add midpoint method to IPartitioner. patch by Stu Hood; reviewed by j…

…bellis for CASSANDRA-242

git-svn-id: https://svn.apache.org/repos/asf/incubator/cassandra/trunk@808206 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/cassandra/dht/CollatingOrderPreservingPartitioner.java

@@ -71,6 +71,84 @@ public Comparator<String> getReverseDecoratedKeyComparator()
         return reverseComparator;
     }
 
+    /**
+     * @return A new byte array that will compare (via compareByteArrays)
+     * approximately halfway between the parameters.
+     */
+    private static byte[] midpoint(byte[] lbytes, byte[] rbytes)
+    {
+        // pad the arrays to equal length, for convenience
+        int inlength;
+        int comparison = FBUtilities.compareByteArrays(lbytes, rbytes);
+        if (comparison < 0)
+        {
+            inlength = Math.max(lbytes.length, rbytes.length);
+            if (lbytes.length < inlength)
+                lbytes = Arrays.copyOf(lbytes, inlength);
+            else if (rbytes.length < inlength)
+                rbytes = Arrays.copyOf(rbytes, inlength);
+        }
+        else
+        {
+            // wrapping range must involve the minimum token
+            assert FBUtilities.isEqualBits(MINIMUM.token, rbytes);
+
+            inlength = Math.max(lbytes.length, 1);
+            if (lbytes.length < inlength)
+                lbytes = Arrays.copyOf(lbytes, inlength);
+            rbytes = new byte[inlength];
+            Arrays.fill(rbytes, (byte)0xFF);
+        }
+
+        // if the lsbits of the two inputs are not equal we have to extend
+        // the result array to make room for a carried bit during the right shift
+        int outlength = (((int)lbytes[inlength-1] & 0x01) == ((int)rbytes[inlength-1] & 0x01))
+                        ? inlength
+                        : inlength+1;
+        byte[] result = new byte[outlength];
+        boolean carrying = false;
+
+        // perform the addition
+        for (int i = inlength-1; i >= 0; i--)
+        {
+            // initialize the lsbit if we're carrying
+            int sum = carrying ? 1 : 0;
+
+            // remove the sign bit, and sum left and right
+            sum += (lbytes[i] & 0xFF) + (rbytes[i] & 0xFF);
+
+            // see if we'll need to carry
+            carrying = sum > 0xFF;
+
+            // set to the sum (truncating the msbit)
+            result[i] = (byte)sum;
+        }
+        // the carried bit from addition will be shifted in as the msbit
+
+        // perform the division (as a right shift)
+        for (int i = 0; i < inlength; i++)
+        {
+            // initialize the msbit if we're carrying
+            byte shifted = (byte)(carrying ? 0x80 : 0x00);
+
+            // check the lsbit to see if we'll need to continue carrying
+            carrying = (result[i] & 0x01) == 0x01;
+
+            // OR the right shifted value into the result byte
+            result[i] = (byte)(shifted | ((result[i] & 0xFF) >>> 1));
+        }
+
+        if (carrying)
+            // the last byte in the result array
+            result[inlength] |= 0x80;
+        return result;
+    }
+
+    public BytesToken midpoint(BytesToken ltoken, BytesToken rtoken)
+    {
+        return new BytesToken(midpoint(ltoken.token, rtoken.token));
+    }
+
     public BytesToken getMinimumToken()
     {
         return MINIMUM;

src/java/org/apache/cassandra/dht/IPartitioner.java

@@ -37,6 +37,18 @@ public interface IPartitioner<T extends Token>
 
     public Comparator<String> getReverseDecoratedKeyComparator();
 
+    /**
+     * Calculate a Token representing the approximate "middle" of the given
+     * range.
+	 *
+	 * The Tokens must have been generated by previous calls to midpoint,
+	 * or be equal to this.getMinimumToken(). The range may not wrap unless it
+	 * involves this.getMinimumToken().
+     *
+     * @return The approximate midpoint between left and right.
+     */
+    public T midpoint(T left, T right);
+
 	/**
 	 * @return The minimum possible Token in the range that is being partitioned.
 	 */

src/java/org/apache/cassandra/dht/OrderPreservingPartitioner.java

@@ -65,6 +65,104 @@ public Comparator<String> getReverseDecoratedKeyComparator()
         return reverseComparator;
     }
 
