random_score function - Added the index name and shard id to the randomization, and improved the PRNG itself

Closes #3559

src/main/java/org/elasticsearch/common/lucene/search/function/RandomScoreFunction.java

@@ -64,9 +64,10 @@ public class RandomScoreFunction implements ScoreFunction {
         return exp;
     }
 
+
     /**
-     * Algorithm based on {@link java.util.Random} except this one is not
-     * thread safe
+     * Algorithm largely based on {@link java.util.Random} except this one is not
+     * thread safe and it incorporates the doc id on next();
      */
     static class PRNG {
 
@@ -84,15 +85,20 @@ public class RandomScoreFunction implements ScoreFunction {
 
         public float random(int doc) {
             if (doc == 0) {
-                doc = -17;
-            }
-            return nextFloat() * (doc ^ 0xCAFEBAB);
+                doc = 0xCAFEBAB;
             }
 
-        public float nextFloat() {
+            long rand = doc;
+            rand |= rand << 32;
+            rand ^= rand;
+            return nextFloat(rand);
+        }
+
+        public float nextFloat(long rand) {
             seed = (seed * multiplier + addend) & mask;
-            int r = (int)(seed >>> 24);
-            return r / ((float)(1 << 24));
+            rand ^= seed;
+            double result = rand / (double)(1L << 54);
+            return (float) result;
         }
 
     }

src/main/java/org/elasticsearch/index/query/functionscore/random/RandomScoreFunctionParser.java

@@ -24,15 +24,14 @@ package org.elasticsearch.index.query.functionscore.random;
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.lucene.search.function.RandomScoreFunction;
 import org.elasticsearch.common.lucene.search.function.ScoreFunction;
-import org.elasticsearch.common.lucene.search.function.ScriptScoreFunction;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.index.query.QueryParseContext;
 import org.elasticsearch.index.query.QueryParsingException;
 import org.elasticsearch.index.query.functionscore.ScoreFunctionParser;
-import org.elasticsearch.script.SearchScript;
+import org.elasticsearch.index.shard.ShardId;
+import org.elasticsearch.search.internal.SearchContext;
 
 import java.io.IOException;
-import java.util.Map;
 
 /**
  *
@@ -73,6 +72,17 @@ public class RandomScoreFunctionParser implements ScoreFunctionParser {
             seed = parseContext.nowInMillis();
         }
 
+        ShardId shardId = SearchContext.current().indexShard().shardId();
+        seed = salt(seed, shardId.index().name(), shardId.id());
+
         return new RandomScoreFunction(seed);
     }
+
+    public static long salt(long seed, String index, int shardId) {
+        long salt = index.hashCode();
+        salt = salt << 32;
+        salt |= shardId;
+        return salt^seed;
+    }
+
 }

src/test/java/org/elasticsearch/test/integration/search/functionscore/RandomScoreFunctionTests.java

@@ -107,15 +107,19 @@ public class RandomScoreFunctionTests extends AbstractSharedClusterTest {
 
         int filled = 0;
         int maxRepeat = 0;
+        int sumRepeat = 0;
         for (int i = 0; i < matrix.length; i++) {
             int value = matrix[i];
+            sumRepeat += value;
             maxRepeat = Math.max(maxRepeat, value);
             if (value > 0) {
                 filled++;
             }
         }
+
         System.out.println();
         System.out.println("max repeat: " +  maxRepeat);
+        System.out.println("avg repeat: " +  sumRepeat / (double)filled);
         System.out.println("distribution: " +  filled/(double)count);
 
         int percentile50 = filled / 2;