CodeBuffer: implement .subSequence(), plus tests

There is no reason why CodeBuffer isn't able to support it.

Just use a CharBuffer to .wrap() the buffer, and use this CharBuffer's
.subSequence() with the indices adjusted according to the buffer's current
position.

sslr-core/src/main/java/org/sonar/sslr/channel/CodeBuffer.java

@@ -27,6 +27,7 @@
 import java.io.IOException;
 import java.io.Reader;
 import java.io.StringReader;
+import java.nio.CharBuffer;
 
 /**
  * The CodeBuffer class provides all the basic features required to manipulate a source code character stream. Those features are :
@@ -43,6 +44,7 @@ public class CodeBuffer implements CharSequence {
   private int lastChar = -1;
   private Cursor cursor;
   private char[] buffer;
+  private CharBuffer cb;
   private int bufferPosition = 0;
   private static final char LF = '\n';
   private static final char CR = '\r';
@@ -73,6 +75,7 @@ protected CodeBuffer(Reader initialCodeReader, CodeReaderConfiguration configura
       }
 
       buffer = CharStreams.toString(reader).toCharArray();
+      cb = CharBuffer.wrap(buffer);
     } catch (IOException e) {
       throw new ChannelException(e.getMessage(), e);
     } finally {
@@ -203,7 +206,9 @@ public final int length() {
 
   @Override
   public final CharSequence subSequence(int start, int end) {
-    throw new UnsupportedOperationException();
+      final int realStart = bufferPosition + start;
+      final int realEnd = bufferPosition + end;
+      return cb.subSequence(realStart, realEnd);
   }
 
   @Override

sslr-core/src/test/java/org/sonar/sslr/channel/CodeBufferTest.java

@@ -264,6 +264,29 @@ public void testChannelCodeReaderFilter() throws Exception {
     assertThat(code.pop(), is( -1));
   }
 
+    @Test
+    public void testSubSequence()
+    {
+        final CodeReaderConfiguration cfg = new CodeReaderConfiguration();
+        final String input = "some pretty short input\njust for kicks";
+        final CodeBuffer code = new CodeBuffer(input, cfg);
+
+        assertThat(code.subSequence(0, 4).toString(), is("some"));
+        assertThat(code.subSequence(24, 38).toString(), is("just for kicks"));
+
+        // Shift by one; the subsequence should shift accordingly
+        code.pop();
+        assertThat(code.subSequence(0, 4).toString(), is("ome "));
+
+        // Shift four more
+        code.pop();
+        code.pop();
+        code.pop();
+        code.pop();
+        assertThat(code.subSequence(0, 12).toString(), is("pretty short"));
+        assertThat(code.subSequence(13, 18).toString(), is("input"));
+    }
+
   /**
    * Backward compatibility with a COBOL plugin: filter returns 0 instead of -1, when end of the stream has been reached.
    */