numeric.c: Fix round_half_even for specific values (#7023)

Handle the integert and the float parts separately in round_half_even
to prevent error occursions in floating point calculation.

numeric.c

@@ -144,31 +144,37 @@ round_half_down(double x, double s)
 static double
 round_half_even(double x, double s)
 {
-    double f, d, xs = x * s;
+    double u, v, us, vs, f, d, uf;
+
+    v = modf(x, &u);
+    us = u * s;
+    vs = v * s;
 
     if (x > 0.0) {
-        f = floor(xs);
-        d = xs - f;
+        f = floor(vs);
+        uf = us + f;
+        d = vs - f;
         if (d > 0.5)
             d = 1.0;
-        else if (d == 0.5 || ((double)((f + 0.5) / s) <= x))
-            d = fmod(f, 2.0);
+        else if (d == 0.5 || ((double)((uf + 0.5) / s) <= x))
+            d = fmod(uf, 2.0);
         else
             d = 0.0;
         x = f + d;
     }
     else if (x < 0.0) {
-        f = ceil(xs);
-        d = f - xs;
+        f = ceil(vs);
+        uf = us + f;
+        d = f - vs;
         if (d > 0.5)
             d = 1.0;
-        else if (d == 0.5 || ((double)((f - 0.5) / s) >= x))
-            d = fmod(-f, 2.0);
+        else if (d == 0.5 || ((double)((uf - 0.5) / s) >= x))
+            d = fmod(-uf, 2.0);
         else
             d = 0.0;
         x = f - d;
     }
-    return x;
+    return us + x;
 }
 
 static VALUE fix_lshift(long, unsigned long);

spec/ruby/core/float/round_spec.rb

@@ -136,7 +136,7 @@ describe "Float#round" do
     -4.809999999999999.round(5, half: :even).should eql(-4.81)
   end
 
-  ruby_bug "", ""..."3.4" do
+  ruby_bug "", ""..."3.3" do
     # These numbers are neighbouring floating point numbers round a
     # precise value. They test that the rounding modes work correctly
     # round that value and precision is not lost which might cause

test/ruby/test_float.rb

@@ -486,6 +486,17 @@ class TestFloat < Test::Unit::TestCase
     assert_equal(-1.26, -1.255.round(2))
   end
 
+  def test_round_half_even_with_precision
+    assert_equal(767573.18759, 767573.1875850001.round(5, half: :even))
+    assert_equal(767573.18758, 767573.187585.round(5, half: :even))
+    assert_equal(767573.18758, 767573.1875849998.round(5, half: :even))
+    assert_equal(767573.18758, 767573.187575.round(5, half: :even))
+    assert_equal(-767573.18759, -767573.1875850001.round(5, half: :even))
+    assert_equal(-767573.18758, -767573.187585.round(5, half: :even))
+    assert_equal(-767573.18758, -767573.1875849998.round(5, half: :even))
+    assert_equal(-767573.18758, -767573.187575.round(5, half: :even))
+  end
+
   def test_floor_with_precision
     assert_equal(+0.0, +0.001.floor(1))
     assert_equal(-0.1, -0.001.floor(1))