fix naming in function_score

- "boost" should be "boost_factor"
    - "mult" should be "multiply"

Also, store combine function names in ImmutableMap instead of iterating
over all possible names each time.

closes #3872 for master

docs/reference/query-dsl/queries/function-score-query.asciidoc

@@ -448,7 +448,7 @@ and the <<query-dsl-custom-filters-score-query>>
 "custom_filters_score": {
     "filters": [
         {
-            "boost": "3",
+            "boost_factor": "3",
             "filter": {...}
         },
         {
@@ -472,7 +472,7 @@ becomes:
 "function_score": {
     "functions": [
         {
-            "boost": "3",
+            "boost_factor": "3",
             "filter": {...}
         },
         {

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

@@ -31,7 +31,7 @@ public enum CombineFunction {
 
         @Override
         public String getName() {
-            return "mult";
+            return "multiply";
         }
 
         @Override

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

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.index.query.functionscore;
 
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableMap.Builder;
 import org.apache.lucene.search.Filter;
 import org.apache.lucene.search.Query;
 import org.elasticsearch.common.Strings;
@@ -54,6 +56,17 @@ public class FunctionScoreQueryParser implements QueryParser {
     public String[] names() {
         return new String[] { NAME, Strings.toCamelCase(NAME) };
     }
+    
+    private static final ImmutableMap<String, CombineFunction> combineFunctionsMap;
+
+    static {
+        CombineFunction[] values = CombineFunction.values();
+        Builder<String, CombineFunction> combineFunctionMapBuilder = ImmutableMap.<String, CombineFunction>builder();
+        for (CombineFunction combineFunction : values) {
+            combineFunctionMapBuilder.put(combineFunction.getName(), combineFunction);
+        }
+        combineFunctionsMap = combineFunctionMapBuilder.build();
+    }
 
     @Override
     public Query parse(QueryParseContext parseContext) throws IOException, QueryParsingException {
@@ -179,11 +192,10 @@ public class FunctionScoreQueryParser implements QueryParser {
 
     private CombineFunction parseBoostMode(QueryParseContext parseContext, XContentParser parser) throws IOException {
         String boostMode = parser.text();
-        for (CombineFunction cf : CombineFunction.values()) {
-            if (cf.getName().equals(boostMode)) {
-                return cf;
-            }
+        CombineFunction cf = combineFunctionsMap.get(boostMode);
+        if (cf == null) {
+            throw new QueryParsingException(parseContext.index(), NAME + " illegal boost_mode [" + boostMode + "]");
         }
-        throw new QueryParsingException(parseContext.index(), NAME + " illegal boost_mode [" + boostMode + "]");
+        return cf;
     }
 }