Index: src/test/org/apache/lucene/index/TestDoc.java
===================================================================
--- src/test/org/apache/lucene/index/TestDoc.java	(revision 1072815)
+++ src/test/org/apache/lucene/index/TestDoc.java	(working copy)
@@ -204,8 +204,8 @@
       r2.close();
       
       final SegmentInfo info = new SegmentInfo(merged, si1.docCount + si2.docCount, si1.dir,
-                                               false, merger.fieldInfos().hasProx(), merger.getSegmentCodecs(),
-                                               merger.fieldInfos().hasVectors());
+                                               false, merger.getSegmentCodecs(),
+                                               merger.fieldInfos());
       
       if (useCompoundFile) {
         Collection<String> filesToDelete = merger.createCompoundFile(merged + ".cfs", info);
Index: src/test/org/apache/lucene/index/TestDocumentWriter.java
===================================================================
--- src/test/org/apache/lucene/index/TestDocumentWriter.java	(revision 1072815)
+++ src/test/org/apache/lucene/index/TestDocumentWriter.java	(working copy)
@@ -25,20 +25,20 @@
 import org.apache.lucene.analysis.MockTokenizer;
 import org.apache.lucene.analysis.TokenFilter;
 import org.apache.lucene.analysis.TokenStream;
+import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
 import org.apache.lucene.analysis.tokenattributes.PayloadAttribute;
 import org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute;
-import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Field;
-import org.apache.lucene.document.Fieldable;
 import org.apache.lucene.document.Field.Index;
 import org.apache.lucene.document.Field.Store;
 import org.apache.lucene.document.Field.TermVector;
+import org.apache.lucene.document.Fieldable;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.util.AttributeSource;
+import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.LuceneTestCase;
 import org.apache.lucene.util._TestUtil;
-import org.apache.lucene.util.BytesRef;
 
 public class TestDocumentWriter extends LuceneTestCase {
   private Directory dir;
@@ -98,8 +98,7 @@
 
     // test that the norms are not present in the segment if
     // omitNorms is true
-    for (int i = 0; i < reader.core.fieldInfos.size(); i++) {
-      FieldInfo fi = reader.core.fieldInfos.fieldInfo(i);
+    for (FieldInfo fi : reader.core.fieldInfos) {
       if (fi.isIndexed) {
         assertTrue(fi.omitNorms == !reader.hasNorms(fi.name));
       }
Index: src/test/org/apache/lucene/index/TestIndexFileDeleter.java
===================================================================
--- src/test/org/apache/lucene/index/TestIndexFileDeleter.java	(revision 1072815)
+++ src/test/org/apache/lucene/index/TestIndexFileDeleter.java	(working copy)
@@ -92,10 +92,9 @@
     CompoundFileReader cfsReader = new CompoundFileReader(dir, "_2.cfs");
     FieldInfos fieldInfos = new FieldInfos(cfsReader, "_2.fnm");
     int contentFieldIndex = -1;
-    for(i=0;i<fieldInfos.size();i++) {
-      FieldInfo fi = fieldInfos.fieldInfo(i);
+    for (FieldInfo fi : fieldInfos) {
       if (fi.name.equals("content")) {
-        contentFieldIndex = i;
+        contentFieldIndex = fi.number;
         break;
       }
     }
Index: src/test/org/apache/lucene/index/TestCodecs.java
===================================================================
--- src/test/org/apache/lucene/index/TestCodecs.java	(revision 1072815)
+++ src/test/org/apache/lucene/index/TestCodecs.java	(working copy)
@@ -23,14 +23,14 @@
 
 import org.apache.lucene.analysis.MockAnalyzer;
 import org.apache.lucene.document.Document;
+import org.apache.lucene.document.Field;
 import org.apache.lucene.document.Field.Store;
-import org.apache.lucene.document.Field;
 import org.apache.lucene.index.codecs.CodecProvider;
 import org.apache.lucene.index.codecs.FieldsConsumer;
 import org.apache.lucene.index.codecs.FieldsProducer;
 import org.apache.lucene.index.codecs.PostingsConsumer;
+import org.apache.lucene.index.codecs.TermStats;
 import org.apache.lucene.index.codecs.TermsConsumer;
-import org.apache.lucene.index.codecs.TermStats;
 import org.apache.lucene.index.codecs.mocksep.MockSepCodec;
 import org.apache.lucene.index.codecs.preflex.PreFlexCodec;
 import org.apache.lucene.search.DocIdSetIterator;
@@ -238,9 +238,9 @@
     final FieldData[] fields = new FieldData[] {field};
 
     final Directory dir = newDirectory();
+    FieldInfos clonedFieldInfos = (FieldInfos) fieldInfos.clone();
     this.write(fieldInfos, dir, fields, true);
-    final SegmentInfo si = new SegmentInfo(SEGMENT, 10000, dir, false, true, SegmentCodecs.build(fieldInfos, CodecProvider.getDefault()), fieldInfos.hasVectors());
-    si.setHasProx(false);
+    final SegmentInfo si = new SegmentInfo(SEGMENT, 10000, dir, false, SegmentCodecs.build(clonedFieldInfos, CodecProvider.getDefault()), clonedFieldInfos);
 
     final FieldsProducer reader = si.getSegmentCodecs().codec().fieldsProducer(new SegmentReadState(dir, si, fieldInfos, 64, IndexReader.DEFAULT_TERMS_INDEX_DIVISOR));
 
@@ -289,8 +289,10 @@
     if (VERBOSE) {
       System.out.println("TEST: now write postings");
     }
+
+    FieldInfos clonedFieldInfos = (FieldInfos) fieldInfos.clone();
     this.write(fieldInfos, dir, fields, false);
-    final SegmentInfo si = new SegmentInfo(SEGMENT, 10000, dir, false, true, SegmentCodecs.build(fieldInfos, CodecProvider.getDefault()), fieldInfos.hasVectors());
+    final SegmentInfo si = new SegmentInfo(SEGMENT, 10000, dir, false, SegmentCodecs.build(clonedFieldInfos, CodecProvider.getDefault()), clonedFieldInfos);
 
     if (VERBOSE) {
       System.out.println("TEST: now read postings");
@@ -440,7 +442,7 @@
         final FieldData field = fields[TestCodecs.random.nextInt(fields.length)];
         final TermsEnum termsEnum = termsDict.terms(field.fieldInfo.name).iterator();
 
-        if (si.getSegmentCodecs().codecs[field.fieldInfo.codecId] instanceof PreFlexCodec) {
+        if (si.getSegmentCodecs().codecs[field.fieldInfo.getCodecId()] instanceof PreFlexCodec) {
           // code below expects unicode sort order
           continue;
         }
@@ -594,7 +596,7 @@
     final FieldsConsumer consumer = state.segmentCodecs.codec().fieldsConsumer(state);
     Arrays.sort(fields);
     for (final FieldData field : fields) {
-      if (!allowPreFlex && codecInfo.codecs[field.fieldInfo.codecId] instanceof PreFlexCodec) {
+      if (!allowPreFlex && codecInfo.codecs[field.fieldInfo.getCodecId()] instanceof PreFlexCodec) {
         // code below expects unicode sort order
         continue;
       }
Index: src/test/org/apache/lucene/index/TestSegmentMerger.java
===================================================================
--- src/test/org/apache/lucene/index/TestSegmentMerger.java	(revision 1072815)
+++ src/test/org/apache/lucene/index/TestSegmentMerger.java	(working copy)
@@ -79,8 +79,8 @@
     int docsMerged = merger.merge();
     assertTrue(docsMerged == 2);
     //Should be able to open a new SegmentReader against the new directory
-    SegmentReader mergedReader = SegmentReader.get(false, mergedDir, new SegmentInfo(mergedSegment, docsMerged, mergedDir, false, merger.fieldInfos().hasProx(),
-                                                                                     merger.getSegmentCodecs(), merger.fieldInfos().hasVectors()),
+    SegmentReader mergedReader = SegmentReader.get(false, mergedDir, new SegmentInfo(mergedSegment, docsMerged, mergedDir, false,
+                                                                                     merger.getSegmentCodecs(), merger.fieldInfos()),
                                                    BufferedIndexInput.BUFFER_SIZE, true, IndexReader.DEFAULT_TERMS_INDEX_DIVISOR);
 
     assertTrue(mergedReader != null);
Index: src/test/org/apache/lucene/index/TestPerFieldCodecSupport.java
===================================================================
--- src/test/org/apache/lucene/index/TestPerFieldCodecSupport.java	(revision 1072815)
+++ src/test/org/apache/lucene/index/TestPerFieldCodecSupport.java	(working copy)
@@ -216,7 +216,7 @@
               IndexFileNames.FIELD_INFOS_EXTENSION));
       FieldInfo fieldInfo = fieldInfos.fieldInfo(field);
       assertEquals("faild for segment index: " + i, codec[i],
-          codecInfo.codecs[fieldInfo.codecId]);
+          codecInfo.codecs[fieldInfo.getCodecId()]);
     }
   }
 
