Implement expansion of curly braces

These patterns are used in DependencyPattern, such as in conflict
declarations, as well as in doc/pkg-vulnerabilities.

v23/util.go

@@ -1018,6 +1018,42 @@ func hasBalancedBraces(text string) bool {
 	return n == 0
 }
 
+// expandCurlyBraces expands "a{b,c}d" to ["abd", "acd"].
+// The braces in s must be balanced, see hasBalancedBraces.
+func expandCurlyBraces(s string) []string {
+
+	find := func(i int, b byte) int {
+		n := 0
+		for ; i < len(s); i++ {
+			switch {
+			case (s[i] == '}' || s[i] == b) && n == 0:
+				return i
+			case s[i] == '{':
+				n++
+			case s[i] == '}':
+				n--
+			}
+		}
+		return i
+	}
+
+	lbrace := strings.IndexByte(s, '{')
+	rbrace := find(lbrace+1, '}')
+	if lbrace == -1 || rbrace == len(s) {
+		return []string{s}
+	}
+
+	var expanded []string
+	pieceStart := lbrace + 1
+	for pieceStart < rbrace+1 {
+		pieceEnd := find(pieceStart, ',')
+		word := s[0:lbrace] + s[pieceStart:pieceEnd] + s[rbrace+1:]
+		expanded = append(expanded, expandCurlyBraces(word)...)
+		pieceStart = pieceEnd + 1
+	}
+	return expanded
+}
+
 var pathMatchers = make(map[string]*pathMatcher)
 
 type pathMatcher struct {

v23/util_test.go

@@ -908,6 +908,30 @@ func (s *Suite) Test_shquote(c *check.C) {
 	test("~", "'~'")
 }
 
+func Test_expandCurlyBraces(t *testing.T) {
+	tests := []struct {
+		s    string
+		want []string
+	}{
+		{"}", []string{"}"}},
+		{"{}{}{}{}{}{}{}", []string{""}},
+		{"{}}", []string{"}"}},
+		{"{}{}{}{}{}{}{}}", []string{"}"}},
+		{"{thir,f{our,if}}teen", []string{"thirteen", "fourteen", "fifteen"}},
+		{"pkgname<=1.0", []string{"pkgname<=1.0"}},
+		{"{pkgname,pkgname-client}<=1.0", []string{"pkgname<=1.0", "pkgname-client<=1.0"}},
+		{"a{b,c,{d,e}}f", []string{"abf", "acf", "adf", "aef"}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.s, func(t *testing.T) {
+			got := expandCurlyBraces(tt.s)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("got %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
 func (s *Suite) Test_LazyStringBuilder_WriteByte__exact_match(c *check.C) {
 	t := s.Init(c)