util.c: round nearly middle value

* util.c (ruby_dtoa): [EXPERIMENTAL] adjust the case that the
  Float value is close to the exact but unrepresentable middle
  value of two values in the given precision, as r55604.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55621 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

3 files changed, 14 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 8269e6d74b..bf613e88d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sun Jul 10 14:27:25 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* util.c (ruby_dtoa): [EXPERIMENTAL] adjust the case that the
+	  Float value is close to the exact but unrepresentable middle
+	  value of two values in the given precision, as r55604.
+
 Sun Jul 10 08:57:20 2016  SHIBATA Hiroshi  <hsbt@ruby-lang.org>
 
 	* thread.c: Fixed implicit conversion error with Apple clang-800.0.31
diff --git a/test/ruby/test_sprintf.rb b/test/ruby/test_sprintf.rb
index 5732cecbf9..440cd1d6ab 100644
--- a/test/ruby/test_sprintf.rb
+++ b/test/ruby/test_sprintf.rb
@@ -282,6 +282,10 @@ class TestSprintf < Test::Unit::TestCase
     assert_equal("          0x1.000p+0", sprintf("%20.3a",  1), bug3979)
   end
 
+  def test_float_prec
+    assert_equal("5.03", sprintf("%.2f",5.025))
+  end
+
   BSIZ = 120
 
   def test_skip
diff --git a/util.c b/util.c
index 8c428b57f0..0b0c935832 100644
--- a/util.c
+++ b/util.c
@@ -3452,8 +3452,12 @@ ruby_dtoa(double d_, int mode, int ndigits, int *decpt, int *sign, char **rve)
                     ilim = i;
                 *s++ = '0' + (int)L;
                 if (i == ilim) {
+                    double x;
                     if (dval(d) > 0.5 + dval(eps))
                         goto bump_up;
+                    else if (!isinf(x = d_ * tens[ilim-1] + 0.5) &&
+                             dval(d) > modf(x, &x))
+                        goto bump_up;
                     else if (dval(d) < 0.5 - dval(eps)) {
                         while (*--s == '0') ;
                         s++;