Index: src/test/org/apache/lucene/index/TestConsistentFieldNumbers.java
===================================================================
--- src/test/org/apache/lucene/index/TestConsistentFieldNumbers.java	(revision 0)
+++ src/test/org/apache/lucene/index/TestConsistentFieldNumbers.java	(revision 0)
@@ -0,0 +1,222 @@
+package org.apache.lucene.index;
+
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import org.apache.lucene.analysis.MockAnalyzer;
+import org.apache.lucene.document.Document;
+import org.apache.lucene.document.Field;
+import org.apache.lucene.document.Field.Index;
+import org.apache.lucene.document.Field.Store;
+import org.apache.lucene.document.Field.TermVector;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.util.LuceneTestCase;
+import org.junit.Test;
+
+public class TestConsistentFieldNumbers extends LuceneTestCase {
+
+  @Test
+  public void testSameFieldNumbersAcrossSegments() throws Exception {
+    for (int i = 0; i < 2; i++) {
+      Directory dir = newDirectory();
+      IndexWriter writer = new IndexWriter(dir, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()).setMergePolicy(NoMergePolicy.COMPOUND_FILES));
+
+      Document d1 = new Document();
+      d1.add(new Field("f1", "first field", Store.YES, Index.ANALYZED, TermVector.NO));
+      d1.add(new Field("f2", "second field", Store.YES, Index.ANALYZED, TermVector.NO));
+      writer.addDocument(d1);
+
+      if (i == 1) {
+        writer.close();
+        writer = new IndexWriter(dir, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()).setMergePolicy(NoMergePolicy.COMPOUND_FILES));
+      } else {
+        writer.commit();
+      }
+
+      Document d2 = new Document();
+      d2.add(new Field("f2", "second field", Store.YES, Index.ANALYZED, TermVector.NO));
+      d2.add(new Field("f1", "first field", Store.YES, Index.ANALYZED, TermVector.YES));
+      d2.add(new Field("f3", "third field", Store.YES, Index.ANALYZED, TermVector.NO));
+      d2.add(new Field("f4", "fourth field", Store.YES, Index.ANALYZED, TermVector.NO));
+      writer.addDocument(d2);
+
+      writer.close();
+
+      SegmentInfos sis = new SegmentInfos();
+      sis.read(dir);
+      assertEquals(2, sis.size());
+
+      FieldInfos fis1 = sis.info(0).getFieldInfos();
+      FieldInfos fis2 = sis.info(1).getFieldInfos();
+
+      assertEquals("f1", fis1.fieldInfo(0).name);
+      assertEquals("f2", fis1.fieldInfo(1).name);
+      assertEquals("f1", fis2.fieldInfo(0).name);
+      assertEquals("f2", fis2.fieldInfo(1).name);
+      assertEquals("f3", fis2.fieldInfo(2).name);
+      assertEquals("f4", fis2.fieldInfo(3).name);
+
+      writer = new IndexWriter(dir, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()));
+      writer.optimize();
+      writer.close();
+
+      sis = new SegmentInfos();
+      sis.read(dir);
+      assertEquals(1, sis.size());
+
+      FieldInfos fis3 = sis.info(0).getFieldInfos();
+
+      assertEquals("f1", fis3.fieldInfo(0).name);
+      assertEquals("f2", fis3.fieldInfo(1).name);
+      assertEquals("f3", fis3.fieldInfo(2).name);
+      assertEquals("f4", fis3.fieldInfo(3).name);
+
+
+      dir.close();
+    }
+  }
+
+  @Test
+  public void testAddIndexes() throws Exception {
+    Directory dir1 = newDirectory();
+    Directory dir2 = newDirectory();
+    IndexWriter writer = new IndexWriter(dir1, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()).setMergePolicy(NoMergePolicy.COMPOUND_FILES));
+
+    Document d1 = new Document();
+    d1.add(new Field("f1", "first field", Store.YES, Index.ANALYZED, TermVector.NO));
+    d1.add(new Field("f2", "second field", Store.YES, Index.ANALYZED, TermVector.NO));
+    writer.addDocument(d1);
+
+    writer.close();
+    writer = new IndexWriter(dir2, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()).setMergePolicy(NoMergePolicy.COMPOUND_FILES));
+
+    Document d2 = new Document();
+    d2.add(new Field("f2", "second field", Store.YES, Index.ANALYZED, TermVector.NO));
+    d2.add(new Field("f1", "first field", Store.YES, Index.ANALYZED, TermVector.YES));
+    d2.add(new Field("f3", "third field", Store.YES, Index.ANALYZED, TermVector.NO));
+    d2.add(new Field("f4", "fourth field", Store.YES, Index.ANALYZED, TermVector.NO));
+    writer.addDocument(d2);
+
+    writer.close();
+
+    writer = new IndexWriter(dir1, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()).setMergePolicy(NoMergePolicy.COMPOUND_FILES));
+    writer.addIndexes(dir2);
+    writer.close();
+
+    SegmentInfos sis = new SegmentInfos();
+    sis.read(dir1);
+    assertEquals(2, sis.size());
+
+    FieldInfos fis1 = sis.info(0).getFieldInfos();
+    FieldInfos fis2 = sis.info(1).getFieldInfos();
+
+    assertEquals("f1", fis1.fieldInfo(0).name);
+    assertEquals("f2", fis1.fieldInfo(1).name);
+    // make sure the ordering of the "external" segment is preserved
+    assertEquals("f2", fis2.fieldInfo(0).name);
+    assertEquals("f1", fis2.fieldInfo(1).name);
+    assertEquals("f3", fis2.fieldInfo(2).name);
+    assertEquals("f4", fis2.fieldInfo(3).name);
+
+    writer = new IndexWriter(dir1, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()));
+    writer.optimize();
+    writer.close();
+
+    sis = new SegmentInfos();
+    sis.read(dir1);
+    assertEquals(1, sis.size());
+
+    FieldInfos fis3 = sis.info(0).getFieldInfos();
+
+    // after merging the ordering should be identical to the first segment
+    assertEquals("f1", fis3.fieldInfo(0).name);
+    assertEquals("f2", fis3.fieldInfo(1).name);
+    assertEquals("f3", fis3.fieldInfo(2).name);
+    assertEquals("f4", fis3.fieldInfo(3).name);
+
+    dir1.close();
+    dir2.close();
+  }
+
+  @Test
+  public void testManyFields() throws Exception {
+    final int NUM_DOCS = 2000;
+    final int MAX_FIELDS = 50;
+
+    int[][] docs = new int[NUM_DOCS][4];
+    for (int i = 0; i < docs.length; i++) {
+      for (int j = 0; j < docs[i].length;j++) {
+        docs[i][j] = random.nextInt(MAX_FIELDS);
+      }
+    }
+
+    Directory dir = newDirectory();
+    IndexWriter writer = new IndexWriter(dir, newIndexWriterConfig( TEST_VERSION_CURRENT, new MockAnalyzer()));
+
+    for (int i = 0; i < NUM_DOCS; i++) {
+      Document d = new Document();
+      for (int j = 0; j < docs[i].length; j++) {
+        d.add(getField(docs[i][j]));
+      }
+
+      writer.addDocument(d);
+    }
+
+    writer.optimize();
+    writer.close();
+
+    SegmentInfos sis = new SegmentInfos();
+    sis.read(dir);
+    for (SegmentInfo si : sis) {
+      FieldInfos fis = si.getFieldInfos();
+
+      for (FieldInfo fi : fis) {
+        Field expected = getField(Integer.parseInt(fi.name));
+        assertEquals(expected.isIndexed(), fi.isIndexed);
+        assertEquals(expected.isTermVectorStored(), fi.storeTermVector);
+        assertEquals(expected.isStorePositionWithTermVector(), fi.storePositionWithTermVector);
+        assertEquals(expected.isStoreOffsetWithTermVector(), fi.storeOffsetWithTermVector);
+      }
+    }
+
+    dir.close();
+  }
+
+  private Field getField(int number) {
+    int mode = number % 16;
+    String fieldName = "" + number;
+    switch (mode) {
+      case 0: return new Field(fieldName, "some text", Store.YES, Index.ANALYZED, TermVector.NO);
+      case 1: return new Field(fieldName, "some text", Store.NO, Index.ANALYZED, TermVector.NO);
+      case 2: return new Field(fieldName, "some text", Store.YES, Index.NOT_ANALYZED, TermVector.NO);
+      case 3: return new Field(fieldName, "some text", Store.NO, Index.NOT_ANALYZED, TermVector.NO);
+      case 4: return new Field(fieldName, "some text", Store.YES, Index.ANALYZED, TermVector.WITH_OFFSETS);
+      case 5: return new Field(fieldName, "some text", Store.NO, Index.ANALYZED, TermVector.WITH_OFFSETS);
+      case 6: return new Field(fieldName, "some text", Store.YES, Index.NOT_ANALYZED, TermVector.WITH_OFFSETS);
+      case 7: return new Field(fieldName, "some text", Store.NO, Index.NOT_ANALYZED, TermVector.WITH_OFFSETS);
+      case 8: return new Field(fieldName, "some text", Store.YES, Index.ANALYZED, TermVector.WITH_POSITIONS);
+      case 9: return new Field(fieldName, "some text", Store.NO, Index.ANALYZED, TermVector.WITH_POSITIONS);
+      case 10: return new Field(fieldName, "some text", Store.YES, Index.NOT_ANALYZED, TermVector.WITH_POSITIONS);
+      case 11: return new Field(fieldName, "some text", Store.NO, Index.NOT_ANALYZED, TermVector.WITH_POSITIONS);
+      case 12: return new Field(fieldName, "some text", Store.YES, Index.ANALYZED, TermVector.WITH_POSITIONS_OFFSETS);
+      case 13: return new Field(fieldName, "some text", Store.NO, Index.ANALYZED, TermVector.WITH_POSITIONS_OFFSETS);
+      case 14: return new Field(fieldName, "some text", Store.YES, Index.NOT_ANALYZED, TermVector.WITH_POSITIONS_OFFSETS);
+      case 15: return new Field(fieldName, "some text", Store.NO, Index.NOT_ANALYZED, TermVector.WITH_POSITIONS_OFFSETS);
+      default: return null;
+    }
+  }
+}
Index: src/test/org/apache/lucene/index/TestBackwardsCompatibility.java
===================================================================
--- src/test/org/apache/lucene/index/TestBackwardsCompatibility.java	(revision 1072815)
+++ src/test/org/apache/lucene/index/TestBackwardsCompatibility.java	(working copy)
@@ -528,10 +528,9 @@
       CompoundFileReader cfsReader = new CompoundFileReader(dir, "_0.cfs");
       FieldInfos fieldInfos = new FieldInfos(cfsReader, "_0.fnm");
       int contentFieldIndex = -1;
-      for(int i=0;i<fieldInfos.size();i++) {
-        FieldInfo fi = fieldInfos.fieldInfo(i);
+      for (FieldInfo fi : fieldInfos) {
         if (fi.name.equals("content")) {
-          contentFieldIndex = i;
+          contentFieldIndex = fi.number;
           break;
         }
       }
Index: src/java/org/apache/lucene/index/FieldInfos.java
===================================================================
--- src/java/org/apache/lucene/index/FieldInfos.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/FieldInfos.java	(working copy)
@@ -17,6 +17,16 @@
  * limitations under the License.
  */
 
+import java.io.IOException;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.SortedMap;
+import java.util.TreeMap;
+
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Fieldable;
 import org.apache.lucene.store.Directory;
@@ -24,9 +34,6 @@
 import org.apache.lucene.store.IndexOutput;
 import org.apache.lucene.util.StringHelper;
 
-import java.io.IOException;
-import java.util.*;
-
 /** Access to the Fieldable Info file that describes document fields and whether or
  *  not they are indexed. Each segment has a separate Fieldable Info file. Objects
  *  of this class are thread-safe for multiple readers, but only one thread can
@@ -34,8 +41,73 @@
  *  accessing this object.
  *  @lucene.experimental
  */
-public final class FieldInfos {
+public final class FieldInfos implements Iterable<FieldInfo> {
+  private static final class FieldNumberBiMap {
+    private final Map<Integer,String> numberToName;
+    private final Map<String,Integer> nameToNumber;
 
+    private FieldNumberBiMap() {
+      this.nameToNumber = new HashMap<String, Integer>();
+      this.numberToName = new HashMap<Integer, String>();
+    }
+
+    synchronized int addOrGet(String fieldName, FieldInfoBiMap fieldInfoMap, int preferredFieldNumber) {
+      Integer fieldNumber = nameToNumber.get(fieldName);
+      if (fieldNumber == null) {
+        if (!numberToName.containsKey(preferredFieldNumber)) {
+          // cool - we can use this number globally
+          fieldNumber = preferredFieldNumber;
+        } else {
+          fieldNumber = findNextAvailableFieldNumber(preferredFieldNumber + 1, numberToName.keySet());
+        }
+
+        numberToName.put(fieldNumber, fieldName);
+        nameToNumber.put(fieldName, fieldNumber);
+      }
+
+      return fieldNumber;
+    }
+
+    synchronized void setIfNotSet(int fieldNumber, String fieldName) {
+     if (!numberToName.containsKey(fieldNumber) && !nameToNumber.containsKey(fieldName)) {
+        numberToName.put(fieldNumber, fieldName);
+        nameToNumber.put(fieldName, fieldNumber);
+      }
+    }
+  }
+
+  private static final class FieldInfoBiMap implements Iterable<FieldInfo> {
+    private final SortedMap<Integer,FieldInfo> byNumber = new TreeMap<Integer,FieldInfo>();
+    private final HashMap<String,FieldInfo> byName = new HashMap<String,FieldInfo>();
+    private int nextAvailableNumber = 0;
+
+    public void put(FieldInfo fi) {
+      assert !byNumber.containsKey(fi.number);
+      assert !byName.containsKey(fi.name);
+
+      byNumber.put(fi.number, fi);
+      byName.put(fi.name, fi);
+    }
+
+    public FieldInfo get(String fieldName) {
+      return byName.get(fieldName);
+    }
+
+    public FieldInfo get(int fieldNumber) {
+      return byNumber.get(fieldNumber);
+    }
+
+    public int size() {
+      assert byNumber.size() == byName.size();
+      return byNumber.size();
+    }
+
+    @Override
+    public Iterator<FieldInfo> iterator() {
+      return byNumber.values().iterator();
+    }
+  }
+
   // First used in 2.9; prior to 2.9 there was no format header
   public static final int FORMAT_START = -2;
   public static final int FORMAT_PER_FIELD_CODEC = -3;
@@ -53,13 +125,20 @@
   static final byte STORE_PAYLOADS = 0x20;
   static final byte OMIT_TERM_FREQ_AND_POSITIONS = 0x40;
   
