Add BoundedOverheadBlockStore. This is suitable for usage on constrained devices like Android phones that cannot afford to store the chain headers in RAM.

On a MacBook pro with this store, the chain can be downloaded at about 2000 blocks per second. Difficulty transitions are not optimized but take around 50 msec. On a Google Nexus S the chain can be downloaded (via wifi) at 200 blocks per second and difficulty traversals take 2 seconds. At this time the profiles on both devices are dominated by verification costs, not storage.

tests/com/google/bitcoin/core/BoundedOverheadBlockStoreTest.java

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+/**
+ * Copyright 2011 Google Inc.
+ *
+ * Licensed 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.
+ */
+package com.google.bitcoin.core;
+
+import org.junit.Test;
+import java.io.File;
+import static org.junit.Assert.assertEquals;
+
+public class BoundedOverheadBlockStoreTest {
+    @Test
+    public void testStorage() throws Exception {
+        File temp = File.createTempFile("bitcoinj-test", null, null);
+        System.out.println(temp.getAbsolutePath());
+        temp.deleteOnExit();
+
+        NetworkParameters params = NetworkParameters.unitTests();
+        Address to = new ECKey().toAddress(params);
+        BoundedOverheadBlockStore store = new BoundedOverheadBlockStore(params, temp);
+        // Check the first block in a new store is the genesis block.
+        StoredBlock genesis = store.getChainHead();
+        assertEquals(params.genesisBlock, genesis.getHeader());
+
+        // Build a new block.
+        StoredBlock b1 = genesis.build(genesis.getHeader().createNextBlock(to).cloneAsHeader());
+        store.put(b1);
+        store.setChainHead(b1);
+        // Check we can get it back out again if we rebuild the store object.
+        store = new BoundedOverheadBlockStore(params, temp);
+        StoredBlock b2 = store.get(b1.getHeader().getHash());
+        assertEquals(b1, b2);
+        // Check the chain head was stored correctly also.
+        assertEquals(b1, store.getChainHead());
+    }
+}