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/internal/qstring.go

@@ -0,0 +1,112 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package internal
+
+import (
+	"io"
+	"strings"
+)
+
+type BoolOpt int
+
+const (
+	BoolOptMust BoolOpt = iota
+	BoolOptShould
+	BoolOptNot
+)
+
+type Token struct {
+	Term  string
+	Kind  BoolOpt
+	Fuzzy bool
+}
+
+type Tokenizer struct {
+	in *strings.Reader
+}
+
+func (t *Tokenizer) next() (tk Token, err error) {
+	var (
+		sb strings.Builder
+		r  rune
+	)
+	tk.Kind = BoolOptShould
+	tk.Fuzzy = true
+
+	// skip all leading white space
+	for {
+		if r, _, err = t.in.ReadRune(); err == nil && r == ' ' {
+			//nolint:staticcheck,wastedassign // SA4006 the variable is used after the loop
+			r, _, err = t.in.ReadRune()
+			continue
+		}
+		break
+	}
+	if err != nil {
+		return tk, err
+	}
+
+	// check for +/- op, increment to the next rune in both cases
+	switch r {
+	case '+':
+		tk.Kind = BoolOptMust
+		r, _, err = t.in.ReadRune()
+	case '-':
+		tk.Kind = BoolOptNot
+		r, _, err = t.in.ReadRune()
+	}
+	if err != nil {
+		return tk, err
+	}
+
+	// parse the string, escaping special characters
+	for esc := false; err == nil; r, _, err = t.in.ReadRune() {
+		if esc {
+			if !strings.ContainsRune("+-\\\"", r) {
+				sb.WriteRune('\\')
+			}
+			sb.WriteRune(r)
+			esc = false
+			continue
+		}
+		switch r {
+		case '\\':
+			esc = true
+		case '"':
+			if !tk.Fuzzy {
+				goto nextEnd
+			}
+			tk.Fuzzy = false
+		case ' ', '\t':
+			if tk.Fuzzy {
+				goto nextEnd
+			}
+			sb.WriteRune(r)
+		default:
+			sb.WriteRune(r)
+		}
+	}
+nextEnd:
+
+	tk.Term = sb.String()
+	if err == io.EOF {
+		err = nil
+	} // do not consider EOF as an error at the end
+	return tk, err
+}
+
+// Tokenize the keyword
+func (o *SearchOptions) Tokens() (tokens []Token, err error) {
+	in := strings.NewReader(o.Keyword)
+	it := Tokenizer{in: in}
+
+	for token, err := it.next(); err == nil; token, err = it.next() {
+		tokens = append(tokens, token)
+	}
+	if err != nil && err != io.EOF {
+		return nil, err
+	}
+
+	return tokens, nil
+}