Added support for random_score function:

 * can be used to return matching results in random order

 Closes #1170

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

@@ -0,0 +1,99 @@
+/*
+ * Licensed to ElasticSearch and Shay Banon under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. ElasticSearch licenses this
+ * file to you 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 org.elasticsearch.common.lucene.search.function;
+
+import org.apache.lucene.index.AtomicReaderContext;
+import org.apache.lucene.search.Explanation;
+
+/**
+ *
+ */
+public class RandomScoreFunction implements ScoreFunction {
+
+    private final PRNG prng;
+    private int docBase;
+
+    public RandomScoreFunction(long seed) {
+        this.prng = new PRNG(seed);
+    }
+
+    @Override
+    public void setNextReader(AtomicReaderContext context) {
+        this.docBase = context.docBase;
+    }
+
+    @Override
+    public double score(int docId, float subQueryScore) {
+        return prng.random(docBase + docId);
+    }
+
+    @Override
+    public double factor(int docId) {
+        return prng.seed;
+    }
+
+    @Override
+    public Explanation explainScore(int docId, Explanation subQueryExpl) {
+        Explanation exp = new Explanation();
+        exp.setDescription("random score function (seed: " + prng.originalSeed + ")");
+        exp.addDetail(subQueryExpl);
+        return exp;
+    }
+
+    @Override
+    public Explanation explainFactor(int docId) {
+        Explanation exp = new Explanation();
+        exp.setDescription("seed: " + prng.originalSeed + ")");
+        return exp;
+    }
+
+    /**
+     * Algorithm based on {@link java.util.Random} except this one is not
+     * thread safe
+     */
+    static class PRNG {
+
+        private static final long multiplier = 0x5DEECE66DL;
+        private static final long addend = 0xBL;
+        private static final long mask = (1L << 48) - 1;
+
+        final long originalSeed;
+        long seed;
+
+        PRNG(long seed) {
+            this.originalSeed = seed;
+            this.seed = (seed ^ multiplier) & mask;
+        }
+
+        public float random(int doc) {
+            if (doc == 0) {
+                doc = -17;
+            }
+            return nextFloat() * (doc ^ 0xCAFEBAB);
+        }
+
+        public float nextFloat() {
+            seed = (seed * multiplier + addend) & mask;
+            int r = (int)(seed >>> 24);
+            return r / ((float)(1 << 24));
+        }
+
+    }
+}

src/main/java/org/elasticsearch/index/query/QueryBuilders.java

@@ -545,7 +545,6 @@ public static CustomFiltersScoreQueryBuilder customFiltersScoreQuery(QueryBuilde
      * A query that allows to define a custom scoring function.
      *
      * @param queryBuilder The query to custom score
-     * @param scoreFunctionBuilder The score function used to re-score the query
      */
     public static FunctionScoreQueryBuilder functionScoreQuery(QueryBuilder queryBuilder) {
         return new FunctionScoreQueryBuilder(queryBuilder);
@@ -555,7 +554,6 @@ public static FunctionScoreQueryBuilder functionScoreQuery(QueryBuilder queryBui
      * A query that allows to define a custom scoring function.
      *
      * @param filterBuilder The query to custom score
-     * @param scoreFunctionBuilder The score function used to re-score the query
      */
     public static FunctionScoreQueryBuilder functionScoreQuery(FilterBuilder filterBuilder) {
         return new FunctionScoreQueryBuilder(filterBuilder);

src/main/java/org/elasticsearch/index/query/functionscore/FunctionScoreModule.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.index.query.functionscore.factor.FactorParser;
 import org.elasticsearch.index.query.functionscore.gauss.GaussDecayFunctionParser;
 import org.elasticsearch.index.query.functionscore.lin.LinearDecayFunctionParser;
+import org.elasticsearch.index.query.functionscore.random.RandomScoreFunctionParser;
 import org.elasticsearch.index.query.functionscore.script.ScriptScoreFunctionParser;
 
 import java.util.List;
@@ -42,6 +43,7 @@ public FunctionScoreModule() {
         registerParser(GaussDecayFunctionParser.class);
         registerParser(LinearDecayFunctionParser.class);
         registerParser(ExponentialDecayFunctionParser.class);
+        registerParser(RandomScoreFunctionParser.class);
     }
 
     public void registerParser(Class<? extends ScoreFunctionParser> parser) {

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

@@ -0,0 +1,62 @@
+/*
+ * Licensed to ElasticSearch and Shay Banon under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. ElasticSearch licenses this
+ * file to you 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 org.elasticsearch.index.query.functionscore.random;
+
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.index.query.functionscore.ScoreFunctionBuilder;
+
+import java.io.IOException;
+
+/**
+ * A function that computes a random score for the matched documents
+ */
+public class RandomScoreFunctionBuilder implements ScoreFunctionBuilder {
+
+    private Long seed = null;
+
+    public RandomScoreFunctionBuilder() {
+    }
+
+    @Override
+    public String getName() {
+        return RandomScoreFunctionParser.NAMES[0];
+    }
+
+    /**
+     * Sets the seed based on which the random number will be generated. Using the same seed is guaranteed to generate the same
+     * random number for a specific doc.
+     *
+     * @param seed The seed.
+     */
+    public RandomScoreFunctionBuilder seed(long seed) {
+        this.seed = seed;
+        return this;
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject(getName());
+        if (seed != null) {
+            builder.field("seed", seed.longValue());
+        }
+        return builder.endObject();
+    }
+
+}

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

@@ -0,0 +1,78 @@
+/*
+ * Licensed to ElasticSearch and Shay Banon under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. ElasticSearch licenses this
+ * file to you 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 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 java.io.IOException;
+import java.util.Map;
+
+/**
+ *
+ */
+public class RandomScoreFunctionParser implements ScoreFunctionParser {
+
+    public static String[] NAMES = { "random_score", "randomScore" };
+
+    @Inject
+    public RandomScoreFunctionParser() {
+    }
+
+    @Override
+    public String[] getNames() {
+        return NAMES;
+    }
+
+    @Override
+    public ScoreFunction parse(QueryParseContext parseContext, XContentParser parser) throws IOException, QueryParsingException {
+
+        long seed = -1;
+
+        String currentFieldName = null;
+        XContentParser.Token token;
+        while ((token = parser.nextToken()) != XContentParser.Token.END_OBJECT) {
+            if (token == XContentParser.Token.FIELD_NAME) {
+                currentFieldName = parser.currentName();
+            } else if (token.isValue()) {
+                if ("seed".equals(currentFieldName)) {
+                    seed = parser.longValue();
+                } else {
+                    throw new QueryParsingException(parseContext.index(), NAMES[0] + " query does not support [" + currentFieldName + "]");
+                }
+            }
+        }
+
+        if (seed == -1) {
+            seed = parseContext.nowInMillis();
+        }
+
+        return new RandomScoreFunction(seed);
+    }
+}

src/main/java/org/elasticsearch/index/query/functionscore/script/ScriptScoreFunctionBuilder.java

@@ -28,7 +28,7 @@
 import java.util.Map;
 
 /**
- * A query that uses a script to compute or influence the score of documents
+ * A function that uses a script to compute or influence the score of documents
  * that match with the inner query or filter.
  */
 public class ScriptScoreFunctionBuilder implements ScoreFunctionBuilder {