Index: src/test/org/apache/lucene/index/TestIndexReader.java
===================================================================
--- src/test/org/apache/lucene/index/TestIndexReader.java	(revision 759487)
+++ src/test/org/apache/lucene/index/TestIndexReader.java	(working copy)
@@ -44,6 +44,7 @@
 import org.apache.lucene.search.ScoreDoc;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.search.Query;
+import org.apache.lucene.search.FieldCache;
 import org.apache.lucene.store.AlreadyClosedException;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.FSDirectory;
@@ -1692,4 +1693,112 @@
     s.close();
     r.close();
   }
+
+  // LUCENE-1579: Ensure that on a cloned reader, segments
+  // reuse the doc values arrays in FieldCache
+  public void testFieldCacheReuseAfterClone() throws Exception {
+    Directory dir = new MockRAMDirectory();
+    IndexWriter writer = new IndexWriter(dir, new WhitespaceAnalyzer(), IndexWriter.MaxFieldLength.UNLIMITED);
+    Document doc = new Document();
+    doc.add(new Field("number", "17", Field.Store.NO, Field.Index.NOT_ANALYZED));
+    writer.addDocument(doc);
+    writer.close();
+
+    // Open reader
+    IndexReader r = IndexReader.open(dir);
+    assertTrue(r instanceof SegmentReader);
+    final int[] ints = FieldCache.DEFAULT.getInts(r, "number");
+    assertEquals(1, ints.length);
+    assertEquals(17, ints[0]);
+
+    // Clone reader
+    IndexReader r2 = (IndexReader) r.clone();
+    r.close();
+    assertTrue(r2 != r);
+    assertTrue(r2 instanceof SegmentReader);
+    final int[] ints2 = FieldCache.DEFAULT.getInts(r2, "number");
+    r2.close();
+
+    assertEquals(1, ints2.length);
+    assertEquals(17, ints2[0]);
+    assertTrue(ints == ints2);
+
+    dir.close();
+  }
+
+  // LUCENE-1579: Ensure that on a reopened reader, that any
+  // shared segments reuse the doc values arrays in
+  // FieldCache
+  public void testFieldCacheReuseAfterReopen() throws Exception {
+    Directory dir = new MockRAMDirectory();
+    IndexWriter writer = new IndexWriter(dir, new WhitespaceAnalyzer(), IndexWriter.MaxFieldLength.UNLIMITED);
+    Document doc = new Document();
+    doc.add(new Field("number", "17", Field.Store.NO, Field.Index.NOT_ANALYZED));
+    writer.addDocument(doc);
+    writer.commit();
+
+    // Open reader1
+    IndexReader r = IndexReader.open(dir);
+    assertTrue(r instanceof SegmentReader);
+    final int[] ints = FieldCache.DEFAULT.getInts(r, "number");
+    assertEquals(1, ints.length);
+    assertEquals(17, ints[0]);
+
+    // Add new segment
+    writer.addDocument(doc);
+    writer.commit();
+
+    // Reopen reader1 --> reader2
+    IndexReader r2 = (IndexReader) r.reopen();
+    r.close();
+    assertTrue(r2 instanceof MultiSegmentReader);
+    IndexReader sub0 = r2.getSequentialSubReaders()[0];
+    assertTrue(sub0 instanceof SegmentReader);
+    final int[] ints2 = FieldCache.DEFAULT.getInts(sub0, "number");
+    r2.close();
+    assertTrue(ints == ints2);
+
+    dir.close();
+  }
+
+  // LUCENE-1579: Make sure all SegmentReaders are new when
+  // reopen switches readOnly
+  public void testReopenChangeReadonly() throws Exception {
+    Directory dir = new MockRAMDirectory();
+    IndexWriter writer = new IndexWriter(dir, new WhitespaceAnalyzer(), IndexWriter.MaxFieldLength.UNLIMITED);
+    Document doc = new Document();
+    doc.add(new Field("number", "17", Field.Store.NO, Field.Index.NOT_ANALYZED));
+    writer.addDocument(doc);
+    writer.commit();
+
+    // Open reader1