-  private final ArrayList<FieldInfo> byNumber = new ArrayList<FieldInfo>();
-  private final HashMap<String,FieldInfo> byName = new HashMap<String,FieldInfo>();
+  private final FieldNumberBiMap globalFieldNumbers;
+  private final FieldInfoBiMap localFieldInfos;
+
   private int format;
 
   public FieldInfos() {
+    this(new FieldNumberBiMap());
   }
 
+  private FieldInfos(FieldNumberBiMap globalFieldNumbers) {
+    this.globalFieldNumbers = globalFieldNumbers;
+    this.localFieldInfos = new FieldInfoBiMap();
+  }
+
   /**
    * Construct a FieldInfos object using the directory and the name of the file
    * IndexInput
@@ -68,6 +147,7 @@
    * @throws IOException
    */
   public FieldInfos(Directory d, String name) throws IOException {
+    this(new FieldNumberBiMap());
     IndexInput input = d.openInput(name);
     try {
       read(input, name);
@@ -76,17 +156,27 @@
     }
   }
 
+  private static final int findNextAvailableFieldNumber(int nextPreferredNumber, Set<Integer> unavailableNumbers) {
+    while (unavailableNumbers.contains(nextPreferredNumber)) {
+      nextPreferredNumber++;
+    }
+
+    return nextPreferredNumber;
+  }
+
+  public FieldInfos newFieldInfosWithGlobalFieldNumberMap() {
+    return new FieldInfos(this.globalFieldNumbers);
+  }
+
   /**
    * Returns a deep clone of this FieldInfos instance.
    */
   @Override
   synchronized public Object clone() {
-    FieldInfos fis = new FieldInfos();
-    final int numField = byNumber.size();
-    for(int i=0;i<numField;i++) {
-      FieldInfo fi = (FieldInfo) ( byNumber.get(i)).clone();
-      fis.byNumber.add(fi);
-      fis.byName.put(fi.name, fi);
+    FieldInfos fis = new FieldInfos(globalFieldNumbers);
+    for (FieldInfo fi : this) {
+      FieldInfo clone = (FieldInfo) (fi).clone();
+      fis.localFieldInfos.put(clone);
     }
     return fis;
   }
@@ -102,9 +192,7 @@
 
   /** Returns true if any fields do not omitTermFreqAndPositions */
   public boolean hasProx() {
-    final int numFields = byNumber.size();
-    for(int i=0;i<numFields;i++) {
-      final FieldInfo fi = fieldInfo(i);
+    for (FieldInfo fi : this) {
       if (fi.isIndexed && !fi.omitTermFreqAndPositions) {
         return true;
       }
@@ -215,9 +303,28 @@
   synchronized public FieldInfo add(String name, boolean isIndexed, boolean storeTermVector,
                        boolean storePositionWithTermVector, boolean storeOffsetWithTermVector,
                        boolean omitNorms, boolean storePayloads, boolean omitTermFreqAndPositions) {
+    return addOrUpdateInternal(name, -1, isIndexed, storeTermVector, storePositionWithTermVector,
+                               storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
+  }
+
+  synchronized private FieldInfo addOrUpdateInternal(String name, int preferredFieldNumber, boolean isIndexed,
+      boolean storeTermVector, boolean storePositionWithTermVector, boolean storeOffsetWithTermVector,
+      boolean omitNorms, boolean storePayloads, boolean omitTermFreqAndPositions) {
+
     FieldInfo fi = fieldInfo(name);
     if (fi == null) {
-      return addInternal(name, isIndexed, storeTermVector, storePositionWithTermVector, storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
+      if (preferredFieldNumber == -1) {
+        preferredFieldNumber = findNextAvailableFieldNumber(localFieldInfos.nextAvailableNumber, localFieldInfos.byNumber.keySet());
+        localFieldInfos.nextAvailableNumber = preferredFieldNumber;
+      }
+
+      // get a global number for this field
+      int fieldNumber = globalFieldNumbers.addOrGet(name, localFieldInfos, preferredFieldNumber);
+      if (localFieldInfos.get(fieldNumber) != null) {
+        // fall back if the global number is already taken
+        fieldNumber = preferredFieldNumber;
+      }
+      return addInternal(name, fieldNumber, isIndexed, storeTermVector, storePositionWithTermVector, storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
     } else {
       fi.update(isIndexed, storeTermVector, storePositionWithTermVector, storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
     }
@@ -225,20 +332,27 @@
   }
 
   synchronized public FieldInfo add(FieldInfo fi) {
-    return add(fi.name, fi.isIndexed, fi.storeTermVector,
+    int preferredFieldNumber = fi.number;
+    FieldInfo other = localFieldInfos.get(preferredFieldNumber);
+    if (other == null || !other.name.equals(fi.name)) {
+      preferredFieldNumber = -1;
+    }
+    return addOrUpdateInternal(fi.name, preferredFieldNumber, fi.isIndexed, fi.storeTermVector,
                fi.storePositionWithTermVector, fi.storeOffsetWithTermVector,
                fi.omitNorms, fi.storePayloads,
                fi.omitTermFreqAndPositions);
   }
 
-  private FieldInfo addInternal(String name, boolean isIndexed,
+  private FieldInfo addInternal(String name, int fieldNumber, boolean isIndexed,
                                 boolean storeTermVector, boolean storePositionWithTermVector, 
                                 boolean storeOffsetWithTermVector, boolean omitNorms, boolean storePayloads, boolean omitTermFreqAndPositions) {
     name = StringHelper.intern(name);
-    FieldInfo fi = new FieldInfo(name, isIndexed, byNumber.size(), storeTermVector, storePositionWithTermVector,
+    globalFieldNumbers.setIfNotSet(fieldNumber, name);
+    FieldInfo fi = new FieldInfo(name, isIndexed, fieldNumber, storeTermVector, storePositionWithTermVector,
                                  storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
-    byNumber.add(fi);
-    byName.put(name, fi);
+
+    assert localFieldInfos.get(fi.number) == null;
+    localFieldInfos.put(fi);
     return fi;
   }
 
@@ -248,7 +362,7 @@
   }
 
   public FieldInfo fieldInfo(String fieldName) {
-    return  byName.get(fieldName);
+    return localFieldInfos.get(fieldName);
   }
 
   /**
@@ -270,16 +384,20 @@
    * doesn't exist.
    */  
   public FieldInfo fieldInfo(int fieldNumber) {
-	return (fieldNumber >= 0) ? byNumber.get(fieldNumber) : null;
+	return (fieldNumber >= 0) ? localFieldInfos.get(fieldNumber) : null;
   }
 
+  public Iterator<FieldInfo> iterator() {
+    return localFieldInfos.iterator();
+  }
+
   public int size() {
-    return byNumber.size();
+    return localFieldInfos.size();
   }
 
   public boolean hasVectors() {
-    for (int i = 0; i < size(); i++) {
-      if (fieldInfo(i).storeTermVector) {
+    for (FieldInfo fi : this) {
+      if (fi.storeTermVector) {
         return true;
       }
     }
@@ -287,8 +405,8 @@
   }
 
   public boolean hasNorms() {
-    for (int i = 0; i < size(); i++) {
-      if (!fieldInfo(i).omitNorms) {
+    for (FieldInfo fi : this) {
+      if (!fi.omitNorms) {
         return true;
       }
     }
@@ -307,8 +425,7 @@
   public void write(IndexOutput output) throws IOException {
     output.writeVInt(FORMAT_CURRENT);
     output.writeVInt(size());
-    for (int i = 0; i < size(); i++) {
-      FieldInfo fi = fieldInfo(i);
+    for (FieldInfo fi : this) {
       byte bits = 0x0;
       if (fi.isIndexed) bits |= IS_INDEXED;
       if (fi.storeTermVector) bits |= STORE_TERMVECTOR;
@@ -318,7 +435,8 @@
       if (fi.storePayloads) bits |= STORE_PAYLOADS;
       if (fi.omitTermFreqAndPositions) bits |= OMIT_TERM_FREQ_AND_POSITIONS;
       output.writeString(fi.name);
-      output.writeInt(fi.codecId);
+      output.writeInt(fi.number);
+      output.writeInt(fi.getCodecId());
       output.writeByte(bits);
     }
   }
@@ -338,6 +456,7 @@
     for (int i = 0; i < size; i++) {
       String name = StringHelper.intern(input.readString());
       // if this is a previous format codec 0 will be preflex!
+      final int fieldNumber = format <= FORMAT_PER_FIELD_CODEC? input.readInt():i;
       final int codecId = format <= FORMAT_PER_FIELD_CODEC? input.readInt():0;
       byte bits = input.readByte();
       boolean isIndexed = (bits & IS_INDEXED) != 0;
@@ -347,8 +466,8 @@
       boolean omitNorms = (bits & OMIT_NORMS) != 0;
       boolean storePayloads = (bits & STORE_PAYLOADS) != 0;
       boolean omitTermFreqAndPositions = (bits & OMIT_TERM_FREQ_AND_POSITIONS) != 0;
-      final FieldInfo addInternal = addInternal(name, isIndexed, storeTermVector, storePositionsWithTermVector, storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
-      addInternal.codecId = codecId;
+      final FieldInfo addInternal = addInternal(name, fieldNumber, isIndexed, storeTermVector, storePositionsWithTermVector, storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
+      addInternal.setCodecId(codecId);
     }
 
     if (input.getFilePointer() != input.length()) {
Index: src/java/org/apache/lucene/index/FieldsWriter.java
===================================================================
--- src/java/org/apache/lucene/index/FieldsWriter.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/FieldsWriter.java	(working copy)
@@ -45,14 +45,12 @@
   // If null - we were supplied with streams, if notnull - we manage them ourselves
   private Directory directory;
   private String segment;
-  private FieldInfos fieldInfos;
   private IndexOutput fieldsStream;
   private IndexOutput indexStream;
 
