Merge -r 540341:540342 from trunk to 0.13 branch. Fixes: HADOOP-1385.

git-svn-id: https://svn.apache.org/repos/asf/lucene/hadoop/branches/branch-0.13@540344 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -407,6 +407,9 @@ Branch 0.13 (unreleased changes)
 122. HADOOP-1388.  Fix a potential NullPointerException in web ui.
      (Devaraj Das via cutting)
 
+123. HADOOP-1385.  Fix MD5Hash#hashCode() to generally hash to more
+     than 256 values.  (omalley via cutting)
+
 
 Release 0.12.3 - 2007-04-06
 

src/java/org/apache/hadoop/io/MD5Hash.java

@@ -116,6 +116,17 @@ public class MD5Hash implements WritableComparable {
     return value;
   }
 
+  /**
+   * Return a 32-bit digest of the MD5.
+   * @return the first 4 bytes of the md5
+   */
+  public int quarterDigest() {
+    int value = 0;
+    for (int i = 0; i < 4; i++)
+      value |= ((digest[i] & 0xff) << (8*(3-i)));
+    return value;    
+  }
+
   /** Returns true iff <code>o</code> is an MD5Hash whose digest contains the
    * same values.  */
   public boolean equals(Object o) {
@@ -125,13 +136,11 @@ public class MD5Hash implements WritableComparable {
     return Arrays.equals(this.digest, other.digest);
   }
 
-  /** Returns a hash code value for this object.*/
+  /** Returns a hash code value for this object.
+   * Only uses the first 4 bytes, since md5s are evenly distributed.
+   */
   public int hashCode() {
-    return                                        // xor four ints
-      (digest[ 0] | (digest[ 1]<<8) | (digest[ 2]<<16) | (digest[ 3]<<24)) ^
-      (digest[ 4] | (digest[ 5]<<8) | (digest[ 6]<<16) | (digest[ 7]<<24)) ^
-      (digest[ 8] | (digest[ 9]<<8) | (digest[10]<<16) | (digest[11]<<24)) ^
-      (digest[12] | (digest[13]<<8) | (digest[14]<<16) | (digest[15]<<24));
+    return quarterDigest();
   }
 
 

src/test/org/apache/hadoop/io/TestMD5Hash.java

@@ -46,6 +46,16 @@ public class TestMD5Hash extends TestCase {
     MD5Hash md5HashFF
       = new MD5Hash(new byte[] {-1, -1, -1, -1, -1, -1, -1, -1,
                                 -1, -1, -1, -1, -1, -1, -1, -1});
+    
+    MD5Hash orderedHash = new MD5Hash(new byte[]{1,2,3,4,5,6,7,8,9,10,11,12,
+                                                 13,14,15,16});
+    MD5Hash backwardHash = new MD5Hash(new byte[]{-1,-2,-3,-4,-5,-6,-7,-8,
+                                                  -9,-10,-11,-12, -13, -14,
+                                                  -15,-16});
+    MD5Hash closeHash1 = new MD5Hash(new byte[]{-1,0,0,0,0,0,0,0,
+                                                0,0,0,0,0,0,0,0});
+    MD5Hash closeHash2 = new MD5Hash(new byte[]{-1,1,0,0,0,0,0,0,
+                                                0,0,0,0,0,0,0,0});
 
     // test i/o
     TestWritable.testWritable(md5Hash);
@@ -67,6 +77,13 @@ public class TestMD5Hash extends TestCase {
     assertEquals(md5Hash00, new MD5Hash(md5Hash00.toString()));
     assertEquals(md5HashFF, new MD5Hash(md5HashFF.toString()));
 
+    assertEquals(0x01020304, orderedHash.quarterDigest());
+    assertEquals(0xfffefdfc, backwardHash.quarterDigest());
+    
+    assertEquals(0x0102030405060708L, orderedHash.halfDigest());
+    assertEquals(0xfffefdfcfbfaf9f8L, backwardHash.halfDigest());
+    assertTrue("hash collision", 
+               closeHash1.hashCode() != closeHash2.hashCode());
   }
 	
 }