download pieces sequentially if over a threshold

core/src/main/groovy/com/muwire/core/download/Pieces.groovy

@@ -3,16 +3,29 @@ package com.muwire.core.download
 class Pieces {
     private final BitSet bitSet
     private final int nPieces
+    private final float ratio
     private final Random random = new Random()
     
     Pieces(int nPieces) {
+        this(nPieces, 1.0f)
+    }
+    
+    Pieces(int nPieces, float ratio) {
         this.nPieces = nPieces
+        this.ratio = ratio
         bitSet = new BitSet(nPieces)
     }
     
     synchronized int getRandomPiece() {
-        if (isComplete())
+        int cardinality = bitSet.cardinality()
+        if (cardinality == nPieces)
             return -1
+        
+        // if fuller than ratio just do sequential
+        if ( (1.0f * cardinality) / nPieces > ratio) {
+            return bitSet.nextClearBit(0)
+        }
+        
         while(true) {
             int start = random.nextInt(nPieces)
             while(bitSet.get(start) && ++start < nPieces);