-  FieldsWriter(Directory directory, String segment, FieldInfos fn) throws IOException {
+  FieldsWriter(Directory directory, String segment) throws IOException {
     this.directory = directory;
     this.segment = segment;
-    fieldInfos = fn;
 
     boolean success = false;
     try {
@@ -70,10 +68,9 @@
     }
   }
 
-  FieldsWriter(IndexOutput fdx, IndexOutput fdt, FieldInfos fn) {
+  FieldsWriter(IndexOutput fdx, IndexOutput fdt) {
     directory = null;
     segment = null;
-    fieldInfos = fn;
     fieldsStream = fdt;
     indexStream = fdx;
   }
@@ -166,7 +163,7 @@
     assert fieldsStream.getFilePointer() == position;
   }
 
-  final void addDocument(Document doc) throws IOException {
+  final void addDocument(Document doc, FieldInfos fieldInfos) throws IOException {
     indexStream.writeLong(fieldsStream.getFilePointer());
 
     int storedCount = 0;
Index: src/java/org/apache/lucene/index/SegmentInfo.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentInfo.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/SegmentInfo.java	(working copy)
@@ -17,22 +17,23 @@
  * limitations under the License.
  */
 
-import org.apache.lucene.store.Directory;
-import org.apache.lucene.store.IndexOutput;
-import org.apache.lucene.store.IndexInput;
-import org.apache.lucene.util.Constants;
-import org.apache.lucene.index.codecs.Codec;
-import org.apache.lucene.index.codecs.CodecProvider;
-import org.apache.lucene.index.codecs.DefaultSegmentInfosWriter;
 import java.io.IOException;
-import java.util.Arrays;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Map.Entry;
 import java.util.Set;
-import java.util.HashSet;
-import java.util.HashMap;
-import java.util.ArrayList;
 
+import org.apache.lucene.index.codecs.Codec;
+import org.apache.lucene.index.codecs.CodecProvider;
+import org.apache.lucene.index.codecs.DefaultSegmentInfosWriter;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.store.IndexInput;
+import org.apache.lucene.store.IndexOutput;
+import org.apache.lucene.util.Constants;
+
 /**
  * Information about a segment such as it's name, directory, and files related
  * to the segment.
@@ -41,6 +42,9 @@
  */
 public final class SegmentInfo {
 
+  @Deprecated
+  // remove with hasVector and hasProx
+  static final int CHECK_FIELDINFOS = -2;  // hasVector and hasProx use this for bw compatibility
   static final int NO = -1;          // e.g. no norms; no deletes;
   static final int YES = 1;          // e.g. have norms; have deletes;
   static final int WITHOUT_GEN = 0;  // a file name that has no GEN in it. 
@@ -62,7 +66,7 @@
    * - NO says this field has no separate norms
    * >= YES says this field has separate norms with the specified generation
    */
-  private long[] normGen;                         
+  private Map<Integer,Long> normGen;
 
   private boolean isCompoundFile;         
 
@@ -80,10 +84,16 @@
 
   private int delCount;                           // How many deleted docs in this segment
 
-  private boolean hasProx;                        // True if this segment has any fields with omitTermFreqAndPositions==false
+  @Deprecated
+  // remove when we don't have to support old indexes anymore that had this field
+  private int hasProx = CHECK_FIELDINFOS;         // True if this segment has any fields with omitTermFreqAndPositions==false
 
-  private boolean hasVectors;                     // True if this segment wrote term vectors
+  @Deprecated
+  // remove when we don't have to support old indexes anymore that had this field
+  private int hasVectors = CHECK_FIELDINFOS;      // True if this segment wrote term vectors
 
+  private FieldInfos fieldInfos;
+
   private SegmentCodecs segmentCodecs;
 
   private Map<String,String> diagnostics;
@@ -100,7 +110,7 @@
   private long bufferedDeletesGen;
   
   public SegmentInfo(String name, int docCount, Directory dir, boolean isCompoundFile,
-                     boolean hasProx, SegmentCodecs segmentCodecs, boolean hasVectors) {
+                     SegmentCodecs segmentCodecs, FieldInfos fieldInfos) {
     this.name = name;
     this.docCount = docCount;
     this.dir = dir;
@@ -108,18 +118,17 @@
     this.isCompoundFile = isCompoundFile;
     this.docStoreOffset = -1;
     this.docStoreSegment = name;
-    this.hasProx = hasProx;
     this.segmentCodecs = segmentCodecs;
-    this.hasVectors = hasVectors;
     delCount = 0;
     version = Constants.LUCENE_MAIN_VERSION;
+    this.fieldInfos = fieldInfos;
   }
 
   /**
    * Copy everything from src SegmentInfo into our instance.
    */
   void reset(SegmentInfo src) {
-    clearFiles();
+    clearFilesCache();
     version = src.version;
     name = src.name;
     docCount = src.docCount;
@@ -130,12 +139,15 @@
     docStoreIsCompoundFile = src.docStoreIsCompoundFile;
     hasVectors = src.hasVectors;
     hasProx = src.hasProx;
+    fieldInfos = src.fieldInfos == null ? null : (FieldInfos) src.fieldInfos.clone();
     if (src.normGen == null) {
       normGen = null;
     } else {
-      normGen = new long[src.normGen.length];
-      System.arraycopy(src.normGen, 0, normGen, 0, src.normGen.length);
+      normGen = new HashMap<Integer, Long>(src.normGen.size());
+      for (Entry<Integer,Long> entry : src.normGen.entrySet()) {
+        normGen.put(entry.getKey(), entry.getValue());
     }
+    }
     isCompoundFile = src.isCompoundFile;
     delCount = src.delCount;
     segmentCodecs = src.segmentCodecs;
@@ -184,17 +196,35 @@
     if (numNormGen == NO) {
       normGen = null;
     } else {
-      normGen = new long[numNormGen];
+      normGen = new HashMap<Integer, Long>();
       for(int j=0;j<numNormGen;j++) {
-        normGen[j] = input.readLong();
+        int fieldNumber = j;
+        if (format <= DefaultSegmentInfosWriter.FORMAT_4_0) {
+          fieldNumber = input.readInt();
       }
+
+        normGen.put(fieldNumber, input.readLong());
     }
+    }
     isCompoundFile = input.readByte() == YES;
 
+    Directory dir0 = dir;
+    if (isCompoundFile) {
+      dir0 = new CompoundFileReader(dir, IndexFileNames.segmentFileName(name, "", IndexFileNames.COMPOUND_FILE_EXTENSION));
+    }
+
+    try {
+      fieldInfos = new FieldInfos(dir0, IndexFileNames.segmentFileName(name, "", IndexFileNames.FIELD_INFOS_EXTENSION));
+    } finally {
+      if (dir != dir0) {
+        dir0.close();
+      }
+    }
+
     delCount = input.readInt();
     assert delCount <= docCount;
 
-    hasProx = input.readByte() == YES;
+    hasProx = input.readByte();
     
     // System.out.println(Thread.currentThread().getName() + ": si.read hasProx=" + hasProx + " seg=" + name);
     segmentCodecs = new SegmentCodecs(codecs);
@@ -208,7 +238,7 @@
     diagnostics = input.readStringStringMap();
     
     if (format <= DefaultSegmentInfosWriter.FORMAT_HAS_VECTORS) {
-      hasVectors = input.readByte() == 1;
+      hasVectors = input.readByte();
     } else {
       final String storesSegment;
       final String ext;
@@ -229,7 +259,11 @@
         dirToTest = dir;
       }
       try {
-        hasVectors = dirToTest.fileExists(IndexFileNames.segmentFileName(storesSegment, "", IndexFileNames.VECTORS_INDEX_EXTENSION));
+        if (dirToTest.fileExists(IndexFileNames.segmentFileName(storesSegment, "", IndexFileNames.VECTORS_INDEX_EXTENSION))) {
+          hasVectors = YES;
+        } else {
+          hasVectors = NO;
+        }
       } finally {
         if (isCompoundFile) {
           dirToTest.close();
@@ -274,13 +308,13 @@
     }
   }
 
-  public boolean getHasVectors() throws IOException {
-    return hasVectors;
+  public boolean getHasVectors() {
+    return hasVectors == CHECK_FIELDINFOS ?
+        (fieldInfos == null ? true : fieldInfos.hasVectors()) : hasVectors == YES;
   }
 
-  public void setHasVectors(boolean v) {
-    hasVectors = v;
-    clearFiles();
+  public FieldInfos getFieldInfos() {
+    return fieldInfos;
   }
 
   public boolean hasDeletions() {
@@ -298,17 +332,18 @@
     } else {
       delGen++;
     }
-    clearFiles();
+    clearFilesCache();
   }
 
   void clearDelGen() {
     delGen = NO;
-    clearFiles();
+    clearFilesCache();
   }
 
   @Override
   public Object clone() {
-    SegmentInfo si = new SegmentInfo(name, docCount, dir, isCompoundFile, hasProx, segmentCodecs, false);
+    SegmentInfo si = new SegmentInfo(name, docCount, dir, isCompoundFile, segmentCodecs,
+        fieldInfos == null ? null : (FieldInfos) fieldInfos.clone());
     si.docStoreOffset = docStoreOffset;
     si.docStoreSegment = docStoreSegment;
     si.docStoreIsCompoundFile = docStoreIsCompoundFile;
@@ -316,8 +351,12 @@
     si.delCount = delCount;
     si.diagnostics = new HashMap<String, String>(diagnostics);
     if (normGen != null) {
-      si.normGen = normGen.clone();
+      si.normGen = new HashMap<Integer, Long>();
+      for (Entry<Integer,Long> entry : normGen.entrySet()) {
+        si.normGen.put(entry.getKey(), entry.getValue());
     }
+    }
+    si.hasProx = hasProx;
     si.hasVectors = hasVectors;
     si.version = version;
     return si;
@@ -339,9 +378,14 @@
    * @param fieldNumber the field index to check
    */
   public boolean hasSeparateNorms(int fieldNumber) {
-    return normGen != null && normGen[fieldNumber] != NO;
+    if (normGen == null) {
+      return false;
   }
 
+    Long gen = normGen.get(fieldNumber);
+    return gen != null && gen.longValue() != NO;
+  }
+
   /**
    * Returns true if any fields in this segment have separate norms.
    */
@@ -349,7 +393,7 @@
     if (normGen == null) {
       return false;
     } else {
-      for (long fieldNormGen : normGen) {
+      for (long fieldNormGen : normGen.values()) {
         if (fieldNormGen >= YES) {
           return true;
         }
@@ -359,10 +403,9 @@
     return false;
   }
 
-  void initNormGen(int numFields) {
+  void initNormGen() {
     if (normGen == null) { // normGen is null if this segments file hasn't had any norms set against it yet
-      normGen = new long[numFields];
-      Arrays.fill(normGen, NO);
+      normGen = new HashMap<Integer, Long>();
     }
   }
 
@@ -373,12 +416,13 @@
    * @param fieldIndex field whose norm file will be rewritten
    */
   void advanceNormGen(int fieldIndex) {
-    if (normGen[fieldIndex] == NO) {
-      normGen[fieldIndex] = YES;
+    Long gen = normGen.get(fieldIndex);
+    if (gen == null || gen.longValue() == NO) {
+      normGen.put(fieldIndex, new Long(YES));
     } else {
-      normGen[fieldIndex]++;
+      normGen.put(fieldIndex, gen+1);
     }
-    clearFiles();
+    clearFilesCache();
   }
 
