numeric.c: round nearly middle value

* numeric.c (flo_round): [EXPERIMENTAL] adjust the case that the
  receiver is close to the exact but unrepresentable middle value
  of two values in the given precision.
  http://d.hatena.ne.jp/hnw/20160702

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55604 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 9 insertions, 2 deletions

diff --git a/numeric.c b/numeric.c
index c92b7b430b..b806ceca3c 100644
--- a/numeric.c
+++ b/numeric.c
@@ -1990,7 +1990,7 @@ rb_int_truncate(VALUE num, int ndigits)
 static VALUE
 flo_round(int argc, VALUE *argv, VALUE num)
 {
-    double number, f;
+    double number, f, x;
     int ndigits = 0;
 
     if (rb_check_arity(argc, 0, 1)) {
@@ -2005,7 +2005,14 @@ flo_round(int argc, VALUE *argv, VALUE num)
     }
     if (float_invariant_round(number, ndigits, &num)) return num;
     f = pow(10, ndigits);
-    return DBL2NUM(round(number * f) / f);
+    x = round(number * f);
+    if (x > 0) {
+	if ((x + 0.5) / f <= number) x += 1;
+    }
+    else {
+	if ((x - 0.5) / f >= number) x -= 1;
+    }
+    return DBL2NUM(x / f);
 }
 
 static int