Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1228907)
+++ lucene/CHANGES.txt	(working copy)
@@ -714,6 +714,11 @@
   contrib/queryparser. If you have used those classes in your code
   just add the lucene-queryparser.jar file to your classpath.
   (Uwe Schindler)
+
+* LUCENE-3681: FST now stores labels for BYTE2 input type as 2 bytes
+  instead of vInt; this can make FSTs smaller and faster, but it is a
+  break in the binary format so if you had built and saved any FSTs
+  then you need to rebuild them. (Robert Muir, Mike McCandless)
   
 Security fixes
 
Index: lucene/src/java/org/apache/lucene/util/fst/FST.java
===================================================================
--- lucene/src/java/org/apache/lucene/util/fst/FST.java	(revision 1228907)
+++ lucene/src/java/org/apache/lucene/util/fst/FST.java	(working copy)
@@ -92,8 +92,11 @@
   /** Changed numBytesPerArc for array'd case from byte to int. */
   private final static int VERSION_INT_NUM_BYTES_PER_ARC = 1;
 
-  private final static int VERSION_CURRENT = VERSION_INT_NUM_BYTES_PER_ARC;
+  /** Write BYTE2 labels as 2-byte short, not vInt. */
+  private final static int VERSION_SHORT_BYTE2_LABELS = 2;
 
+  private final static int VERSION_CURRENT = VERSION_SHORT_BYTE2_LABELS;
+
   // Never serialized; just used to represent the virtual
   // final node w/ no arcs:
   private final static int FINAL_END_NODE = -1;
@@ -199,7 +202,9 @@
   public FST(DataInput in, Outputs<T> outputs) throws IOException {
     this.outputs = outputs;
     writer = null;
-    CodecUtil.checkHeader(in, FILE_FORMAT_NAME, VERSION_INT_NUM_BYTES_PER_ARC, VERSION_INT_NUM_BYTES_PER_ARC);
+    // NOTE: only reads most recent format; we don't have
+    // back-compat promise for FSTs (they are experimental):
+    CodecUtil.checkHeader(in, FILE_FORMAT_NAME, VERSION_SHORT_BYTE2_LABELS, VERSION_SHORT_BYTE2_LABELS);
     if (in.readByte() == 1) {
       // accepts empty string
       int numBytes = in.readVInt();
@@ -389,7 +394,7 @@
       writer.writeByte((byte) v);
     } else if (inputType == INPUT_TYPE.BYTE2) {
       assert v <= 65535: "v=" + v;
-      writer.writeVInt(v);
+      writer.writeShort((short) v);
     } else {
       //writeInt(v);
       writer.writeVInt(v);
@@ -399,7 +404,11 @@
   int readLabel(DataInput in) throws IOException {
     final int v;
     if (inputType == INPUT_TYPE.BYTE1) {
+      // Unsigned byte:
       v = in.readByte()&0xFF;
+    } else if (inputType == INPUT_TYPE.BYTE2) {
+      // Unsigned short:
+      v = in.readShort()&0xFFFF;
     } else { 
       v = in.readVInt();
     }