numeric.c: round to nearest even

* numeric.c (flo_round, int_round): support round-to-nearest-even
  semantics of IEEE 754 to match sprintf behavior, and add `half:`
  optional keyword argument for the old behavior.
  [ruby-core:76273] [Bug #12548]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56590 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 36 insertions, 2 deletions

diff --git a/rational.c b/rational.c
index f0e8d56cbf..8362075ab4 100644
--- a/rational.c
+++ b/rational.c
@@ -1250,7 +1250,7 @@ nurat_truncate(VALUE self)
 }
 
 static VALUE
-nurat_round(VALUE self)
+nurat_round_half_up(VALUE self)
 {
     VALUE num, den, neg;
 
@@ -1274,6 +1274,33 @@ nurat_round(VALUE self)
 }
 
 static VALUE
+nurat_round_half_even(VALUE self)
+{
+    VALUE num, den, neg, qr;
+
+    get_dat1(self);
+
+    num = dat->num;
+    den = dat->den;
+    neg = f_negative_p(num);
+
+    if (neg)
+	num = f_negate(num);
+
+    num = f_add(f_mul(num, TWO), den);
+    den = f_mul(den, TWO);
+    qr = rb_funcall(num, rb_intern("divmod"), 1, den);
+    num = RARRAY_AREF(qr, 0);
+    if (f_zero_p(RARRAY_AREF(qr, 1)))
+	num = rb_funcall(num, '&', 1, LONG2FIX(((int)~1)));
+
+    if (neg)
+	num = f_negate(num);
+
+    return num;
+}
+
+static VALUE
 f_round_common(int argc, VALUE *argv, VALUE self, VALUE (*func)(VALUE))
 {
     VALUE n, b, s;
@@ -1403,7 +1430,14 @@ nurat_truncate_n(int argc, VALUE *argv, VALUE self)
 static VALUE
 nurat_round_n(int argc, VALUE *argv, VALUE self)
 {
-    return f_round_common(argc, argv, self, nurat_round);
+    VALUE opt;
+    enum ruby_num_rounding_mode mode = (
+	argc = rb_scan_args(argc, argv, "*:", NULL, &opt),
+	rb_num_get_rounding_option(opt));
+    VALUE (*round_func)(VALUE) =
+	ROUND_TO(mode,
+		 nurat_round_half_up, nurat_round_half_even);
+    return f_round_common(argc, argv, self, round_func);
 }
 
 /*