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Prevent exponential precision growth in division #325

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Prevent exponential precision growth in division #325

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27 changes: 21 additions & 6 deletions ext/bigdecimal/bigdecimal.c
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Original file line number Diff line number Diff line change
Expand Up @@ -116,6 +116,9 @@ static struct {
#define BIGDECIMAL_POSITIVE_P(bd) ((bd)->sign > 0)
#define BIGDECIMAL_NEGATIVE_P(bd) ((bd)->sign < 0)

/* Maximum precision to add in division, to ensure at most linear growth */
#define BIGDECIMAL_MAX_DIVISION_PRECISION_GROWTH 100

/*
* ================== Memory allocation ============================
*/
Expand Down Expand Up @@ -1822,10 +1825,10 @@ static VALUE
BigDecimal_divide(VALUE self, VALUE r, Real **c, Real **res, Real **div)
/* For c = self.div(r): with round operation */
{
ENTER(5);
ENTER(7);
Real *a, *b;
ssize_t a_prec, b_prec;
size_t mx;
size_t mx, double_mx, max_mx;

TypedData_Get_Struct(self, Real, &BigDecimal_data_type, a);
SAVE(a);
Expand Down Expand Up @@ -1855,7 +1858,13 @@ 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;

/* Try a reasonable precision for division, but prevent exponential growth */
double_mx = mx * 2;

max_mx = mx + BIGDECIMAL_MAX_DIVISION_PRECISION_GROWTH;

mx = (double_mx < max_mx) ? double_mx : max_mx;

if (2*BIGDECIMAL_DOUBLE_FIGURES > mx)
mx = 2*BIGDECIMAL_DOUBLE_FIGURES;
Expand Down Expand Up @@ -1946,11 +1955,11 @@ BigDecimal_quo(int argc, VALUE *argv, VALUE self)
static VALUE
BigDecimal_DoDivmod(VALUE self, VALUE r, Real **div, Real **mod)
{
ENTER(8);
ENTER(10);
Real *c=NULL, *d=NULL, *res=NULL;
Real *a, *b;
ssize_t a_prec, b_prec;
size_t mx;
size_t mx, double_mx, max_mx;

TypedData_Get_Struct(self, Real, &BigDecimal_data_type, a);
SAVE(a);
Expand Down Expand Up @@ -2011,7 +2020,13 @@ BigDecimal_DoDivmod(VALUE self, VALUE r, Real **div, Real **mod)
BigDecimal_count_precision_and_scale(rr, &b_prec, NULL);

mx = (a_prec > b_prec) ? a_prec : b_prec;
mx *= 2;

/* Try a reasonable precision for division, but prevent exponential growth */
double_mx = mx * 2;

max_mx = mx + BIGDECIMAL_MAX_DIVISION_PRECISION_GROWTH;

mx = (double_mx < max_mx) ? double_mx : max_mx;

if (2*BIGDECIMAL_DOUBLE_FIGURES > mx)
mx = 2*BIGDECIMAL_DOUBLE_FIGURES;
Expand Down
14 changes: 14 additions & 0 deletions test/bigdecimal/test_bigdecimal.rb
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Expand Up @@ -986,6 +986,20 @@ def test_div_gh220
assert_equal(c, x / y, "[GH-220]")
end

def test_div_gh222
# Prevent exponential precision growth

high_prec = BigDecimal("1.0")
10.times { high_prec /= BigDecimal("1.000001") } # Accumulate precision

result = high_prec / BigDecimal("1.03")

# Test that we don't get exponential precision growth
assert(result.precision < 2 * high_prec.precision,
"Division precision should not grow exponentially. " \
"Got #{result.precision}, expected < #{2 * high_prec.precision} [GH-222]")
end

def test_div_precision
bug13754 = '[ruby-core:82107] [Bug #13754]'
a = BigDecimal('101')
Expand Down