   /**
@@ -388,7 +432,7 @@
    */
   public String getNormFileName(int number) {
     if (hasSeparateNorms(number)) {
-      return IndexFileNames.fileNameFromGeneration(name, "s" + number, normGen[number]);
+      return IndexFileNames.fileNameFromGeneration(name, "s" + number, normGen.get(number));
     } else {
       // single file for all norms 
       return IndexFileNames.fileNameFromGeneration(name, IndexFileNames.NORMS_EXTENSION, WITHOUT_GEN);
@@ -403,7 +447,7 @@
    */
   void setUseCompoundFile(boolean isCompoundFile) {
     this.isCompoundFile = isCompoundFile;
-    clearFiles();
+    clearFilesCache();
   }
 
   /**
@@ -433,7 +477,7 @@
   
   void setDocStoreIsCompoundFile(boolean v) {
     docStoreIsCompoundFile = v;
-    clearFiles();
+    clearFilesCache();
   }
   
   public String getDocStoreSegment() {
@@ -446,14 +490,14 @@
   
   void setDocStoreOffset(int offset) {
     docStoreOffset = offset;
-    clearFiles();
+    clearFilesCache();
   }
 
   void setDocStore(int offset, String segment, boolean isCompoundFile) {        
     docStoreOffset = offset;
     docStoreSegment = segment;
     docStoreIsCompoundFile = isCompoundFile;
-    clearFiles();
+    clearFilesCache();
   }
   
   /** Save this segment's info. */
@@ -474,27 +518,24 @@
     if (normGen == null) {
       output.writeInt(NO);
     } else {
-      output.writeInt(normGen.length);
-      for (long fieldNormGen : normGen) {
-        output.writeLong(fieldNormGen);
+      output.writeInt(normGen.size());
+      for (Entry<Integer,Long> entry : normGen.entrySet()) {
+        output.writeInt(entry.getKey());
+        output.writeLong(entry.getValue());
       }
     }
     
     output.writeByte((byte) (isCompoundFile ? YES : NO));
     output.writeInt(delCount);
-    output.writeByte((byte) (hasProx ? 1:0));
+    output.writeByte((byte) hasProx);
     segmentCodecs.write(output);
     output.writeStringStringMap(diagnostics);
-    output.writeByte((byte) (hasVectors ? 1 : 0));
+    output.writeByte((byte) hasVectors);
   }
 
-  void setHasProx(boolean hasProx) {
-    this.hasProx = hasProx;
-    clearFiles();
-  }
-
   public boolean getHasProx() {
-    return hasProx;
+    return hasProx == CHECK_FIELDINFOS ?
+        (fieldInfos == null ? true : fieldInfos.hasProx()) : hasProx == YES;
   }
 
   /** Can only be called once. */
@@ -550,7 +591,7 @@
       } else {
         fileSet.add(IndexFileNames.segmentFileName(docStoreSegment, "", IndexFileNames.FIELDS_INDEX_EXTENSION));
         fileSet.add(IndexFileNames.segmentFileName(docStoreSegment, "", IndexFileNames.FIELDS_EXTENSION));
-        if (hasVectors) {
+        if (getHasVectors()) {
           fileSet.add(IndexFileNames.segmentFileName(docStoreSegment, "", IndexFileNames.VECTORS_INDEX_EXTENSION));
           fileSet.add(IndexFileNames.segmentFileName(docStoreSegment, "", IndexFileNames.VECTORS_DOCUMENTS_EXTENSION));
           fileSet.add(IndexFileNames.segmentFileName(docStoreSegment, "", IndexFileNames.VECTORS_FIELDS_EXTENSION));
@@ -559,7 +600,7 @@
     } else if (!useCompoundFile) {
       fileSet.add(IndexFileNames.segmentFileName(name, "", IndexFileNames.FIELDS_INDEX_EXTENSION));
       fileSet.add(IndexFileNames.segmentFileName(name, "", IndexFileNames.FIELDS_EXTENSION));
-      if (hasVectors) {
+      if (getHasVectors()) {
         fileSet.add(IndexFileNames.segmentFileName(name, "", IndexFileNames.VECTORS_INDEX_EXTENSION));
         fileSet.add(IndexFileNames.segmentFileName(name, "", IndexFileNames.VECTORS_DOCUMENTS_EXTENSION));
         fileSet.add(IndexFileNames.segmentFileName(name, "", IndexFileNames.VECTORS_FIELDS_EXTENSION));
@@ -572,11 +613,11 @@
     }
 
     if (normGen != null) {
-      for (int i = 0; i < normGen.length; i++) {
-        long gen = normGen[i];
+      for (Entry<Integer,Long> entry : normGen.entrySet()) {
+        long gen = entry.getValue();
         if (gen >= YES) {
           // Definitely a separate norm file, with generation:
-          fileSet.add(IndexFileNames.fileNameFromGeneration(name, IndexFileNames.SEPARATE_NORMS_EXTENSION + i, gen));
+          fileSet.add(IndexFileNames.fileNameFromGeneration(name, IndexFileNames.SEPARATE_NORMS_EXTENSION + entry.getKey(), gen));
         }
       }
     }
@@ -588,7 +629,7 @@
 
   /* Called whenever any change is made that affects which
    * files this segment has. */
-  private void clearFiles() {
+  void clearFilesCache() {
     files = null;
     sizeInBytesNoStore = -1;
     sizeInBytesWithStore = -1;
@@ -623,7 +664,7 @@
     if (this.dir != dir) {
       s.append('x');
     }
-    if (hasVectors) {
+    if (getHasVectors()) {
       s.append('v');
     }
     s.append(docCount);
Index: src/java/org/apache/lucene/index/InvertedDocConsumer.java
===================================================================
--- src/java/org/apache/lucene/index/InvertedDocConsumer.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/InvertedDocConsumer.java	(working copy)
@@ -35,10 +35,4 @@
   /** Attempt to free RAM, returning true if any RAM was
    *  freed */
   abstract boolean freeRAM();
-
-  FieldInfos fieldInfos;
-
-  void setFieldInfos(FieldInfos fieldInfos) {
-    this.fieldInfos = fieldInfos;
   }
-}
Index: src/java/org/apache/lucene/index/SegmentReader.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentReader.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/SegmentReader.java	(working copy)
@@ -22,23 +22,22 @@
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.HashSet;
-
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-
 import java.util.concurrent.atomic.AtomicInteger;
+
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.FieldSelector;
+import org.apache.lucene.index.codecs.FieldsProducer;
 import org.apache.lucene.store.BufferedIndexInput;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.IndexInput;
 import org.apache.lucene.store.IndexOutput;
 import org.apache.lucene.util.BitVector;
 import org.apache.lucene.util.Bits;
-import org.apache.lucene.util.CloseableThreadLocal;
-import org.apache.lucene.index.codecs.FieldsProducer;
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.CloseableThreadLocal;
 
 /**
  * @lucene.experimental
@@ -120,7 +119,7 @@
         }
         cfsDir = dir0;
 
-        fieldInfos = new FieldInfos(cfsDir, IndexFileNames.segmentFileName(segment, "", IndexFileNames.FIELD_INFOS_EXTENSION));
+        fieldInfos = si.getFieldInfos();
         
         this.termsIndexDivisor = termsIndexDivisor;
         
@@ -598,12 +597,12 @@
                                   && (!si.hasDeletions() || this.si.getDelFileName().equals(si.getDelFileName()));
     boolean normsUpToDate = true;
     
-    boolean[] fieldNormsChanged = new boolean[core.fieldInfos.size()];
-    final int fieldCount = core.fieldInfos.size();
-    for (int i = 0; i < fieldCount; i++) {
-      if (!this.si.getNormFileName(i).equals(si.getNormFileName(i))) {
+    Set<Integer> fieldNormsChanged = new HashSet<Integer>();
+    for (FieldInfo fi : core.fieldInfos) {
+      int fieldNumber = fi.number;
+      if (!this.si.getNormFileName(fieldNumber).equals(si.getNormFileName(fieldNumber))) {
         normsUpToDate = false;
-        fieldNormsChanged[i] = true;
+        fieldNormsChanged.add(fieldNumber);
       }
     }
 
@@ -659,11 +658,10 @@
       clone.norms = new HashMap<String,Norm>();
 
       // Clone norms
-      for (int i = 0; i < fieldNormsChanged.length; i++) {
-
+      for (FieldInfo fi : core.fieldInfos) {
         // Clone unchanged norms to the cloned reader
-        if (doClone || !fieldNormsChanged[i]) {
-          final String curField = core.fieldInfos.fieldInfo(i).name;
+        if (doClone || !fieldNormsChanged.contains(fi.number)) {
+          final String curField = fi.name;
           Norm norm = this.norms.get(curField);
           if (norm != null)
             clone.norms.put(curField, (Norm) norm.clone());
@@ -735,7 +733,7 @@
     }
 
     if (normsDirty) {               // re-write norms
-      si.initNormGen(core.fieldInfos.size());
+      si.initNormGen();
       for (final Norm norm : norms.values()) {
         if (norm.dirty) {
           norm.reWrite(si);
@@ -880,8 +878,7 @@
     ensureOpen();
 
     Set<String> fieldSet = new HashSet<String>();
-    for (int i = 0; i < core.fieldInfos.size(); i++) {
-      FieldInfo fi = core.fieldInfos.fieldInfo(i);
+    for (FieldInfo fi : core.fieldInfos) {
       if (fieldOption == IndexReader.FieldOption.ALL) {
         fieldSet.add(fi.name);
       }
@@ -959,8 +956,7 @@
   private void openNorms(Directory cfsDir, int readBufferSize) throws IOException {
     long nextNormSeek = SegmentMerger.NORMS_HEADER.length; //skip header (header unused for now)
     int maxDoc = maxDoc();
-    for (int i = 0; i < core.fieldInfos.size(); i++) {
-      FieldInfo fi = core.fieldInfos.fieldInfo(i);
+    for (FieldInfo fi : core.fieldInfos) {
       if (norms.containsKey(fi.name)) {
         // in case this SegmentReader is being re-opened, we might be able to
         // reuse some norm instances and skip loading them here
Index: src/java/org/apache/lucene/index/DocFieldConsumer.java
===================================================================
--- src/java/org/apache/lucene/index/DocFieldConsumer.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/DocFieldConsumer.java	(working copy)
@@ -22,9 +22,6 @@
 import java.util.Map;
 
 abstract class DocFieldConsumer {
-
-  FieldInfos fieldInfos;
-
   /** Called when DocumentsWriter decides to create a new
    *  segment */
   abstract void flush(Map<DocFieldConsumerPerThread,Collection<DocFieldConsumerPerField>> threadsAndFields, SegmentWriteState state) throws IOException;
@@ -39,8 +36,4 @@
    *  The consumer should free RAM, if possible, returning
    *  true if any RAM was in fact freed. */
   abstract boolean freeRAM();
-
-  void setFieldInfos(FieldInfos fieldInfos) {
-    this.fieldInfos = fieldInfos;
   }
-}
Index: src/java/org/apache/lucene/index/DocFieldProcessorPerThread.java
===================================================================
--- src/java/org/apache/lucene/index/DocFieldProcessorPerThread.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/DocFieldProcessorPerThread.java	(working copy)
@@ -41,14 +41,13 @@
   float docBoost;
   int fieldGen;
   final DocFieldProcessor docFieldProcessor;
-  final FieldInfos fieldInfos;
   final DocFieldConsumerPerThread consumer;
 
