Handle min_doc_freq in phrase suggester (#40840)

The phrase suggesters have an option to remove terms that have
a frequency lower than a provided min_doc_freq. However this value is
overwritten by the frequency of the original term in the popular mode.
This change ensures that we keep the maximum value between the provided
min_doc_value and the original term frequency as a threshold to select
candidates.

Fixes #16764

server/src/main/java/org/elasticsearch/search/suggest/phrase/DirectCandidateGenerator.java

@@ -132,31 +132,41 @@ public final class DirectCandidateGenerator extends CandidateGenerator {
     public CandidateSet drawCandidates(CandidateSet set) throws IOException {
         Candidate original = set.originalTerm;
         BytesRef term = preFilter(original.term, spare, byteSpare);
-        if (suggestMode != SuggestMode.SUGGEST_ALWAYS) {
+        float origThreshold = spellchecker.getThresholdFrequency();
-            /**
+        try {
-             * We use the {@link TermStats#docFreq} to compute the frequency threshold
+            if (suggestMode != SuggestMode.SUGGEST_ALWAYS) {
-             * because that's what {@link DirectSpellChecker#suggestSimilar} expects
+                /**
-             * when filtering terms.
+                 * We use the {@link TermStats#docFreq} to compute the frequency threshold
-             */
+                 * because that's what {@link DirectSpellChecker#suggestSimilar} expects
-            int threshold = thresholdTermFrequency(original.termStats.docFreq);
+                 * when filtering terms.
-            if (threshold == Integer.MAX_VALUE) {
+                 */
-                // the threshold is the max possible frequency so we can skip the search
+                int threshold = thresholdTermFrequency(original.termStats.docFreq);
-                return set;
+                if (threshold == Integer.MAX_VALUE) {
+                    // the threshold is the max possible frequency so we can skip the search
+                    return set;
+                }
+                // don't override the threshold if the provided min_doc_freq is greater
+                // than the original term frequency.
+                if (spellchecker.getThresholdFrequency() < threshold) {
+                    spellchecker.setThresholdFrequency(threshold);
+                }
             }
-            spellchecker.setThresholdFrequency(threshold);
-        }
 
-        SuggestWord[] suggestSimilar = spellchecker.suggestSimilar(new Term(field, term), numCandidates, reader, this.suggestMode);
+            SuggestWord[] suggestSimilar = spellchecker.suggestSimilar(new Term(field, term), numCandidates, reader, this.suggestMode);
-        List<Candidate> candidates = new ArrayList<>(suggestSimilar.length);
+            List<Candidate> candidates = new ArrayList<>(suggestSimilar.length);
-        for (int i = 0; i < suggestSimilar.length; i++) {
+            for (int i = 0; i < suggestSimilar.length; i++) {
-            SuggestWord suggestWord = suggestSimilar[i];
+                SuggestWord suggestWord = suggestSimilar[i];
-            BytesRef candidate = new BytesRef(suggestWord.string);
+                BytesRef candidate = new BytesRef(suggestWord.string);
-            TermStats termStats = internalTermStats(candidate);
+                TermStats termStats = internalTermStats(candidate);
-            postFilter(new Candidate(candidate, termStats,
+                postFilter(new Candidate(candidate, termStats,
-                suggestWord.score, score(termStats, suggestWord.score, sumTotalTermFreq), false), spare, byteSpare, candidates);
+                    suggestWord.score, score(termStats, suggestWord.score, sumTotalTermFreq), false), spare, byteSpare, candidates);
+            }
+            set.addCandidates(candidates);
+            return set;
+        } finally {
+            // restore the original value back
+            spellchecker.setThresholdFrequency(origThreshold);
         }
-        set.addCandidates(candidates);
-        return set;
     }
 
     protected BytesRef preFilter(final BytesRef term, final CharsRefBuilder spare, final BytesRefBuilder byteSpare) throws IOException {

server/src/test/java/org/elasticsearch/search/suggest/SuggestSearchIT.java

@@ -1005,6 +1005,52 @@ public class SuggestSearchIT extends ESIntegTestCase {
         assertSuggestion(searchSuggest, 0, "suggestion", "apple");
     }
 
+    public void testPhraseSuggestMinDocFreq() throws Exception {
+        XContentBuilder mapping = XContentFactory.jsonBuilder()
+            .startObject()
+                .startObject("type")
+                    .startObject("properties")
+                        .startObject("text")
+                            .field("type", "keyword")
+                        .endObject()
+                    .endObject()
+                .endObject()
+            .endObject();
+        assertAcked(prepareCreate("test")
+            .setSettings(Settings.builder().put("index.number_of_shards", 1).build())
+            .addMapping("type", mapping));
+
+        List<IndexRequestBuilder> builders = new ArrayList<>();
+        builders.add(client().prepareIndex("test", "type").setSource("text", "apple"));
+        builders.add(client().prepareIndex("test", "type").setSource("text", "apple"));
+        builders.add(client().prepareIndex("test", "type").setSource("text", "apple"));
+        builders.add(client().prepareIndex("test", "type").setSource("text", "appfle"));
+        indexRandom(true, false, builders);
+
+        PhraseSuggestionBuilder phraseSuggest = phraseSuggestion("text").text("appple")
+            .size(2)
+            .addCandidateGenerator(new DirectCandidateGeneratorBuilder("text")
+                .suggestMode("popular"));
+
+        Suggest searchSuggest = searchSuggest("suggestion", phraseSuggest);
+        assertSuggestion(searchSuggest, 0, "suggestion", 2, "apple", "appfle");
+
+        phraseSuggest = phraseSuggestion("text").text("appple")
+            .addCandidateGenerator(new DirectCandidateGeneratorBuilder("text")
+                .suggestMode("popular")
+                .minDocFreq(2));
+
+        searchSuggest = searchSuggest("suggestion", phraseSuggest);
+        assertSuggestion(searchSuggest, 0, "suggestion", 1,"apple");
+
+        phraseSuggest = phraseSuggestion("text").text("appple")
+            .addCandidateGenerator(new DirectCandidateGeneratorBuilder("text")
+                .suggestMode("popular")
+                .minDocFreq(2));
+        searchSuggest = searchSuggest("suggestion", phraseSuggest);
+        assertSuggestion(searchSuggest, 0, "suggestion", 1,"apple");
+    }
+
     @Override
     protected Collection<Class<? extends Plugin>> nodePlugins() {
         return Collections.singleton(DummyTemplatePlugin.class);