[ruby/bigdecimal] Use larger precision in divide for irrational or recurring results

mrkn · mrkn · commit 41ff5ddeb821 · 2021-12-23T19:15:12.000+09:00
Just in case for irrational or recurring results, the precision of the quotient is set to at least more than 2*Float::DIG plus alpha. [Bug ruby#13754] [Fix rubyGH-94] ruby/bigdecimal@99442c75d3
diff --git a/ext/bigdecimal/bigdecimal.c b/ext/bigdecimal/bigdecimal.c
@@ -1508,13 +1508,12 @@ BigDecimal_divide(VALUE self, VALUE r, Real **c, Real **res, Real **div)
     SAVE(b);
 
     *div = b;
-    mx = a->Prec + vabs(a->exponent);
-    if (mx < b->Prec + vabs(b->exponent)) mx = b->Prec + vabs(b->exponent);
-    mx++; /* NOTE: An additional digit is needed for the compatibility to
-                   the version 1.2.1 and the former.  */
-    mx = (mx + 1) * VpBaseFig();
-    GUARD_OBJ((*c), VpCreateRbObject(mx, "#0", true));
-    GUARD_OBJ((*res), VpCreateRbObject((mx+1) * 2 +(VpBaseFig() + 1), "#0", true));
+    mx = (a->Prec > b->Prec) ? a->Prec : b->Prec;
+    mx *= BASE_FIG;
+    if (2*BIGDECIMAL_DOUBLE_FIGURES > mx)
+        mx = 2*BIGDECIMAL_DOUBLE_FIGURES;
+    GUARD_OBJ((*c), VpCreateRbObject(mx + 2*BASE_FIG, "#0", true));
+    GUARD_OBJ((*res), VpCreateRbObject(mx*2 + 2*BASE_FIG, "#0", true));
     VpDivd(*c, *res, a, b);
     return Qnil;
 }
diff --git a/test/bigdecimal/test_bigdecimal.rb b/test/bigdecimal/test_bigdecimal.rb
@@ -953,9 +953,13 @@ def test_div
     assert_equal(2, BigDecimal("2") / 1)
     assert_equal(-2, BigDecimal("2") / -1)
 
-    assert_equal(BigDecimal('1486.868686869'), BigDecimal('1472.0') / BigDecimal('0.99'), '[ruby-core:59365] [#9316]')
+    assert_equal(BigDecimal('1486.868686869'),
+                 (BigDecimal('1472.0') / BigDecimal('0.99')).round(9),
+                 '[ruby-core:59365] [#9316]')
 
-    assert_equal(4.124045235, BigDecimal('0.9932') / (700 * BigDecimal('0.344045') / BigDecimal('1000.0')), '[#9305]')
+    assert_in_delta(4.124045235,
+                    (BigDecimal('0.9932') / (700 * BigDecimal('0.344045') / BigDecimal('1000.0'))).round(9, half: :up),
+                    10**Float::MIN_10_EXP, '[#9305]')
 
     BigDecimal.mode(BigDecimal::EXCEPTION_INFINITY, false)
     assert_positive_zero(BigDecimal("1.0")  / BigDecimal("Infinity"))
@@ -969,6 +973,15 @@ def test_div
     assert_raise_with_message(FloatDomainError, "Computation results in '-Infinity'") { BigDecimal("-1") / 0 }
   end
 
+  def test_dev_precision
+    bug13754 = '[ruby-core:82107] [Bug #13754]'
+    a = BigDecimal('101')
+    b = BigDecimal('0.9163472602589686')
+    c = a/b
+    assert(c.precision > b.precision,
+           "(101/0.9163472602589686).precision >= (0.9163472602589686).precision #{bug13754}")
+  end
+
   def test_div_with_float
     assert_kind_of(BigDecimal, BigDecimal("3") / 1.5)
     assert_equal(BigDecimal("0.5"), BigDecimal(1) / 2.0)