   // Holds all fields seen in current doc
   DocFieldProcessorPerField[] fields = new DocFieldProcessorPerField[1];
   int fieldCount;
 
-  // Hash table for all fields ever seen
+  // Hash table for all fields seen in current segment
   DocFieldProcessorPerField[] fieldHash = new DocFieldProcessorPerField[2];
   int hashMask = 1;
   int totalFieldCount;
@@ -60,7 +59,6 @@
   public DocFieldProcessorPerThread(DocumentsWriterThreadState threadState, DocFieldProcessor docFieldProcessor) throws IOException {
     this.docState = threadState.docState;
     this.docFieldProcessor = docFieldProcessor;
-    this.fieldInfos = docFieldProcessor.fieldInfos;
     this.consumer = docFieldProcessor.consumer.addThread(this);
     fieldsWriter = docFieldProcessor.fieldsWriter.addThread(docState);
   }
@@ -75,6 +73,7 @@
         field = next;
       }
     }
+    doAfterFlush();
     fieldsWriter.abort();
     consumer.abort();
   }
@@ -92,45 +91,15 @@
     return fields;
   }
 
-  /** If there are fields we've seen but did not see again
-   *  in the last run, then free them up. */
-
-  void trimFields(SegmentWriteState state) {
-
-    for(int i=0;i<fieldHash.length;i++) {
-      DocFieldProcessorPerField perField = fieldHash[i];
-      DocFieldProcessorPerField lastPerField = null;
-
-      while (perField != null) {
-
-        if (perField.lastGen == -1) {
-
-          // This field was not seen since the previous
-          // flush, so, free up its resources now
-
-          // Unhash
-          if (lastPerField == null)
-            fieldHash[i] = perField.next;
-          else
-            lastPerField.next = perField.next;
-
-          if (state.infoStream != null) {
-            state.infoStream.println("  purge field=" + perField.fieldInfo.name);
+  /** In flush we reset the fieldHash to not maintain per-field state
+   *  across segments */
+  @Override
+  void doAfterFlush() {
+    fieldHash = new DocFieldProcessorPerField[2];
+    hashMask = 1;
+    totalFieldCount = 0;
           }
 
-          totalFieldCount--;
-
-        } else {
-          // Reset
-          perField.lastGen = -1;
-          lastPerField = perField;
-        }
-
-        perField = perField.next;
-      }
-    }
-  }
-
   private void rehash() {
     final int newHashSize = (fieldHash.length*2);
     assert newHashSize > fieldHash.length;
@@ -155,7 +124,7 @@
   }
 
   @Override
-  public DocumentsWriter.DocWriter processDocument() throws IOException {
+  public DocumentsWriter.DocWriter processDocument(FieldInfos fieldInfos) throws IOException {
 
     consumer.startDocument();
     fieldsWriter.startDocument();
Index: src/java/org/apache/lucene/index/InvertedDocEndConsumer.java
===================================================================
--- src/java/org/apache/lucene/index/InvertedDocEndConsumer.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/InvertedDocEndConsumer.java	(working copy)
@@ -25,5 +25,4 @@
   abstract InvertedDocEndConsumerPerThread addThread(DocInverterPerThread docInverterPerThread);
   abstract void flush(Map<InvertedDocEndConsumerPerThread,Collection<InvertedDocEndConsumerPerField>> threadsAndFields, SegmentWriteState state) throws IOException;
   abstract void abort();
-  abstract void setFieldInfos(FieldInfos fieldInfos);
 }
Index: src/java/org/apache/lucene/index/TermsHashConsumer.java
===================================================================
--- src/java/org/apache/lucene/index/TermsHashConsumer.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/TermsHashConsumer.java	(working copy)
@@ -25,10 +25,4 @@
   abstract TermsHashConsumerPerThread addThread(TermsHashPerThread perThread);
   abstract void flush(Map<TermsHashConsumerPerThread,Collection<TermsHashConsumerPerField>> threadsAndFields, final SegmentWriteState state) throws IOException;
   abstract void abort();
-
-  FieldInfos fieldInfos;
-
-  void setFieldInfos(FieldInfos fieldInfos) {
-    this.fieldInfos = fieldInfos;
   }
-}
Index: src/java/org/apache/lucene/index/StoredFieldsWriterPerThread.java
===================================================================
--- src/java/org/apache/lucene/index/StoredFieldsWriterPerThread.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/StoredFieldsWriterPerThread.java	(working copy)
@@ -32,7 +32,7 @@
   public StoredFieldsWriterPerThread(DocumentsWriter.DocState docState, StoredFieldsWriter storedFieldsWriter) throws IOException {
     this.storedFieldsWriter = storedFieldsWriter;
     this.docState = docState;
-    localFieldsWriter = new FieldsWriter((IndexOutput) null, (IndexOutput) null, storedFieldsWriter.fieldInfos);
+    localFieldsWriter = new FieldsWriter((IndexOutput) null, (IndexOutput) null);
   }
 
   public void startDocument() {
Index: src/java/org/apache/lucene/index/StoredFieldsWriter.java
===================================================================
--- src/java/org/apache/lucene/index/StoredFieldsWriter.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/StoredFieldsWriter.java	(working copy)
@@ -27,15 +27,13 @@
 
   FieldsWriter fieldsWriter;
   final DocumentsWriter docWriter;
-  final FieldInfos fieldInfos;
   int lastDocID;
 
   PerDoc[] docFreeList = new PerDoc[1];
   int freeCount;
 
-  public StoredFieldsWriter(DocumentsWriter docWriter, FieldInfos fieldInfos) {
+  public StoredFieldsWriter(DocumentsWriter docWriter) {
     this.docWriter = docWriter;
-    this.fieldInfos = fieldInfos;
   }
 
   public StoredFieldsWriterPerThread addThread(DocumentsWriter.DocState docState) throws IOException {
@@ -62,7 +60,7 @@
 
   private synchronized void initFieldsWriter() throws IOException {
     if (fieldsWriter == null) {
-      fieldsWriter = new FieldsWriter(docWriter.directory, docWriter.getSegment(), fieldInfos);
+      fieldsWriter = new FieldsWriter(docWriter.directory, docWriter.getSegment());
       lastDocID = 0;
     }
   }
Index: src/java/org/apache/lucene/index/SegmentInfos.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentInfos.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/SegmentInfos.java	(working copy)
@@ -17,26 +17,26 @@
  * limitations under the License.
  */
 
-import org.apache.lucene.store.Directory;
-import org.apache.lucene.store.IndexInput;
-import org.apache.lucene.store.IndexOutput;
-import org.apache.lucene.store.NoSuchDirectoryException;
-import org.apache.lucene.index.codecs.CodecProvider;
-import org.apache.lucene.index.codecs.SegmentInfosReader;
-import org.apache.lucene.index.codecs.SegmentInfosWriter;
-import org.apache.lucene.util.ThreadInterruptedException;
-
 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.io.PrintStream;
-import java.util.Vector;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.HashSet;
-import java.util.HashMap;
 import java.util.Map;
+import java.util.Vector;
 
+import org.apache.lucene.index.codecs.CodecProvider;
+import org.apache.lucene.index.codecs.SegmentInfosReader;
+import org.apache.lucene.index.codecs.SegmentInfosWriter;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.store.IndexInput;
+import org.apache.lucene.store.IndexOutput;
+import org.apache.lucene.store.NoSuchDirectoryException;
+import org.apache.lucene.util.ThreadInterruptedException;
+
 /**
  * A collection of segmentInfo objects with methods for operating on
  * those segments in relation to the file system.
Index: src/java/org/apache/lucene/index/DocConsumerPerThread.java
===================================================================
--- src/java/org/apache/lucene/index/DocConsumerPerThread.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/DocConsumerPerThread.java	(working copy)
@@ -27,7 +27,8 @@
    *  DocumentsWriter.DocWriter and return it.
    *  DocumentsWriter then calls finish() on this object
    *  when it's its turn. */
-  abstract DocumentsWriter.DocWriter processDocument() throws IOException;
+  abstract DocumentsWriter.DocWriter processDocument(FieldInfos fieldInfos) throws IOException;
 
+  abstract void doAfterFlush();
   abstract void abort();
 }
Index: src/java/org/apache/lucene/index/TermsHash.java
===================================================================
--- src/java/org/apache/lucene/index/TermsHash.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/TermsHash.java	(working copy)
@@ -57,12 +57,6 @@
   }
 
   @Override
-  void setFieldInfos(FieldInfos fieldInfos) {
-    this.fieldInfos = fieldInfos;
-    consumer.setFieldInfos(fieldInfos);
-  }
-
-  @Override
   public void abort() {
     consumer.abort();
     if (nextTermsHash != null)
Index: src/java/org/apache/lucene/index/SegmentMerger.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentMerger.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/SegmentMerger.java	(working copy)
@@ -26,16 +26,16 @@
 import org.apache.lucene.document.Document;
 import org.apache.lucene.index.IndexReader.FieldOption;
 import org.apache.lucene.index.MergePolicy.MergeAbortedException;
-import org.apache.lucene.index.codecs.CodecProvider;
 import org.apache.lucene.index.codecs.Codec;
-import org.apache.lucene.index.codecs.MergeState;
+import org.apache.lucene.index.codecs.CodecProvider;
 import org.apache.lucene.index.codecs.FieldsConsumer;
+import org.apache.lucene.index.codecs.MergeState;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.IndexInput;
 import org.apache.lucene.store.IndexOutput;
 import org.apache.lucene.util.Bits;
-import org.apache.lucene.util.ReaderUtil;
 import org.apache.lucene.util.MultiBits;
+import org.apache.lucene.util.ReaderUtil;
 
 /**
  * The SegmentMerger class combines two or more Segments, represented by an IndexReader ({@link #add},
@@ -75,8 +75,8 @@
     this.payloadProcessorProvider = payloadProcessorProvider;
     directory = dir;
     this.codecs = codecs;
-    this.fieldInfos = fieldInfos;
     segment = name;
+    this.fieldInfos = fieldInfos;
     if (merge != null) {
       checkAbort = new MergeState.CheckAbort(merge, directory);
     } else {
@@ -180,9 +180,8 @@
         SegmentReader segmentReader = (SegmentReader) reader;
         boolean same = true;
         FieldInfos segmentFieldInfos = segmentReader.fieldInfos();
-        int numFieldInfos = segmentFieldInfos.size();
-        for (int j = 0; same && j < numFieldInfos; j++) {
-          same = fieldInfos.fieldName(j).equals(segmentFieldInfos.fieldName(j));
+        for (FieldInfo fi : segmentFieldInfos) {
+          same = fieldInfos.fieldName(fi.number).equals(fi.name);
         }
         if (same) {
           matchingSegmentReaders[i] = segmentReader;
@@ -208,9 +207,8 @@
       if (reader instanceof SegmentReader) {
         SegmentReader segmentReader = (SegmentReader) reader;
         FieldInfos readerFieldInfos = segmentReader.fieldInfos();
-        int numReaderFieldInfos = readerFieldInfos.size();
-        for (int j = 0; j < numReaderFieldInfos; j++) {
-          fieldInfos.add(readerFieldInfos.fieldInfo(j));
+        for (FieldInfo fi : readerFieldInfos) {
+          fieldInfos.add(fi);
         }
       } else {
         addIndexed(reader, fieldInfos, reader.getFieldNames(FieldOption.TERMVECTOR_WITH_POSITION_OFFSET), true, true, true, false, false);
@@ -224,13 +222,13 @@
       }
     }
     final SegmentCodecs codecInfo = SegmentCodecs.build(fieldInfos, this.codecs);
-    fieldInfos.write(directory, segment + ".fnm");
+    fieldInfos.write(directory, segment + "." + IndexFileNames.FIELD_INFOS_EXTENSION);
 
     int docCount = 0;
 
     setMatchingSegmentReaders();
 
