2 files changed, 92 insertions, 0 deletions
diff --git a/internal/text/substring.go b/internal/text/substring.go
new file mode 100644
index 000000000..659b4ab50
--- /dev/null
+++ b/internal/text/substring.go
@@ -0,0 +1,45 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package text
+
+import (
+	"github.com/rivo/uniseg"
+)
+
+// FirstNBytesByWords produces a prefix substring of up to n bytes from a given string, respecting Unicode grapheme and
+// word boundaries. The substring may be empty, and may include leading or trailing whitespace.
+func FirstNBytesByWords(s string, n int) string {
+	substringEnd := 0
+
+	graphemes := uniseg.NewGraphemes(s)
+	for graphemes.Next() {
+
+		if !graphemes.IsWordBoundary() {
+			continue
+		}
+
+		_, end := graphemes.Positions()
+		if end > n {
+			break
+		}
+
+		substringEnd = end
+	}
+
+	return s[0:substringEnd]
+}
diff --git a/internal/text/substring_test.go b/internal/text/substring_test.go
new file mode 100644
index 000000000..f85688218
--- /dev/null
+++ b/internal/text/substring_test.go
@@ -0,0 +1,47 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package text_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/suite"
+	"github.com/superseriousbusiness/gotosocial/internal/text"
+)
+
+type SubstringTestSuite struct {
+	suite.Suite
+}
+
+func (suite *SubstringTestSuite) TestText() {
+	suite.Equal(
+		"Sphinx of black quartz, ",
+		text.FirstNBytesByWords("Sphinx of black quartz, judge my vow!", 25),
+	)
+}
+
+func (suite *SubstringTestSuite) TestEmoji() {
+	suite.Equal(
+		"🏳️‍⚧️ ",
+		text.FirstNBytesByWords("🏳️‍⚧️ 🙈", 20),
+	)
+}
+
+func TestSubstringTestSuite(t *testing.T) {
+	suite.Run(t, new(SubstringTestSuite))
+}