avoid allocation in stop token filter

the token stream resulting from the removal of stop words must
be shorter or the same length as the original, so we just
reuse it and truncate it at the end.

analysis/token_filters/stop_tokens_filter/stop_tokens_filter.go

@@ -36,16 +36,16 @@ func NewStopTokensFilter(stopTokens analysis.TokenMap) *StopTokensFilter {
 }
 
 func (f *StopTokensFilter) Filter(input analysis.TokenStream) analysis.TokenStream {
-	rv := make(analysis.TokenStream, 0, len(input))
-
+	j := 0
 	for _, token := range input {
 		_, isStopToken := f.stopTokens[string(token.Term)]
 		if !isStopToken {
-			rv = append(rv, token)
+			input[j] = token
+			j++
 		}
 	}
 
-	return rv
+	return input[:j]
 }
 
 func StopTokensFilterConstructor(config map[string]interface{}, cache *registry.Cache) (analysis.TokenFilter, error) {

analysis/token_filters/stop_tokens_filter/stop_tokens_filter_test.go

@@ -71,3 +71,49 @@ func TestStopWordsFilter(t *testing.T) {
 		t.Errorf("expected %#v got %#v", expectedTokenStream, ouputTokenStream)
 	}
 }
+
+func BenchmarkStopWordsFilter(b *testing.B) {
+
+	inputTokenStream := analysis.TokenStream{
+		&analysis.Token{
+			Term: []byte("a"),
+		},
+		&analysis.Token{
+			Term: []byte("walk"),
+		},
+		&analysis.Token{
+			Term: []byte("in"),
+		},
+		&analysis.Token{
+			Term: []byte("the"),
+		},
+		&analysis.Token{
+			Term: []byte("park"),
+		},
+	}
+
+	cache := registry.NewCache()
+	stopListConfig := map[string]interface{}{
+		"type":   token_map.Name,
+		"tokens": []interface{}{"a", "in", "the"},
+	}
+	_, err := cache.DefineTokenMap("stop_test", stopListConfig)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	stopConfig := map[string]interface{}{
+		"type":           "stop_tokens",
+		"stop_token_map": "stop_test",
+	}
+	stopFilter, err := cache.DefineTokenFilter("stop_test", stopConfig)
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		stopFilter.Filter(inputTokenStream)
+	}
+
+}