 200]
exponents = [0, 5]
a_precs.product(exponents, b_precs, exponents, c_precs).each do |prec_a, ex_a, prec_b, ex_b, prec_c|
a = BigDecimal('7.' + '1' * (prec_a - 1) + "e#{ex_a}")
b = BigDecimal('3.' + '1' * (prec_b - 1) + "e#{ex_b}")
c = a.div(b, prec_c)
assert_in_delta(prec_c, c.n_significant_digits, 2)
assert_in_delta(a, c * b, a * 10**(1 - prec_c))
end
end

def test_div_with_float
assert_kind_of(BigDecimal, BigDecimal("3") / 1.5)
assert_equal(BigDecimal("0.5"), BigDecimal(1) / 2.0)
Expand Down