feat(issue search): query string for boolean operators and phrase search (#6952)

closes #6909

related to forgejo/design#14

# Description

Adds the following boolean operators for issues when using an indexer (with minor caveats)

- `+term`: `term` MUST be present for any result
- `-term`: negation; exclude results that contain `term`
- `"this is a term"`: matches the exact phrase `this is a term`

In all cases the special characters may be escaped by the prefix `\`

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6952
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Shiny Nematoda <snematoda.751k2@aleeas.com>
Co-committed-by: Shiny Nematoda <snematoda.751k2@aleeas.com>

modules/indexer/issues/elasticsearch/elasticsearch.go

@@ -23,6 +23,10 @@ const (
 	// Reference: https://www.elastic.co/guide/en/elasticsearch/reference/7.0/query-dsl-multi-match-query.html#multi-match-types
 	esMultiMatchTypeBestFields   = "best_fields"
 	esMultiMatchTypePhrasePrefix = "phrase_prefix"
+
+	// fuzziness options
+	// Reference: https://www.elastic.co/guide/en/elasticsearch/reference/7.0/common-options.html#fuzziness
+	esFuzzyAuto = "AUTO"
 )
 
 var _ internal.Indexer = &Indexer{}
@@ -145,12 +149,30 @@ func (b *Indexer) Search(ctx context.Context, options *internal.SearchOptions) (
 	query := elastic.NewBoolQuery()
 
 	if options.Keyword != "" {
-		searchType := esMultiMatchTypePhrasePrefix
-		if options.IsFuzzyKeyword {
-			searchType = esMultiMatchTypeBestFields
+		q := elastic.NewBoolQuery()
+		tokens, err := options.Tokens()
+		if err != nil {
+			return nil, err
 		}
+		for _, token := range tokens {
+			innerQ := elastic.NewMultiMatchQuery(token.Term, "title", "content", "comments")
+			if token.Fuzzy {
+				// If the term is not a phrase use fuzziness set to AUTO
+				innerQ = innerQ.Type(esMultiMatchTypeBestFields).Fuzziness(esFuzzyAuto)
+			} else {
+				innerQ = innerQ.Type(esMultiMatchTypePhrasePrefix)
+			}
 
-		query.Must(elastic.NewMultiMatchQuery(options.Keyword, "title", "content", "comments").Type(searchType))
+			switch token.Kind {
+			case internal.BoolOptMust:
+				q.Must(innerQ)
+			case internal.BoolOptShould:
+				q.Should(innerQ)
+			case internal.BoolOptNot:
+				q.MustNot(innerQ)
+			}
+		}
+		query.Must(q)
 	}
 
 	if len(options.RepoIDs) > 0 {