Implement Java-specific ATN data optimization (-2) shift

Add ATNDataReader, ATNDataWriter
Clean up ATN serializer/deserializer code

runtime-testsuite/test/org/antlr/v4/test/runtime/BaseRuntimeTestSupport.java

@@ -219,7 +219,7 @@ protected ATN createATN(Grammar g, boolean useSerializer) {
 
 		ATN atn = g.atn;
 		if ( useSerializer ) {
-			char[] serialized = ATNSerializer.getSerializedAsChars(atn);
+			char[] serialized = ATNSerializer.getSerializedAsChars(atn, g.getLanguage());
 			return new ATNDeserializer().deserialize(serialized);
 		}
 

runtime-testsuite/test/org/antlr/v4/test/runtime/java/TestInterpreterDataReader.java

@@ -76,7 +76,7 @@ public void testParseFile() throws IOException, NoSuchFieldException, IllegalAcc
 		Assert.assertNull(channels);
 		Assert.assertNull(modes);
 
-        char[] atnChars = ATNSerializer.getSerializedAsChars(atn);
+        char[] atnChars = ATNSerializer.getSerializedAsChars(atn, g.getLanguage());
 		Assert.assertEquals(ATNDeserializer.SERIALIZED_VERSION, atnChars[0]);
     }
 

runtime/Java/src/org/antlr/v4/runtime/atn/ATNDataReader.java

@@ -0,0 +1,45 @@
+package org.antlr.v4.runtime.atn;
+
+public class ATNDataReader {
+	private final static int JavaOptimizeOffset2 = ((1 << 16) - 1) - ATNDataWriter.JavaOptimizeOffset + 1;
+
+	private final char[] data;
+	private int p;
+
+	public ATNDataReader(char[] data) {
+		this.data = data;
+	}
+
+	public int readUInt32() {
+		return readUInt16() | (readUInt16() << 16);
+	}
+
+	public int readUInt16() {
+		return readUInt16(true);
+	}
+
+	public int readUInt16(boolean normalize) {
+		int result = data[p++];
+		// Each char value in data is shifted by +2 at the entry to this method.
+		// This is an encoding optimization targeting the serialized values 0
+		// and -1 (serialized to 0xFFFF), each of which are very common in the
+		// serialized form of the ATN. In the modified UTF-8 that Java uses for
+		// compiled string literals, these two character values have multi-byte
+		// forms. By shifting each value by +2, they become characters 2 and 1
+		// prior to writing the string, each of which have single-byte
+		// representations. Since the shift occurs in the tool during ATN
+		// serialization, each target is responsible for adjusting the values
+		// during deserialization.
+		//
+		// As a special case, note that the first element of data is not
+		// adjusted because it contains the major version number of the
+		// serialized ATN, which was fixed at 3 at the time the value shifting
+		// was implemented.
+		if (normalize) {
+			return result >= ATNDataWriter.JavaOptimizeOffset
+					? result - ATNDataWriter.JavaOptimizeOffset
+					: result + JavaOptimizeOffset2;
+		}
+		return result;
+	}
+}

runtime/Java/src/org/antlr/v4/runtime/atn/ATNDataWriter.java

@@ -0,0 +1,36 @@
+package org.antlr.v4.runtime.atn;
+
+import org.antlr.v4.runtime.misc.IntegerList;
+
+public class ATNDataWriter {
+	public static final int JavaOptimizeOffset = 2;
+
+	private final IntegerList data;
+	private final String language;
+	private final boolean isJava;
+
+	public ATNDataWriter(IntegerList data, String language) {
+		this.data = data;
+		this.language = language;
+		this.isJava = language.equals("Java");
+	}
+
+	public void writeUInt32(int value) {
+		writeUInt16((char)value);
+		writeUInt16((char)(value >> 16));
+	}
+
+	public void writeUInt16(int value) {
+		writeUInt16(value, true);
+	}
+
+	public void writeUInt16(int value, boolean optimize) {
+		if (value < Character.MIN_VALUE || value > Character.MAX_VALUE) {
+			throw new UnsupportedOperationException("Serialized ATN data element "+
+					data.size() + " element " + value + " out of range "+
+					(int)Character.MIN_VALUE + ".." + (int)Character.MAX_VALUE);
+		}
+
+		data.add(isJava && optimize ? (value + JavaOptimizeOffset) & 0xFFFF : value);
+	}
+}