JRUBY-5455: StringIO 1.9 ungetc

* Could probably do with more specs for ungetc around codepoints
  outside the BMP.
* getc passes.
* readchar passes.
* Extract common 1.8 / 1.9 behaviour for ungetc.

Signed-off-by: Charles Oliver Nutter <headius@headius.com>

Author: jabley

src/org/jruby/RubyStringIO.java

@@ -34,7 +34,6 @@
 import java.util.Arrays;
 import java.util.List;
 
-import org.jcodings.Encoding;
 import org.jruby.anno.FrameField;
 import org.jruby.anno.JRubyClass;
 import org.jruby.anno.JRubyMethod;
@@ -744,6 +743,15 @@ public IRubyObject readchar() {
         return c;
     }
 
+    @JRubyMethod(name = "readchar", compat = CompatVersion.RUBY1_9)
+    public IRubyObject readchar19(ThreadContext context) {
+        IRubyObject c = getc19(context);
+
+        if (c.isNil()) throw getRuntime().newEOFError();
+
+        return c;
+    }
+
     @JRubyMethod(name = "readline", optional = 1, writes = FrameField.LASTLINE)
     public IRubyObject readline(ThreadContext context, IRubyObject[] args) {
         IRubyObject line = gets(context, args);
@@ -883,16 +891,7 @@ public IRubyObject ungetc(IRubyObject arg) {
 
         int c = RubyNumeric.num2int(arg);
         if (data.pos == 0) return getRuntime().getNil();
-        data.internal.modify();
-        data.pos--;
-
-        ByteList bytes = data.internal.getByteList();
-
-        if (bytes.length() <= data.pos) {
-            bytes.length((int)data.pos + 1);
-        }
-
-        bytes.set((int) data.pos, c);
+        ungetcCommon(c);
         return getRuntime().getNil();
     }
 
@@ -901,17 +900,33 @@ public IRubyObject ungetc19(ThreadContext context, IRubyObject arg) {
         checkReadable();
 
         if (!arg.isNil()) {
+            int c;
             if (arg instanceof RubyFixnum) {
-                int codepoint = RubyNumeric.fix2int(arg);
-                Encoding encoding = data.internal.getEncoding();
+                c = RubyNumeric.fix2int(arg);
             } else {
-                RubyString s = arg.convertToString();
+                RubyString str = arg.convertToString();
+                c = str.getEncoding().mbcToCode(str.getBytes(), 0, 1);
             }
+
+            ungetcCommon(c);
         }
 
         return getRuntime().getNil();
     }
 
+    private void ungetcCommon(int c) {
+        data.internal.modify();
+        data.pos--;
+
+        ByteList bytes = data.internal.getByteList();
+
+        if (bytes.length() <= data.pos) {
+            bytes.length((int)data.pos + 1);
+        }
+
+        bytes.set((int) data.pos, c);
+    }
+
     @JRubyMethod(name = {"write", "syswrite"}, required = 1)
     public IRubyObject write(ThreadContext context, IRubyObject arg) {
         return context.getRuntime().newFixnum(writeInternal(context, arg));