-    final FieldsWriter fieldsWriter = new FieldsWriter(directory, segment, fieldInfos);
+    final FieldsWriter fieldsWriter = new FieldsWriter(directory, segment);
 
     try {
       int idx = 0;
@@ -312,7 +310,7 @@
         // NOTE: it's very important to first assign to doc then pass it to
         // termVectorsWriter.addAllDocVectors; see LUCENE-1282
         Document doc = reader.document(j);
-        fieldsWriter.addDocument(doc);
+        fieldsWriter.addDocument(doc, fieldInfos);
         docCount++;
         checkAbort.work(300);
       }
@@ -339,7 +337,7 @@
         // NOTE: it's very important to first assign to doc then pass it to
         // termVectorsWriter.addAllDocVectors; see LUCENE-1282
         Document doc = reader.document(docCount);
-        fieldsWriter.addDocument(doc);
+        fieldsWriter.addDocument(doc, fieldInfos);
         checkAbort.work(300);
       }
     }
@@ -574,8 +572,7 @@
   private void mergeNorms() throws IOException {
     IndexOutput output = null;
     try {
-      for (int i = 0, numFieldInfos = fieldInfos.size(); i < numFieldInfos; i++) {
-        final FieldInfo fi = fieldInfos.fieldInfo(i);
+      for (FieldInfo fi : fieldInfos) {
         if (fi.isIndexed && !fi.omitNorms) {
           if (output == null) { 
             output = directory.createOutput(IndexFileNames.segmentFileName(segment, "", IndexFileNames.NORMS_EXTENSION));
Index: src/java/org/apache/lucene/index/DocFieldProcessor.java
===================================================================
--- src/java/org/apache/lucene/index/DocFieldProcessor.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/DocFieldProcessor.java	(working copy)
@@ -34,16 +34,13 @@
 final class DocFieldProcessor extends DocConsumer {
 
   final DocumentsWriter docWriter;
-  final FieldInfos fieldInfos;
   final DocFieldConsumer consumer;
   final StoredFieldsWriter fieldsWriter;
 
   public DocFieldProcessor(DocumentsWriter docWriter, DocFieldConsumer consumer) {
     this.docWriter = docWriter;
     this.consumer = consumer;
-    fieldInfos = docWriter.getFieldInfos();
-    consumer.setFieldInfos(fieldInfos);
-    fieldsWriter = new StoredFieldsWriter(docWriter, fieldInfos);
+    fieldsWriter = new StoredFieldsWriter(docWriter);
   }
 
   @Override
@@ -53,7 +50,6 @@
     for ( DocConsumerPerThread thread : threads) {
       DocFieldProcessorPerThread perThread = (DocFieldProcessorPerThread) thread;
       childThreadsAndFields.put(perThread.consumer, perThread.fields());
-      perThread.trimFields(state);
     }
     fieldsWriter.flush(state);
     consumer.flush(childThreadsAndFields, state);
@@ -63,7 +59,7 @@
     // FreqProxTermsWriter does this with
     // FieldInfo.storePayload.
     final String fileName = IndexFileNames.segmentFileName(state.segmentName, "", IndexFileNames.FIELD_INFOS_EXTENSION);
-    fieldInfos.write(state.directory, fileName);
+    state.fieldInfos.write(state.directory, fileName);
   }
 
   @Override
Index: src/java/org/apache/lucene/index/SegmentCodecs.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentCodecs.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/SegmentCodecs.java	(working copy)
@@ -74,22 +74,20 @@
   }
 
   static SegmentCodecs build(FieldInfos infos, CodecProvider provider) {
-    final int size = infos.size();
     final Map<Codec, Integer> codecRegistry = new IdentityHashMap<Codec, Integer>();
     final ArrayList<Codec> codecs = new ArrayList<Codec>();
 
-    for (int i = 0; i < size; i++) {
-      final FieldInfo info = infos.fieldInfo(i);
-      if (info.isIndexed) {
+    for (FieldInfo fi : infos) {
+      if (fi.isIndexed) {
         final Codec fieldCodec = provider.lookup(provider
-            .getFieldCodec(info.name));
+            .getFieldCodec(fi.name));
         Integer ord = codecRegistry.get(fieldCodec);
         if (ord == null) {
           ord = Integer.valueOf(codecs.size());
           codecRegistry.put(fieldCodec, ord);
           codecs.add(fieldCodec);
         }
-        info.codecId = ord.intValue();
+        fi.setCodecId(ord.intValue());
       }
     }
     return new SegmentCodecs(provider, codecs.toArray(Codec.EMPTY));
Index: src/java/org/apache/lucene/index/FieldInfo.java
===================================================================
--- src/java/org/apache/lucene/index/FieldInfo.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/FieldInfo.java	(working copy)
@@ -32,7 +32,7 @@
   public boolean omitTermFreqAndPositions;
 
   public boolean storePayloads; // whether this field stores payloads together with term positions
-  int codecId = 0; // set inside SegmentCodecs#build() during segment flush - this is used to identify the codec used to write this field 
+  private int codecId = -1; // set inside SegmentCodecs#build() during segment flush - this is used to identify the codec used to write this field
 
   FieldInfo(String na, boolean tk, int nu, boolean storeTermVector, 
             boolean storePositionWithTermVector,  boolean storeOffsetWithTermVector, 
@@ -57,10 +57,21 @@
     }
   }
 
+  public void setCodecId(int codecId) {
+    assert this.codecId == -1 : "CodecId can only be set once.";
+    this.codecId = codecId;
+  }
+
+  public int getCodecId() {
+    return codecId;
+  }
+
   @Override
   public Object clone() {
-    return new FieldInfo(name, isIndexed, number, storeTermVector, storePositionWithTermVector,
+    FieldInfo clone = new FieldInfo(name, isIndexed, number, storeTermVector, storePositionWithTermVector,
                          storeOffsetWithTermVector, omitNorms, storePayloads, omitTermFreqAndPositions);
+    clone.codecId = this.codecId;
+    return clone;
   }
 
   void update(boolean isIndexed, boolean storeTermVector, boolean storePositionWithTermVector, 
Index: src/java/org/apache/lucene/index/PerFieldCodecWrapper.java
===================================================================
--- src/java/org/apache/lucene/index/PerFieldCodecWrapper.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/PerFieldCodecWrapper.java	(working copy)
@@ -67,7 +67,7 @@
 
     @Override
     public TermsConsumer addField(FieldInfo field) throws IOException {
-      final FieldsConsumer fields = consumers.get(field.codecId);
+      final FieldsConsumer fields = consumers.get(field.getCodecId());
       return fields.addField(field);
     }
 
@@ -100,18 +100,16 @@
     public FieldsReader(Directory dir, FieldInfos fieldInfos, SegmentInfo si,
         int readBufferSize, int indexDivisor) throws IOException {
 
-      final int fieldCount = fieldInfos.size();
       final Map<Codec, FieldsProducer> producers = new HashMap<Codec, FieldsProducer>();
       boolean success = false;
       try {
-        for (int i = 0; i < fieldCount; i++) {
-          FieldInfo fi = fieldInfos.fieldInfo(i);
+        for (FieldInfo fi : fieldInfos) {
           if (fi.isIndexed) { // TODO this does not work for non-indexed fields
             fields.add(fi.name);
-            Codec codec = segmentCodecs.codecs[fi.codecId];
+            Codec codec = segmentCodecs.codecs[fi.getCodecId()];
             if (!producers.containsKey(codec)) {
               producers.put(codec, codec.fieldsProducer(new SegmentReadState(dir,
-                                                                             si, fieldInfos, readBufferSize, indexDivisor, ""+fi.codecId)));
+                                                                             si, fieldInfos, readBufferSize, indexDivisor, ""+fi.getCodecId())));
             }
             codecs.put(fi.name, producers.get(codec));
           }
Index: src/java/org/apache/lucene/index/DocumentsWriter.java
===================================================================
--- src/java/org/apache/lucene/index/DocumentsWriter.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/DocumentsWriter.java	(working copy)
@@ -279,12 +279,13 @@
   private int maxBufferedDocs = IndexWriterConfig.DEFAULT_MAX_BUFFERED_DOCS;
 
   private boolean closed;
-  private final FieldInfos fieldInfos;
+  private FieldInfos fieldInfos;
 
   private final BufferedDeletesStream bufferedDeletesStream;
   private final IndexWriter.FlushControl flushControl;
 
-  DocumentsWriter(Directory directory, IndexWriter writer, IndexingChain indexingChain, int maxThreadStates, FieldInfos fieldInfos, BufferedDeletesStream bufferedDeletesStream) throws IOException {
+  DocumentsWriter(Directory directory, IndexWriter writer, IndexingChain indexingChain, int maxThreadStates, FieldInfos fieldInfos,
+      BufferedDeletesStream bufferedDeletesStream) throws IOException {
     this.directory = directory;
     this.writer = writer;
     this.similarityProvider = writer.getConfig().getSimilarityProvider();
@@ -350,10 +351,6 @@
     return doFlush;
   }
 
-  public FieldInfos getFieldInfos() {
-    return fieldInfos;
-  }
-
   /** If non-null, various details of indexing are printed
    *  here. */
   synchronized void setInfoStream(PrintStream infoStream) {
@@ -482,9 +479,14 @@
   private void doAfterFlush() throws IOException {
     // All ThreadStates should be idle when we are called
     assert allThreadsIdle();
+    for (DocumentsWriterThreadState threadState : threadStates) {
+      threadState.consumer.doAfterFlush();
+    }
+
     threadBindings.clear();
     waitQueue.reset();
     segment = null;
+    fieldInfos = fieldInfos.newFieldInfosWithGlobalFieldNumberMap();
     numDocs = 0;
     nextDocID = 0;
     bufferIsFull = false;
@@ -602,7 +604,7 @@
         pendingDeletes.docIDs.clear();
       }
 
-      newSegment = new SegmentInfo(segment, numDocs, directory, false, fieldInfos.hasProx(), flushState.segmentCodecs, false);
+      newSegment = new SegmentInfo(segment, numDocs, directory, false, flushState.segmentCodecs, fieldInfos);
 
       Collection<DocConsumerPerThread> threads = new HashSet<DocConsumerPerThread>();
       for (DocumentsWriterThreadState threadState : threadStates) {
@@ -613,7 +615,7 @@
 
       consumer.flush(threads, flushState);
 
