Fix some rounding corner cases

Author: chrisseaton

spec/ruby/core/integer/round_spec.rb

@@ -67,12 +67,15 @@
     25.round(-1, half: :up).should      eql(30)
     25.round(-1, half: :down).should    eql(20)
     25.round(-1, half: :even).should    eql(20)
+    25.round(-1, half: nil).should      eql(30)
     35.round(-1, half: :up).should      eql(40)
     35.round(-1, half: :down).should    eql(30)
     35.round(-1, half: :even).should    eql(40)
+    35.round(-1, half: nil).should      eql(40)
     (-25).round(-1, half: :up).should   eql(-30)
     (-25).round(-1, half: :down).should eql(-20)
     (-25).round(-1, half: :even).should eql(-20)
+    (-25).round(-1, half: nil).should   eql(-30)
   end
 
   ruby_version_is "2.4"..."2.5" do
@@ -90,4 +93,9 @@
       35.round(1, half: :even).should    eql(35)
     end
   end
+
+  it "raises ArgumentError for an unknown rounding mode" do
+    lambda { 42.round(-1, half: :foo) }.should raise_error(ArgumentError, /invalid rounding mode: foo/)
+    lambda { 42.round(1, half: :foo) }.should raise_error(ArgumentError, /invalid rounding mode: foo/)
+  end
 end

src/main/ruby/truffleruby/core/integer.rb

@@ -161,7 +161,7 @@ def chr(enc=undefined)
     String.from_codepoint self, enc
   end
 
-  def round(ndigits=undefined, half: nil)
+  def round(ndigits=undefined, half: :up)
     return self if undefined.equal? ndigits
 
     if Float === ndigits && ndigits.infinite?
@@ -172,7 +172,12 @@ def round(ndigits=undefined, half: nil)
     Truffle::Type.check_int(ndigits)
 
     if ndigits >= 0
-      self
+      case half
+      when :up, nil, :down, :even
+        self
+      else
+        raise ArgumentError, "invalid rounding mode: #{half}"
+      end
     else
       ndigits = -ndigits
 
@@ -185,12 +190,17 @@ def round(ndigits=undefined, half: nil)
 
       if kind_of? Integer and f.kind_of? Integer
         x = self < 0 ? -self : self
-        x = if (half == :down)
-              x / f
-            else # :up or nil
-              (x + f / 2) / f
-            end
-        x = (x / 2) * 2 if half == :even
+        case half
+        when :up, nil
+          x = (x + (f / 2)) / f
+        when :down
+          x = x / f
+        when :even
+          x = (x + (f / 2)) / f
+          x = (x / 2) * 2
+        else
+          raise ArgumentError, "invalid rounding mode: #{half}"
+        end
         x = x * f
         x = -x if self < 0
         return x