+    /**
+     * Copies the given string into a char array, padding the end
+     * with empty chars up to length.
+     */
+    private static char[] getChars(String str, int length)
+    {
+        char[] chars;
+        if (str.length() < length)
+        {
+            chars = new char[length];
+            str.getChars(0, str.length(), chars, 0);
+        }
+        else if (str.length() == length)
+        {
+            chars = str.toCharArray();
+        }
+        else
+            throw new RuntimeException("Cannot truncate string of length " + str.length() + " to length " + length);
+        return chars;
+    }
+
+    /**
+     * @return A new String array that will compare
+     * approximately halfway between the parameters.
+     */
+    private static String midpoint(String left, String right)
+    {
+        int inlength;
+        char[] lchars;
+        char[] rchars;
+        int comparison = left.compareTo(right);
+        if (comparison < 0)
+        {
+            inlength = Math.max(left.length(), right.length());
+            lchars = getChars(left, inlength);
+            rchars = getChars(right, inlength);
+        }
+        else
+        {
+            // wrapping range must involve the minimum token
+            assert MINIMUM.token.equals(right);
+
+            inlength = Math.max(left.length(), 1);
+            lchars = getChars(left, inlength);
+            rchars = new char[inlength];
+            Arrays.fill(rchars, (char)0xFFFF);
+        }
+
+
+        // if the lsbits of the two inputs are not equal we have to extend
+        // the result array to make room for a carried bit during the right shift
+        int outlength = (((int)lchars[inlength-1] & 0x0001) == ((int)rchars[inlength-1] & 0x0001))
+                        ? inlength
+                        : inlength+1;
+        char[] result = new char[outlength];
+        boolean carrying = false;
+
+        // perform the addition
+        for (int i = inlength-1; i >= 0; i--)
+        {
+            // initialize the lsbit if we're carrying
+            int sum = carrying ? 0x0001 : 0x0000;
+
+            // remove the sign bit, and sum left and right
+            sum += (lchars[i] & 0xFFFF) + (rchars[i] & 0xFFFF);
+
+            // see if we'll need to carry
+            carrying = sum > 0xFFFF;
+
+            // set to the sum (truncating the msbit)
+            result[i] = (char)sum;
+        }
+        // the carried bit from addition will be shifted in as the msbit
+
+        // perform the division (as a right shift)
+        for (int i = 0; i < inlength; i++)
+        {
+            // initialize the msbit if we're carrying
+            char shifted = (char)(carrying ? 0x8000 : 0x0000);
+
+            // check the lsbit to see if we'll need to continue carrying
+            carrying = (result[i] & 0x0001) == 0x0001;
+
+            // OR the right shifted value into the result char
+            result[i] = (char)(shifted | ((result[i] & 0xFFFF) >>> 1));
+        }
+
+        if (carrying)
+            // the last char in the result array
+            result[inlength] |= 0x8000;
+        return new String(result);
+    }
+
+    public StringToken midpoint(StringToken ltoken, StringToken rtoken)
+    {
+        return new StringToken(midpoint(ltoken.token, rtoken.token));
+    }
+
     public StringToken getMinimumToken()
     {
         return MINIMUM;

src/java/org/apache/cassandra/dht/RandomPartitioner.java

@@ -31,6 +31,9 @@
  */
 public class RandomPartitioner implements IPartitioner<BigIntegerToken>
 {
+    public static final BigInteger TWO = new BigInteger("2");
+    public static final BigInteger MD5_MAX = TWO.pow(127);
+
     public static final BigIntegerToken MINIMUM = new BigIntegerToken("0");
 
     private static final Comparator<String> comparator = new Comparator<String>()
@@ -81,6 +84,26 @@ public Comparator<String> getReverseDecoratedKeyComparator()
         return rcomparator;
     }
 
+    public BigIntegerToken midpoint(BigIntegerToken ltoken, BigIntegerToken rtoken)
+    {
+        BigInteger left = ltoken.token;
+        BigInteger right = rtoken.token;
+
+        BigInteger midpoint;
+        if (left.compareTo(right) < 0)
+        {
+            midpoint = left.add(right).divide(TWO);
+        }
+        else
+        {
+            // wrapping case
+            BigInteger distance = MD5_MAX.add(right).subtract(left);
+            BigInteger unchecked = distance.divide(TWO).add(left);
+            midpoint = (unchecked.compareTo(MD5_MAX) > 0) ? unchecked.subtract(MD5_MAX) : unchecked;
+        }
+        return new BigIntegerToken(midpoint);
+    }
+
 	public BigIntegerToken getMinimumToken()
     {
         return MINIMUM;

test/conf/storage-conf.xml

@@ -22,7 +22,7 @@
    <FlushIndexBufferSizeInMB>0.1</FlushIndexBufferSizeInMB>
    <CommitLogSync>batch</CommitLogSync>
    <CommitLogSyncBatchWindowInMS>1.0</CommitLogSyncBatchWindowInMS>
-   <Partitioner>org.apache.cassandra.dht.OrderPreservingPartitioner</Partitioner>
+   <Partitioner>org.apache.cassandra.dht.CollatingOrderPreservingPartitioner</Partitioner>
    <EndPointSnitch>org.apache.cassandra.locator.EndPointSnitch</EndPointSnitch>
    <ReplicaPlacementStrategy>org.apache.cassandra.locator.RackUnawareStrategy</ReplicaPlacementStrategy>
    <ReplicationFactor>1</ReplicationFactor>

test/unit/org/apache/cassandra/dht/PartitionerTestCase.java

@@ -50,6 +50,38 @@ public void testCompare()
         assert tok("asdz").compareTo(tok("asdf")) > 0;
     }
 
+    public void assertMidpoint(T left, T right, int depth)
+    {
+        T mid = this.part.midpoint(left, right);
+        assert new Range(left, right).contains(mid)
+                : "For " + tos(left) + "," + tos(right) + ": range did not contain mid:" + tos(mid);
+        if (depth > 0)
+            assertMidpoint(left, mid, depth-1);
+        if (depth > 0)
+            assertMidpoint(mid, right, depth-1);
+    }
+
+    @Test
+    public void testMidpoint()
+    {
+        assertMidpoint(tok("a"), tok("b"), 16);
+        assertMidpoint(tok("a"), tok("bbb"), 16);
+    }
+
+    @Test
+    public void testMidpointMinimum()
+    {
+        assertMidpoint(tok(""), tok("a"), 16);
+        assertMidpoint(tok(""), tok("aaa"), 16);
+    }
+
+    @Test
+    public void testMidpointWrapping()
+    {
+        assertMidpoint(tok(""), tok(""), 16);
+        assertMidpoint(tok("a"), tok(""), 16);
+    }
+
     @Test
     public void testTokenFactoryBytes()
     {