-      newSegment.setHasVectors(flushState.hasVectors);
+      newSegment.clearFilesCache();
 
       if (infoStream != null) {
         message("new segment has " + (flushState.hasVectors ? "vectors" : "no vectors"));
@@ -796,7 +798,7 @@
       // work
       final DocWriter perDoc;
       try {
-        perDoc = state.consumer.processDocument();
+        perDoc = state.consumer.processDocument(fieldInfos);
       } finally {
         docState.clear();
       }
Index: src/java/org/apache/lucene/index/IndexWriter.java
===================================================================
--- src/java/org/apache/lucene/index/IndexWriter.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/IndexWriter.java	(working copy)
@@ -38,7 +38,6 @@
 import org.apache.lucene.index.IndexWriterConfig.OpenMode;
 import org.apache.lucene.index.PayloadProcessorProvider.DirPayloadProcessor;
 import org.apache.lucene.index.codecs.CodecProvider;
-import org.apache.lucene.index.codecs.DefaultSegmentInfosWriter;
 import org.apache.lucene.search.Query;
 import org.apache.lucene.store.AlreadyClosedException;
 import org.apache.lucene.store.BufferedIndexInput;
@@ -221,6 +220,7 @@
   volatile long pendingCommitChangeCount;
 
   final SegmentInfos segmentInfos;       // the segments
+  final FieldInfos fieldInfos;
 
   private DocumentsWriter docWriter;
   private IndexFileDeleter deleter;
@@ -791,7 +791,10 @@
 
       setRollbackSegmentInfos(segmentInfos);
 
-      docWriter = new DocumentsWriter(directory, this, conf.getIndexingChain(), conf.getMaxThreadStates(), getCurrentFieldInfos(), bufferedDeletesStream);
+      // start with previous field numbers, but new FieldInfos
+      fieldInfos = getCurrentFieldInfos();
+      docWriter = new DocumentsWriter(directory, this, conf.getIndexingChain(), conf.getMaxThreadStates(),
+          fieldInfos.newFieldInfosWithGlobalFieldNumberMap(), bufferedDeletesStream);
       docWriter.setInfoStream(infoStream);
 
       // Default deleter (for backwards compatibility) is
@@ -854,23 +857,14 @@
   private FieldInfos getCurrentFieldInfos() throws IOException {
     final FieldInfos fieldInfos;
     if (segmentInfos.size() > 0) {
-      if (segmentInfos.getFormat() > DefaultSegmentInfosWriter.FORMAT_HAS_VECTORS) {
-        // Pre-4.0 index.  In this case we sweep all
-        // segments, merging their FieldInfos:
         fieldInfos = new FieldInfos();
         for(SegmentInfo info : segmentInfos) {
           final FieldInfos segFieldInfos = getFieldInfos(info);
-          final int fieldCount = segFieldInfos.size();
-          for(int fieldNumber=0;fieldNumber<fieldCount;fieldNumber++) {
-            fieldInfos.add(segFieldInfos.fieldInfo(fieldNumber));
+        for (FieldInfo fi : segFieldInfos) {
+          fieldInfos.add(fi);
           }
         }
       } else {
-        // Already a 4.0 index; just seed the FieldInfos
-        // from the last segment
-        fieldInfos = getFieldInfos(segmentInfos.info(segmentInfos.size()-1));
-      }
-    } else {
       fieldInfos = new FieldInfos();
     }
     return fieldInfos;
@@ -2272,7 +2266,7 @@
       String mergedName = newSegmentName();
       SegmentMerger merger = new SegmentMerger(directory, termIndexInterval,
                                                mergedName, null, codecs, payloadProcessorProvider,
-                                               ((FieldInfos) docWriter.getFieldInfos().clone()));
+                                               fieldInfos.newFieldInfosWithGlobalFieldNumberMap());
       
       for (IndexReader reader : readers)      // add new indexes
         merger.add(reader);
@@ -2280,8 +2274,8 @@
       int docCount = merger.merge();                // merge 'em
       
       SegmentInfo info = new SegmentInfo(mergedName, docCount, directory,
-                                         false, merger.fieldInfos().hasProx(), merger.getSegmentCodecs(),
-                                         merger.fieldInfos().hasVectors());
+                                         false, merger.getSegmentCodecs(),
+                                         merger.fieldInfos());
       setDiagnostics(info, "addIndexes(IndexReader...)");
 
       boolean useCompoundFile;
@@ -3014,7 +3008,7 @@
     // Bind a new segment name here so even with
     // ConcurrentMergePolicy we keep deterministic segment
     // names.
-    merge.info = new SegmentInfo(newSegmentName(), 0, directory, false, false, null, false);
+    merge.info = new SegmentInfo(newSegmentName(), 0, directory, false, null, fieldInfos.newFieldInfosWithGlobalFieldNumberMap());
 
     // Lock order: IW -> BD
     final BufferedDeletesStream.ApplyDeletesResult result = bufferedDeletesStream.applyDeletes(readerPool, merge.segments);
@@ -3165,7 +3159,7 @@
 
     SegmentMerger merger = new SegmentMerger(directory, termIndexInterval, mergedName, merge,
                                              codecs, payloadProcessorProvider,
-                                             ((FieldInfos) docWriter.getFieldInfos().clone()));
+                                             merge.info.getFieldInfos());
 
     if (infoStream != null) {
       message("merging " + merge.segString(directory) + " mergeVectors=" + merger.fieldInfos().hasVectors());
@@ -3174,8 +3168,9 @@
     merge.readers = new ArrayList<SegmentReader>();
     merge.readerClones = new ArrayList<SegmentReader>();
 
-    merge.info.setHasVectors(merger.fieldInfos().hasVectors());
+    merge.info.clearFilesCache();
 
+
     // This is try/finally to make sure merger's readers are
     // closed:
     boolean success = false;
@@ -3230,7 +3225,7 @@
       // because codec must know if prox was written for
       // this segment:
       //System.out.println("merger set hasProx=" + merger.hasProx() + " seg=" + merge.info.name);
-      merge.info.setHasProx(merger.fieldInfos().hasProx());
+      merge.info.clearFilesCache();
 
       boolean useCompoundFile;
       synchronized (this) { // Guard segmentInfos
Index: src/java/org/apache/lucene/index/codecs/preflex/PreFlexFields.java
===================================================================
--- src/java/org/apache/lucene/index/codecs/preflex/PreFlexFields.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/codecs/preflex/PreFlexFields.java	(working copy)
@@ -19,14 +19,15 @@
 
 import java.io.IOException;
 import java.util.Collection;
-import java.util.Iterator;
-import java.util.TreeMap;
+import java.util.Comparator;
 import java.util.HashMap;
+import java.util.Iterator;
 import java.util.Map;
-import java.util.Comparator;
+import java.util.TreeMap;
 
-import org.apache.lucene.index.DocsEnum;
+import org.apache.lucene.index.CompoundFileReader;
 import org.apache.lucene.index.DocsAndPositionsEnum;
+import org.apache.lucene.index.DocsEnum;
 import org.apache.lucene.index.FieldInfo;
 import org.apache.lucene.index.FieldInfos;
 import org.apache.lucene.index.FieldsEnum;
@@ -35,7 +36,6 @@
 import org.apache.lucene.index.Term;
 import org.apache.lucene.index.Terms;
 import org.apache.lucene.index.TermsEnum;
-import org.apache.lucene.index.CompoundFileReader;
 import org.apache.lucene.index.codecs.FieldsProducer;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.IndexInput;
@@ -94,13 +94,11 @@
       // so that if an index update removes them we'll still have them
       freqStream = dir.openInput(info.name + ".frq", readBufferSize);
       boolean anyProx = false;
-      final int numFields = fieldInfos.size();
-      for(int i=0;i<numFields;i++) {
-        final FieldInfo fieldInfo = fieldInfos.fieldInfo(i);
-        if (fieldInfo.isIndexed) {
-          fields.put(fieldInfo.name, fieldInfo);
-          preTerms.put(fieldInfo.name, new PreTerms(fieldInfo));
-          if (!fieldInfo.omitTermFreqAndPositions) {
+      for (FieldInfo fi : fieldInfos) {
+        if (fi.isIndexed) {
+          fields.put(fi.name, fi);
+          preTerms.put(fi.name, new PreTerms(fi));
+          if (!fi.omitTermFreqAndPositions) {
             anyProx = true;
           }
         }
Index: src/java/org/apache/lucene/index/DocInverter.java
===================================================================
--- src/java/org/apache/lucene/index/DocInverter.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/DocInverter.java	(working copy)
@@ -40,13 +40,6 @@
   }
 
   @Override
-  void setFieldInfos(FieldInfos fieldInfos) {
-    super.setFieldInfos(fieldInfos);
-    consumer.setFieldInfos(fieldInfos);
-    endConsumer.setFieldInfos(fieldInfos);
-  }
-
-  @Override
   void flush(Map<DocFieldConsumerPerThread, Collection<DocFieldConsumerPerField>> threadsAndFields, SegmentWriteState state) throws IOException {
 
     Map<InvertedDocConsumerPerThread,Collection<InvertedDocConsumerPerField>> childThreadsAndFields = new HashMap<InvertedDocConsumerPerThread,Collection<InvertedDocConsumerPerField>>();
Index: src/java/org/apache/lucene/index/NormsWriter.java
===================================================================
--- src/java/org/apache/lucene/index/NormsWriter.java	(revision 1072815)
+++ src/java/org/apache/lucene/index/NormsWriter.java	(working copy)
@@ -36,7 +36,6 @@
 
 final class NormsWriter extends InvertedDocEndConsumer {
 
-  private FieldInfos fieldInfos;
   @Override
   public InvertedDocEndConsumerPerThread addThread(DocInverterPerThread docInverterPerThread) {
     return new NormsWriterPerThread(docInverterPerThread, this);
@@ -48,11 +47,6 @@
   // We only write the _X.nrm file at flush
   void files(Collection<String> files) {}
 
-  @Override
-  void setFieldInfos(FieldInfos fieldInfos) {
-    this.fieldInfos = fieldInfos;
-  }
-
   /** Produce _X.nrm if any document had a field with norms
    *  not disabled */
   @Override
@@ -60,7 +54,7 @@
 
     final Map<FieldInfo,List<NormsWriterPerField>> byField = new HashMap<FieldInfo,List<NormsWriterPerField>>();
 
-    if (!fieldInfos.hasNorms()) {
+    if (!state.fieldInfos.hasNorms()) {
       return;
     }
 
@@ -96,15 +90,10 @@
     try {
       normsOut.writeBytes(SegmentMerger.NORMS_HEADER, 0, SegmentMerger.NORMS_HEADER.length);
 
-      final int numField = fieldInfos.size();
-
       int normCount = 0;
 
-      for(int fieldNumber=0;fieldNumber<numField;fieldNumber++) {
-
-        final FieldInfo fieldInfo = fieldInfos.fieldInfo(fieldNumber);
-
-        List<NormsWriterPerField> toMerge = byField.get(fieldInfo);
+      for (FieldInfo fi : state.fieldInfos) {
+        List<NormsWriterPerField> toMerge = byField.get(fi);
         int upto = 0;
         if (toMerge != null) {
 
@@ -158,7 +147,7 @@
           // Fill final hole with defaultNorm
           for(;upto<state.numDocs;upto++)
             normsOut.writeByte((byte) 0);
-        } else if (fieldInfo.isIndexed && !fieldInfo.omitNorms) {
+        } else if (fi.isIndexed && !fi.omitNorms) {
           normCount++;
           // Fill entire field with default norm:
           for(;upto<state.numDocs;upto++)