add three level hashpath to reduce path depth

src/main/java/bio/guoda/preston/cmd/CmdCopyTo.java

@@ -83,8 +83,8 @@ public void on(Triple statement) {
             }
         }
         stopWatch.stop();
-        System.out.println("\tcopying [100%].");
-        System.out.println("copying [" + IRIStrings.size() + "] datasets from [" + source.getAbsolutePath() + "] to [" + targetDir + "] completed in [" + stopWatch.getTime(TimeUnit.MINUTES) + "] min.");
+        System.out.println("\tcopying done.");
+        System.out.println("Copied [" + IRIStrings.size() + "] datasets from [" + source.getAbsolutePath() + "] to [" + targetDir + "] in [" + stopWatch.getTime(TimeUnit.MINUTES) + "] minutes.");
     }
 
     static String formatProgress(long i, long total) {

src/main/java/bio/guoda/preston/store/HashKeyUtil.java

@@ -0,0 +1,14 @@
+package bio.guoda.preston.store;
+
+import bio.guoda.preston.Hasher;
+import org.apache.commons.lang3.StringUtils;
+
+public class HashKeyUtil {
+    public static void validateHashKey(String hashKey) {
+        int offset = Hasher.getHashPrefix().length();
+        int expectedLength = 8 + offset;
+        if (StringUtils.length(hashKey) < expectedLength) {
+            throw new IllegalArgumentException("expected id [" + hashKey + "] of at least [" + expectedLength + "] characters, instead got [" + hashKey.length() + "] characters.");
+        }
+    }
+}

src/main/java/bio/guoda/preston/store/KeyTo3LevelPath.java

@@ -0,0 +1,19 @@
+package bio.guoda.preston.store;
+
+import bio.guoda.preston.Hasher;
+import org.apache.commons.lang3.StringUtils;
+
+import java.util.Arrays;
+
+public class KeyTo3LevelPath implements KeyToPath {
+    @Override
+    public String toPath(String key) {
+        HashKeyUtil.validateHashKey(key);
+
+        int offset = Hasher.getHashPrefix().length();
+        String u0 = key.substring(offset + 0, offset + 2);
+        String u1 = key.substring(offset + 2, offset + 4);
+        return StringUtils.join(Arrays.asList(u0, u1, key.substring(offset)), "/");
+    }
+
+}

src/main/java/bio/guoda/preston/store/KeyTo5LevelPath.java

@@ -8,11 +8,9 @@
 public class KeyTo5LevelPath implements KeyToPath {
     @Override
     public String toPath(String key) {
+        HashKeyUtil.validateHashKey(key);
+
         int offset = Hasher.getHashPrefix().length();
-        int expectedLength = 8 + offset;
-        if (StringUtils.length(key) < expectedLength) {
-            throw new IllegalArgumentException("expected id [" + key + "] of at least [" + expectedLength + "] characters");
-        }
         String u0 = key.substring(offset + 0, offset + 2);
         String u1 = key.substring(offset + 2, offset + 4);
         String u2 = key.substring(offset + 4, offset + 6);

src/test/java/bio/guoda/preston/store/KeyTo3LevelPathTest.java

@@ -0,0 +1,27 @@
+package bio.guoda.preston.store;
+
+import bio.guoda.preston.Hasher;
+import org.apache.commons.rdf.api.IRI;
+import org.hamcrest.core.Is;
+import org.junit.Test;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+
+public class KeyTo3LevelPathTest {
+
+    @Test
+    public void toPath() {
+        IRI hash = Hasher.calcSHA256("bla");
+        assertThat(hash.getIRIString(), is("hash://sha256/4df3c3f68fcc83b27e9d42c90431a72499f17875c81a599b566c9889b9696703"));
+
+        String actualPath = new KeyTo3LevelPath().toPath(hash.getIRIString());
+        assertThat(actualPath, Is.is("4d/f3/4df3c3f68fcc83b27e9d42c90431a72499f17875c81a599b566c9889b9696703"));
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void toPathTooShort() {
+        new KeyTo3LevelPath().toPath("too:short");
+    }
+
+}