From 9bb27fa318928464f5ab66649a75bbd034307baa Mon Sep 17 00:00:00 2001
From: nobu <nobu@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Thu, 7 Jul 2016 07:37:55 +0000
Subject: [PATCH] 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
---
 ChangeLog               |  7 +++++++
 numeric.c               | 11 +++++++++--
 test/ruby/test_float.rb |  5 +++++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 5dbd688284..59bff9542a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Thu Jul  7 16:37:53 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* 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
+
 Thu Jul  7 16:31:07 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* io.c (rb_io_s_foreach, rb_io_s_readlines): convert arguments
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
diff --git a/test/ruby/test_float.rb b/test/ruby/test_float.rb
index 06b1d85242..2684cd8eab 100644
--- a/test/ruby/test_float.rb
+++ b/test/ruby/test_float.rb
@@ -447,6 +447,11 @@ class TestFloat < Test::Unit::TestCase
     assert_raise(TypeError) {1.0.round(nil)}
     def (prec = Object.new).to_int; 2; end
     assert_equal(1.0, 0.998.round(prec))
+
+    assert_equal(+5.02, +5.015.round(2))
+    assert_equal(-5.02, -5.015.round(2))
+    assert_equal(+1.26, +1.255.round(2))
+    assert_equal(-1.26, -1.255.round(2))
   end
 
   def test_floor_with_precision