Add preserve_original setting in ngram token filter

docs/reference/analysis/tokenfilters/ngram-tokenfilter.asciidoc

@@ -185,6 +185,10 @@ Maximum length of characters in a gram. Defaults to `2`.
 (Optional, integer)
 Minimum length of characters in a gram. Defaults to `1`.
 
+`preserve_original`::
+(Optional, boolean)
+Emits original token when set to `true`. Defaults to `false`.
+
 You can use the <<index-max-ngram-diff,`index.max_ngram_diff`>> index-level
 setting to control the maximum allowed difference between the `max_gram` and
 `min_gram` values.

modules/analysis-common/src/main/java/org/elasticsearch/analysis/common/NGramTokenFilterFactory.java

@@ -29,10 +29,10 @@
 
 
 public class NGramTokenFilterFactory extends AbstractTokenFilterFactory {
-
     private final int minGram;
-
     private final int maxGram;
+    private final boolean preserveOriginal;
+    private static final String PRESERVE_ORIG_KEY = "preserve_original";
 
     NGramTokenFilterFactory(IndexSettings indexSettings, Environment environment, String name, Settings settings) {
         super(indexSettings, name, settings);
@@ -46,12 +46,12 @@ public class NGramTokenFilterFactory extends AbstractTokenFilterFactory {
                     + maxAllowedNgramDiff + "] but was [" + ngramDiff + "]. This limit can be set by changing the ["
                     + IndexSettings.MAX_NGRAM_DIFF_SETTING.getKey() + "] index level setting.");
         }
+        preserveOriginal = settings.getAsBoolean(PRESERVE_ORIG_KEY, false);
     }
 
     @Override
     public TokenStream create(TokenStream tokenStream) {
-        // TODO: Expose preserveOriginal
-        return new NGramTokenFilter(tokenStream, minGram, maxGram, false);
+        return new NGramTokenFilter(tokenStream, minGram, maxGram, preserveOriginal);
     }
 
     @Override

modules/analysis-common/src/test/java/org/elasticsearch/analysis/common/NGramTokenFilterFactoryTests.java

@@ -0,0 +1,66 @@
+/*
+ * Licensed to Elasticsearch 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.analysis.common;
+
+import org.apache.lucene.analysis.Tokenizer;
+import org.apache.lucene.analysis.standard.StandardTokenizer;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.env.Environment;
+import org.elasticsearch.index.analysis.AnalysisTestsHelper;
+import org.elasticsearch.index.analysis.TokenFilterFactory;
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.test.ESTokenStreamTestCase;
+
+import java.io.IOException;
+import java.io.StringReader;
+
+public class NGramTokenFilterFactoryTests extends ESTokenStreamTestCase {
+
+    public void testDefault() throws IOException {
+        ESTestCase.TestAnalysis analysis = AnalysisTestsHelper.createTestAnalysisFromSettings(
+            Settings.builder()
+                .put(Environment.PATH_HOME_SETTING.getKey(), createTempDir().toString())
+                .put("index.analysis.filter.my_ngram.type", "ngram")
+                .build(),
+            new CommonAnalysisPlugin());
+        TokenFilterFactory tokenFilter = analysis.tokenFilter.get("my_ngram");
+        String source = "foo";
+        String[] expected = new String[]{"f", "fo", "o", "oo", "o"};
+        Tokenizer tokenizer = new StandardTokenizer();
+        tokenizer.setReader(new StringReader(source));
+        assertTokenStreamContents(tokenFilter.create(tokenizer), expected);
+    }
+
+    public void testPreserveOriginal() throws IOException {
+        ESTestCase.TestAnalysis analysis = AnalysisTestsHelper.createTestAnalysisFromSettings(
+            Settings.builder()
+                .put(Environment.PATH_HOME_SETTING.getKey(), createTempDir().toString())
+                .put("index.analysis.filter.my_ngram.type", "ngram")
+                .put("index.analysis.filter.my_ngram.preserve_original", true)
+                .build(),
+            new CommonAnalysisPlugin());
+        TokenFilterFactory tokenFilter = analysis.tokenFilter.get("my_ngram");
+        String source = "foo";
+        String[] expected = new String[]{"f", "fo", "o", "oo", "o", "foo"};
+        Tokenizer tokenizer = new StandardTokenizer();
+        tokenizer.setReader(new StringReader(source));
+        assertTokenStreamContents(tokenFilter.create(tokenizer), expected);
+    }
+}