+    IndexReader r = IndexReader.open(dir);
+    assertTrue(r instanceof SegmentReader);
+    final int[] ints = FieldCache.DEFAULT.getInts(r, "number");
+    assertEquals(1, ints.length);
+    assertEquals(17, ints[0]);
+
+    // Reopen to readonly w/ no chnages
+    IndexReader r3 = (IndexReader) r.reopen(true);
+    assertTrue(r3 instanceof ReadOnlySegmentReader);
+    r3.close();
+
+    // Add new segment
+    writer.addDocument(doc);
+    writer.commit();
+
+    // Reopen reader1 --> reader2
+    IndexReader r2 = (IndexReader) r.reopen(true);
+    r.close();
+    assertTrue(r2 instanceof MultiSegmentReader);
+    IndexReader[] subs = r2.getSequentialSubReaders();
+    final int[] ints2 = FieldCache.DEFAULT.getInts(subs[0], "number");
+    r2.close();
+
+    assertTrue(subs[0] instanceof ReadOnlySegmentReader);
+    assertTrue(subs[1] instanceof ReadOnlySegmentReader);
+    assertTrue(ints == ints2);
+
+    dir.close();
+  }
 }
Index: src/java/org/apache/lucene/search/FieldCacheImpl.java
===================================================================
--- src/java/org/apache/lucene/search/FieldCacheImpl.java	(revision 759487)
+++ src/java/org/apache/lucene/search/FieldCacheImpl.java	(working copy)
@@ -50,11 +50,13 @@
     public Object get(IndexReader reader, Object key) throws IOException {
       Map innerCache;
       Object value;
+      final Object wrapper = reader.getFieldCacheWrapper();
+
       synchronized (readerCache) {
-        innerCache = (Map) readerCache.get(reader);
+        innerCache = (Map) readerCache.get(wrapper);
         if (innerCache == null) {
           innerCache = new HashMap();
-          readerCache.put(reader, innerCache);
+          readerCache.put(wrapper, innerCache);
           value = null;
         } else {
           value = innerCache.get(key);
Index: src/java/org/apache/lucene/index/SegmentReader.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentReader.java	(revision 759487)
+++ src/java/org/apache/lucene/index/SegmentReader.java	(working copy)
@@ -1250,4 +1250,32 @@
       norm.dirty = norm.rollbackDirty;
     }
   }
+
+  private final static class FieldCacheWrapper {
+    final Object obj;
+    FieldCacheWrapper(Object obj) {
+      this.obj = obj;
+    }
+    public int hashCode() {
+      return obj.hashCode();
+    }
+    public boolean equals(Object other) {
+      return obj.equals(((FieldCacheWrapper) other).obj);
+    }
+  }
+  
+  private FieldCacheWrapper fieldCacheWrapper;
+
+  // This is necessary so that cloned SegmentReaders (which
+  // share the underlying postings data) will map to the
+  // same entry in the FieldCache.  See LUCENE-1579.
+  public final Object getFieldCacheWrapper() {
+    if (fieldCacheWrapper== null) {
+      // It's very important that we retain a reference to
+      // the wrapper, so that FieldCache, which uses a
+      // WeakHashMap, does not prematurely evict entries:
+      fieldCacheWrapper = new FieldCacheWrapper(freqStream);
+    }
+    return fieldCacheWrapper;
+  }
 }
Index: src/java/org/apache/lucene/index/IndexReader.java
===================================================================
--- src/java/org/apache/lucene/index/IndexReader.java	(revision 759487)
+++ src/java/org/apache/lucene/index/IndexReader.java	(working copy)
@@ -1248,4 +1248,10 @@
   public IndexReader[] getSequentialSubReaders() {
     return null;
   }
+
+  /** Expert    
+   *  @deprecated */
+  public Object getFieldCacheWrapper() {
+    return this;
+  }
 }