diff --git a/core/src/main/groovy/com/muwire/core/download/Pieces.groovy b/core/src/main/groovy/com/muwire/core/download/Pieces.groovy
new file mode 100644
index 00000000..4692525d
--- /dev/null
+++ b/core/src/main/groovy/com/muwire/core/download/Pieces.groovy
@@ -0,0 +1,30 @@
+package com.muwire.core.download
+
+class Pieces {
+    private final BitSet bitSet
+    private final int nPieces
+    private final Random random = new Random()
+    
+    Pieces(int nPieces) {
+        this.nPieces = nPieces
+        bitSet = new BitSet(nPieces)
+    }
+    
+    synchronized int getRandomPiece() {
+        if (isComplete())
+            return -1
+        while(true) {
+            int start = random.nextInt(nPieces)
+            while(bitSet.get(start) && ++start < nPieces);
+            return start
+        }
+    }
+    
+    synchronized void markDownloaded(int piece) {
+        bitSet.set(piece)
+    }
+    
+    synchronized boolean isComplete() {
+        bitSet.cardinality() == nPieces
+    }
+}
diff --git a/core/src/test/groovy/com/muwire/core/download/PiecesTest.groovy b/core/src/test/groovy/com/muwire/core/download/PiecesTest.groovy
new file mode 100644
index 00000000..7dc31b78
--- /dev/null
+++ b/core/src/test/groovy/com/muwire/core/download/PiecesTest.groovy
@@ -0,0 +1,23 @@
+package com.muwire.core.download
+
+import org.junit.Test
+
+class PiecesTest {
+    
+    Pieces pieces
+    
+    @Test
+    public void testEmpty() {
+        pieces = new Pieces(20)
+        assert !pieces.isComplete()
+    }
+    
+    @Test
+    public void testSinglePiece() {
+        pieces = new Pieces(1)
+        assert !pieces.isComplete()
+        assert pieces.getRandomPiece() == 0
+        pieces.markDownloaded(0)
+        assert pieces.isComplete()
+    }
+}